Ketones and the Heart: Metabolic Principles and Therapeutic Implications

Abstract

The ketone bodies beta-hydroxybutyrate (ßOHB) and acetoacetate (AcAc) are hepatically produced metabolites catabolized in extrahepatic organs. Ketone bodies are a critical cardiac fuel and have diverse roles in the regulation of cellular processes such as metabolism, inflammation, and cellular crosstalk in multiple organs that mediate disease. This review focuses on the role of cardiac ketone metabolism in health and disease with an emphasis on the therapeutic potential of ketosis as a treatment for heart failure (HF). Cardiac metabolic reprogramming, characterized by diminished mitochondrial oxidative metabolism, contributes to cardiac dysfunction and pathologic remodeling during the development of HF. Growing evidence supports an adaptive role for ketone metabolism in HF to promote normal cardiac function and attenuate disease progression. Enhanced cardiac ketone utilization during HF is mediated by increased availability due to systemic ketosis and a cardiac autonomous upregulation of ketolytic enzymes. Therapeutic strategies designed to restore high-capacity fuel metabolism in the heart show promise to address fuel metabolic deficits that underpin the progression of HF. However, the mechanisms involved in the beneficial effects of ketone bodies in HF have yet to be defined and represent important future lines of inquiry. In addition to use as an energy substrate for cardiac mitochondrial oxidation, ketone bodies modulate myocardial utilization of glucose and fatty acids, two vital energy substrates that regulate cardiac function and hypertrophy. The salutary effects of ketone bodies during HF may also include extra-cardiac roles in modulating immune responses, reducing fibrosis, and promoting angiogenesis and vasodilation. Additional pleotropic signaling properties of ßOHB and AcAc are discussed including epigenetic regulation and protection against oxidative stress. Evidence for the benefit and feasibility of therapeutic ketosis is examined in preclinical and clinical studies. Finally, ongoing clinical trials are reviewed for perspective on translation of ketone therapeutics for the treatment of HF.

Tremendous interest in ketone metabolism has emerged for the past several years driven by a flurry of discoveries demonstrating that ketone bodies not only serve as critical fuel sources but also act as signaling molecules affecting cellular metabolism, inflammation, cell-cell crosstalk, and bioenergetics relevant to almost every organ. In addition, increasing evidence indicates that ketone metabolism may prove to be an exciting new therapeutic target for a wide array of diseases including cancer, obesity, aging, neurodegeneration, inflammatory disorders, and heart failure (HF). General reviews on this topic have recently been published.^1–4 In this review, we focus on the current understanding of ketone metabolism relevant to the heart and vasculature in health and disease.

OVERVIEW OF KETONE BODY METABOLISM

The ketone bodies acetoacetate (AcAc) and D-ß-hydroxybutyrate (D-ßOHB) are endogenously produced metabolites that are particularly abundant during periods of physiological or nutritional stress such as pregnancy, post-exercise, the neonatal period, fasting, long-term starvation, and high fat diets (HFDs), and has been reviewed in detail elsewhere.¹ Circulating ketone bodies are produced almost entirely by hepatocyte mitochondria from fatty acid-derived acetyl-CoA and are transported to extrahepatic tissues primarily for terminal oxidation, although emerging evidence indicates distinct cell-specific roles for each ketone body including intracellular signaling, as well as roles in lipogenic and cholesterologenic pathways.⁵ The primary circulating ketone bodies, AcAc and its redox partner D-ßOHB, exchange in a near equilibrium.

Ketone bodies are synthesized in the liver largely from circulating free (non-esterified) fatty acids.⁶ Hepatocyte mitochondrial ß-oxidation-derived acetoacetyl-CoA (AcAc-CoA) condenses with acetyl-CoA through a reaction catalyzed by the fate-committing mitochondrial enzyme 3-hydroxymethylglutaryl-CoA synthase 2 (HMGCS2) (Figure 1). The 3-hydroxyglutaryl-CoA (HMG-CoA) product is further cleaved by HMG-CoA lyase (HMGCL) to acetyl-CoA and AcAc. Mitochondrial D-ß-hydroxybutyrate dehydrogenase 1 (BDH1) catalyzes the equilibration of AcAc and D-ßOHB in an NAD⁺/NADH coupled reaction. Extrahepatic organs, including heart and kidney, are capable of ketogenesis but in almost all contexts only the liver contributes to endogenously produced circulating ketone bodies.

Figure 1. Hepatic ketogenesis and cardiac ketolysis.

Pathways of hepatic mitochondrial ketogenesis and cardiac mitochondrial oxidation of the ketone bodies acetoacetate (AcAc) and D-ß-hydroxybutyrate (ßOHB). CoA, coenzyme A; TCA, tricarboxylic acid; HMGCS2, 3-hydroxymethylglutaryl-CoA synthase 2; HMG-CoA, 3-hydroxyglutaryl-CoA; HMGCL, 3-hydroxymethylglutaryl-CoA lyase; BDH1, D-ß-hydroxybutyrate dehydrogenase 1; SCOT, succinyl-CoA:3-oxoacid-CoA transferase; NAD, nicotinamide adenine dinucleotide; Lys, lysine; Leu, leucine; CS, citrate synthase; ETC, electron transport chain.

Both ketone bodies are transported across the plasma membrane via monocarboxylate transporters 1 and 2 (MCT 1 and MCT2, encoded by the SLC16A family) into the circulation and further imported back via MCTs into any extrahepatic tissue.⁷ The first reaction in extrahepatic tissue, including the heart, is a reversal of the BDH1 reaction in which D-ßOHB is converted back to AcAc via BDH1 (Figure 1). The fate committing enzyme succinyl-CoA:3-oxoacid-CoA transferase (SCOT, encoded by OXCT1) catalyzes a conversion of AcAc to AcAc-CoA in a succinyl-CoA/succinate coupled reaction. AcAc-CoA further produces two molecules of acetyl-CoA via AcAc-CoA thiolase reaction catalyzed in majority by acetyl-CoA acetyltransferase 1 (e.g., ACAT1). Ketone oxidation occurs due to mass action and rapid turnover of downstream acetyl-CoA in the TCA cycle via citrate synthase, serving as a fuel for the ultimate production of ATP (Figure 1). Importantly, while HMGCS2 is localized in the mitochondria of hepatocytes, SCOT is present in mitochondria ubiquitously, with the exception of hepatocyte mitochondria, ensuring net production by the liver and net consumption in the extrahepatic periphery, including in the heart. Although HMGCS2 expression has been detected in heart, its exact function is unknown given that the heart remains primarily ketolytic (ketone body-disposing) in nearly all circumstances.^8–10

Ketone body oxidation becomes a primary contributor to bioenergetic homeostasis during ketotic (~1 mM or higher) states, during which ketone bodies are oxidized in proportion to their delivery in heart, brain, and muscle until saturation of either uptake or oxidation occurs.¹¹ Although touted as energy-efficient substrates, an energetic requirement for ketone body oxidation at the cellular level has never been demonstrated, even in states of diminished carbohydrate supply. Nonetheless, human inborn errors of ketone body oxidation do result in clinically significant disease.¹² Moreover, ketone bodies also serve as ‘fuel additives’ that fine tune mitochondrial utilization of other nutrients in the heart, which will be developed further below.

KETONE METABOLISM IN THE NORMAL HEART

The mammalian heart has tremendous energy demands given its function as a constant pump throughout the life of the organism. The human heart turns over kilogram quantities of ATP per day.¹³ Cardiomyocytes satisfy energy demands through a dense network of mitochondria capable of high-capacity oxidative phosphorylation.¹⁴ Notably, the fuel and energy storage capacity of the heart is limited. Accordingly, high-capacity fuel import and oxidation machinery is precisely coordinated and dynamically regulated, which is necessary for continuous generation of reducing equivalents for mitochondrial ATP production. The complex fuel and energy machinery involved in cardiac energy homeostasis has been described in detail in a recent comprehensive review by Lopaschuk et al.¹⁵ Fatty acids serve as the main fuel source for the normal adult heart.^16,17 The importance of high-capacity mitochondrial fatty acid ß-oxidation (FAO) for cardiac function is underscored by the occurrence of cardiomyopathy in humans with genetic defects of enzymes within the FAO pathway.¹⁸ In addition, the capacity for FAO is reduced during the development of acquired forms of HF, as described below.

While fatty acids serve as the chief fuel source, the normal adult heart is omnivorous and relies on additional fuels including glucose, lactate, and ketone bodies depending on physiological and nutritional state which in turn influence the concentration of substrate delivered to the cardiomyocyte. For example, under well-fed conditions in which insulin levels are increased, glucose can become a significant substrate via glycolysis and glucose-derived pyruvate oxidation. Conversely, fatty acids, and to a lesser extent ketone bodies, serve as the primary cardiac fuels in the acute fasting state. Recent work by several groups provides key evidence for the importance of glucose oxidation for cardiac function in genetically altered mice.^19–21 Specifically, mice with cardiac restricted loss of the mitochondrial pyruvate carrier (MPC), a critical transporter for mitochondrial glucose oxidation, develop HF.^22–24 Thus, a balance of fuel substrates provides energy for maintenance of cardiac function. The omnivorous nature of cardiac fuel utilization was recently shown in humans by simultaneously obtaining systemic arterial and coronary sinus blood samples to assess metabolite levels.²⁵ This important study demonstrated that multiple substrates are used in the normal human heart including fatty acids, lactate, amino acids, and ketone bodies. Interestingly, glucose was not observed as a key substrate in this study given the subjects were in the fasted state, further demonstrating: (i) the important influence of nutritional state in determining cardiac fuel preferences, and (ii) the governing role that glycolytic intermediate homeostasis may serve in the heart.¹⁹

Studies in many organisms including humans have shown that the heart has a high capacity for oxidation of ßOHB and AcAc. Pre-clinical studies suggested that the rate of myocardial ßOHB oxidation is dependent largely on circulating concentrations. For example, at circulating levels of 2 mM, ßOHB becomes the primary metabolic fuel in healthy isolated working hearts, though ßOHB oxidation did not enhance cardiac efficiency.²⁶ The importance of circulating ketone body concentration for uptake and utilization was also demonstrated in humans under fasted conditions and after ketone supplementation.^25,27 These collective results indicate that the heart has the capacity to voraciously consume ketone bodies.

Cardiac-specific perturbations of key ketolytic enzymes in genetically engineered mice have provided evidence for the importance of ketone body oxidation in heart. Cardiac-specific (cs) deletion of the genes encoding BDH1 and SCOT have been generated.^28,29 Both lines are without cardiac phenotype under normal conditions. However, with nutritional stress (prolonged fasting) csBdh1 knockout (KO) mice develop cardiac dysfunction. In addition, both the csBdh1 and csOxct1 KO mice develop worsened HF compared to wild-type counterparts following pressure overload and ischemia. These results underscore the importance of ketone oxidation under conditions of stress that may alter fatty acid and glucose utilization. In addition, the stress-induced cardiac phenotype of the csBdh1 KO mice indicates that AcAc alone cannot provide sufficient alternate fuel for the heart under stress conditions.

The ketogenic enzyme, HMGCS2, is expressed in cardiomyocytes and upregulated under ketogenic and HFD-fed conditions, although its function in the heart is unclear given that the heart has limited capacity for true de novo ketogenesis.^8,30 Recent studies suggest a role for cardiomyocyte HMGCS2 in cardiac maturation and repair. HMGCS2 was uniquely upregulated in cardiomyocytes programmed to overexpress Yamanaka factors.³¹ Cardiomyocyte-specific overexpression of HMGCS2 phenocopied de-differentiation observed with the Yamanaka factors. In addition, csHMGCS2 KO exacerbated dysfunction and increased infarct size following myocardial infarction (MI) in mice. Notably, cardiac expression of HMGCS2 is upregulated in the postnatal period and is critical for cardiac maturation.¹⁰ Global loss of HMGCS2 drove perinatal mortality and diminished cardiac function that could be rescued by ßOHB supplementation. These observations highlight a potential role for locally sourced ketone bodies as a fuel or paracrine signal, interesting areas for future investigation.

In addition to actions in cardiomyocytes, ketone bodies also influence the function of other cell types that may contribute to cardiovascular health. For example, macrophages lack the ability to oxidize ßOHB due to the absence of BDH1 protein, yet readily oxidize AcAc in a manner that directly influences hepatic fibrosis.^32,33 Whether this pathway is relevant in cardiac macrophages, a highly dynamic and heterogeneous population of cells important for acute and chronic responses to injury, remains to be determined. Absence of ßOHB oxidation by macrophages also preserves the ßOHB pool for signaling roles in these and other immune cells. For example, ketone bodies influenced the formation and maintenance of memory T-cells through upregulation of FOXO1 and PGC-1α via beta-hydroxybutyrylation of H3K9.³⁴ ßOHB is also an agonist for the niacin receptor, GPR109A, and may have salutary anti-atherogenic effects through this target in macrophages.³⁵ ßOHB in cell culture and a low-carbohydrate ketogenic diet (KD) in vivo enhanced T-cell immune capacity in part due to metabolic rewiring of T-cells toward increased oxidative metabolism.³⁶ ßOHB also strengthens the immune response of primary T-cells from severely diseased COVID-19 patients, possibly by promoting the survival and effector functions of CD8+ cells, and the production of interferon-γ by CD4⁺ T cells.^37–39 In many contexts, ßOHB, but not AcAc or butyrate, inhibits NLRP3 inflammasome formation in myeloid cells.⁴⁰ Ketone metabolism may also influence cardiac health through its impact on epithelial and endothelial cells. In intestinal epithelial cells, HMGCS2 is regulated by Wnt/ß-catenin signaling via PPARγ.⁴¹ HMGCS2-derived ßOHB, available locally within the intestinal mucosa, suppressed glycolysis and regulated differentiation of intestinal epithelial cells. In the heart, vascular endothelial cells oxidized both ßOHB and AcAc to generate ATP supporting homeostatic in vitro endothelial cell proliferation and angiogenesis.⁴² Additionally, KD transiently increased cardiac vascular endothelial cell proliferation in vivo to prevent capillary rarefication in a transverse aortic constriction (TAC) pressure-overload model of cardiac hypertrophy. Together, these studies suggest ketone bodies favorably augment inflammatory, immune, and vascular responses in a variety of cell types. The extent to which these mechanisms extend to myocardium and vasculature during cardiac disease remain open questions for future investigation. With the increasing availability of single-cell and single-nuclear sequencing, cell-type specific transcriptional responses to ketosis and interactions between cell populations may be further unveiled in the heart.

KETONE METABOLISM IN THE HYPERTROPHIED AND FAILING HEART

Cardiac Ketone Utilization is Increased in the Hypertrophied and Failing Heart

During the development of HF, metabolic flexibility declines and deficits in FAO and glucose oxidation contribute to impaired energy homeostasis.^{13,15,43–45} These metabolic changes begin during compensated hypertrophy and are accentuated in overt HF.⁴⁶ It is in this setting of alterations in fuel metabolism coupled to pathophysiological stresses that alternative fuels, including ketone bodies, may provide critical support for myocardial energy demands. Two parallel studies, one in mice and one in humans, provided key evidence for enhanced ketone utilization in the hypertrophied and failing heart.^47,48 Unbiased metabolomic, transcriptomic and proteomic profiling of failing mouse and human hearts revealed common signatures for reduced FAO and enhanced ketolysis. Failing mouse hearts had increased abundance of Bdh1 transcript, the enzyme responsible for the first step of ßOHB oxidation, and failing human hearts upregulated expression of OXCT1, the rate limiting ketolytic enzyme. Metabolite changes, including increased C4OH-acylcarnitine and succinate together with reduced succinyl-CoA, were consistent with increased oxidative flux of D-ßOHB. In addition, ¹³C isotope-NMR studies demonstrated increased contribution of ßOHB to the TCA cycle in pressure overload-induced hypertrophied hearts.⁴⁷ Taken together these results demonstrated that the failing heart increases reliance on ketone oxidation in the context of reduced capacity for fatty acid oxidation (Figure 2).

Figure 2. Enhanced cardiac ketone utilization in the failing heart.

Schematic of organ-organ cross talk during heart failure mediating enhanced hepatic ketogenesis and cardiac ketolysis in the setting of diminished fatty acid oxidation. The precise mechanism(s) through which the failing heart induces ketogenesis is unknown, represented as Factor ? in the schematic. Similarly, the precise regulation of cardiac ketolysis and its relationship to non-ketone substrate oxidation is yet to be determined.

Cardiac utilization of ketone bodies is determined largely by systemic availability, although regulation of ketolytic enzymes may also play a role in determining the full capacity of oxidation. Multiple studies have identified increased circulating levels and myocardial uptake of ketone bodies in HF.^25,49 HF is a modestly ketotic state recognized by an increase in exhaled acetone and circulating ßOHB in human patients.^49–54 Given that cardiac uptake of ketone bodies is proportional to circulating concentrations, augmented hepatic ketogenesis is a likely contributor to increased cardiac utilization in HF via increased supply (Figure 2).^25,27 Extraction of ßOHB proportional to supply is increased in patients with reduced LVEF.²⁵ The proclivity of the myocardium to upregulate ketolytic enzymes may also contribute to increased ketone metabolism in HF.^47,48,55 Increased multimerization among oligomeric units of SCOT in mouse hearts exposed to TAC, which could augment SCOT enzymatic activity, suggests that multiple myocardium-autonomous mechanisms contribute to enhanced ketone oxidation in the failing heart.⁵⁶ This and other prospective mechanisms require further validation.

In addition to HF with reduced ejection fraction (HFrEF), increased cardiac ketone utilization has also been observed in conditions of LV hypertrophy and stable ejection fraction secondary to aortic stenosis.⁵⁷ This observation raises the question of whether ketone bodies are also readily utilized in the hypertrophied hearts of patients with HF with preserved ejection fraction (HFpEF). The degree of cardiac ketone utilization in HFpEF may be sensitive to disease etiology and requires additional study.⁵⁸ The role of cardiac ketone metabolism in diabetic cardiac dysfunction is also an area of active investigation. In the setting of impaired glucose utilization in the diabetic heart, ketone bodies could represent an important ancillary cardiac fuel.⁵⁸ In support of this notion, cardiac substrate utilization in patients with type 2 diabetes undergoing cardiac catheterization for cardiac disease, measured by aortic to coronary sinus concentration gradient, demonstrated increased ßOHB and AcAc uptake with no change in FFA and decreased uptake of glucose, lactate, and pyruvate.⁵⁹ This is consistent with blunted glucose utilization in insulin resistant myocardium and compensatory ketone body uptake.

Evidence is also emerging that circulating ketone bodies may have predictive value in HF. Elevated concentrations of exhaled breath acetone are associated with poor outcomes in HFrEF.^60,61 Elevated ßOHB concentration also predicts outcomes in patients with arrhythmogenic cardiomyopathy.⁹ In a prospective study, risk of HFrEF and HFpEF correlated with elevated ßOHB concentrations.⁶² Higher plasma ßOHB was most closely associated with future incidence of HFrEF in women. These results suggest that ketosis may serve as a biomarker of HF severity. However, additional evidence is needed regarding the specificity and sensitivity of ßOHB as a biomarker. For instance, females are generally more prone to ketosis than men even in the absence of HF.^63–65 Additionally, the relationship between endogenous ketosis and response to ketone supplementation is unknown. Future investigations are warranted to carefully delineate circulating metabolite profiles and their prognostic value in the diagnosis and treatment of HF of diverse etiologies.

The observation that ketone body levels are increased in the context of HF raises the intriguing mechanistic question as to how the failing heart communicates with the liver to increase ketogenesis. The answer remains unknown but several potential mechanisms should be considered (Figure 2). One possibility involves centrally mediated effects such as the increased sympathetic drive that occurs in HF. Noradrenaline increases adipose tissue lipolysis which in turn releases free fatty acids that drive hepatic ketogenesis.⁵¹ Additionally, reduced myocardial FAO could shunt fatty acids typically destined for cardiac oxidation toward substrate for additional hepatic ketogenesis. Future studies aimed at testing these and related mechanisms will provide new insight into the control of ketogenesis in disease states and have potential therapeutic implications.

Increased Ketone Utilization in the Failing Heart is an Adaptive Response: The Benefits of Increasing Ketone Delivery in Heart Failure

The finding that the hypertrophied and failing heart increase utilization of ketone bodies set the stage for subsequent inquiries to determine whether this fuel shift is adaptive, maladaptive, or of no consequence to the outcome of HF. Several lines of evidence support the conclusion that the shift to increased ketone oxidation in the failing heart is adaptive and that further increases in cardiac ketone delivery show therapeutic promise. First, mice with either csOxct1 KO or csBdh1 KO have exacerbated cardiac dysfunction following pressure overload.^28,29 In addition, a KD reduced TAC/MI-induced pathologic cardiac remodeling in wild-type mice.²⁸ Second, cardiomyocyte-restricted over-expression of BDH1 attenuated cardiac fibrosis, oxidative stress, and improved systolic function in pressure overload-induced HFrEF.⁵⁵ Third, chronic infusion of ßOHB increased myocardial uptake of ßOHB, reduced pathologic cardiac remodeling, and improved systolic function in a canine tachypacing model of HF.²⁸ Additionally, two distinct ketone ester (KE) diets reduced hypertrophic remodeling and improved systolic function in two separate murine models of HFrEF.⁶⁶ Lastly, ßOHB oxidation contributed significantly to total energy production in pressure overload HF, though cardiac efficiency was not enhanced with ketone oxidation.⁶⁷ These latter results suggest that ketone bodies are not necessarily a preferred fuel but rather a salvage for diminished flux of primary fuels. In summary, the results of studies to date conducted in pre-clinical HFrEF models demonstrate that increased myocardial ketone utilization is adaptive and increasing delivery of ketone bodies to the heart is cardioprotective.

A specific role for ketone metabolism has not been fully established in HFpEF. This is an active area of inquiry aided by the advent of recently developed preclinical models of HFpEF that incorporate pathologic drivers of the human syndrome.⁶⁸ Early work in these models demonstrate therapeutic value of ketone supplementation. A KE gavage increased ßOHB and attenuated inflammation, hyperacetylation and fibrosis in a 3-hit murine model of HFpEF in a similar fashion to a sodium glucose cotransporter 2 inhibitor (SGLT2i).⁶⁹ In a separate HFpEF model of HFD plus N^ω-nitro-l-arginine methyl ester (L-NAME), weekly IP doses of ßOHB mitigated diastolic dysfunction, fibrosis, pathologic remodeling, and inflammation via increased T regulatory cells.⁷⁰ In diabetic db/db mice, ßOHB improved cardiac function associated with increased mitochondrial biogenesis, quality control, O₂ consumption, and resistance to oxidative stress and mPTP opening.⁷¹ Subcutaneous ßOHB administration attenuated microvascular collagen deposition and oxidative stress in STZ diabetic rats.⁷² Several additional indirect lines of evidence argue for a beneficial role of cardiac ketone metabolism in HFpEF. First, ketone bodies increase during treatment with SGLT2i which were recently demonstrated to be beneficial for reducing mortality and HF hospital admissions in HFpEF.^73,74 Second, ketone bodies may serve as an ancillary fuel to rebalance cardiac energetics as demonstrated by uptake in hypertrophied hearts with stable ejection fraction.⁵⁷ In metabolic forms of HFpEF, insulin resistance results in diminished cardiac glucose uptake and a reliance on fat as a primary fuel.⁵⁸ Accumulation of lipotoxic species exacerbates cardiac dysfunction.^75,76 Ketone supplementation could therefore improve cardiac function by alleviating reliance on overactive FAO. Finally, ketone bodies can improve satiety and decrease weight gain and thus mitigate metabolic derangements that contribute to diastolic dysfunction.^77,78 Additional evidence is required to identify the full benefits and limitations of ketosis in HFpEF.

Cardioprotective effects of ketone bodies may also extend to the vasculature. Glomerular filtration rate was increased in healthy and diabetic humans receiving D,L- ßOHB infusion.⁷⁹ Autophagy-dependent ßOHB synthesis induced vasodilation in rats via potassium channels independent of GPR109A.⁸⁰ Circulating ßOHB was suppressed in hypertensive Dahl salt-sensitive rats, and hypertension was ameliorated after restoration of ßOHB by administration of the precursor 1,3-butanediol.⁸¹ Intravascular administration of sodium D-ßOHB dramatically reduced systemic vascular resistance in a tachypacing canine model of HFrEF and in human HFrEF patients.^28,82 Sodium ßOHB infusion also increased myocardial blood flow in healthy humans.⁸³ Cardiac endothelial cells oxidize ßOHB and AcAc to support angiogenesis after pressure overload hypertrophy.⁴² Taken together, ketone bodies may exert protective effects in cardiovascular disease by enhancing vasodilation and improving blood flow.

POTENTIAL MECHANISMS OF KETONE-MEDIATED CARDIOPROTECTION

The demonstration that increasing ketone delivery to the heart has beneficial effects in HF raises the important question of the mechanism(s) of this cardioprotective effect. At least part of the benefit is likely derived from the provision of an ancillary fuel. However, substantial evidence indicates that βOHB and AcAc exert additional effects that could prove beneficial in the setting of HF. The mechanism of the cardioprotective effects of ketone bodies has not been fully defined. This section will outline potential mechanisms (Figure 3) and published results to date.

Figure 3. Potential mechanisms of ketone-mediated cardioprotection.

The ketone bodies acetoacetate (AcAc), D-ß-hydroxybutyrate (D-ßOHB), and L-ß-hydroxybutyrate (L-ßOHB), depicted in center, have pleiotropic effects that act directly on the heart and systemically. A subset or combination of these mechanisms underlie the salutary effects of ketone administration in the setting of heart failure. ATP, adenosine triphosphate; HDAC, histone deacetylase; SIRT, sirtuin; Ac, acetyl; Bhb, beta-hydroxybutyrate; Ca²⁺, calcium; MvO₂, myocardial oxygen consumption.

Ketone Bodies as a Cardiac Fuel

Several lines of evidence support a role for ketone bodies in providing an ancillary fuel for the failing heart in the context of reduced utilization of the chief fuels, fatty acids and glucose. Mice rendered unable to oxidize ketone bodies in the heart developed more severe HF whereas csBdh1 overexpression reduced cardiac dysfunction in pressure overload models.^28,29,55 In isolated cardiac mitochondria, ßOHB improved respiratory efficiency by maintaining a greater membrane potential per O₂ consumed.²⁸ Myocardial ATP content was improved with KE administration in an MI model of HFrEF.⁶⁶ The benefit of ßOHB infusion in a canine tachypacing model of HF was associated with increased cardiac ßOHB extraction and evidence for competition with glucose oxidation.²⁸ This latter observation indicates that the effects of ketone bodies on HF may also involve competition with other myocardial energy substrates that may be less effective in the failing heart. Unlike the well-defined substrate competition between glucose and fatty acids (Randle Cycle), the effects of ketosis on cardiac fuel utilization appears context dependent. Ketone oxidation can supplement or replace oxidation of fatty acids or glucose. In isolated working mouse heart following pressure overload HF, ßOHB increased total energy production without interrupting glucose or palmitate oxidation rates.⁶⁷ In anesthetized swine, ßOHB infusion reduced myocardial oxidation of fatty acids independent of changes in malonyl-CoA arguing for a CPT1-independent mechanism.⁸⁴ In the canine model of tachypacing HF, infusion of sodium ßOHB reduced exuberant glucose oxidation and hypertrophic remodeling,²⁸ raising the possibility that diminished glucose metabolism may contribute to reduced hypertrophy. Consistent with this notion, high FAO flux via ACC2 KO in mice reduced glucose oxidation and ablated pathologic remodeling in phenylephrine (PE) or TAC-induced hypertrophy, suggesting a predilection for glucose-derived carbons to be diverted toward anabolic growth.⁸⁵ ßOHB infusion suppresses myocardial glucose uptake in healthy adults and ketone administration lowers circulating glucose concentrations.^83,86 Together these findings raise the possibility that ketone bodies mitigate pathologic cardiac hypertrophy through antagonizing glucose utilization. Competition between ketone bodies and other fuel substrates can also work in reverse. Murine models of perturbed glucose utilization highlight competition whereby increased glucose import and O-GlcNAcylation diminished BDH1 expression and activity.⁸⁷ In isolated mitochondria or LV myofibers, ketone respiration was minimal when other substrates were available.⁸⁸ The exact hierarchy of fuel substrates may depend on disease etiology and severity, with the intriguing possibility that ketone bodies may reset fuel metabolism by replacing a diminished fuel source or outcompeting excessive pathologic substrate utilization- a fuel additive effect. In the setting of reduced FAO and/or glucose oxidation, ketone bodies may thus provide critical support for myocardial energy demands.

The impact of ketone oxidation on cataplerotic and anaplerotic TCA cycle flux should also be considered. Succinyl-CoA is siphoned for heme synthesis and by SCOT during ketolysis.⁸⁹ Consumption of succinyl-CoA was increased following MI, potentially due to increased ketone oxidation. Restoration of succinyl-CoA levels with 5-aminolevulinic acid improved oxidative phosphorylation capacity and reduced HF after MI. Indeed, ketone bodies cannot be the sole metabolic substrate due to the requirement for an anaplerotic substrate to maintain TCA flux,⁹⁰ suggesting that a combination of fuel substrates may optimally support ketone utilization in the failing heart.

Ketone bodies may also provide a fuel additive effect to failing cardiomyocytes. As an example, the activities of enzymes within the mitochondrial FAO spiral are downregulated during the development of cardiac hypertrophy and HF. This sets the stage for ‘bottlenecks’ within the β-oxidation pathway as well as in the routing of reducing equivalents (NADH, FADH) to complex I versus complex II of the electron transport chain. Such bottlenecking not only contributes to a reduction in the generation of NADH but also could lead to sequestration of key cofactors such as CoA that are necessary for complete oxidation and trafficking of fatty acids. This FAO bottleneck could potentially be alleviated by the more direct route of ßOHB oxidation to acetyl-CoA. Evidence to support this fuel additive effect was provided in isolated mitochondrial studies by the Muoio lab in which addition of ßOHB to fatty acid substrates increased respiratory efficiency.²⁸ Additional studies are required to delineate the cooperative and competitive interactions between ketone bodies and other metabolites in the healthy and failing heart.

Cellular Signaling and Epigenetic Effects

Ketone bodies act through a variety of pathways to improve cell survival, reduce dysfunction, and attenuate cellular proliferation. ßOHB and AcAc have shared and unique signaling properties as has been recently reviewed.¹ ßOHB antagonizes the sympathetic nervous system through interaction with GPR41 and has the potential to reduce atherosclerosis via niacin receptor GPR109A signaling.^91,92 Additionally, ßOHB induced vasodilation through activation of potassium channels in Dahl SS rats.⁸⁰ Potentially through these mechanisms, ketone supplementation reduced total peripheral resistance in the context of increased cardiac output in both animals and humans with reduced ejection fraction.^28,82 Decreased peripheral vascular resistance and reduced afterload may be an important mediator of improved cardiac function and reduced pathologic remodeling in HF. As discussed previously, ßOHB and AcAc also attenuate immune, inflammatory, and fibrotic responses in non-cardiac cells.^{32,34–38,40} Provision of 1,3-butanediol, a ßOHB precursor, attenuated renal NLRP3 inflammasome activity, reduced hypertension, and decreased renal fibrosis in salt-sensitive rats.⁸¹ Cardiac vascular endothelial cells oxidized ßOHB and AcAc to support angiogenesis, and KD transiently prevented capillary rarefication after pressure overload.⁴² In healthy human volunteers, infusion of sodium ßOHB increased myocardial blood flow.⁸³ Ketosis may thus assert additional cardioprotective effects via systemic reduction of inflammation and fibrosis and improved blood flow.

ßOHB was recently demonstrated to reduce tumorigenesis in colorectal cancer via activation of the transcription factor Hopx downstream of GPR109A.⁹³ Hopx is implicated in the regulation of cardiomyocyte proliferation and growth via interactions with serum response factor and the cardiogenic factor GATA4.^94,95 Intriguingly, Hopx is downregulated during pathologic hypertrophy in isolated neonatal rat ventricular myocytes (NRVM) after isoproterenol stress and in mouse hearts following pressure overload, and Hopx expression is decreased in failing human hearts.⁹⁶ As yet unproven, regulation of Hopx via GPR109A could represent a direct signaling action of ßOHB on cardiomyocytes to reduce pathologic cardiac hypertrophy akin to observations with KE administration in HFrEF.⁶⁶

ßOHB has been shown to alter gene expression via several epigenetic effects. At high concentrations, ßOHB inhibits class I histone deacetylases (HDAC) contributing to transcriptional changes that include upregulation of oxidative stress resistance factors.⁹⁷ Class I and II HDAC inhibitors blunted pressure overload cardiac hypertrophy in mice,⁹⁸ which suggests a potential role for ketone-mediated HDAC inhibition. Sirtuins, NAD⁺-sensitive class III HDACs, mediate beneficial metabolic effects of calorie restriction, KD, and SGLT2i in the heart.^99–101 Sirt1 activity in peripheral blood mononuclear cells was positively correlated with circulating ßOHB in HFpEF patients after an exercising training regimen.¹⁰² Cardiac-specific Sirt3 KO induced diastolic dysfunction in mice, while NAD⁺ repletion with nicotinamide riboside improved cardiac remodeling and diastolic dysfunction in a 2-hit HFpEF model.¹⁰³ Interestingly, Sirt3 boosted ketone utilization via increased HMGCS2-dependent hepatic ketogenesis and increased ketone oxidation in the brain through activity of OXCT1 and ACAT1.¹⁰⁴ However, the deacetylase effect of ketosis was independent of Sirt3 in a 3-hit HFpEF model indicating sirtuin-independent regulation of histone deacetylation attributed to downregulation of FAO.⁶⁹ In pressure overload-induced HF, ßOHB was increased in hearts in the absence of ketonemia.¹⁰⁵ Tissue accumulation of ßOHB may also facilitate epigenetic modification via beta-hydroxybutyrylation. Metabolic pathways related to amino acids, redox, and PPAR signaling are regulated by histone lysine beta-hydroxybutyrylation in mouse liver.¹⁰⁶ Additional consequences of histone beta-hydroxybutyrylation are under investigation.¹⁰⁷

A series of studies describe the potential for ketone bodies to attenuate oxidative stress. ßOHB suppressed oxidative stress via HDAC inhibition in HEK293 cells.⁹⁷ Cardiac-specific overexpression of Bdh1 attenuated oxidative stress and apoptosis in pressure overload HFrEF.⁵⁵ ßOHB upregulated thioredoxin 1 in cardiomyocytes, an important antioxidant for protecting the diabetic heart, to reduce oxidative stress.¹⁰⁸ ßOHB reduced oxidative stress by inducing SOD2 and catalase expression via FOXO3a in cardiomyocytes.¹⁰⁵ These responses lead to speculation that transient ketosis is a hormetic response, likely evolved from starvation, to promote resistance to oxidative and inflammatory stress.¹⁰⁹

Taken together, the balance between ketone oxidation and ketone signaling in mediating cardiovascular benefits of ketosis likely depends on cardiac energy requirements and disease etiology. That ketone bodies exhibit an array of pleiotropic effects may prove to be an asset for therapeutic efficacy. Future studies aimed at delineating the relative role of ketone cellular actions in HF will be important.

THE POTENTIAL FOR KETONE THERAPEUTICS: RATIONALE AND IMPLICATIONS

Pre-clinical Evidence Supporting Ketone Supplementation Therapy for Chronic Heart Failure

A variety of ketone supplementation strategies have been investigated to assess efficacy in HF (Figure 4). Ketone augmentation can be implemented via dietary modifications and exogenous KE supplementation or administration of ketone salts. KD consisting of low carbohydrate and high fat induce endogenous ketogenesis by depleting carbohydrates and supplying fatty acid substrate. KDs result in modest ketosis in humans and in preclinical models.^28,110 The disadvantage of KD are the potential deleterious effects of chronic HFD and reduced protein intake. Intermittent fasting could also be considered although this has not been tested in pre-clinical models of HF. Although ßOHB containing diets are not palatable, diets can be supplemented with KEs from which ßOHB or AcAc are liberated after ingestion by endogenous esterases. Formulations of KEs include ßOHB, AcAc, or the ßOHB precursor 1,3-butanediol esterified with molecules generating R-ßOHB (also called D-ßOHB), that are themselves selected for favorable ketogenic and kinetic properties. The most extensively used KE, (R)-3-hydroxybutyl (R)-3-hydroxybutyrate, has been studied for over a decade and has been demonstrated to safely induce ketonemia in humans and preclinical models.^111,112 This KE was found to induce more durable ketosis than ingestion of ketone salts.¹¹³ Generally, ketosis induced by KE may also be preferred to avoid high salt associated with ketone salts. The kinetics of a novel bis-hexanoyl (R)-1,3-butanediol KE were recently investigated to improve dose and duration of ketosis.¹¹⁴ The diester had more sustained ketosis than the monoester, presumably due to slow release of ßOHB following endogenous oxidation of C6 and subsequent hepatic ketogenesis.

Figure 4. Ketone therapeutic strategies.

Methods and considerations for inducing therapeutic ketosis. This figure was created with Biorender.com.

Ketone supplementation has been shown to ameliorate cardiac dysfunction in a variety of preclinical models. Intravenous ßOHB infusion prevented myocardial damage after coronary occlusion in rats.¹¹⁵ Subcutaneous infusion of ßOHB reduced infarct size and protected function after ischemia reperfusion.¹¹⁶ Ketone bodies also directly affect the function and structure of cardiomyocytes. In isolated NRVM, addition of ßOHB or AcAc to culture media reduced pathologic remodeling after PE agonism.¹¹⁷ ßOHB augmented excitation-contraction coupling in NRVM and iPSC-CMs in a manner sensitive to ßOHB enantiomer.^118,119 In a tachypacing model of HF, chronic infusion of sodium ßOHB increased myocardial uptake of ßOHB, reduced overexuberant glucose oxidation, improved cardiac function and remodeling, and lowered systemic vascular resistance.²⁸ In translational experiments, two KE diet formulations, utilizing either a R/S-ßOHB-C6 diester or (R)-3-hydroxybutyl (R)-3-hydroxybutyrate monoester, reduced hypertrophic remodeling and improved systolic function in both mouse and rat models of HFrEF.⁶⁶ The ketone monoester diet improved cardiac dynamics when administered either prior to MI or 2 days after MI, suggesting a therapeutic window following cardiac ischemia. In both models, ketosis fluctuated in a diurnal manner consistent with feeding patterns of mice and rats. The optimal dose of ketone supplementation in terms of peak concentration and duration has not been determined. However, these studies provide evidence that modest diurnal ketosis is sufficient for therapeutic benefit. Future studies should investigate threshold criteria for therapeutic ketosis during HF.

Compared to ketone salts or KE wherein ketosis is achieved through direct administration of exogenous ketone bodies, low carbohydrate KDs rely exclusively on boosting endogenous hepatic ketogenesis. Notably, circulating levels of both βOHB and AcAc are increased by KDs. KDs include a broader spectrum of effects and their efficacy will depend, in part, on properties independent of ketosis such as glucose sparing, low insulin, and high fat. Glucose oxidation has been identified as a critical regulator of myocardial hypertrophy and adaptive response to pathologic stress. KD reversed HF progression in mice with glucose oxidation defect due to MPC deletion.²⁴ KD also reduced pathologic remodeling in a murine TAC/MI model of HFrEF.²⁸ Two formulations of low-carbohydrate diet, one ketogenic and one not, reduced pathologic cardiac remodeling and HF severity following pressure overload.¹¹⁷

Evidence Supporting Ketone Supplementation in Humans to Ameliorate HF

Acute ketone supplementation has been administered to humans in a variety of settings. Infusion of sodium ßOHB robustly increased cardiac output in humans with HFrEF.⁸² The increase in cardiac output correlated with ketonemia and was associated with a dramatic decrease in systemic vascular resistance. These results suggest that acute ßOHB infusion may be effective as a treatment for severe acute HF. Transmyocardial extraction of energy substrates was assessed in patients with moderate HFrEF and non-failing controls before and after ingestion of (R)-3-hydroxybutyl (R)-3-hydroxybutyrate KE.²⁷ Cardiac extraction of ßOHB was greatly increased after KE in proportion to circulating ßOHB concentrations and primarily displaced free fatty acid uptake in the failing hearts. Bis-hexanoyl (R)-1,3-butanediol KE ingestion increased circulating ketone bodies in a dose-dependent manner in healthy adults.¹¹³ In this study, a ketone salt containing racemic ßOHB was also administered. The L-ßOHB enantiomer (also called S-ßOHB), which cannot be oxidized by BDH1, had a longer half-life than the D-ßOHB enantiomer. This could be valuable for dosing paradigms targeting signaling properties that are unrelated to provision of ßOHB as a metabolic fuel. The bis-hexanoyl (R)-1,3-butanediol KE was determined to have a safe toxicology profile in healthy adults though it has yet to be tested in patients with cardiovascular disease.¹²⁰ Relevant to HF, particularly metabolic HFpEF with skeletal muscle insulin resistance, the (R)-3-hydroxybutyl (R)-3-hydroxybutyrate KE robustly induced circulating ßOHB and enhanced VO₂ max in healthy exercising adults.¹²¹ This KE also reduced glycemic response to OGTT in both healthy and obese individuals,^122,123 a finding that could antagonize a pathologic risk factor for metabolic HFpEF. This supports prior evidence from healthy adults in which sodium ßOHB infusion displaced myocardial glucose uptake and increased myocardial blood flow.⁸³ KE ingestion acutely augmented LVEF, global longitudinal strain, heart rate, LA strain, and decreased systemic vascular resistance in healthy volunteers.¹²⁴ However, limitations to KE ingestion were shown by inferiority compared to 72-hour KD for suppression of myocardial glucose uptake in healthy volunteers despite robust ketonemia.¹²⁵ Taken together, several strategies for ketone supplementation show promise in humans. The consequences and compliance of long-term ketone supplementation remain to be evaluated. Future studies of exogenous ketone preparations will resolve the independent impact of the BDH1 substrate D-ßOHB from the non-BDH1 substrate L-ßOHB. L-ßOHB is cleared much less rapidly and may have an extensive number of signaling targets that remain incompletely distinguished from those of D-ßOHB.

Potential Adverse Effects of Therapeutic Ketosis

Benefits of therapeutic ketosis must be balanced with potential risks of endogenous and exogenous mediators of ketone body metabolism. KD is used in the treatment of seizures in pediatric populations with a generally safe profile.^126,127 The most common adverse events are gastrointestinal disturbances, while severe adverse events are rare. In contrast to beneficial findings in multiple preclinical studies, KD-fed diabetic rats had lower myocardial ketone and glucose oxidation than chow-fed diabetic rats.¹²⁸ KD was associated with a trend toward increased myocardial lipid content, collagen deposition, oxidative stress, and hypertrophy. In the context of potential lipotoxic risk in diabetic cardiomyopathy and metabolic HFpEF, KD may be poorly tolerated and exogenous ketone bodies, such as KE, may be the preferred supplementation strategy. Additionally, long durations of KD reduce myocardial ketolytic enzymes and may attenuate efficiency of ketone oxidation.⁸

KE have also been evaluated for safety. The (R)-3-hydroxybutyl (R)-3-hydroxybutyrate KE is safe in rats when administered as 30% of total dietary calories, a substantially higher dose than proposed for therapeutic ketosis in humans.¹¹² Similarly, the (R,S)-1,3-butanediol AcAc diester is tolerated in mice up to 25% of dietary calories, beyond which energy consumption and lean body mass declines.¹²⁹ In humans, the primary adverse event associated with ingestion of (R)-3-hydroxybutyl (R)-3-hydroxybutyrate or (R,S)-1,3-butanediol AcAc diester is gastrointestinal discomfort.^111,130 Bis-hexanoyl (R)-1,3-butanediol KE administered to rats at 12 g/kg/day for 90 days was identified to be without adverse effects.¹³¹ In healthy adults, 25 g/day of bis-hexanoyl (R)-1,3-butanediol KE was safely tolerated for three weeks.¹²⁰ These studies provide evidence for safe administration of KE within a therapeutic range and identify gastrointestinal distress as the most likely adverse event. Long term trials in humans will be necessary to determine the safety, efficacy, and compliance of ketone supplementation strategies for chronic disease management.

Ketosis as a Potential Mediator of SGLT2i Therapy Efficacy

SGLT2i induce a mild ketosis suggesting a possible mechanism of action for their salutary benefits in HF for both diabetic and non-diabetic patients.^132,133 ßOHB and related C4-OH were elevated after 12 weeks of SGLT2i treatment in a placebo controlled trial evaluating circulating metabolites in HFrEF.¹³⁴ SGLT2i induced ketolytic genes, CD36, AMPK-P, and decreased mTOR and insulin signaling while reducing cardiac fibrosis and hypertrophy after TAC in mice.¹³⁵ Two hours of intravascular balloon occlusion induced an increase in myocardial glycolysis and reduced FA uptake in swine.¹³⁶ In this model, SGLT2i induced a switch toward enhanced ketone, FFA and BCAA uptake. In response to SGLT2i, increased ßOHB and decreased insulin attenuated inflammation via reduced NLRP3 activity in macrophages in patients with T2DM and high cardiovascular disease risk.¹³⁷

However, mechanisms other than ketosis may also mediate the cardiometabolic effects of SGLT2i. While empagliflozin increased cardiac energy production in db/db mice, the increase was attributed to enhanced glucose and FAO rather than ketone oxidation.¹³⁸ Indeed, ketosis could be a biomarker rather than mechanism of SGLT2i. Induction of starvation and autophagy programs have been identified as potential mediators of SGLT2i benefit which could occur in a ketogenic state.¹³⁹ That SGLT2i affect nutrient signaling in cell culture, independent of ketosis, argues for at least a partial mechanism of action unrelated to ketone bodies.¹⁴⁰ Additional evidence in knockout models of perturbed ketogenesis or ketolysis are needed to parse the role of ketosis in mediating the beneficial effects of SGLT2i in vivo.

Short-chain Fatty Acids as an Alternative to Ketone Supplementation

Theoretically, fatty acids of chain length C8 or less could bypass a potential CPT1 import bottleneck during HF to provide energy via oxidative metabolism. Oxidation of butyrate, a four-carbon fatty acid, was investigated in isolated perfused rat hearts 14 weeks after sham or TAC surgery.¹⁴¹ Oxidation of butyrate outpaced ßOHB, and both were readily oxidized in the heart at the expense of long-chain fatty acids. However, a functional benefit of this fuel switch was not detected in the isolated perfused hearts. Unlike ketone bodies, there is no readily available dietary manipulation to robustly increase circulating butyrate. Butyrate is endogenously produced at low levels via gut microbiota. Dietary fiber promotes butyrate production, though, due to rapid mucosal and hepatic clearance, peripherally circulating butyrate levels are typically substantially lower than that of ketone bodies. Thus, despite the capacity for high myocardial oxidation rates, butyrate supplementation has yet to be applied to humans at doses consistent with use as an ancillary cardiac fuel. Butyrate shares overlapping signaling properties with ketone bodies but differences may have specific advantages in certain contexts.^1,91,97,142 High affinity for signaling may support a therapeutic role for butyrate even at low circulating concentrations. The consequences of butyrate as a signaling metabolite should be explored further, as should the effects of other short-chain fuels, such as octanoate, as ancillary cardiac fuels during HF.¹⁴³

Clinical Trials Investigating Ketosis as a Mediator of Cardiac Function in HF

Clinical trials are ongoing to explore the relationship between ketosis and cardiac function in HF. ßOHB infusion will be administered across a physiologic range of concentrations to determine the effect of ketosis on cardiac function and glucose uptake in diabetic HFrEF patients (Clinical Trial ID NCT03560323). Cardiovascular effects of niacin ingestion and ßOHB infusion are being compared to investigate the role of GPR109A in mediating previously observed increases in cardiac output after ßOHB infusion (NCT04703361). A KE is being evaluated for its effect on cardiac output and LVEF during acute and stable HFrEF (NCT04442555, NCT04594265). A single dose of KE is being evaluated for its effect on exercise performance and phosphocreatine concentration in HFrEF patients utilizing a ³¹P magnetic resonance spectroscopy (MRS) exercise protocol and placebo cross over design (NCT05348460). Two trials are designed to define the consequence of 14-day KE treatment on cardiac output, exercise capacity, and systemic metabolic changes in non-diabetic HFrEF patients (NCT05161650, NCT05161676). The acute effect of a ketone salt on LVEF and circulating metabolites will be evaluated in non-diabetic HFrEF patients (NCT05651529). Additionally, commercially available ketone salts and an oral KE will be compared for their safety and effect on cardiac output in non-diabetic HFrEF patients (NCT04443426). Finally, a head-to-head comparison of endogenous versus exogenous ketosis is being conducted in HFrEF patients to evaluate changes in LVEF after 10-days of KD versus standard diet supplemented with KE (NCT04921293). These comparison studies will be important to define optimal methods for ketone body supplementation in the setting of HFrEF.

In HFpEF patients, a KE drink is being tested for its ability to improve exercise capacity (NCT04633460). In addition to characterizing peak and submaximal exercise, metrics of systemic vascular resistance, substrate utilization, echocardiographic parameters, safety, and tolerability will be obtained. In type 2 diabetic HFpEF patients, 14 days of KE administration will be evaluated for changes in cardiac output, exercise tolerance, and skeletal muscle palmitate flux and glucose kinetics (NCT05236335, NCT05159570). In obesity-related HFpEF, a low-carbohydrate KD is being evaluated for patient reported outcomes and metabolic characteristics associated with improved HFpEF symptoms (NCT04942548).

Several ongoing trials are investigating the cardiometabolic effects of SGLT2i. Changes in skeletal muscle energetics after SGLT2i are being characterized by ³¹P-MRS in HF patients with type 2 diabetes (NCT05057806). Patients will be evaluated before and after empagliflozin for abundance of high energy phosphates in skeletal muscle, circulating ketone bodies, cardiopulmonary capacity, exercise tolerance, and patient reported outcomes to identify a role of SGLT2i in modulating skeletal muscle exercise capacity via ketosis. Similarly, the effects of ertugliflozin on peak VO₂, cardiac dynamics during exercise, and serum ketone bodies are being evaluated in HFpEF patients (NCT04071626). The goal is to determine the effect of ketosis and peripheral glucose flux on cardiopulmonary fitness following short term SGLT2i therapy. Finally, the efficacy of a combination therapy of empagliflozin and potassium nitrate is under investigation in HFpEF patients (NCT05138575). The rationale is to simultaneously augment energy fuel metabolism and tissue perfusion to improve exercise performance via enhanced oxidative metabolism in skeletal muscle. This intriguing combination hints at future synergies wherein ketone metabolism may be used as an adjuvant for existing therapies targeting inotropy and neurohormonal regulation. Additional investigation will be required to determine the effects of chronic ketosis on HF progression and the potential for reverse remodeling in established disease.

CONCLUSIONS AND FUTURE DIRECTIONS

We are witnessing an explosion of new information regarding the role of ketone metabolism in the healthy and diseased heart. Early in the development of HF, cardiac ketone utilization increases due, at least in part, to increased hepatic ketogenesis. This shift towards increased cardiac ketone utilization is adaptive as evidenced by studies conducted in mice rendered unable to oxidize ketone bodies in the heart and the cardioprotective effects of increasing ßOHB to the failing heart. The mechanisms involved in the cardioprotective effects of ßOHB are currently an area of intense investigation but likely involve provision of an ancillary fuel in the context of reduced capacity for FAO and possibly additional independent cytoprotective effects. First in human studies to assess the safety and efficacy of acute doses of ketone esters in patients with HF are ongoing. Exciting future investigative directions include delineation of the cardioprotective mechanisms involved in ketone body supplementation, the mechanism for increased hepatic ketogenesis triggered by HF, the mechanisms underlying increased utilization of ketone bodies by failing cardiomyocytes, and new approaches to increase ketone body delivery to the heart.

Non-standard Abbreviations and Acronyms

ßOHB

beta-hydroxybutyrate

AcAc

acetoacetate

HF

heart failure

HFrEF

heart failure with reduced ejection fraction

HFpEF

heart failure with preserved ejection fraction

KD

ketogenic diet

KE

ketone ester

SGLT2i

Sodium glucose cotransporter 2 inhibitors

TAC

transaortic constriction

MI

myocardial infarction

HFD

high fat diet

BDH1

D-ß-hydroxybutyrate dehydrogenase 1

SCOT

succinyl-CoA:3-oxoacid-CoA transferase

HMGCS2

3-hydroxymethylglutaryl-CoA synthase 2

FAO

fatty acid ß-oxidation

cs

cardiac-specific

KO

knockout