Dietary Protein, Chronic Salt-Sensitive Hypertension, and Kidney Damage

Abstract

It has been estimated that over a fifth of deaths worldwide can be attributed to dietary risk factors. A particularly serious condition is salt-sensitive (SS) hypertension and renal damage, participants of which demonstrate increased morbidity and mortality. Notably, a large amount of evidence from humans and animals has demonstrated that other components of the diet can also modulate hypertension and associated end-organ damage. Evidence presented in this review provides support for the view that immunity and inflammation serve to amplify the development of SS hypertension and leads to malignant disease accompanied by tissue damage. Interestingly, SS hypertension is modulated by changes in dietary protein intake, which also influences immune mechanisms. Together, the evidence presented in this review from animal and human studies indicates that changes in dietary protein source have profound effects on the gut microbiota, microbiota-derived metabolites, gene expression, immune cell activation, the production of cytokines and other factors, and the development of SS hypertension and kidney damage.

Introduction

Diseases of the heart, stroke, and kidney comprise the most common cause of death among all sexes, ages, and races in the United States. In 2021, these diseases accounted for over 17,000 deaths per 100,000 individuals in the population.¹ High BP, or hypertension, is a primary modifiable risk factor for cardiovascular, cerebrovascular, and kidney disease.² Concurrently, hypertension has been reported to be the largest individual contributing factor to disease and mortality worldwide,^3–5 and participants aged 30 years with BP >140/90 mm Hg lose approximately five cardiovascular disease-free years during their lives compared with age-matched normotensive participants.⁶ Hypertension is also a primary risk factor for CKD; approximately 37 million people in the United States, more than one in seven, are estimated to have CKD.⁷

The American College of Cardiology/American Heart Association BP guidelines indicate that over 45% of the United States population is hypertensive.⁸ Multiple factors contribute to increased BP,⁵ but the influence of diet is gaining increased recognition.⁹ A comparative risk assessment estimated over 20% of worldwide deaths in 2017 were attributable to dietary risk factors; remarkably, cardiovascular disease was the leading cause of diet-related deaths.¹⁰ Accordingly, evidence-based dietary guidance indicates that the consumption of fruits and vegetables, whole-grain foods, and healthy sources of protein can promote cardiovascular health.⁹

These observations highlight the potential of dietary modifications to prevent and treat high BP and associated kidney disease. Experimental studies from our laboratory have shown that dietary interventions (e.g. changes in dietary protein) can alter the severity of chronic salt-induced hypertension and renal damage in a process dependent on the gut microbiota and the immune system. This review examines the potential interplay between these novel mechanisms as potential links between diet, hypertension, and renal damage.

Influence of Dietary Protein and Other Macronutrients on Hypertension and Renal Damage

Epidemiological studies correlated the consumption of diets with elevated sodium chloride (salt), carbohydrate, saturated fat, and cholesterol with hypertension.^11–13 The effects of diets with elevated protein content are somewhat controversial, with evidence supporting protein-induced decreases^13,14 and increases¹⁵ in BP. In humans with preexisting renal insufficiency, it is clear that high protein diets accelerate a decline in renal function^16–18 while a diet with reduced protein improves outcomes related to total kidney failure.¹⁹ It is unclear, however, if diets with reduced protein improve clinical outcomes related to CKD.²⁰ Furthermore, the source of dietary protein is also important because different proteins are associated with varying degrees of disease susceptibility. Observational studies compared the influence of animal versus plant protein consumption on overall cardiovascular health. Vegetarians have lower BP than omnivores²¹ while vegetable protein intake inversely correlates with BP.²² This observation was confirmed by the Optimal Macronutrient Intake Trial to Prevent Heart Disease and the interventional dietary approaches to stop hypertension trial which demonstrated the BP benefit of diets rich in plant protein.^23,24 It is important to note that the generic animal versus plant distinction is an oversimplification with the concomitant contribution of other environmental factors including the consumption of nonprotein dietary components²⁵ and genetic influences.²⁶

BP has been shown to be modulated by the amount of dietary fat,^27–30 carbohydrate,^30–32 and protein³³ in animal models of hypertension. Recent investigations in our laboratory have used the Dahl salt–sensitive (SS) rat as a model of chronic high BP and kidney disease to examine the importance of animal-based and grain-based diets in hypertension and associated end-organ damage. Studies demonstrated that Dahl SS fed an animal-based diet had a significantly greater increase in BP and renal damage when fed high salt in comparison with Dahl SS fed a grain-based diet.³⁴ Additional studies attributed the effect of the animal versus grain diet specifically to the difference in the source of dietary protein (casein versus wheat gluten),³⁵ revealing a distinct contribution for both the immune system^36–38 and the maternal environment.^39,40 The association data in humans, together with experimental data in animals, provide compelling evidence that dietary protein is an important determinant of SS hypertension and kidney damage.

Dietary Protein and Immune Mechanisms

Immune mechanisms amplify vascular disease, kidney disease, and hypertension in humans and experimental animal models.^41–44 A potentially effective approach to modulate inflammatory disease is through dietary modification. A randomized interventional study compared the relationship between dietary protein intake and inflammation. The Metabolic Syndrome Reduction in Navarra study compared a high protein diet (30% energy from protein) with a diet based on American Heart Association guidelines (15% of energy from protein) and revealed a direct relationship between dietary protein intake and inflammation. Moreover, the relationship was specific for increases in animal/meat protein, but not for increases in fish or vegetable protein intake.⁴⁵ Additional evidence linked increased dietary protein from animal sources to an enhanced incidence of inflammatory bowel disease⁴⁶ while increased protein in the diet, particularly from red and processed meats, increases potential relapse of ulcerative colitis.⁴⁷ It seems clear that increased dietary protein enhances gut inflammation, but less is known regarding the influence on other organ systems.

The link between dietary protein and inflammation holds great interest because immune mechanisms are implicated in the pathology of many different cardiovascular diseases. Altered oral tolerance, the mechanisms whereby the immune system does not normally respond to orally administered antigens,⁴⁸ could link dietary protein and inflammatory responses. When oral tolerance does not occur, immune reactivity to components of the diet can lead to food allergy. An example illustrating the potential role of dietary protein to influence oral tolerization was provided in mice pretolerized by oral administration of ovalbumin. Mice pretolerized to ovalbumin by oral administration demonstrated an attenuated inflammatory response as assessed by reduced serum immunoglobulin E (IgE). Interestingly though, placing these mice on a high protein diet resulted in increased IgE titers, enhanced splenocyte production of cytokines driving T-lymphocyte differentiation, and increased LPS-induced proliferation.⁴⁹ This example illustrates that dietary proteins may enhance systemic immune responses. Further demonstration of the importance of dietary protein and oral tolerance was provided when germ-free mice were fed an elemental, antigen-free (i.e., lacking whole protein) diet. Compared with mice fed solid food, these animals have lower levels of serum immunoglobulins and fewer intestinal lymphocytes—primarily because of a depression in CD4+ memory T cells accompanied by a reduction in Foxp3⁺ CD4⁺ T-regulatory cells (Tregs). A restoration of these immune cell populations occurred when mice were switched to a solid chow containing whole proteins, but not when switched to an amino acid diet.⁵⁰ Combined, these studies indicate that antigens in the diet, in the form of whole proteins, may play a role in the differentiation of immune cell subsets, particularly T lymphocytes, and the proper production of intestinal Tregs. This mechanism is important to consider when investigating the relationship between dietary protein, inflammation, and cardiovascular disease.

Dietary Protein and the Gut Microbiota

In addition to mechanisms whereby ingested antigens may directly lead to activation of immune mechanisms, diet can also affect intestinal bacteria. The consumption of different foods can drive the gut microbial composition; the resultant microbial metabolites can then have systemic effects. Comparing the fecal microbiota of Western diet–fed, European children with that of children in rural Africa, who consumed a high-fiber diet, demonstrated a greater Firmicutes-to-Bacteroidetes ratio, unique bacterial speciation, and greater short-chain fatty acid release in children fed the diet enriched in fiber.⁵¹ Furthermore, the human microbiota rapidly responded to a switch between an herbivorous and carnivorous bacterial profile in adults placed on an exclusive plant-based or animal-based diet.⁵² These data highlight the impact of diet to drive the microbial composition, and the microbiota can serve as a major determinant of the physiologic effects of diet. For dietary protein, the microbiota participate in proteolysis,⁵³ and the ability of the microbiota to ferment dietary proteins is essential for amino acid balance, utilization, and bioavailability.^54,55 The composition of the gut microbiota and the diet play reciprocal roles critical for maintenance of host health. Interestingly, proteolytic fermentation involves multiple metabolic pathways and results in the production of diverse metabolites, including short-chained and branch-chained fatty acids, ammonia, amines, hydrogen sulfide, phenols, and indoles.⁵⁶

The microbial products may have (patho)physiological effects. For example, short-chain fatty acids may modulate host physiology by stimulating olfactory receptor Olf78 and G protein-coupled receptors GPR41, GPR43, and GPR109A.^57–62 Changes in gut microbial composition, and presumably altered metabolites, are also linked to changes in DNA methylation in metabolic syndrome,⁶³ inflammatory bowel disease,⁶⁴ and colorectal cancer.⁶⁵ Such epigenetic changes would presumably alter gene transcription resulting in altered function. The source of dietary protein is an additional variable which has been demonstrated to influence protein digestion and microbiota composition.^66–68 Interestingly, casein has been shown to increase microbial density and decrease microbial diversity, leading to worsened inflammatory bowel disease in a mouse model of dextran sulfate sodium-induced colitis.⁶⁹ The emerging role of immunity and inflammation is a likely link between the diet, the microbiota, and the pathology of disease.

Experimental animal and human data demonstrate a relationship between the microbiota and hypertensive disease. Both hypertensive humans and animal models exhibit gut dysbiosis, decreases in microbial diversity, and the hallmark deleterious increase in Firmicutes-to-Bacteroidetes ratio compared with normotensive controls.^70,71 Experimental data indicate that pathogenic factors are derived directly from the microbiota of hypertensives because normotensive rats receiving cecal transplant from spontaneously hypertensive stroke-prone rats developed hypertension and increased the Firmicutes-to-Bacteroidetes ratio on transfer.⁷² In addition, fecal transplant from hypertensive humans elevated BP in germ-free mice.⁷³ Recently, immune mechanisms have been linked to the microbiota and disease development. SS hypertension in a mouse disease model was linked to a depletion of Lactobacillus murinus and occurs in a T-helper 17 (Th17) cell–dependent manner.⁷⁴ Moreover, gut dysbiosis observed in hypertensive humans and experimental mice has been associated with increased intestinal inflammation and activation of antigen presenting cells.⁷⁵ The links between dietary protein consumption, the microbiota, and immune activation provide a potential mechanism for modulation of hypertension and renal damage.

Role of Immune Mechanisms as Mediators of SS Hypertension and Renal Damage

The importance of immune mechanisms as modulators of vascular disease, kidney disease, and hypertension in humans and experimental animal models has been appreciated for many years.^41–44 Of particular interest is the role of immunity in SS hypertension. Work in our laboratory has been focused on the Dahl SS rat.^41,42 Similar to SS humans, the Dahl SS develops hypertension, renal histological damage, and albuminuria when fed high salt. The renal damage parallels the albuminuria observed in SS hypertensive humans when compared with salt-resistant participants. Interestingly, the damaged kidneys of hypertensive Dahl SS rats after high salt feeding contain increased macrophages, T cells, and B cells which surround glomeruli, vessels, and tubules in the kidney.^41,42 Importantly, the localization of immune cells in the renal interstitial spaces adjacent to damaged tubules, vessels, and glomeruli of the Dahl SS is similar to that observed in the kidneys of hypertensive participants.⁷⁶ The treatment of Dahl SS with immunosuppressive agents prevented infiltration of T cells in the kidney and attenuated SS hypertension and renal damage in the Dahl SS rat.^41,42 This finding is consistent with the protective effects of immunosuppressive treatment in multiple other models of hypertension in animals^41–44 and observations in humans.⁷⁷ Subsequently, genetic editing strategies in Dahl SS targeted individual immune cell types to examine their importance in SS hypertension. Of note, Dahl SS with a selective deletion of T cells demonstrated an attenuation of SS hypertension and renal damage and replacement of T cells by adoptive transfer of splenocytes reproduced the hypertensive and kidney disease phenotype.^41,42 These studies thus demonstrated that adaptive immune mechanisms, mediated by T cells, amplify SS hypertension and kidney damage.

The mechanisms whereby immune cells infiltrate the kidney and mediate tissue damage and hypertension are the subject of intense study. Multiple factors, including increased sympathetic nerve stimulation and increased intrarenal antigens/neoantigens, have been implicated as mediators of immune cell infiltration into the kidney. We hypothesize that immune infiltration is a secondary response to an elevation of arterial pressure mediated by nonimmune mechanisms.^41,42,78 Based on work in hypertensive rats, we hypothesize that the primary elevation of BP is transmitted to the renal vasculature and mediates the infiltration of immune cells into the kidney.^41,42,78 The mechanism is unknown, but we speculate that elevated perfusion pressure results in barotrauma triggering the migration of innate and adaptive immune cells into the kidney tissue near damaged renal tubules and blood vessels. Evidence indicates that infiltrating immune cells release proinflammatory cytokines, free radicals, or other factors that amplify hypertension by increasing tubular epithelial sodium and water reabsorption, constricting the renal vasculature, and/or directly mediating further tissue damage.^{41,42,44,78–80}

Dietary Protein, the Gut Microbiota, Immune Activation, and SS Hypertension/Renal Damage

Studies in the Dahl SS interrogated the mechanistic relationship between dietary protein intake, the gut microbiota, immune activation, and SS hypertension/renal damage. Experiments illustrated that changing the source or the amount of protein in the diet, independent of sodium chloride, altered the magnitude of SS hypertension and related kidney damage.^{34,35,38,81,82} Specifically, Dahl SS fed an animal-based diet develop more severe SS hypertension and renal damage than Dahl SS fed a grain-based diet. Notably, the altered severity of SS disease positively correlated with infiltration of immune cells (predominantly T cells) into the kidney. In addition, pharmacologic or genetic suppression of the immune system blunted the effects of an elevated content of animal-based protein to amplify SS hypertension and renal damage.^36,38 The profound influence of dietary protein consumption to modulate salt sensitivity led us to hypothesize that changes in the gut microbiota, potentially mediated by altered immune activation, can modulate SS hypertension and renal damage.

As described above, Dahl SS fed animal-based and grain-based diets demonstrated significant differences in SS disease.^34,35,37,40 Interestingly, large differences in the composition of fecal microbiota were observed between Dahl SS fed the different diets.⁸² To test the hypothesis that the differences in the microbiota participate in disease development, a fecal material transplant (FMT) was performed from the Dahl SS fed the animal diet to Dahl SS fed the grain diet. The FMT from rats fed the animal-based, prohypertensive diet into the grain-fed rats demonstrated an increase in T cells infiltrating the kidney as well as amplified systolic arterial BP and albuminuria. Because Dahl SS hypertension and renal damage are amplified by T cells,^41,42 the observed alteration in immune cell infiltration in the FMT group may provide the mechanistic link between dietary protein, the microbiota, and SS disease.

Although the mechanisms whereby changes in dietary protein influence immune function are unclear, such changes are consistent with recent observations demonstrating that changes in sodium or other components of the diet may affect immune activation. In both mouse and human, an increase in extracellular sodium concentration induced naïve T cells to polarize to Th17 cells.^83,84 Interestingly, consumption of a high salt diet also altered the gut microbiota and enhanced the induction of Th17 cells in mice and humans.⁷⁴ Experiments indicate that changes in protein intake may also alter immune activation. A marked upregulation of proinflammatory genes was observed in T cells isolated from the kidneys of Dahl SS fed an animal-based diet when compared with T cells isolated from the kidneys of Dahl SS fed a grain-based diet. Interestingly, a downregulation of genes related to metabolism was observed in the T cells in the kidneys of Dahl SS fed a grain diet.³⁷ Together, these studies demonstrate the potential influence of diet-induced changes in the microbiota and gene expression to modulate immune cell function with downstream effects on renal end-organ damage and hypertension.

These animal studies are consistent with findings in humans; vegetable and plant protein–enriched diets reduce BP.^22–24 The influence of elevated intake of animal protein on inflammation provides a potential mechanism to explain these observations.⁴⁵ Similarly, reduced sodium intake in normotensive individuals was shown to associate with decreased peripheral monocytes,⁸⁵ decreased proinflammatory cytokines (IL-6 and IL-23), and increased protective cytokines (IL-10).⁸⁵ Observations in humans and experimental animals thus demonstrate that changes in dietary protein and sodium can alter the microbiota, gene expression, immune activation, and the progression of SS hypertension and related renal end-organ damage.

Summary

It is estimated that 20% of worldwide mortality is attributable to dietary risk factors including high intake of sodium and low intake of fruits and whole grains.¹⁰ The present review briefly summarized the influence of altered sources of dietary protein on the development of SS hypertension and renal end-organ damage. A schematic depicting a hypothesized mechanism whereby dietary protein may modify the development of SS disease is depicted in Figure 1. We propose that an animal-based compared with a plant-based diet leads to differences in the composition of the gut microbiota and thus microbial metabolites. The metabolites can have multiple biological effects but are associated with marked changes in gene expression in immune cells, particularly T lymphocytes. The altered gene expression leads to a proinflammatory or anti-inflammatory profile which subsequently modulates the development of SS hypertension and kidney damage. Much work remains to establish the cause-and-effect relationships in this proposed mechanism. The mechanism(s) whereby changes in dietary protein alter biological function, the role of the microbiota and metabolites, and the effects of immune cell activation or inactivation in disease remain to be explored. A multidisciplinary approach will likely be required to assess and understand this complex relationship in health and disease. It is clear, however, that each of these components is involved in the modulation of SS hypertension and renal damage.

Figure 1.

Hypothesized role of plant-based and animal-based diets to influence the development of hypertension and associated renal damage. Prohypertensive protein sources (i.e., animal-based) lead to gut dysbiosis and the release of microbial-derived metabolites which activate inflammatory pathways. Activated immune mechanisms subsequently led to an amplification of kidney disease and hypertension. Created with BioRender.com.

Disclosures

All authors have nothing to disclose.

Funding

The authors are supported by NIH HL161231, HL166458, and the Georgia Research Alliance.

Author Contributions

Writing – original draft: Justine M. Abais-Battad, John Henry Dasinger, David L. Mattson.

Writing – review & editing: Justine M. Abais-Battad, John Henry Dasinger, David L. Mattson.