Rethinking Obesity Through the Lens of Lipid Spillover

Summary

Conventional models frame obesity as excess caloric intake but do not explain why cardiometabolic disease develops in only a subset of individuals. Building on Unger’s seminal lipotoxicity hypothesis and subsequent adipose expandability frameworks, the lipid spillover concept emphasizes adipose tissue dysfunction—rather than total fat mass—as the driver of disease. When subcutaneous adipose tissue reaches its safe storage capacity, adipocyte hypertrophy, inflammation, and insulin resistance increase lipolysis, raising circulating free fatty acids and promoting ectopic lipid deposition. Accumulation of fat in organs such as the liver, heart, skeletal muscle, pancreas, vasculature, and kidneys initiates organ-specific injury and systemic metabolic–vascular dysfunction that may occur independently of body mass index. Visceral, perivascular, and epicardial fat depots act as inflammatory reservoirs that exacerbate vascular disease and myocardial dysfunction. Advances in imaging and biomarker profiling now enable quantification of ectopic fat burden and activity, supporting risk stratification beyond body size alone. Therapeutic strategies that preferentially reduce ectopic fat through lifestyle intervention, pharmacotherapy, or bariatric surgery, offer targeted cardiometabolic risk reduction.

Contemporary models illustrate obesity merely as caloric excess; however, they fail to explain why only a proportion develop atherosclerosis or type 2 diabetes mellitus and others remain metabolically healthy. The lipid spillover hypothesis, building on Unger’s lipotoxicity paradigm¹ and Shulman’s seminal work² linking ectopic lipid accumulation to insulin resistance, conceptualizes obesity as a disorder of adipose tissue function rather than total fat mass. When the storage capacity of subcutaneous adipose tissue (SAT) is exceeded, because of adipocyte hypertrophy and inflammation, excess lipids leak into the circulation and accumulate in nonadipose organs.^1,2 This ectopic deposition initiates insulin resistance, inflammation, and organ dysfunction, which ultimately results in cardiometabolic disease.^1,2 Under normal conditions, adipose tissue acts as an energy buffer, storing excess energy as triglycerides after meals and mobilizing these stores during fasting through lipolysis, releasing free fatty acids (FFAs) into the circulation.³ With chronic caloric surplus, this buffering capacity becomes saturated: adipocytes enlarge, develop hypoxia, attract macrophages, and become insulin resistant.³ This is followed by lipolysis; resulting in an oversupply of circulating FFAs, which are redirected to systemic organs, resulting in metabolic injury. In this translational perspective, we synthesize foundational lipotoxicity and adipose expandability paradigms with emerging clinical evidence to present an integrated framework of adipose dysfunction and “lipid spillover.” We further discuss how this model may refine cardiometabolic risk stratification and therapeutic targeting.

Adipose Tissue Expandability

Adipose tissue “expandability” refers to its ability to accommodate excess lipid safely, thereby preventing downstream metabolic dysfunction. SAT, particularly in gluteofemoral regions, can increase by forming new adipocytes.⁴ Once this adaptive reserve is exhausted, further storage occurs through hypertrophy, precipitating hypoxia and fibrosis. This is the basis of pathogenesis which results in lipolysis and raised plasma FFA levels, initiating ectopic fat deposition. Importantly, lower body SAT is metabolically protective, whereas upper body and abdominal SAT release more FFAs into the circulation.^3,4 Failure of lower body expansion therefore amplifies the spillover state. This has been shown in populations with type 2 diabetes mellitus, in which Almandoz et al⁵ used dual-isotope infusions to quantify dietary fat spillover into plasma FFAs, showing that fractional spillover correlated inversely with leg fat and positively with trunk-to-leg fat ratio. Individuals with less lower body fat exhibit greater systemic lipid overflow (Central Illustration).

Central Illustration.

The Lipid Spillover Hypothesis and Ectopic Fat Disease

The lipid spillover mechanism outlines how adipose dysfunction leads to ectopic fat accumulation across multiple organs including the liver, heart, vasculature, pancreas, skeletal muscle, and kidneys. These depots contribute to various organ-specific pathology, including hepatic steatosis, myocardial fibrosis and diastolic dysfunction, endothelial injury and atherosclerosis, β cell dysfunction, insulin resistance, and hypertension. All these mechanisms collectively drive cardiometabolic disease. FFA = free fatty acid; RAAS = renin-angiotensin-aldosterone system.

Ectopic Fat and Organ Dysfunction

There are several key organs that are affected by ectopic fat deposition, including the heart, liver, pancreas, kidneys, vasculature, and skeletal muscle.⁶ With regard to the liver, elevated portal FFAs and reduced β-oxidation promote hepatic triglyceride accumulation, leading to what is well known as metabolic dysfunction–associated steatotic liver disease.⁶ Hepatic steatosis worsens insulin resistance through excess gluconeogenesis, linking obesity to diabetes mellitus and cardiovascular disease independent of body mass index (BMI).⁶ Visceral adipose tissue and perivascular adipose tissue (PVAT) are highly vascularized and secrete proinflammatory adipokines such as tumor necrosis factor α, interleukin-6, and resistin.⁶ Under spillover conditions, PVAT loses its anti-inflammatory functions, contributing to endothelial dysfunction and plaque formation.⁶ Epicardial adipose tissue (EAT) demonstrates this process: continuous with the myocardium and coronary arteries, it transforms from an energy reservoir into an inflammatory depot promoting plaque instability and myocardial fibrosis.⁶ Numerous imaging studies confirm associations between EAT volume, coronary plaque burden, atrial fibrillation, and heart failure.⁷ In skeletal muscle, lipid oversupply produces intramyocellular triglyceride accumulation that impairs insulin signaling via several pathways.⁶ Similarly, spillover into the pancreas causes β cell lipotoxicity, reducing insulin secretion, while renal sinus fat compresses intrarenal vessels and activates the renin-angiotensin-aldosterone system, contributing to hypertension.⁶ These organ-specific consequences transform lipid overflow into systemic disease processes.

Spillover and Cardiovascular Disease

Systemic excess of FFAs and ectopic triglycerides induce reactive oxygen species, which in turn activate nuclear factor κB signaling and cytokine release.⁸ These processes lead to endothelial dysfunction, characterized by reduced nitric oxide bioavailability and increased monocyte adhesion, all early hallmarks of atherogenesis.⁸ The myocardium, which normally depends on fatty acid oxidation, becomes overwhelmed, leading to lipid accumulation, mitochondrial injury, and contractile dysfunction.⁸ Accordingly, “cardiac lipotoxicity” describes the mismatch between fatty acid delivery and myocardial oxidative capacity, leading to mitochondrial injury, lipid accumulation, calcium dysregulation, and impaired diastolic and electrical function.⁸ Perivascular inflammation further accelerates foam cell formation and smooth muscle proliferation, causing plaque vulnerability.⁸ These changes align insulin resistance, dyslipidemia, and atherosclerosis along a shared continuum of metabolic–vascular injury initiated by lipid overflow.

Quantifying Lipid Spillover

Advances in noninvasive imaging have allowed for quantification of lipid spillover. Specifically, magnetic resonance imaging and proton magnetic resonance spectroscopy quantify intracellular triglyceride content in the liver, skeletal muscle, and myocardium,⁹ while computed tomography precisely assesses visceral, epicardial, and perivascular fat.¹⁰ Low-attenuation values identify lipid-rich, inflamed depots linked to adverse outcomes.¹⁰ Positron emission tomography can identify fatty acid turnover and inflammatory activity within these depots. In addition, biochemical profiling such as elevated fasting FFAs, acylcarnitine, and ceramides indicate heightened lipid flux and oxidative stress.^8,11 These tools allow deep phenotyping of spillover severity and its cardiometabolic consequences.

Therapeutic Implications

There are several therapeutic methods to reduce ectopic fat disease; namely lifestyle modifications, pharmacotherapy, and bariatric surgery. Even modest weight loss through caloric restriction or physical activity redistributes fat away from visceral depots and improves insulin sensitivity.¹² Exercise has been shown to enhance mitochondrial oxidation and promotes lipid utilization independent of total weight change.¹³ Pharmacologically, drugs such as thiazolidinediones, sodium-glucose cotransporter-2 inhibitors and glucagon-like peptide-1 receptor agonists preferentially reduce visceral and EAT while improving cardiac structure and outcomes.¹⁴ Further, statins and fibrates lower triglycerides and improve endothelial function, indirectly mitigating spillover-related vascular injury. Gaborit and colleagues¹⁵ have shown that bariatric surgery provides the most rapid reversal of ectopic fat, normalizing hepatic and epicardial lipid content even before major weight loss driving metabolic recovery. There are also several experimental approaches, such as activation of brown and beige fat, and anti-inflammatory therapies targeting NLRP3 or interleukin-1β, which all aim to restore metabolic health.¹⁶

Ethnic Perspectives

Ethnic variations also modulate vulnerability to lipid spillover and cardiometabolic disease. South Asian populations consistently exhibit greater visceral and hepatic fat for a given BMI. This phenotype likely reflects a combination of reduced subcutaneous adipose tissue expandability, genetically patterned body fat distribution, lower skeletal muscle mass, and heightened insulin resistance, all of which may promote earlier ectopic lipid deposition. Large imaging cohorts such as MASALA (Mediators of Atherosclerosis in South Asians Living in America) confirm that ectopic fat volumes predict cardiovascular and mortality outcomes independently of BMI, establishing spillover as a clinically relevant construct.

Future Directions

Future research should integrate metabolomics, lipidomics, and advanced imaging to better map lipid flow and biology. Enhancing adipose biology, through improved angiogenesis, mitochondrial efficiency, and insulin sensitivity, may prevent the threshold that precipitates spillover. Incorporating measures of ectopic fat into risk prediction algorithms could refine prevention strategies, shifting clinical focus from how much fat a person carries to where it is stored.

Conclusions

The “lipid spillover” framework integrates established lipotoxicity and adipose expandability paradigms to explain the transition from metabolically healthy to unhealthy obesity. Cardiometabolic disease therefore reflects the failure of adipose containment, when fat escapes its reservoirs and invades organs designed for energy turnover. Quantifying this spillover may open doors to therapeutics and personalized cardiometabolic prevention.

Funding Support and Author Disclosures

Dr Khanna is a doctoral student supported by the Australian National Health and Medical Research Council and National Heart Foundation postgraduate scholarships. All other authors have reported that they have no relationships relevant to the contents of this paper to disclose.