Obesity-Initiated Metabolic Syndrome and the Kidney
Authors: Susan P. Bagby
Journal: Journal of the American Society of Nephrology (2004)

Metabolic syndrome, originally described in 1988 as "syndrome X" by Reaven et al. (1), has evolved in our collective thinking from a vague association of common chronic disease states to a formally defined cluster of clinical traits with adverse impact on cardiovascular risk (2). The cause is incompletely understood but represents a complex interaction among genetic, environmental, and metabolic factors, clearly including diet (3,4) and level of physical activity (4,5). These abnormalities are mediated by—and interconnected by—complex pathways that affect energy homeostasis at cellular, organ, and whole-body levels. This review focuses on obesity-initiated metabolic syndrome, first to provide a pathogenetic overview of extrarenal metabolic derangements; second to consider predisposing conditions shaped by genetic or environmental factors, including growth constraints in utero; and finally to consider the impact of metabolic syndrome on the kidney in its prediabetic phase. The pathogenesis of hypertension in the context of metabolic syndrome is considered separately in this series. Similarly, central nervous system pathways that contribute to disordered energy homeostasis is addressed in detail by others. The mechanisms of irreversible renal injury from hypertension and overt diabetes are well documented and are beyond the scope of this review; nonetheless, they loom large in the long-term renal future of the patient with metabolic syndrome. The current worldwide epidemic of obesity-initiated metabolic syndrome, with its potential for renal damage, mandates our commitment to early renal protection in the obese and to vigorous prevention of obesity in both pediatric and adult populations. Metabolic Syndrome Defined: A Work in Progress The Adult Treatment Panel III (ATPIII) of the National Cholesterol Education Program (NCEP) (2) defines metabolic syndrome clinically as any three of the following five traits (Table 1): abdominal obesity, impaired fasting glucose (reflecting insulin resistance), hypertension, hypertriglyceridemia, and low HDL cholesterol. In addition, the NCEP ATPIII recognizes prothrombotic and proinflammatory states as characteristic of metabolic syndrome (2). Importantly, as subsequent paragraphs detail, these simple clinical criteria for diagnosis belie the emerging complexity of the underlying metabolic derangements (6). Thus, insulin resistance is viewed as the essential common denominator of metabolic syndrome, regardless of cause. Abdominal obesity, now identified solely by waist circumference criteria (Table 1), is the single most common cause of insulin resistance, and key mechanisms that mediate this pathway are becoming clear (6). Hypertension [defined in this high-risk context as ≥130/85 (2)] and the typical pattern of atherogenic dyslipidemia—hypertriglyceridemia, low HDL cholesterol, and increase in small dense LDL particles (2)—are also likely downstream consequences of insulin resistance with identifiable contributions from specific organs. Clinical criteria also do not emphasize the role of disordered skeletal muscle metabolism in this syndrome or highlight for the clinician the therapeutic power of regular exercise to offset insulin resistance. Finally, concepts now evolving from research advances suggest that the current clinical definition identifies individuals at a relatively advanced stage, well beyond onset of irreversible organ/tissue injury. Consequently, one immediate research challenge is to define, in the temporal evolution of metabolic syndrome, where interventions can both reverse metabolic derangements and prevent the tissue damage that conveys long-term risk. Table 1: Clinical criteria for diagnosis of metabolic syndromePrevalence and Cardiovascular Risks of Metabolic Syndrome On the basis of the Third National Health and Nutrition Examination Survey (NHANES III; 1988 to 1994), the prevalence of metabolic syndrome in the U.S. population ≥20 yr of age is 23.7% (7), rising to >40% in those ≥60 yr of age and in those from specific geographic regions (e.g., south Texas) (8). This compares with a 30.5% prevalence of obesity (body mass index [BMI] ≥30) and a 64.5% prevalence of overweight (BMI ≥25) in the NHANES III U.S. population sample, reflecting marked increases of 7 to 10%, respectively, in the previous decade (9). Among non-U.S. populations, prevalence ranges from 49.4% of 1625 hypertensive individuals in Spain (10), 19.8% in Greece (11), and 17.8% in older Italians (12). The last cohort exhibited a stepwise increase in insulin resistance severity (by Homeostasis Model Assessment) with increasing numbers of metabolic syndrome features (12). Importantly, overt metabolic syndrome is not limited to adults (13). Cruz et al. (14) examined 126 overweight Hispanic children who were aged 8 to 13 and had a family history of type 2 diabetes; 62% exhibited abdominal obesity, and 30% met criteria for metabolic syndrome. As in adults, obesity in the pediatric population is increasing, with Hispanic and non-Hispanic black adolescents at greatest risk (15). The 2002 NCEP ATPIII panel rated metabolic syndrome equivalent to cigarette smoking in magnitude of risk for premature coronary heart disease (2). In epidemiologic studies, metabolic syndrome increases risk of developing overt diabetes (16), cardiovascular disease (17,18), and cardiovascular mortality (17). In a prospective Finnish cohort, both NCEP ATPIII and World Health Organization criteria for defining metabolic syndrome predicted a five- to ninefold increase in risk of new diabetes over 4 yr (16). Lakka et al. (17), in a cohort of 1209 disease-free Finnish men who were aged 42 to 60 yr and followed for >11 yr, found that the presence of metabolic syndrome conferred a three- to fourfold increased risk for death from coronary heart disease. Using NHANES III data, Ninomiya et al. (7) described an approximately twofold increase in myocardial infarction and stroke risk in the presence of metabolic syndrome. Pathogenesis of Obesity-Initiated Metabolic Syndrome The NCEP panel identifies the root causes of metabolic syndrome as overweight/obesity, physical inactivity, and genetic factors (2). Unraveling underlying mechanisms has been complicated by the unique multiorgan complexity of this trait cluster. Fundamentally, the metabolic syndrome reflects disordered energy homeostasis. Just as evolution prepared us well for surviving hypotension but poorly for combating hypertension, it has apparently equipped us for surviving the fast but not the feast. Unger (19,20) described metabolic syndrome as "a failure of the system of intracellular lipid homeostasis which prevents lipotoxicity in organs of overnourished individuals," a system that normally acts "by confining the lipid overload to cells specifically designed to store large quantities of surplus calories, the white adipocytes." Central to the breakdown of this system are (1) exogenous fuel overload, (2) ectopic accumulation of lipid in nonadipose cells (21), and (3) insulin resistance (3,16). To summarize concepts to be detailed below, the evolution of metabolic syndrome seems to proceed not as a linear sequence of events but along a matrix of interconnected pathways that mediate interactions among multiple organs and also link these organs as a functional unit to regulate total-body energy homeostasis (Figure 1). Each organ/cell type is typically both a target and an effector within this matrix. Furthermore, disturbance within this matrix of pathways can be initiated by stimuli acting at any one of multiple sites in the matrix (e.g., in adipocytes, in hepatocytes, in skeletal myocytes), each independently capable of disturbing whole-body fuel homeostasis. However, initiation of metabolic syndrome by obesity, in keeping with the now-recognized role of adipose tissue as an endocrine organ, is characterized by powerful systemic stimuli that together impair energy homeostasis in multiple organs simultaneously, leaving no room for protective compensation. This multiplicity of pathways and targets likely explains the efficacy of obesity as the major generator of metabolic syndrome. Figure 1. : Pathogenesis of obesity-initiated metabolic syndrome. Increased abdominal fat mass yields high circulating free fatty acids (FFA), which drives increased cellular FFA uptake. Reduced release of adiponectin from expanding abdominal white adipose tissue (WAT) reduces mitochondrial FA uptake/oxidation in multiple tissues. Despite increased release of leptin from WAT, which normally also enhances FA oxidation, tissue resistance to leptin further promotes cytosolic FA build-up. As a result, excess intracellular FA and its metabolites (fatty acyl CoA, diacylglyceride) accumulate, causing insulin resistance (see pathway, Figure 2). Organ-specific consequences include increased hepatic gluconeogenesis and reduced skeletal muscle glucose uptake; the latter raises plasma glucose content and stimulates pancreatic insulin release, and hyperinsulinemia ensues. The newly available glucose plus high insulin now comes back full circle to stimulate further WAT lipogenesis. Increasing fat cell size induces release of chemotactic molecules (e.g., monocyte chemoattractant protein-1) with macrophage infiltration plus TNF-α and IL-6 generation. These cytokines generate an inflammatory reaction and enhance adipocyte insulin resistance in WAT.Figure 2. : Intracellular pathways of insulin resistance. Accumulation of FA and its metabolites (fatty acyl CoA and diacylglycerol) induce protein kinase C isoforms, leading to serine/threonine phosphorylation of insulin receptor substrate-1 (IRS-1) on serine 302. This renders the IRS-1 resistant to tyrosine phosphorylation by the activated insulin receptor. As a result, downstream effects of insulin receptor activation—Akt activation and translocation of the glucose transporter Glut 4 to the plasma membrane—is reduced. Glucose uptake is thereby diminished, secondarily decreasing both glucose-derived glycogen synthesis and glucose-dependent lipogenesis. Activation of the JNK pathway by elevated cytoplasmic FA may provide an additional pathway for induction of insulin resistance. Adapted from reference 52.Generation of Obesity-Associated Metabolic Syndrome: Reversible Derangements of the Metabolic Matrix Obesity-initiated metabolic syndrome is consistently associated with specific metabolic abnormalities: high circulating free fatty acids (FFA) (22); increased intracellular lipid content of not only white adipose tissue (WAT) but also hepatocytes, skeletal myocytes, pancreatic β cells, cardiomyocytes, gastrointestinal enterocytes, and vascular endothelial cells (23,24); insulin resistance in (at least) the same list of tissues; and reduced functional activity of two insulin-sensitizing adipokines that promote tissue fuel oxidation, adiponectin (25–27), and leptin (27). As abdominal fat expands, adiponectin is progressively reduced (25) while leptin levels are progressively elevated (28), the latter reflecting tissue leptin resistance (20,29). Although not yet included in formal clinical definitions, additional features are increasingly considered integral to obesity-initiated metabolic syndrome: macrocytic infiltration of WAT (30,31), increase in local and circulating inflammatory markers [C-reactive protein (32), TNF-α (33), plasminogen activator inhibitor-1 (34,35), and IL-6 (36)] and hyperhomocysteinemia (37). How do we integrate these diverse elements into a coherent process that permits rational clinical interventions? New data suggest that excess visceral fat mass alone is sufficient to generate all elements of the metabolic syndrome. Role of Abdominal Obesity Most studies support the view that the metabolic syndrome that now confronts U.S. physicians in epidemic proportions is largely initiated by abdominal obesity. The four most crucial elements that link abdominal obesity to other features of the metabolic syndrome seem to be elevated FFA, reduction in circulating insulin-sensitizing adiponectin, peripheral-tissue resistance to the insulin-sensitizing actions of leptin, and enhanced macrophage infiltration in fat tissue with release of proinflammatory cytokines (Figure 1). Abdominal fat is unique in its metabolic features as compared with peripheral fat depots, exhibiting larger adipocytes that contain more triglyceride (TG) and exhibit greater insulin resistance than smaller adipocytes. Adipocyte resistance to the lipogenic effect of insulin yields higher basal rates of lipolysis with increased release of FFA into the portal venous system. This direct access to the liver (see below) may also contribute to the unique impact of visceral fat on energy homeostasis. Abdominal fat may also secrete less leptin than subcutaneous fat. Thus Cnop et al. (28), comparing lean-insulin resistant, lean insulin-sensitive, and obese insulin-resistant adults, found that leptin levels correlated with increasing subcutaneous—but not visceral—fat mass, proposing yet another metabolic distinction between these two compartments. Abdominal fat mass expansion is also coupled with reciprocally reduced release of adiponectin, a multifunctional collagen-like molecule with potent capacity to stimulate fuel oxidation in peripheral tissues (25). Abdominal fat additionally expresses higher levels of renin-angiotensin system components: increased angiotensinogen and increased angiotensin II (Ang II) AT1 receptors (38). Finally, epidemiologic studies confirm the unique significance of abdominal fat mass in predicting microalbuminuria, diabetes, and overall cardiovascular risk (39). It was this compelling evidence for a unique role of central or visceral obesity—in contradistinction to subcutaneous obesity—that prompted the NCEP ATPIII decision to specify abdominal obesity in the clinical definition of metabolic syndrome. These phenomena originating in abdominal adipose tissue generate the clinical picture that we recognize as metabolic syndrome. The roles of FFA excess and adiponectin deficiency are reviewed below in the context of their actions in individual organs; the role of leptin is addressed subsequently to compare and integrate prevailing views of how insulin resistance evolves. Liver Under conditions of normal energy homeostasis, the liver serves as a short-term energy reservoir, taking up absorbed dietary glucose and FFA, synthesizing/storing glycogen, synthesizing/storing TG, and packaging TG into VLDL. During fasting, the liver must sustain a continuous supply of plasma glucose, acutely by glycogenolysis and later in the fasting period by gluconeogenesis. Secreted VLDL provides ongoing TG and ultimately FA fuel to skeletal muscle, heart, and other peripheral tissues via lipoprotein lipase activity in the vascular space. Effect of Excess FFA on Liver Intracellular FFA content is a function of substrate delivery from the plasma and FFA utilization (efflux into mitochondria for oxidation or cytosolic synthesis of intracellular lipids). With abdominal obesity, the increased FFA released into the portal vein from excess visceral fat lipolysis have direct access to the liver. Because cellular FA uptake is substrate dependent, increased hepatocyte FFA uptake ensues (23). Elevated cytoplasmic FA content leads to hepatic insulin resistance. This process involves competition of FA and glucose for access to mitochondrial oxidative metabolism. The molecular mechanism was recently described by Shulman et al. (40,41) (Figure 2), wherein elevated intracellular fatty acyl CoA activates protein kinase Cθ (PKCθ), causing phosphorylation of serine-302 of insulin receptor substrate-1 (IRS-1). This renders IRS-1 unavailable for tyrosine phosphorylation by the activated insulin receptor and reduces all downstream actions of insulin. As a result, the fasting state is simulated and hepatocyte enzymatic machinery is shifted to favor enhanced hepatic gluconeogenesis at the expense of glycogen synthesis. The consequent increase in liver-derived glucose in plasma leads to hyperinsulinemia, a hallmark of metabolic syndrome in its earliest stage and a marker of insulin resistance. The capacity of the insulin-resistant liver to impair secondarily systemic energy homeostasis is illustrated by transgenic studies introducing an insulin-resistant form of the rate-limiting enzyme of liver gluconeogenesis: phosphoenolpyruvate carboxykinase (42). Creating isolated hepatic insulin resistance led to systemic hyperglycemia, hyperinsulinemia, and a moderate increase in fat mass (42). The last reflects WAT utilization of surplus circulating glucose for insulin-induced lipogenesis. In effect, this represents a redistribution of fuel away from the liver to adipose fat stores. These findings emphasize the potential for activating the abnormal metabolic matrix simply by inducing hepatic insulin resistance and also illustrate the dual role of the liver as target and effector in metabolic syndrome derangements. In dogs that were fed an isocaloric moderate-fat diet, striking visceral obesity was associated with marked reduction in the ability of insulin to suppress hepatic gluconeogenesis, even before any reduction in insulin-stimulated glucose uptake appeared; investigators concluded that hepatic insulin resistance plays a dominant role in the pathophysiologic cascade initiated by abdominal obesity (43). FFA overload also provides substrate for increased hepatic TG synthesis and for TG-rich VLDL assembly and secretion. Although details are beyond the scope of this review, the peripheral metabolism of these VLDL generate a small, dense form of highly atherogenic LDL [reviewed by Avramoglu et al. (44)] along with an increase in plasma TG. In addition, increased hepatic lipase activity in the insulin-resistant state reduces levels of protective HDL-2 cholesterol (45), which is essential to the transport of cholesterol from tissues back to the liver. Thus, hepatic insulin resistance, high plasma TG, and low plasma HDL are pathogenetically linked manifestations of altered lipid regulation in metabolic syndrome. Effect of Adiponectin Deficiency on Liver In addition to the effects of elevated FFA load, the energy-related functions of the liver are profoundly affected by the reduced circulating levels of the adipokine adiponectin. The actions of this multifunctional protein are organ specific and uniformly insulin sensitizing. Adiponectin normally promotes insulin sensitivity in liver in part by enhancing FA oxidation (46); this reduces accumulation of cytoplasmic FA, thereby reducing intracellular FA levels and enhancing insulin action via IRS-1 availability to the insulin receptor. Second, like insulin, adiponectin normally suppresses hepatic gluconeogenic enzymes and induces glycogenetic enzymes. Increase in 5′-AMP-activated kinase mediates these effects of adiponectin (46,47). Conversely, deficiency of adiponectin in states of abdominal obesity directly contributes to insulin resistance by further enhancing accumulation of intracellular FA and FA metabolites and by stimulating hepatic glucose output. The impact of insulin-sensitizing adipokines is apparent from transgenic mouse models that completely lack fat (and thus both adiponectin and leptin) (48). Animals are insulin resistant; the provision of physiologic levels of both adiponectin and leptin fully restores normal energy homeostasis, whereas either alone is only partially effective (48). These studies underscore the regulatory role of fat-derived adipokines and lend logic to the seeming paradox that either too little or too much adipose tissue can lead to insulin resistance (21). Skeletal Muscle Increased circulating FFA also have an impact on skeletal muscle energy homeostasis. Skeletal muscle is normally a major site of glucose and FA uptake, accounting for the bulk of total-body glucose utilization and deriving 60% of resting energy from FA. As in the hepatocyte, increase in intramyocellular FA in skeletal muscle has been shown to impair insulin receptor signaling by PKC-dependent serine phosphorylation of IRS-1; this leads to reduced IRS-1 availability for tyrosine phosphorylation, reducing Glut 4 translocation to the myocyte plasma membrane with consequent reduction in glucose uptake (6). Secondarily, glucose-driven lipogenesis and glycogen synthesis in skeletal myocytes are also reduced. Accordingly, elevated circulating FFA contribute to insulin resistance in both liver and skeletal muscle. As in the hepatocyte, reduced adiponectin secretion secondary to increased visceral fat mass augments insulin resistance in skeletal muscle, also in part via reducing FA oxidation rate, further increasing intramyocellular FA content and impairing insulin action (48). Using magnetic resonance spectroscopy in insulin-resistant offspring of patients with type 2 diabetes, Petersen et al. (49) found evidence of a 30% reduction in mitochondrial oxidative phosphorylation together with impaired muscle FA oxidation and an 80% increase in intramyocellular lipid content. Overt diabetes has also been associated with impaired muscle oxidative capacity (50,51). Finally, the insulin resistance of aging is associated with impaired mitochondrial FA oxidative capacity in skeletal muscle (52). Impaired energy production is a particularly important consequence of insulin resistance in skeletal muscle. Diabetic and prediabetic patients have impaired maximal exercise capacity, reduced maximal oxygen consumption, and slower oxygen uptake at initiation of low-level exercise, potentially contributing to the fatigue and reduced physical activity typical of obesity/insulin resistance (53). Exercise stimulates skeletal muscle oxidative enzymes and activates mitochondrial biogenesis (54). Inactivity would be predicted to reduce basal metabolic rate both by reducing muscle mass and by augmenting defective muscle energy production. The practical physical consequences of these skeletal muscle metabolic abnormalities have not yet been widely studied in metabolic syndrome but are likely to reinforce the vicious cycle of ongoing weight gain and sedentary lifestyle. Pancreas The pancreas is the ultimate arbiter of insulin availability, determining the point at which overt diabetes will occur. Early in the course of metabolic syndrome, hepatic gluconeogenesis stimulates the pancreas to hypersecrete insulin, yielding normoglycemic hyperinsulinemia. Increased FFA uptake by pancreatic cells also increases glucose-induced insulin secretion and modifies expression of peroxisome proliferator–activated receptor-α (PPAR-α), glucokinase, and Glut 2 transporter (23). In the spontaneously obese captive rhesus monkey, hyperinsulinemia is sustained and progressively increases, eventually falling as overt hyperglycemia appears (55). Once hyperglycemia ensues, insulin-secreting β cells become targets of glucotoxicity: reduction in insulin-stimulated insulin secretion, late increase in mitochondrial free-radical production, and lipid overload–induced apoptosis (lipotoxicity; see also below) with progressive loss of β cell mass (56–58). Adverse effects of hyperinsulinemia per se on organ structure and function in the prehyperglycemic phase of metabolic syndrome are not well defined but are relevant to establishing optimum timing of intervention. It is worthy of emphasis that lifestyle interventions that reduce hyperglycemia can markedly decrease progression to overt diabetes (59). Vascular Endothelium Available evidence indicates that the insulin-receptor signaling pathway mediating glucose uptake in vascular endothelium requires stimulation of endothelial nitric oxide (NO) synthase and NO production (60), a potent vasodilatory and antithrombotic stimulus. Comparable endothelial responses—enhanced NO production with vasodilation—are induced by adiponectin (61). These actions mediate a hemodynamic component of energy distribution, enhancing tissue blood flow to optimize nutrient delivery. When insulin resistance ensues, insulin-induced NO production is concomitantly impaired, representing one of several mechanisms linking abdominal obesity/insulin resistance and hypertension. Implications of endothelial dysfunction for hypertension in metabolic syndrome are addressed in detail by Dr. Sowers elsewhere in this issue. Role of Leptin Resistance in Peripheral Organs In addition to FFA excess and adiponectin deficiency, functional leptin deficiency in peripheral tissues is believed to play a significant role in the evolution of obesity-initiated insulin resistance. Leptin is secreted in proportion to body fat mass and, under normal circumstances, signals via central nervous system receptors to attenuate appetite and to enhance sympathetic outflow, the latter stimulating energy utilization and thermogenesis. Increase in fat-derived plasma leptin seen with abdominal obesity states is paradoxically coupled with poorly understood leptin resistance to central appetite-suppressing and to peripheral insulin-sensitizing effects of leptin (see below), thus a functional leptin deficiency. Central pathways of adipocytokines are addressed separately in this series. Unger and colleagues (62) proposed, on the basis of compelling experimental evidence, that peripheral-tissue leptin resistance is a crucial factor leading to insulin resistance in metabolic syndrome (19). They contended that leptin's major role in normal energy homeostasis is not prevention of obesity, as originally conceived, but rather protection of nonadipocytes against the cytotoxicity of intracellular lipid overload during periods of nutrient excess (21,24). Leptin potently activates cellular fuel consumption by stimulating FA oxidation, reducing lipogenesis, enhancing glucose entry and metabolism, and dramatically shrinking fat stores in adipose tissue (63) as well as in muscle and liver cells (24). Accumulation of cytoplasmic FA thus could reflect functional leptin deficiency acting via impaired mitochondrial oxidative capacity and concomitantly enhanced lipogenesis. The ensuing insulin resistance could be viewed as a compensatory cytoprotective response to prevent further accumulation of intracellular lipid, i.e., reduced glucose entry attenuates glucose-derived lipogenesis (21). The mechanisms of peripheral resistance to the fuel-burning actions of leptin are not yet known. In reality, excess FFA in the circulation, leptin resistance, and adiponectin deficiency are likely acting in concert to generate intracellular FA excess, although their precise sequence and relative importance remain to be determined. The biochemical pathways involved in these intracellular processes have been reviewed in detail by Unger (19,21), Petersen and Shulman (64), and Shulman (6). The crucial participation of PPAR-γ, -α, and -δ in mediating tissue-specific actions of adiponectin and leptin—and their alterations in metabolic syndrome—are addressed separately in this series. Meanwhile, Back to Fat Although excess abdominal fat serves to initiate dysfunctional energy homeostasis in multiple other organs, it eventually becomes also a target tissue. It is first a target of excess glucose in the vascular space. Increased glucose availability from liver-based gluconeogenesis and from muscle-based reduction in glucose uptake, together with pancreas-dependent hyperinsulinemia, promote lipogenesis in WAT (42) (Figure 1). This poses the disturbing possibility that abdominal obesity creates a self-perpetuating cycle. Excess cortisol activity is known to shift fat from peripheral (gluteal and subcutaneous) to central visceral depots and to mimic many aspects of metabolic syndrome. It thus is not surprising that glucocorticoid excess has been suspected in the cause of metabolic syndrome. Recently, increased activity (65) and a twofold increased expression of 11 β hydroxy steroid dehydrogenase 1 (11βHSD1) in adipose tissue of nondiabetic centrally obese women (66) were reported. This enzyme acts predominantly to convert inactive cortisone to the active cortisol form, thus generating glucocorticoid at a tissue level. Expression levels of 11βHSD1 were directly proportional to waist circumference and insulin resistance (66). Supporting the relevance of locally generated glucocorticoid in WAT, transgenic mice overexpressing 11βHSD1 in adipocytes faithfully reproduced the metabolic syndrome (67,68), whereas deficient mice were metabolically resistant to high-fat feeding (69). Once again, a change confined to fat tissue induces systemic metabolic dysregulation. Expanding WAT also becomes the primary target of an inflammatory process. Recent studies in mice and human adipose tissue elegantly documented this process and implicated the role of macrophage infiltration and macrophage-derived inflammatory mediators in obesity and metabolic syndrome (30,31). Weisberg et al. (31) demonstrated that increasing body mass and increasing fat-cell volume (i.e., fat content) each correlates linearly with bone marrow–derived macrophage infiltration in WAT and linearly with increased expression of macrophage-linked proinflammatory genes (Figure 3). Xu et al. (30) similarly found that upregulated genes in WAT of obese mouse models were primarily inflammatory genes linked to macrophage infiltration/activation; furthermore, in diet-induced obesity, this inflammatory process within WAT preceded insulin resistance. TNF-α and IL-6 have been shown to induce insulin resistance in vitro and to contribute to insulin resistance in mouse models of obesity (36,70). This in situ inflammatory reaction within WAT therefore may induce/augment insulin resistance in the adipocytes per se, coming full circle in generation of multiorgan energy dysregulation. Figure 3. : Macrocyte infiltration in WAT correlates linearly with body mass index and adipocyte size/fat content (31). In human subcutaneous adipos