Studies dating back to the early 1900’s by Moreschi and Rous, and later by McCay and Tannenbaum [59
], respectively, were the first to demonstrate that a reduction in food intake was capable of increasing lifespan and inhibiting tumor formation in rats [61
]. Nearly a century later, calorie restriction (CR) remains the only known behavioral intervention capable of delaying the onset of many age-related diseases and extending maximal longevity [62
]. This finding has since been extended to several diverse organisms including yeast, nematode, water flea, fruitfly, dog, and cow [60
The fact that limiting calorie intake has such profound effects on mammalian aging and disease is intriguing, because it suggests that the rate of biological aging is intimately related to energy metabolism. In the laboratory, CR is generally implemented by limiting food intake 20–40% of ad libitum
-fed controls [67
]. Thus, the beneficial effects of CR have historically been attributed to a reduction in food intake [68
]. However, this simplistic view has recently been called into question since CR is not only characterized by less food intake, but also by concurrent changes energy balance, body mass, and body composition [71
]. Since adipose tissue has been historically viewed as an inert storage depot for triglycerides, this robust phenotypic change had been widely discounted as merely a byproduct of reducing food intake [68
]. It was not until 1960 that a reduction in fat stores was proposed as an important mediator of CR [74
]. Since many believed CR worked by slowing metabolism or retarding growth and development, this hypothesis was never fully embraced at the time, although it was neither discounted until 20 years later.
5.1. Evidence against a role for body fat in determining lifespan
In 1980 and 1984, respectively, two historic studies were published which essentially discredited the “Reduction of Body Fat Hypothesis” for many years. The first by Bertrand and colleagues [68
] reported that fat mass was not related to lifespan in ad libitum
-fed rats, but was paradoxically associated with a greater lifespan in calorie-restricted rats. The later study by Harrison et al.
] found that calorie-restricted ob/ob
mice were longer lived than ad libitum
-fed wild types, despite having nearly twice as much body fat. Indeed, these studies made a compelling case against a role for body fat in the determination of lifespan. However, the conclusions of these studies have since been criticized and debated [69
]. There has also been controversy regarding the role of body fat in both spontaneous and transgenic long-lived mutants. For instance, several models of growth hormone (GH) deficiency or resistance including the Ames dwarf, Snell dwarf, and the GHR −/− mouse have reduced body size and live substantially longer than controls. However, these mice all have a high percentage body fat, although to the best of our knowledge, body fat distribution in these models has not been reported.
5.2. A paradigm shift
Since the discovery of leptin in 1994 [75
], the view of adipose tissue as an inert storage depot began to change. Indeed, evidence began to mount that obesity and specifically VF is associated with a low-grade inflammation due to the increased secretion of numerous pro-inflammatory cytokines from adipocytes and their associated macrophages [76
]. Many of these cytokines also referred to as “adipokines” including leptin, TNF-α, IL-6, heparin-binding epidermal growth factor (HB-EGF), and vascular endothelial growth factor (VEGF) among others, may play an important role in many disease pathologies by promoting angiogenesis, inflammation, cell proliferation, and insulin resistance [77
]. Considering this new perspective of fat, coupled with studies from our group linking age-related changes in body composition with insulin resistance, we hypothesized that the ability of CR to improve insulin action is by reducing VF.
5.3. VF removal in rats mimics the benefits of CR on insulin action
Since CR limits the accumulation of VF and preserves hepatic insulin action with aging in mammals [79
] and extends lifespan in a variety of species [80
], we suspected that the ability of CR to prevent insulin resistance with aging was due to the attenuation of VF, rather than other fat depots. To directly test this hypothesis, we studied four groups of rats: VF-, SC- (equivalent SC fat removed), SO (sham-operated controls), and CR (CR+sham operated) [45
]. Post-absorptive plasma insulin levels were nearly 50% greater in the SC- and SO rats as compared to CR and VF- animals. During a glucose clamp, VF- rats had an 80% increase in the rate of glucose infusion, significantly greater glucose uptake, and 50% increase in the ability of insulin to suppress hepatic glucose production compared to SC- and SO rats. Most striking, the dramatic improvement in insulin action with VF, but not SC fat removal, closely resembled the effects of prolonged CR, suggesting that decreased VF could largely account for the beneficial effects of a reduction in food intake. Additionally, our earlier studies had a shown a causal role for VF in age-related metabolic decline [72
] leading us to revisit the body fat and longevity hypothesis by proposing that the ability of CR to improve longevity is by reducing VF.
5.4. VF removal in rats improves longevity
As previously mentioned, VF accretion is a common hallmark of aging, and we have demonstrated metabolic benefits to VF removal [79
], and that decreased VF largely accounts for the improvement in insulin action with CR [45
]. Therefore, it seemed plausible that the beneficial effects of CR on longevity may be due to the attenuation of VF [72
]. Quite surprisingly, very few rodent studies have found a role for body fat in the determination of lifespan. In the most widely cited report by Bluher et al.
in 2003, fat-specific insulin receptor knockout (FIRKO) mice showed a modest reduction in body mass, but were markedly leaner than controls, demonstrating a 50% reduction in fat mass and lived nearly 20% longer than controls. Using a mathematical approach, Wang et al.
] found that CR and body weight had independent effects on mortality rate in male Wistar rats, such that statistically, body weight accounted for approximately 11% of the CR effect on mortality rate. However, this study may have underestimated the importance of body fat and specifically VF, since body composition was not measured.
In order to definitively demonstrate that VF modulates longevity, we prospectively studied lifespan in 3 groups of rats: ad libitum
fed (AL), 40% CR and VF- rats [82
]. CR rats demonstrated the greatest survival among all experimental groups (). Statistical analysis revealed a significant increase in both mean and maximum lifespan for VF- rats as compared to AL fed animals and VF removal accounted for ~20% of the effect of CR on longevity. Furthermore, the hazard rate of death in the VF- group was 0.49 (51% reduction) and in the CR group was 0.13 (87% reduction) compared to the AL-fed group. This effect of VF per se
was even more remarkable when considering that VF- rats had similar food intake, body weight and body fat as AL controls. In fact, VF- animals were 220g heavier, had nearly 115g more body fat and a percentage body fat nearly double that of CR rats. Furthermore, the only fat pads removed were the epididymal and perinephric fat pads, which do not drain into the portal vein in rats. This raises the possibility in humans that VF depletion may be even more beneficial since VF depots in humans have direct portal access and hence a greater potential to harm the liver. An illustration of the proposed link among VF, age-related diseases and lifespan can be found in . In summary, these data provide the first causal evidence implicating VF depletion as an important underlying cause of improved lifespan with CR.
Figure 1 Survival curve of the three groups of rats (AL-fed, dashed line; VF-removed, dotted line; and CR, solid line). CR rats had the greatest mean and maximum lifespan but VF removal only was sufficient to extend lifespan as compared to AL controls and accounted (more ...)
Figure 2 The proposed link among VF, age-related diseases and lifespan. The accumulation of VF, which is a hallmark of aging, is hypothesized to pose a greater risk for the development of insulin resistance and other features of the metabolic syndrome than other (more ...)