Recently, our laboratory presented data demonstrating an increased susceptibility of old rodents to obesity-related insulin resistance (8
). However, we did not address the importance of mesenteric fat, true visceral fat in the rodent, in the manifestation of insulin resistance. Here, we test the susceptibility hypothesis in more depth by examining the response to short-term weight loss in young fat-fed versus old rats. Six weeks of restriction reduced body weight and abdominal adiposity in both young and old groups. Remarkably, reductions in mesenteric fat in old rats exactly mirrored those in young rats, whereas this similarity did not exist for other abdominal fat depots. Whole-body insulin sensitivity was markedly improved in both groups, which significantly coincided with a preferential loss of abdominal fat in both young and old rats. However, improvement in young rats exhibited a more than twofold greater dependence on the mesenteric fat depot versus other abdominal depots. Furthermore, this association was altered by age such that a similar degree of mesenteric fat loss led to much lesser amelioration of insulin resistance in older rats, and slower recovery of insulin sensitivity in older rats was accompanied by a remarkably subdued reduction of hepatic triglyceride. These data support the hypothesis that old animals are at higher risk for development of insulin resistance in the face of visceral obesity, which may be coupled with a defect in hepatic lipid turnover. The corollary is that adiposity, per se, cannot completely account for the insulin resistance of the aging rat.
Short-term caloric restriction has been shown to decrease both weight and adiposity in other models (16
). Interestingly, our data suggest a preferential loss in abdominal depots (30–45%) rather than subcutaneous fat in both young and old rats with 6 weeks of restriction. Studies of lifetime caloric restriction have been less apt to clarify this issue because these animals typically exhibit marked reduction in all fat depots. Studies looking at specific adipose depots have resorted to surgical removal, such as that reported by Barzilai and colleagues (7
). These latter studies have examined the effects of gonadal and perirenal fat removal on SI
, depots that do not fit the specific criteria of visceral fat, namely circulatory drainage into the portal vein. To our knowledge, our present study is the first to examine reductions in true visceral (i.e., mesenteric) fat as they relate to recovery of SI
Other studies examining fat distribution during weight loss have demonstrated a similar preferential diminution of the visceral depot compared with other adipose depots (2
). Data from our laboratory in dogs have suggested that the visceral depot may serve as a favored site of fat accumulation in the early stages of diet-induced obesity with subsequent fat gain appearing in the periphery once the visceral depot's capacity is exhausted (21
) (with further support from [5
]). Greater loss of visceral versus subcutaneous depots, as observed in the current study, might be expected because of the inherently increased lipolytic activity and insulin resistance of visceral fat (23
). And although this characteristic of visceral fat has been used to suggest its importance in the etiology of insulin resistance, it can equally be exploited to help explain a potential preferential loss of the tissue in times of energy deficit.
In contrast to our previous observations, here we find that resistance in young and old obese rats exists primarily at the liver, rather than the periphery (8
). It has been demonstrated that once Rd
and therefore SI Rd
are expressed per LBM, the apparent resistance observed in many studies, especially those examining animals of various sizes (i.e., different ages), disappears (27
). Although in our previous study LBM was only estimated, here we use a more precise measure of LBM using a stable isotope. Furthermore, that fat-fed animals did not exhibit any substantial peripheral resistance is likely explained by the fact that these animals were on HFD for less time (3 vs. 4 weeks).
Despite these differences, we found a similar degree of whole-body insulin resistance as observed previously in both fat-fed young animals and ad libitum–fed old rats (~50%). More importantly, in concert with fat loss, insulin action improved dramatically in both groups when exposed to caloric restriction, albeit refractory in old animals. Our data also demonstrate a highly significant dependence of SI
on abdominal fat of all types in both ages. However reductions in true visceral fat appear to have a much stronger influence on attenuating resistance () at least in young rats. Thörne et al. achieve similar results with surgical removal of visceral fat in obese patients undergoing gastric banding surgery (29
), a result also observed in the nonobese canine model (30
). However, studies of the removal of subcutaneous fat in humans have been less clear (31
). Thus, our data suggest that visceral fat “removal” reverses insulin resistance whether induced by diet or age, further suggesting its causative role in the impairment of insulin action with obesity.
Our data demonstrate a blunted association between visceral fat loss and the recovery of insulin sensitivity in old rats, and although this study was not designed to look at the potential mechanisms for this disparity, several explanations might exist. Although it is possible that there is an adipose-independent effect at work in old rats, regression analysis revealed a significant interaction between age and visceral adiposity for SI
< 0.001) suggesting that “old visceral fat” is different from “young visceral fat.” Although it is possible that inflammatory cytokines, which are elevated with age (33
), could explain this difference, the refractory recovery of SI
in old animals could not be explained by altered regulation of either adiponectin or resistin. Because the primary site of resistance in this study was the liver, we must ask what role the “portal hypothesis” might play in the differences observed between young and old rats. Old animals appeared to have greater resistance to FFA suppression during clamp conditions versus young animals, especially at 2 weeks. However, because this study was not designed to study lipid turnover directly, it is impossible to discern the tissue source of these circulating FFAs.
It has been suggested that HFD, such as that used in this study, induces insulin resistance not through fat depot accretion, but rather ectopic fat storage (e.g., liver and/or muscle) (35
). Here we demonstrate that young and old obese animals did in fact have similar degrees of fat accumulation in the liver. Most remarkable was the finding that liver triglyceride in old animals, like liver insulin resistance, reduced more slowly with caloric restriction versus young fat-fed rats. Our data suggest that old rats do not mobilize liver triglyceride stores as readily as young, contributing to their persistent insulin resistance. To test this, further studies examining hepatic lipid turnover will be required.
Although caloric restriction reduces adipose mass, it also has profound effects on eating patterns and insulin-independent energy balance (37
). Furthermore, although not measured in this study, it cannot be discounted that there are inherent differences in activity levels between young and old animals, and the effect that caloric restriction has on activity in each of these age groups may not be equivalent. Further investigation would be necessary to clarify these potential differences.
In conclusion, our data demonstrate that short-term diet restriction is an effective way to reverse insulin resistance in two obese rat models and this reversal is highly correlated with abdominal fat and hepatic triglyceride loss. In particular, we have shown that a commonly neglected fat depot representing true visceral fat in the rodent, the mesenteric fat pad, has a more than twofold higher influence on the improvements in SI observed with restriction. Yet, old animals appear to have a refractory response to visceral fat loss, suggesting an altered association between obesity and resistance in old versus young animals. Although the potential mechanism(s) of this altered association in young and old animals is not known, our data suggest that differences in lipid handling by the liver may be a candidate. Moreover, further investigation into the potential role of differences in local delivery of adipokines from mesenteric fat will be required.