In this study, we compared glucose tolerance, insulin action, and adipokine production in healthy weight-stable lean men and women, who were consuming a self-imposed CR diet, containing more than 100% of the RDI for all essential nutrients, for 3 to 20 years, with age-, sex-, and body fat- matched endurance runners, and age- and sex-matched sedentary individuals, who were consuming Western diets. Our data show that insulin sensitivity, determined by HOMA-IR index and the Matsuda and DeFronzo ISI, was significantly higher in the CR and EX groups than in the WD group. However, although the average values for the HOMA-IR and the Matsuda ISI indicate that the CR group was more insulin sensitive than the WD group, ~40% of the CR individuals exhibited impaired glucose tolerance in response to a glucose load, which suggests that HOMA-IR is not a good surrogate marker of insulin sensitivity in people practicing CR. Interestingly, serum fructosamine concentration, a marker of glycation, was higher in the CR and WD groups than in the EX group.
Aging and many age-associated diseases in both humans and rodents are associated with progressive increase in fasting insulin concentration and insulin resistance (Meigs et al. 2003
; Basu et al. 2003
; Reaven 1995
). However, the relationship between aging/aging-related diseases and insulin resistance is confounded by associated factors such as excessive abdominal adiposity, decreased physical activity, hyperinsulinemia, dyslipedemia, inflammation, and other metabolic and hormonal components of the metabolic syndrome (Barzilai et al. 1998
; Reaven 1995
). It is intriguing that based on the 2-h OGTT plasma glucose values, 11 of our CR subjects met diagnostic criteria for glucose intolerance (Genuth et al. 2003
), and six of the CR-NGT subjects had high–normal 2-h glucose values, even though all CR subjects were extremely lean and had very low fasting plasma concentrations of glucose and insulin, and an outstanding metabolic profile (very low triglyceride, high HDL-cholesterol, high adiponectin, and extremely low C-reactive protein concentrations; Fontana et al. 2006
). The elevated glucose levels after the glucose load, in the CR-IGT group, compared to the CR-NGT group, cannot reflect acute CR, because all the volunteers were instructed to maintain their usual diet and weight stability. Carbohydrate restriction seems also unlikely as a possible cause for the glucose intolerance in the CR-IGT group, both because 150 g of carbohydrate appears to be sufficient to prevent impairment of glucose tolerance due to carbohydrate deficiency (Wilkerson et al. 1960
; American Diabetes Association 2009
) and because the CR-NGT group carbohydrate intake was similar to that of the CR-IGT group. Finally, the elevated glucose levels after the glucose load, in the CR-IGT group, compared to the CR-NGT group, cannot be explained by a reduced insulin response as their insulin levels were similar during the OGT. Therefore, our data suggest that severe chronic CR, in some individuals, may be associated with a relative peripheral insulin resistance mainly due to a low muscle mass with decreased capacity to take up glucose. This will have to be confirmed by more definitive studies that can directly assess insulin secretion and sensitivity.
One possible explanation for the reduced glucose disposal following a glucose load in the CR practitioners may be a protective physiological adaptation to prevent hypoglycemia, which is considered part of the adaptive response to fasting (Jensen et al. 1987
). This hypothesis is supported by the finding that impaired glucose tolerance in the CR-IGT group was associated with lower circulating levels of IGF-1, total testosterone, leptin, and triiodothyronine, which are key metabolic/hormonal adaptations to CR in rodents (Fontana & Klein 2007
). The combination of decreased fasting levels of serum triiodothyronine, leptin, and anabolic hormones (i.e., IGF-1, testosterone, and insulin), and increased levels of adiponectin is a clear indication that these individuals are in a state of “sensing” severe energy restriction.
During periods of severe food deprivation, increased insulin sensitivity and glucose disposal are clearly detrimental to survival of the organism, because of severe hypoglycemia risk. In contrast, decreased insulin sensitivity and glucose disposal could enhance survival, by preventing hypoglycemia and maintaining circulating glucose for organs that require glucose as a fuel (Cahill 2006
). This hypothesis is supported by the finding that chronic CR or prolonged starvation in normal mice induces impairment of insulin signaling at a receptor and post-receptor level (Rao 1995
; Du et al. 2003
). For example, prolonged starvation causes an elevation of the insulin signaling (Akt) inhibitor TRB3 in the liver, despite the very low levels of circulating insulin (Du et al. 2003
). Moreover, prolonged fasting in insulin-resistant hypoinsulinemic rats causes a reduction in insulin-induced phosphorylation of the protein Shc (an insulin receptor substrate) in the liver and adipose tissue, whereas a significant increase in Shc activation was observed in adipose tissue, skeletal muscle, and liver of insulin-resistant hyperinsulinemic old rats (Páez-Espinosa et al. 1999
). Thus, from an evolutionary point of view, a CR-mediated reduction in insulin signaling could be a protective metabolic response against hypoglycemia when food is scarce.
Interestingly, recent evidence suggests that decreased insulin signaling, and not enhanced insulin sensitivity, is implicated in the delayed aging phenotype of some of the animal models of increased longevity, such as klotho transgenic mice, insulin receptor substrate 1 null mice, brain insulin receptor substrate 2 null mice, and FIRKO mice (Bluher et al. 2003
; Taguchi et al. 2007
; Selman et al. 2008
; Kurosu et al. 2005
). It is also intriguing that glucose tolerance in response to a glucose load was reduced, and insulin sensitivity in response to an intravenous insulin tolerance test was enhanced in long-lived Ames dwarf and growth hormone receptor KO mice, that have extremely low circulating levels of fasting IGF-1 and insulin, and hypothyroidism (Dominici et al. 2002
; Coschigano et al. 1999
). Therefore, it may be possible that severe CR decreases signaling through the insulin pathway in some tissues and this may play a role in mediating expression of anti-aging genes and negatively regulating the expression of pro-aging genes by reducing AKT/protein kinase B (PKB) and class O of forkhead box transcription factors (FoxO) activities (Dominici et al. 2003
; Hsieh and Papaconstantinou 2004
; Salih and Brunet 2008
While the finding that various strains of long-lived mice are insulin resistant is interesting relative to our finding of IGT in the CR-IGT group, it must be kept in mind that there is currently no evidence that CR slows primary aging in humans. Also, it is not known which of the biological effects of CR in rodents are responsible for the increase in maximal life span. So, the idea that the “insulin resistance” in the CR-IGT group might have the effect of slowing aging, based on the finding that a number of insulin-resistant strains of mice are long-lived is just a hypothesis/speculation at this point.
The mechanism responsible for the higher glucose tolerance and insulin action, and lower glycation of plasma proteins in our endurance athletes is likely related to exercise itself, rather than changes in body composition caused by exercise. Even though relative fat mass was low and similar in the EX and CR groups, 2-h glucose and 2-h insulin concentrations were ~22% and ~55% lower in the EX group than in the CR group. Serum fructosamine concentration, a marker of nonenzymatic glycation of serum proteins that integrates plasma glucose fluctuations over the preceding 2–3 weeks (Tahara and Shima 1995
), was higher in the CR group than in the EX group. In addition, soluble RAGE, a decoy receptor that antagonizes the toxic effects of RAGE-mediated signaling, was higher in the EX group than in the CR and WD groups (Koyama et al. 2007
). It is well known that endurance exercise training has positive effects on glucose tolerance, insulin sensitivity, and responsiveness in addition to those that result from a low adiposity (Holloszy 2005
). Physical training causes multiple adaptations in skeletal muscle that contribute to increased insulin action, including up-regulation of muscle GLUT4 protein, and increased enzyme capacities, that disappears after few days of detraining (Ebeling et al. 1993
; Yu et al. 2001
; Hughes et al. 1993
). While exercise induces an acute, insulin-independent activation of glucose transport in muscle, this effect wears off within 3 to 4 h and is replaced by increases in insulin sensitivity and responsiveness (Holloszy 2005
This study has potential limitations. One is the fact that the CR volunteers are a heterogeneous group, and therefore differences in macro- or micronutrient content of their diets as well as in fitness (as evidenced by the lower VO2 max in the IGT-CR group) may be responsible, at least in part, for the differences in glucose tolerance. Another limitation was the lack of use of a glucose clamp to calculate insulin sensitivity. On the other hand, a limitation of the hyperinsulinemic euglycemic clamp is that utilizes steady-state insulin levels that may be supraphysiological, resulting in a reversal of the normal portal to peripheral insulin gradient.
In conclusion, the results of this study demonstrate that endurance exercise training is associated with better glucose tolerance, insulin action, and protein glycation, than severe CR, independent of body fat mass and basal adipokine production. The results of this study provide the new information that long-term severe CR is associated with impaired glucose tolerance in some individuals, presumably because of decreased insulin-mediated glucose disposal. This reduced glucose disposal is associated with lower circulating levels of IGF-1, total testosterone, and triiodothyronine, which are typical adaptations to CR in rodents.