The data from the present study show that long-term CR with adequate nutrition is associated with lower circulating sex hormones in healthy lean men. Serum concentrations of total and free testosterone were significantly lower in CR participants than in body fat-matched endurance runners and non-obese sedentary individuals who were consuming Western diets. Serum concentration of 17-β-estradiol was significantly lower in the CR and EX group than in the WD group. In addition, circulating levels of SHBG, a glycoprotein that binds to sex hormones, was significantly higher in the CR group, and tended to be higher in the EX group than in the WD group. This CR-mediated reduction in sex hormones is consistent with that seen in long-lived CR rodents, and further support the concept that many of the hormonal adaptations to CR are remarkably well conserved among species (Grewall et al., 1971
; Howland, 1975
; Merry & Holehan, 1981
; Govic et al., 2008
). However, no differences in serum DHEA-s concentrations were detected between groups.
Whether the CR-mediated reduction in anabolic sex hormones plays an independent role in modulating the biology of aging and lifespan extension is not known. However, it is well known that in experimental animals CR reduces the production of anabolic hormones and growth factors (Weindruch & Walford, 1988
; Fontana & Klein, 2007
; Kalant et al., 1988
; Grewall et al., 1971
; Breese et al., 1991
). Anabolic/mitogenic processes require energy to synthesize large complex molecules from small simple molecules, promote cell proliferation and growth. When food intake is reduced, cell growth and proliferation decrease as a result of lower availability of substrate and energy. It appears from the data on CR animals and our findings on CR humans that there are feedback mechanisms by which decreased availability of substrates and/or ATP bring about a decrease in production of anabolic hormones that normally stimulate cell growth and proliferation. The results of our study show that long-term CR with adequate nutrition cause a chronic decrease in serum total and free testosterone concentrations in men, independent of body fat mass. It is interesting in this context that body mass index and lean mass were both directly correlated with serum testosterone concentrations. Moreover, circulating levels of triiodothyronine, a hormone typically reduced by CR that regulates cellular metabolism and growth (Braverman & Utiger, 2004
), was also directly correlated with serum testosterone concentrations, further suggesting an integrated hormonal/metabolic control of anabolic processes during CR. The mechanism(s) responsible for the relationship between reduced intake of calories and lower serum concentration of sex hormones is not completely understood, but a decreased production of gonadotrophins by the pituitary is clearly involved (Howland, 1975
; Pugeat et al., 1991
Calorie restriction without malnutrition is also the most potent and reproducible physiological intervention for protecting against cancer in mammals (Longo & Fontana, 2010
). Accumulating evidence suggests that chronically elevated concentrations of growth factors and anabolic hormones exert direct powerful mitogenic effects through several signalling transduction pathways (e.g. phosphatidylinositol-3 kinase/Akt/Tor/S6kinase, and p66Shc
) in many cell types, especially in pre-neoplastic cells (Heinlein & Chang, 2004
; Lee et al., 2004
; Longo & Fontana, 2010
). CR likely exerts its anti-cancer effects by down-regulating a number of anabolic/growth factor molecules and pathways, and by up-regulating several genes and signal transduction molecules that promote DNA and cellular repair, resistance against oxidative stress and apoptosis of damaged cells (Longo and Fontana, 2010
). Whether the CR-mediated reduction in anabolic sex hormones has an independent role in the prevention of cancer is not clear. In women, higher concentrations of estrogens and testosterone are associated with an increased risk of breast and endometrial cancer (Yager & Davidson., 2006
; Davidson et al., 1981
). Recent data from the Baltimore Longitudinal Study of Aging suggest that elevated circulating levels of serum free testosterone are associated with an increased risk of aggressive prostate cancer among older men, but with a reduced risk in men younger than 65 years (Pierorazio et al., 2009
). More studies are needed to elucidate the role of a reduction in anabolic hormones (e.g. testosterone and 17-β-estradiol) in combination with the reduction of other growth factors (e.g. insulin and IGF-1) and inflammatory cytokines (e.g. IL6) in mediating the anti-cancer effects of CR. Further studies are warranted to understand the long-term effects of the CR-induced reduction in sex hormones concentrations on skeletal and bone mass and strength.
The data from the present study also show that long-term CR with adequate nutrition is associated with a marked increase in SHBG, a circulating protein that binds to sex hormones and inhibits the function of these hormones (Pugeat et al., 1991
). SHBG is mainly, but not exclusively, produced by liver cells and is released into the bloodstream. Several nutritional and hormonal factors regulate the hepatic production of SHBG, including monosaccharides, insulin, and IGF-1(Selva et al., 2007
; Plymate et al., 1990
). Weight loss in obese men is associated with an increase in SHBG levels, improvement in insulin sensitivity, and a significant decrease in plasma insulin levels (Niskanen et al., 2004
; Kaukua et al., 2003
). In contrast, overfeeding (1000 kcal energy excess/day) for a period of 100 days was associated with a fall in SHBG in a study of young, sedentary, male monozygotic twins (Pritchard et al., 1998
). In this study, we found that body mass index and serum IGF-1 concentration were both inversely correlated with serum SHBG concentration, whereas serum adiponectin concentration and the Matsuda and DeFronzo insulin sensitivity index were directly correlated with serum SHBG concentration. More studies are needed to understand if this increase in serum SHBG concentration induced by reduced energy intake has clinical implications (e.g. prevention of cardiovascular disease and cancer) or is simply a biological marker of energy status (Davidson et al., 1981
; Laaksonen et al., 2004
; Kalme et al., 2005
In gorillas, chimps and human primates, the sex steroid hormone DHEAS, which is produced by the adrenal cortex, circulates in far higher concentrations than any of the other endogenous sex steroid hormones in serum (Hornsby, 1995
). In contrast, rodents secrete little or no DHEA-S. Data from epidemiological studies have shown that serum concentration of DHEA-S rapidly declines with age so that by age 75 yr serum DHEA-S concentration is ~80% lower than at 20 yr (Orentreich et al., 1992
). It has been reported that CR protects against the decrease in serum DHEA-s concentration with advancing age in rhesus monkeys. Serum DHEA-S concentration declined more than 30% over 4 yr in adult male rhesus monkeys eating an “ad libitum” diet, whereas the decline was only 3% in the CR monkeys (Lane et al., 1997
). In the present study we found that long-term CR without malnutrition does not protect against a decline in DHEAS in humans as there were no differences in serum DHEAS concentration between the individuals practicing CR for an average of 7 years and age-matched controls.
Our study has limitations. First, because of the cross-sectional design of our study, we are only able to show associations with diet, physical activity and circulating sex hormones, and cannot determine true causal relationships. A long-term randomized controlled trial would be needed to determine cause-and-effect relationships. However, this type of trial is extremely difficult to perform, because of the difficulty in achieving long-term dietary and exercise compliance, and because of the cost. In contrast, the ability to utilize specimens from human volunteers engaged in long-term CR (and the comparison with equally lean endurance athletes) is providing valuable information regarding the maximal effects of CR on several important metabolic and hormonal factors that are attainable in response to prolonged, severe CR by highly motivated, disciplined lean individuals. A multicenter randomized controlled trial of the effects of 2 yr of 25% CR in middle-aged non-obese (body mass index ≥ 22 and <28 kg/m2) women and men, currently in progress (CALERIE phase 2), might provide some of this information regarding a cause-and-effect relationship, and the time-course of these CR-mediated hormonal adaptations. Second, women were not included in this study because at this time we have been able to identify and study only four women who are practicing severe CR, and are willing to participate in our study.
Our data show that long-term CR with adequate protein and micro-nutrient intake is associated with low serum total and free testosterone, independent of body fat mass, while serum concentrations of SHBG are elevated in CR men. More studies are needed to elucidate whether this CR-mediated reduction in sex hormones has beneficial or detrimental effects in modulating aging and the pathogenesis of age-associated chronic diseases such as cancer.