In this study, mean 24-hour, day-time and night-time core body temperature were all significantly lower in the CR group than in the WD and EX groups. This reduction in 24-h core body temperature is consistent with findings in CR rodents and monkeys studies [3
], and may contribute at least in part to the anti-aging effects of CR.
Although we found a significant correlation between % body fat and 24 h body temperature, it seems likely that the reduction in core body temperature induced by CR is largely related to CR itself, rather than changes in body composition. Mean 24-hours, day-time and night-time core body temperature were ~0.2 °C lower in the CR than in the EX group, even though percent body fat was similarly low in these groups. However, energy intake was ~37% lower in the CR than in the EX groups. It has been hypothesized that the CR-mediated reduction in body temperature relates to the induction of an energy conservation mechanism during CR [11
]. In humans, monkeys and rodents long-term CR (but not endurance exercise) reduces the circulating levels of triiodothyronine [12
] which controls energy homeostasis, body temperature, and cell respiration [16
]. The decrease in T3 could play a role in mediating the decrease in body temperature, and the reductions in T3 and temperature could influence the rate of aging by reducing metabolic rate and oxidative stress.
It is interesting in this context that body temperature is also reduced in the long-lived dwarf and growth hormone receptor KO mice [18
]. As in CR rodents, circulating levels of IGF-1, insulin and thyroid hormones are reduced in the dwarf and growth hormone receptor KO mice [20
]. In contrast, in humans practicing long-term CR do not have lower circulating IGF-1 levels [21
], suggesting that a down-regulation of the IGF-1 pathway is not involved in mediating the reduction in body temperature. The importance of a reduction of body temperature in modulating longevity has been also supported by the data obtained from the Hcrt-UCP2 mice and the Baltimore Longitudinal Study of Aging (BLSA). Overexpression of the uncoupling protein 2 in hypocretin neurons causes an elevation of hypothalamic temperature that leads to a 0.3-0.5°C reduction in core body temperature, and a significant increase in longevity, independently of caloric intake [7
]. In the Baltimore Longitudinal Study of Aging (BLSA), men with core body temperatures below the median lived significantly longer than men with body temperatures above the median in the absence of CR [8
Data from weight loss studies have shown that short-term (6 mos) CR significantly decreases core body temperature in overweight subjects that are actively losing weight [22
]. However, in overweight men and women who had achieved a “stable” lower body weight using a low-calorie liquid diet, there was no change in core body temperature [22
]. Consistently, no difference in core body temperature between weight-stable obese and normal-weight subjects has been detected, suggesting that in steady-state obese subjects maintain their core body temperature normal probably by increasing heat dissipation from peripheral regions [23
]. In contrast, in our study we have found that long-term CR, but not endurance exercise, chronically reduces core body temperature in weight-stable lean individuals. One possible explanation for the reduced 24-h core body temperature in the CR practitioners may be a protective physiological adaptation to save energy. This hypothesis is supported by the finding that the reduced core body temperature in the CR group is associated with lower circulating levels triiodothyro-nine, insulin, leptin and total testosterone, which are key nutrient-sensing metabolic/anabolic hormones [12
]. The combination of decreased core body temperature and lower circulating levels of serum triiodothyronine, leptin, and anabolic hormones (i.e. testosterone, insulin) is a clear indication that these individuals are in a state of “sensing” severe energy restriction.
In conclusion, the results of this study provide evidence that long-term CR, with adequate intake micronutrients, in healthy, weight-stable individuals is associated with a reduction of mean 24-hour, day-time and night-time core body temperature, similar to that found in calorie-restricted rodents and monkeys.