Calorie restriction (CR) is the only experimental manipulation that is known to extend the lifespan of a number of organisms including yeast, worms, flies, rodents and perhaps non-human primates. In addition, CR has been shown to reduce the incidence of age-related disorders (for example, diabetes, cancer and cardiovascular disorders) in mammals. The mechanisms through which this occurs have been unclear. CR induces metabolic changes, improves insulin sensitivity and alters neuroendocrine function in animals. In this review, we summarize recent findings that are beginning to clarify the mechanisms by which CR results in longevity and robust health, which might open new avenues of therapy for diseases of ageing.

Leptin is a nutritionally regulated hormone that may modulate neuroendocrine function during caloric deficit. We hypothesized that administration of low-dose leptin would prevent changes in neuroendocrine function resulting from short-term caloric restriction. We administered physiologic doses of r-metHuLeptin [(0.05 mg/kg sc daily or identical placebo in divided doses (0800, 1400, 2000, and 0200 h)] to 17 healthy, normal-weight, reproductive-aged women during a 4-d fast. Leptin levels were lower in the placebo-treated group during fasting (3.3 +/- 0.2 vs. 9.6 +/- 1.0 ng/ml, P < 0.001, placebo vs. leptin-treated at end of study). Fat mass decreased more in the leptin than the placebo-treated group (-0.6 +/- 0.1 vs. -0.2 +/- 0.1 kg, P = 0.03). Both overnight LH area (38.9 +/- 21.5 vs. 1.2 +/- 11.1 microIU/ml.min, P = 0.05) and LH peak width increased (15.8 +/- 7.1 vs. -2.3 +/- 6.7 min, P = 0.06) and LH pulsatility decreased (-2.0 +/- 0.9 vs. 1.0 +/- 0.8 peaks/12 h, P = 0.03) more in the leptin vs. placebo group. LH pulse regularity was higher in the leptin-treated group (P = 0.02). Twenty-four-hour mean TSH decreased more in the placebo than the leptin-treated group, respectively (-1.06 +/- 0.27 vs. -0.32 +/- 0.18 microIU/ml, P = 0.03). No differences in 24-h mean GH, cortisol, IGF binding protein-1, and IGF-I were observed between the groups. Hunger was inversely related to leptin levels in the subjects randomized to leptin (r = -0.76, P = 0.03) but not placebo (r = -0.18, P = 0.70) at the end of the study. Diminished hunger was seen among subjects achieving the highest leptin levels. Our data provide new evidence of the important role of physiologic leptin regulation in the neuroendocrine response to acute caloric deprivation.

Reducing the intake of food in rodents inhibits body growth, retards most physiological ageing processes, delays the onset of pathology and prolongs life. Food restriction (FR) reduces pituitary hormone secretion and in consequence has been called 'functional hypophysectomy'. Direct life-long comparisons in the rat showed that hypophysectomy (HYP) (a complete absence of pituitary hormones) has a greater anti-ageing action than FR (a partial lack of pituitary hormones) on collagen, kidney and muscle. This suggests that pituitary hormones accelerate ageing. Recent American research on genetic variants of the mouse indicates that pituitary growth hormone (GH) may accelerate ageing and shorten life. Both the Snell and Ames dwarf mice have a deficiency of pituitary GH and live 50% longer than normal mice. The Snell dwarf mouse has retarded ageing of both collagen and immune functions. The Ames dwarf mouse has high antioxidant enzyme activities in liver and kidney. A transgenic human GH mouse is short lived, has a low activity of antioxidant enzymes in liver and kidney and an early development of disease in these organs. It is postulated that FR by reducing the secretion of pituitary hormones, such as GH, diminishes the oxidative damage of certain tissues, thereby delaying the development of age-related diseases in these tissues and by this means extends life.

Evolutional theories of aging and caloric restriction (CR) in animals predict the presence of neuroendocrine signals to divert the limited energy resources from energy-costly physiologic processes such as reproduction to those essential for survival in response to food shortage. The diversion of energy and subsequent molecular mechanisms might extend the lifespan. A growing body of evidence indicates that leptin, a peptide hormone secreted from adipocytes, has a key role in neuroendocrine adaptation against life-threatening stress such as fasting. The present review discusses the potential role of leptin in the anti-aging action of CR. Although several alternative signaling pathways might also mediate the anti-aging action of CR, leptin signaling could be a substantial pathway in the CR action. Research on neuroendocrine mechanisms of CR is warranted, because such efforts might provide clues to the regulation of the aging process in humans.

Because neuroendocrine mechanisms may contribute to the antiaging effects of food restriction (FR), we measured the effect of FR on mRNAs encoding anterior pituitary (AP) tropic hormones. Slot blots or RNase protection assays were done on AP RNA from 3-, 6-, 12-, 18- and 24-mo-old male F344 rats consuming food ad libitum (AL) or food restricted (FR; to 60% of AL food intake) from 6 wk. Both AL and FR rats gained body weight during the study (P < 0.05), but FR rats weighed approximately 40% less (P < 0.0001). Messenger RNA levels were expressed in two ways, i.e., per total AP and per microgram total AP RNA. Proopiomelanocortin (POMC) mRNA/microg RNA was higher (P < 0.0005) in FR than in AL rats at all ages. Thyroid-stimulating hormone (TSH) beta mRNA declined with age (P < 0.05) in AL but not FR rats and was reduced by FR up to 12 mo (P < 0.01). Growth hormone (GH) mRNA/microg RNA declined with age (P < 0.05) in AL but not FR rats, and total GH mRNA in the AP was reduced by FR at early ages (P < 0.05). FR reduced prolactin (PRL) mRNA and its age-related increase (P < 0.0005). Levels of luteinizing hormone (LH) beta and follicle-stimulating hormone (FSH) beta mRNAs did not differ between AL and FR rats until 12 mo, but thereafter rose in FR (LH beta mRNA; P < 0.01, FSH beta mRNA; P < 0.05). Many of these changes in gene _expression corroborate previously reported hormonal changes in FR rodents and mutant mice with extended life spans, and thus provide further support for the hypothesis that an altered hormonal milieu contributes to the antiaging effects of food restriction.

Cancer that occurs at numerous organ sites, including the colon and breast, is inhibited by energy restriction, and the inhibition is proportional to the degree of restriction imposed. In an effort to identify the mechanism(s) by which energy restriction exerts this effect, a short term model system of experimentally induced mammary carcinogenesis was used. Given that carcinogenesis is known to involve a dysregulation to tissue size homeostasis in which cell proliferation and cell death are in dysequilibrium, we hypothesized that energy restriction exerts its effect by altering one or more aspects of cell cycle regulation. It was observed that energy restriction inhibited cell proliferation and increased cell death due to apoptosis. Thus attention was next focused on aspects of cell cycle regulation that might be affected by energy restriction. It was observed that the amount of p27 protein, one member of the Cip/Kip family of genes that are involved in cell cycle arrest, was increased dose dependently by energy restriction. Based on this and related observations, the hypothesis is advanced that energy restriction inhibits carcinogenesis, at least in part, by delaying cell cycle progression via shifting cell populations into a G(0)/G(1)state. Ongoing work indicates that corticosteroids, which are produced in increased amounts in response to energy restriction, may be involved in mediating this effect.