Aging, GH and longevity
The possible role of GH, and IGF-I in the age-dependent changes in body composition is not yet clarified. Interestingly, high GH concentrations in mice that express too much GH throughout their whole live (GH-transgenic mice) are associated with reduced life expectancy and premature aging. On the other hand, dwarf mice with hereditary GH deficiency live much longer than normal controls [5]. One must realize that these GH deficient mice eat less that their normal littermates, which makes them only small, but not fat. In men, however, many physiological changes in the elderly resemble symptoms of GH deficiency, which can be corrected by GH administration. One of the main features in human adult GH deficiency is that the affected subjects are obese. Also, GH deficient humans have a reduced life expectancy. So possibly, these contradictory observations are related to a negative correlation between body size and life expectancy within a species [6].
The underlying mechanism by which long-term calorie restriction (CR) extends mammalian life spans is unknown. Also in man, studies have shown that certain monastic orders that apply CR almost throughout life have an increased lifespan. Especially a drop in cancer rate and incidence in cardio-vascular diseases accounts for this phenomenon. CR appears to alter the rate of decline in reserve capacity and the exposure to growth stimulus without appreciable alteration of metabolic rate [7].
It is known that the activity of the hypothalamic GH-IGF network declines with age. It is also known that an increased cardiovascular mortality in adults with GH deficiency can be found. Some investigators found that even in an elderly population, further aging is accompanied by a progressive decline in circulating IGF-I, suggesting a continuing diminution of the GH-IGF axis throughout aging. Moreover, some studies suggest that GH might play an important role in lipid metabolism in healthy elderly subjects [8].
Effects of habitual endurance exercise on age-related changes in endocrine function and body composition were also studied. For instance, a comparison was made between IGF-I, sex hormonal status and body composition in physical active men aged 60-70 years and minimally exercising controls. Compared to young adults, total IGF-I was similarly depressed in both exercising, and non-exercising aging subjects. There was no relationship between serum IGF-I concentrations and any body composition, or muscle strength variable. These results indicate that even high levels of regular endurance exercise do not prevent the decline in the GH - IGF-I axis that occurs with aging [9].
