Aging : Hormones, Growth Factors

by National Institute on Aging

(Page 10 of 15)

Hormones and Research on Aging

Produced by glands, organs, and tissues, hormones are the body's chemical messengers, flowing through the blood stream and searching out cells fitted with special receptors. Each receptor, like a lock, can be opened by the specific hormone that fits it and also, to a lesser extent, by closely related hormones. Here are some of the hormones and other growth factors of special interest to gerontologists.

ESTROGEN: Although it is primarily associated with women, men also produce small amounts of this sex hormone. Among its many roles, estrogen slows the bone thinning that accompanies aging. In premenopausal women the ovaries are the main manufacturers of estrogen. After menopause, fat tissue is the major source of smaller amounts and weaker forms of estrogen than that produced by the ovaries. While many women with menopausal symptoms are helped by hormone therapy during and after menopause, some are placed at higher risk for certain diseases if they take it. The results of the WHI are prompting further studies about the usefulness and safety of this therapy when used by younger menopausal and postmenopausal women to control symptoms, such as hot flashes, and to prevent chronic diseases.

GROWTH HORMONE: This product of the pituitary gland appears to play a role in body composition and muscle and bone strength. It is released through the action of another trophic factor called growth hormone releasing hormone, which is produced in the brain. It works, in part, by stimulating the production of insulin-like growth factor, which comes mainly from the liver. All three hormones are being studied for their potential to strengthen muscle and bones and prevent frailty among older people. For now, however, there is no convincing evidence that taking growth hormone will improve the health of those who do not suffer a profound deficiency of this hormone.

MELATONIN: Contrary to some claims, secretion of this hormone, made by the pineal gland, does not necessarily diminish with age. Instead, a number of factors, including light, can affect production of this hormone, which seems to regulate various seasonal changes in the body. Current research does indicate that melatonin in low dosages may help some older individuals with their sleep. However, it is recommended that a physician knowledgeable in sleep medicine be consulted before self-medication. Claims that melatonin can slow or reverse aging are far from proven.

TESTOSTERONE: In men, testosterone is produced in the testes (women also produce small amounts of this hormone). Production peaks in early adulthood. However, the range of normal testosterone production is vast. So while there are some declines in testosterone production with age, most older men stay well within normal limits. The NIA is investigating the role of testosterone supplementation in delaying or preventing frailty. Preliminary results have been inconclusive, and it remains unclear if supplementation of this hormone can sharpen memory or help men maintain stout muscles, sturdy bones, and robust sexual activity. Investigators are also looking at its side effects, which may include an increased risk of certain cancers, particularly prostate cancer. A small percentage of men with profound deficiencies may be helped by prescription testosterone supplements.

DHEA: Short for dehydroepiandrosterone, DHEA is produced in the adrenal glands. It is a precursor to some other hormones, including testosterone and estrogen. Production peaks in the mid-20s, and gradually declines with age. What this drop means or how it affects the aging process, if at all, is unclear. Investigators are working to find more definite answers about DHEA's effects on aging, muscles, and the immune system. DHEA supplements, even when taken briefly, may cause liver damage and have other detrimental effects on the body.

Growth Factors

Some types of hormones can be referred to as growth or trophic factors. These factors include substances such as insulin-like growth factor (IGF-I), which mediates many of the actions of hGH. Another trophic factor of interest to gerontologists is growth hormone releasing hormone, which stimulates the release of hGH. Growth factors might have an important role in longevity determination. In nematodes, for instance, mutations in at least two genes in the IGF-I pathway result in extended lifespan.

The mechanisms - how hormones and growth factors produce their effects - are still a matter of intense speculation and study. Scientists know that these chemical messengers selectively stimulate cell activities, which in turn affect critical events, such as the size and functioning of skeletal muscle. However, the pathway from hormone to muscle is complex and still unclear.

Consider growth hormone. It begins by stimulating production of IGF-I. Produced primarily in the liver, IGF-I enters and flows through the blood stream, seeking out special IGF-I receptors on the surface of various cells, including muscle cells. Through these receptors it signals the muscle cells to increase in size and number, perhaps by stimulating their genes to produce more of special, muscle-specific proteins. Also involved at some point in this process are one or more of the six known proteins that specifically bind with IGF-I; their regulatory roles are still a mystery.

As if the cellular complexities weren't enough, the action of growth hormone also may be intertwined with a cluster of other factors - exercise, for example, which stimulates a certain amount of hGH secretion on its own, and obesity, which depresses production of hGH. Even the way fat is distributed in the body may make a difference; lower levels of hGH have been linked to excess abdominal fat but not to lower body fat.

About the Author

www.nia.nih.gov
NIA, one of the 27 Institutes and Centers of NIH, leads a broad scientific effort to understand the nature of aging and to extend the healthy, active years of life. In 1974, Congress granted authority to form NIA to provide leadership in aging research, training, health information dissemination, and other programs relevant to aging and older people.