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Over the lifespan there is a decline in food intake. This has been termed the physiological anorexia of aging. It has many causes, including alterations in the gastrointestinal satiating system, the effect of elevated leptin levels, especially in men, and a variety of changes in central nervous system neurotransmitters. Beyond the age of 70 years body mass declines. This includes both loss of adipose tissue and muscle mass. The loss of muscle mass in older individuals is termed sarcopenia. There is increasing evidence that this is caused, in men, partly by the decline in testosterone. Illness results in an increase of cytokines that produce both anorexia and cause protein wasting. Many of the causes of cachexia in older individuals are treatable. Depression is the most common cause of weight loss in older individuals. Dieting in older individuals is associated with a loss of skeletal tissue as well as fat mass. This can place older individuals at risk of becoming the 'fat frail'.
John E. Morley (Mon,) studied this question.