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One of the truly age-driven phenomena is the loss of muscle mass and strength, called sarcopenia. It is distinct from muscle loss (cachexia) caused by inflammatory disease or from weight loss and attendant muscle wasting caused by starvation or advanced disease. Regardless of major differences between individuals, aging-related changes in body composition with time are universal. In addition to changes in lean tissue, this also holds for changes in fat mass, body water, and bone mass.

Throughout middle age, body mass tends to increase due to an accumulation of fat, preferentially intra-abdominally. Thereafter, usually after 60 years of age, it declines in association with loss of lean tissue. Diminution of physical activity enhances the changes in body composition occurring with aging, which in turn affect physical function. Ultimately, these processes result in a lower requirement for energy.

The total demand for energy is dominated by the energy needed per day to maintain vital functions, the basal metabolic rate (BMR), representing 60-70% of total energy expenditure. Most of the remainder (approximately 25%) is needed to cover the costs of physical activities. The BMR declines with age by up to 5% per decade. It is the decrease in lean tissue with age that determines this decline. One of the most important preventive measures in this process is the maintenance of physical activity. This helps to maintain lean body mass, physical fitness, and the requirement for energy.

Partly as a response to reduced energy needs, the energy intakes of affluent populations decline with age. This decline in food intake involves a decrease in meal size and a reduction in between-meal snacks. Morley called this physiologic decline in food intake ''anorexia of aging.'' This type of anorexia may be considered partially as a response to reduced energy needs but also partially as a dysregulation of food intake. Roberts et al. studied energy regulation in young and older adults by deliberately overfeeding and underfeeding their subjects. After a period of underfeeding, young people compensated by overeating when fed ad libitum. However, the older adults did not compensate. The same holds following a period of overfeeding; the older adults did not compensate with a reduction in food intake when fed ad libitum.

Anorexia of aging places an increasing number of elderly people at risk for malnutrition because the opportunities for providing an adequate dietary nutrient intake are very limited when total food consumption becomes low (e.g., <6.3 MJ (1500 kcal) (Figure 1). Current recommendations for daily energy intake are approximately 9MJ for elderly men and approximately 8MJ for elderly women. Institutionalized elderly people or the elderly who are sick are especially likely to fail to achieve such intakes.

For health reasons, it is important that elderly people avoid becoming underweight. Although losing weight may be favorable at younger ages and being overweight is a known health risk in adults, there is evidence that low body weight and loss of body weight in the elderly are more strongly associated with risk of mortality (Figure 2). This is clearly shown by data from the Survey in Europe

Women

Figure 1 Prevalence of inadequate intake of at least one nutrient among elderly people whose daily energy intake is less than 6.3 MJ (gray bars) and for those whose energy intake exceeds 7MJ (black bars). (From De Groot CPGM, van Staveren WA, Dirren H et al. (eds.) (1996). SENECA, nutrition and the elderly in Europe. Follow-up study and longitudinal analysis. European Journal of Clinical Nutrition 50(supplement 2):127, with permission from Macmillan Press Limited.)

Women

Figure 1 Prevalence of inadequate intake of at least one nutrient among elderly people whose daily energy intake is less than 6.3 MJ (gray bars) and for those whose energy intake exceeds 7MJ (black bars). (From De Groot CPGM, van Staveren WA, Dirren H et al. (eds.) (1996). SENECA, nutrition and the elderly in Europe. Follow-up study and longitudinal analysis. European Journal of Clinical Nutrition 50(supplement 2):127, with permission from Macmillan Press Limited.)

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Figure 2 Probability of survival for participants from the SENECA study with and without weight change in the first 4years. (Reproduced from Thomas D (ed.) (2002) Undernutrition in older adults. Clinics in Geriatrics 18(4), with permission of WB Saunders.)

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4000

1000 2000 3000 Survival time (days)

Figure 2 Probability of survival for participants from the SENECA study with and without weight change in the first 4years. (Reproduced from Thomas D (ed.) (2002) Undernutrition in older adults. Clinics in Geriatrics 18(4), with permission of WB Saunders.)

on Nutrition and the Elderly, a Concerted Action (SENECA). Weight loss (>5kg over 4years) seemed to be predictive for survival. It is even more important to be slightly overweight than underweight for people older than age 70 years. Therefore, except for those who are obese, elderly people should be encouraged to maintain an adequate energy intake. According to the SENECA study, 20-25% of the relatively healthy participants failed to do so: Approximately 8% lost and 16% gained at least 5 kg of body weight over a period of 4 years. When appetite is reduced, an increase in meal frequency may not only help to promote energy intakes but also prevent blood glucose levels from declining steeply.

Body Water, Dehydration, and Medication

Because lean tissue has a high water content, there is a decrease in total body water—especially extracellular water—with advancing age from 80% at birth to 60-70% after age 70 years. In addition, older people experience diminished sensation of thirst, and urinary concentrating ability declines as a function of age. Thus, older people have an increased risk of dehydration, particularly when diuretic or laxative medicines are used or in the presence of some diseases common in old age, such as diarrhea, renal disease, and infection with fever. Because water is essential to all biological functions, fluid intakes during old age should be at least 1700 ml per day. In the body, water acts as a dilutent for water-soluble drugs. Given the decrease in body water with age, older people may need lower dosages of water-soluble drugs than younger adults to achieve the desired therapeutic effect and to avoid drug toxicity.

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