Читать книгу Pathy's Principles and Practice of Geriatric Medicine - Группа авторов - Страница 337

Many studies have documented that the relative risks for chronic disease related to BMI become less pronounced with ageing. One review examined the effect of an elevated BMI on all‐cause mortality risk in men and women age 65, concluding that overweight (25–29 kg⁻²) elders do not have an increased risk of mortality. Obesity (BMI ≥30 kg⁻²) carries only a modest increase in mortality risk regardless of gender, disease status, and smoking status in older adults.⁴ Several explanations for this phenomenon include selective survival, cohort effects, and/or a ceiling effect of mortality rates. However, another explanation may be the limitation of BMI as an indicator of body composition. The U‐shaped relation between BMI and mortality in younger adults may result from a positive association between body fat mass and mortality and an inverse linear association between fat‐free mass and mortality. As stated above, the ratio between the two compartments (fat and fat‐free mass) changes with ageing. Further, kyphosis in old age makes it difficult to measure height and, therefore, may result in unreliable estimates of BMI. For this reason, changes in weight (rather than changes in BMI) are a preferable measure with consideration of oedema in the overall interpretation.⁵ As a final reason for the less pronounced relative risk of a high BMI, it is suggested that an excess fat mass is less detrimental in old age. However, recent views on pro‐inflammatory factors related to adiposity indicate that fat loss ameliorates some catabolic conditions of ageing since some cytokines may directly affect muscle protein synthesis and breakdown. A voluntary decrease in weight may also ease the mechanical burden on ageing joints and muscles, thus improving mobility. Therefore, only weight‐loss therapy that minimizes muscle and bone mineral density loss is recommended for older obese people. Especially in the case of sarcopenic obesity, prevention of further loss of muscle mass is necessary. Sarcopenic obesity is defined as the coexistence of diminished lean mass and increased fat mass.⁶

Prospective studies have shown that weight loss and decline in BMI can both be markers of and independent contributors to adverse health outcomes. Involuntary weight loss in elderly subjects is likely to reflect sarcopenia, a loss of lean body mass and particularly muscle mass. Cachexia is a disease‐related weight loss, and starvation or undereating reflects a loss of fat mass predominantly. Weight loss may contribute to increased mortality, especially if the initial body weight is relatively low. Prevalence of involuntary weight loss in community‐dwelling elders is 5–15%, 8% of all adult outpatient visits, and 27% of frail people ≥65 years old.^7‐9 Clinically, the observation of weight loss is considered the most important indicator of under‐nutrition. A loss of 10% in six months, 7.5% in three months, or 5% in one month is considered serious, owing to the direct relationship with morbidity and mortality. The reduction in total caloric intake is associated with nutritional depletion that may in part contribute to the mortality risk.¹⁰