There is increasing evidence that the adverse effects of being overweight or obese, as defined by standard BMI criteria, are not as great in the elderly as in younger adults. Ideal weight ranges based on life expectancy are higher for older than young adults. For example, in a 12 year study of 324 000 people in the American Cancer Society Cohort, for people under the age of 75 years there was a significant and progressive increase in subsequent mortality as baseline BMI increased above 21.9 kg m−2. These adverse effects of increasing body weight diminished, however, with increasing age above 45 years and were absent altogether over 75 years.4 Among 4736 people aged 60 or more followed for an average of 4.5 years in the Systolic Hypertension in the Elderly Program (SHEP),5 those whose baseline BMI was in the lowest quintile (<23.6 kg m−2) had the highest subsequent mortality and those within the highest BMI quintile (≥31 kg m−2), corresponding to the conventional criteria of obesity, had the lowest mortality.

Recommendations for ideal weight ranges in older people vary, but there is good evidence that BMI values below about 22 kg m−2 in people over 70 years of age are associated with worse outcomes than higher weights, and BMIs below 18.5 kg m−2 are a particular concern. The optimum BMI for survival in people over age 70 years is probably in the range 25–30 kg m−2 and there is some recent evidence that BMIs above (so-called) normal may be somewhat more protective in women than men.6

Although optimum body weight for older people is probably higher than that for young adults, they in fact tend to weigh less. The decline in body weight with age, as measured by BMI, has been well documented in population-based, cross-sectional and longitudinal studies.7, 8. For example, in the 1997–1998 US National Health Interview Survey of 68 556 adults, more people aged 75 years and older than those aged 45–64 years were ‘underweight’ (BMI <18.5; 5 vs 1.2%) and substantially less were ‘overweight’ (BMI >25; 47.2 vs 63.5%).7

The lower average body weight of older than younger adults is not just because overweight people die earlier from obesity-related diseases, leaving the healthy, thin people behind. On average, people over 75 years of age are more likely to lose than gain weight,9, 10 in part explaining why they weigh less than younger adults. For example, in a study that followed 247 community-dwelling American men aged over 65 years for 2 years, on average these men lost 0.5% of their body weight per year and 13.1% of the group had involuntary weight loss of 4% per annum or more.9 Numerous studies have shown that weight loss in the elderly is associated with poor outcomes, certainly if involuntary, but possibly even when deliberate. The prospective Cardiovascular Health Study10 for example, studied 4714 home-dwelling subjects aged over 65 years without known cancer. In the 3 years after study entry, 17% of the subjects lost 5% or more of their initial body weight, compared with 13% who gained 5% weight or more. The weight-loss group had significant increases in total [2.09 × ↑ (95% confidence interval (CI) 1.67–2.62)] and risk-adjusted mortality [1.67 × ↑ (95% CI 1.29–2.15)] over the following 4 years compared with the stable weight group. The increased mortality occurred irrespective of starting weight and whether or not the weight loss was intentional. The weight gain group had no increase in mortality. In the SHEP study mentioned above,5 those subjects who had a weight loss of 1.6 kg per year or more experienced a 4.9 times greater death rate (95% CI 3.5–6.8) than those without significant weight change. Although mortality was also increased if weight increased more than 0.5 kg per year, the increase was less than that with weight loss (2.4-fold vs 4.9-fold increase). Of particular note, the adverse effects on mortality of weight loss were present even in the subjects who were heaviest at baseline (BMI ≥31) and were independent of baseline weight. The combination of initially low body weight and weight loss is especially bad news for older people. In the SHEP study,5 subjects with a low baseline weight (BMI<23.6 kg m−2 who lost more than 1.6 kg per year had a mortality rate of 22.6%, almost 20 times greater than the mortality rate of those with a baseline BMI of 23.6–28 kg m−2 whose weight remained stable. This interaction is a particular concern as the tendency for older people to lose weight is variable, with lean individuals probably most at risk.11 In an older person, unintentional weight loss of 5% or more over 6–12 months is associated with an increased risk of adverse effects, and a loss of 10% or more very likely means protein-energy malnutrition. There are many reasons why weight loss in older people has adverse effects. It some cases, weight loss is due to an illness, such as a malignancy, which is mainly responsible for the poor outcome and the weight loss is partly an ‘innocent bystander’. Nevertheless, the weight loss and associated undernutrition are themselves often a significant problem. This is because loss of body weight after the age of 60 years is disproportionately of lean body tissue, that is, sarcopenia,12 and individuals lose up to 3 kg of lean body mass per decade after the age of 50 years. Unlike loss of fat tissue, such a loss of lean tissue has adverse effects. Sarcopenia is associated with metabolic, physiological and functional impairments and disability, including increased falls and increased risk of protein-energy malnutrition.

Weight loss and resulting adverse outcomes in older people may be due to cachexia, malnutrition, the physiological anorexia of ageing or some combination of these factors. Although there is often considerable overlap between them, cachexia and malnutrition are not the same. Whereas all cachectic patients are malnourished, not all malnourished patients are cachectic.13 Cachexia is a complex metabolic syndrome associated with underlying illness and characterized by loss of muscle with or without loss of fat mass. The prominent clinical feature of cachexia in adults is weight loss, but inflammation is a key component. Anorexia, insulin resistance and increased muscle protein breakdown are also frequently associated with cachexia. Cachexia is distinct from starvation, age-related loss of muscle mass, primary depression, malabsorption and hyperthyroidism.14 Inflammation plays a major role in the pathogenesis of cachexia, with an absolute or relative increase in levels of inflammatory cytokines such as tumour necrosis factor alpha (TNF-α), interleukin-1 and interleukin-6.13 Conditions which often afflict older people which are frequently associated with cachexia include cancer, cardiac failure, chronic obstructive pulmonary disease and chronic renal failure. Recently, the European Society for Clinical Nutrition and Metabolism subclassified cachexia on the basis of severity into cachexia and pre-cachexia, the latter being present when there is (1) an underlying chronic disease, (2) a systemic inflammatory response, (3) anorexia and (4) unintentional weight loss over the previous 6 months of less than 5% of usual body weight.13

Prevalence

Protein-energy malnutrition is common in the elderly. Reported rates vary, in part because of differing methods used to diagnose this condition, but studies in developed countries have found that up to 15% of community-dwelling and home-bound elderly, between 23 and 62% of hospitalized patients and up to 85% of nursing home residents suffer from the condition.15

Adverse Effects

Reduced Food Intake

Ageing is associated with a decline in energy (food) intake. Energy intake decreases by ∼30% between age 20 and 80 years.17 Elderly people on average consume smaller meals more slowly, and fewer snacks between meals,15 and consistently report that they are less hungry than young adults.18 For example, the 1989 cross-sectional American National Health and Nutrition Examination Survey (NHANES III) study reported a decline in energy intake, between the ages of 20 and 80 years, of 1321 cal per day in men and 629 cal per day in women,19 a 7 year New Mexico longitudinal study of 156 persons aged 64–91 years reported a decrease of 19.3 kcal per day per year in women and 25.1 kcal per day per year in men20 and a Swedish 6 year longitudinal study of 98 people found that between the ages of 70 and 76 years there was a decrease in energy intake of 610 cal per in men and 440 cal per day in women.21

The Physiological ‘Anorexia of Ageing’

The age-related decline in food intake is not just due to the effects of illnesses that become more frequent with increasing age. Numerous studies have documented an age-related decline in appetite and energy intake in healthy, ambulant non-institutionalized people.17 Healthy older persons are less hungry and are more full before and become more rapidly satiated after eating a standard meal than younger persons. Much of this decrease in energy is probably a response to the decline in energy expenditure that also occurs during normal ageing. In many individuals, however, the decrease in energy intake is greater than the decrease in energy expenditure, so body weight is lost (see the preceding text). This physiological, age-related reduction in energy intake has been termed anorexia of ageing.15

Loss of Homeostasis

Old age is associated with diminished homeostatic regulation of many physiological functions, including the regulation of energy intake. For example, Roberts et al.22 underfed 17 young and old men by 3.17 MJ per day (∼750 kcal per day) for 21 days, during which time both the young and old men lost weight. After the underfeeding period, the men were allowed to again eat ad libitum. The young men ate more than at baseline (pre-underfeeding) and quickly returned to normal weight, whereas the old men did not compensate, returned only to their baseline intake and did not regain the weight that they had lost. Older people also have a reduced ability to detect and respond to dehydration. The combination of age-related physiological anorexia and impaired homeostasis means that both older people and young adults do not respond to acute undernutrition. Consequently, after an anorectic insult (for example, major surgery), older people are likely to take longer than young adults to regain the weight lost, remain undernourished longer and be more susceptible to subsequent superimposed illnesses, such as infections.

Pathological Anorexia and Undernutrition in Older People