Body mass index is of limited utility in assessing obesity in older people due to the age-related changes in body composition. The relationship between BMI and cardiovascular risk and mortality in the elderly is flattened and a high BMI carries a lower risk than in younger people. In contrast, a low BMI, which reflects a low lean body mass, confers a higher risk in older people in comparison with younger people. The combination of a low lean body mass and increased fat mass, known as sarcopenic obesity, confers the highest risk of frailty and death. Sarcopenic obesity increases in prevalence with age and is associated with functional decline, and also other causes of morbidity and mortality in the elderly. Various definitions of sarcopenic obesity have been proposed, such as relative skeletal muscle index (muscle mass divided by height squared) less than two standard deviations below the gender-specific mean of young adults and percentage body fat greater than median (27% in men and 38% in women); or muscle mass in the lower two and fat mass in the upper two quintiles of the population.1 Depending on the definition used and populations studied, the prevalence varies from 4.4 to 9.6% in men and from 3.0 to 12.4% in women.1 Identification of sarcopenic obesity requires precise, simultaneous methods of measuring body fat and muscle mass, such as dual-energy X-ray absorptiometry (DEXA). Bioelectrical impedance analysis (BIA) tends to underestimate fat mass in obese subjects11 and the reference ranges for the definition of sarcopenic obesity have not been widely validated. Computed tomography (CT) scanning is of greater precision, but is relatively expensive and may be less accessible.

Waist circumference (WC) correlates with abdominal fat mass as measured by CT and has been proposed as an additional measure for the definition of obesity in the elderly.7 Adults aged 51–72 years with BMI in the normal range (18.5–25 kg m−2) but WC ≥102 cm (men) or ≥88 cm (women) have a 20% higher risk of all-cause mortality than their counterparts with similar BMI but normal WC.12 Waist:hip ratio (WHR) is an alternative measure that provides some assessment of lean body mass and subcutaneous fat mass. A WHR exceeding 1.0 is associated with increased risk of obesity-related comorbidities, particularly cardiovascular disease. WHR is a better predictor of obesity-associated morbidity and mortality than BMI in the elderly.12–14

Overall Mortality

In contrast to young and middle-aged populations, in which the mortality risk increases with BMI, the relationship between BMI and mortality in the elderly may be U-shaped or reverse J-shaped, with increased mortality risk at BMI exceeding 35 kg m−2. The range of BMI which is associated with the lowest risk of mortality in older adults in most retrospective studies ranged from 27 to 30 kg m−2 7, 16 and is 2–5 kg m−2 higher in women than men.7 Several explanations have been proposed for this apparent paradox of overweight elderly having a lower risk of death: overweight people who are more susceptible to the adverse effects of obesity tend to die younger, leaving their peers of similar weight to be included in studies of older subjects; the effects of confounding conditions such as smoking and weight change due to undiagnosed chronic disease are difficult to separate from the effects of obesity per se, and older people may succumb to apparently-unrelated comorbidities. The elderly at greatest risk are those who are simultaneously sarcopenic and abdominally obese.

Body composition, in particular increased abdominal adiposity and reduced muscle mass, as measured by large waist circumference and waist:hip ratio, may be a better predictor of cardiovascular and all-cause mortality than high BMI. In the Rotterdam Study, WC ≥102 cm in men and ≥88 cm in women with normal BMI was associated with 20% higher mortality risk compared with subjects with a combination of normal BMI (18.5–<25 kg m−2) and normal waist circumference12 In a large study of community-dwelling adults aged 75 years and older, increasing WHR was associated with mortality from cardiovascular disease, with WHR exceeding 0.99 conferring the highest risk of mortality in non-smoking men, and WHR >0.90 in non-smoking women was associated with the greatest risk of death.13 In the MacArthur Successful Aging Study, a longitudinal study of high-functioning men and women aged 70–79 years, there was a graded relationship between WHR and all-cause mortality in women (relative risk 1.28 per 0.1 increase in WHR) and a threshold relationship in men (relative risk 1.75 for WHR >1.0 compared with WHR ≤1.0.14

Mobility-related Disability, Functioning and Quality of Life

Aging is associated with reduced muscle strength and mass and increased prevalence of osteoarthritis (OA), which adversely affects mobility, ability to perform activities of daily living and quality of life. Obesity is associated with increased risk of knee OA, possibly due to mechanical strain on weight-bearing joints, although metabolic syndrome is now recognized to be a risk factor for OA. In a community-based study of people aged 65–80 years, up to 96% of obese (BMI ≥30 kg m−2) subjects were frail, as determined by physical performance test scores, peak oxygen consumption and self-reported ability to perform activities of daily living.17 The same study also found that obesity was associated with a lower health-related quality of life, as assessed by the SF-36 physical function scoring. Sarcopenic obesity in particular is associated with high risk of impaired mobility and reduced functioning. The combination of obesity and low muscle strength was associated with the development of mobility-related disability in 930 people aged 65 years and above in the InCHIANTI study in Italy.18 Despite the positive association between weight and bone mineral density, a recent study found that high body weight or BMI increased the risk of vertebral fractures in postmenopausal osteoporotic women.19 Vitamin D deficiency, which is more prevalent in obese sedentary elderly because of reduced exposure to sunlight, impairment of vitamin D synthesis in the skin with ageing and obesity-associated reduction in bioavailability of cutaneous and nutritional vitamin D, is likely to contribute to osteopenia, fractures, muscle weakness and reduced mobility.

Metabolic Syndrome and Type 2 Diabetes Mellitus (T2DM)

The prevalence of the metabolic syndrome (defined by the presence of at least three of the following characteristics: WC >102 cm in men and 88 cm in women, high-density lipoprotein cholesterol <1 mmol l−1 in men and 1.3 mmol l−1 in women, blood pressure >130/85 mmHg, serum glucose >6.1 mmol l−1) increases with age. Increased body fatness and increased abdominal obesity, rather than ageing per se, are thought to be directly linked to the greatly increased incidence of metabolic syndrome and T2DM among the elderly. As in younger people, increased abdominal fat mass is independently associated with the development of metabolic syndrome in older men and women and waist circumference, a surrogate of abdominal adiposity, is positively correlated with dyslipidaemia, fasting glucose and hypertension, independently of BMI and age. Visceral adiposity-mediated inflammation is involved in the pathogenesis of insulin resistance in elderly obese adults. The age-related reduction in insulin sensitivity is likely to be due to increase in adiposity rather than being a consequence of ageing per se.

Fatty Liver