Obesity Epidemiology

68 STUDY DESIGNS AND MEASUREMENTS(A)Muscle mass (kg)5040302010MenWomen(B)Muscle mass (%)55453525MenWomen01525 35 45 55Age65 75 851515 25 35 45 55Age65 75 85Figure 5.1 (A) Relationship between whole-body SM (skeletal muscle) mass and age in men andwomen. Solid lines, regression lines. Men: SM mass = −0.001 (age 2 ) + 35.5; SE of estimate(SEE) = 5.1. Women: SM mass = −0.001 (age 2 ) + 22.5; SEE = 3.6. (B) Relationship betweenrelative SM mass (SM mass/body mass) and age in men and women. Solid lines, regression lines.Note that slope of regression line is greater (P < .01) in men than in women. Men: SM mass =−0.188 (age) + 46.0; SEE = 4.4. Women: SM mass = −0.084 (age) + 34.2; SEE = 5.4.Reproduced with permission from Janssen I, Heymsfield SB, Wang ZM, Ross R. Skeletal musclemass and distribution in 468 men and women aged 18-88 yr. J Appl Physiol. 2000;89:81-88. 99Multiple factors may be responsible for the change in body composition with aging,including decreased testosterone levels and physical activity, and lower consumption ofprotein energy. 84 Because of age-related changes in body composition, between-personvariations in BMI reflect, in part, variations in LBM, reducing the validity of BMI asa marker of body fat in the elderly. 57 Using data from NHANES I and NHANES II,Micozzi and Harris 101 found a stronger correlation between BMI and estimates of bodyfat in younger men and women than in older ones; in contrast, correlations between BMIand muscle mass were stronger in older individuals than in younger ones.Aging is also associated with changes in body fat distribution. Hughes et al. 102 described10-year changes in body composition and the metabolic and physical activity factors associatedwith these changes in 54 men and 75 women aged 60.4 ± 7.8 years at baseline.During the 10 years of follow-up, the amount of subcutaneous fat declined (−17.2%;P < .001), while total FM increased (7.2%; P < .05), primarily due to an increase inintra-abdominal fat. An increase in physical activity was associated with an attenuation ofdecline in FFM (P < .007). Other studies in the elderly have shown increased intramuscularand intrahepatic fat, which is associated with increased insulin resistance. 103The lower validity of BMI as a measure of adiposity in the elderly may explain whythe relationship between BMI and mortality is less pronounced in older adults than inyounger adults (see Chapter 11). In the elderly, a low BMI probably reflects low LBMrather than low FM. 104 A larger WC appears to be a better measure of adiposity, especiallyabdominal fatness, in older people. This may explain why, in some epidemiologicstudies, WC is better than BMI as a predictor of mortality for the elderly. 105Ethnic DifferencesThe interpretation of BMI is complicated by well-recognized ethnic variations in bodycomposition. 93 Wagner and Heyward 106 conducted a detailed comparative review on measuresof body composition in blacks and whites. They noted that body mineral content,