Indirect estimates of body composition are useful for groups but unreliable in individuals

OBJECTIVE: To assess the usefulness of the body mass index (BMI) in identif ying individuals classified as overweight or obese based on estimates of bo dy fat percentage (BF%) obtained by the deuterium dilution (BF%(DD)) method . In addition, to assess the accuracy of bioelectrical impedance analysis ( BIA) and skinfold thickness (SFT) measurements in the estimation of body co mposition of Australians at the individual and group level. DESIGN: Cross-sectional study. SUBJECTS: One hundred and seventeen healthy Australian volunteers of Europe an descent, comprising of 51 males and 66 females, ranging in age from 19 t o 77 y. MEASUREMENTS: BMI was calculated from body weight and height. Fat-free mass (FFM) was estimated from measures of total body water (TBW) using deuteriu m dilution (FFMDD). SFT using the equations of Durnin and Womersley (Br J N utr 1974; 32: 77 - 97) (FFMSFT), and BIA using the equations of Luhaski at al (J Appl Physiol 1986; 60: 1327 - 1332) (FFMLU), Segal et al (Am J Clin N utr 1988; 47: 7 - 14) (FFMSe) and Heitmann (Eur J Clin Nutr 1990; 44: 831 - 837) (FFMHe). Estimates of fat mass (FM) were calculated as the difference between body weight and FFM, while BF% was calculated by expressing FM as a percentage of body weight. RESULTS: BMI had poor sensitivity and positive predictive value in identify ing individuals as being overweight/obese as classified by BF%(DD). Further more, estimates of FFM (and hence FM) from BIA or SFT could not be used int erchangeably with DD, without the risk of considerable error at the individ ual level. At the group level errors were relatively smaller, though statis tically significant. While FFMSFT could be corrected by the addition of the bias (1.2 kg in males and 0.8 kg in females), no simple correction was pos sible with BIA estimates of FFM for any of the equations used. However, an accurate prediction of FFMDD was possible from the combination of FFMHe, bi ceps SFT and mid-arm circumference in both males and females. The bias of t his prediction was small ( < 0.15 kg), statistically non-significant in bot h sexes, and unrelated to the mean FFM obtained by the two methods. The rev ision of Heitmann's estimate of FFM using anthropometric variables describe d in this study had the best sensitivity (79%), specificity (96%) and posit ive predictive value (92%) in identifying overweight/obese individuals in c omparison to the other equations tested. CONCLUSION: BMI was a poor surrogate for body fatness in both males and fem ales. The currently recommended equations for the prediction of body compos ition from SFT and BIA provided inaccurate estimates of FFM both at the ind ividual and group level as compared to estimates from DD. However, Heitmann 's equations, when combined with measures of the biceps SFT and mid-arm cir cumference, provided better estimates of FFM both at the individual and gro up level.