Body composition in adopted and biological siblings.

Body density through underwater weighing and 6 skinfolds (triceps, biceps, subscapular, suprailiac, abdominal and medial calf) were measured in 871 biological and adopted siblings of French descent from the greater Quebec city area. These individuals belonged to 49 sibships of adopted sibs, 71 sibships of unrelated individuals living together including adoptees, 35 «sibships» of first degree cousins, 229 sibships of biological brothers and sisters, 66 sibships of DZ twins and 83 sibships of MZ twins. Age of subjects ranged from 8 to 26 years. Age and sex accounted from 14% to 48% of the variance in body composition measurements, with the exception of fat free weight (71%). Residual scores of age and sex were submitted to analysis of variance and correlation analysis. The ratio of the between sibships to the within sibship means of squares reached significance (p ^ .05) for almost all measurements in the MZ, DZ, biological and cousin sibships, for the skinfolds but not for body density, fat free weight and the ratio of subcutaneous fat to total fat mass in the unrelated sibships, and for none of the measurements in the adoptive sibships. Interclass correlations could be computed in a maximum of 80 pairs of adopted sibs, 120 pairs of unrelated sibs, 95 pairs of cousins, 370 pairs of biological sibs, 69 pairs of DZ twins and 87 pairs of MZ twins. None of the coefficients reached 0.20 (p < 0.05) in the pairs of adopted or unrelated sibs. On the other hand, correlations were often significant in the pairs of cousins, particularly for the skinfold measurements. The same coefficients were generally significant and increasing from the pairs of brothers and sisters to the pairs of DZ twins and MZ twins. This pattern was particularly clear for several individual skinfolds, the extremity and the trunk skinfolds, the ratio of extremity to trunk subcutaneous fat and fat free weight. Genetic effects reaching about 0.40 and above were found for measurements of subcutaneous fat, fat distribution, body density and fat free weight. Twin data yielded higher and often unrealistic estimates of heritability probably because of the differential effects of the common environment in MZ and DZ pairs. Skinfold measurements, body density obtained through underwater weighing and fat free mass derived from the latter are generally used as indicators of body composition in humans (Malina, 1980). Several investigators have studied skinfold measurements in biologically related brothers and sisters (Howells, 1966; Garn et al. 1975; Mueller, 1978; Mueller and Malina, 1980; Bouchard, 1980; Savard et al. 1983). Others have considered skinfold data in pairs of dizygotic and monozygotic twins living 1Physical Activity Sciences Laboratory, Laval University, Quebec, Canada G1K 7P4. Human Biology, February 1985, Vol. 57, No. 1, pp. 61-75 © Wayne State University Press, 1985 This content downloaded from 157.55.39.144 on Wed, 07 Sep 2016 04:31:57 UTC All use subject to http://about.jstor.org/terms 62 Claude Bouchard et al. together (Osborne and DeGeorge, 1959; Brook et al. 1975; Hawk and Brook, 1979), but only Garn and co-workers (1979) have reported such data in pairs of genetically unrelated siblings living together. On the other hand, the studies cited above have used a maximum of four skinfold measurements, with the exception of Savard et al. (1983), who reported on six sites. Moreover, the relationship between subcutaneous fat and total body fat is not a simple one, as it is affected by age, sex and total fat among several factors (Lohman, 1981). To the best of our knowledge, no studies have been published about body density and fat free weight in biological and unrelated siblings. Therefore, the purpose of this report is to analyse the resemblance in unrelated and biological sibships and the covariation in pairs of sibs of various kinds for six skinfold measurements, subcutaneous fat distribution, body density, fat free weight and a ratio of subcutaneous fat to total fat mass. Material and Methods