24-HOUR RESPIRATORY QUOTIENT - THE ROLE OF DIET AND FAMILIAL RESEMBLANCE

Body weight and obesity show familial resemblance that could be the re sult of familial correlation of fat oxidation, low levels of which hav e been implicated in the etiology of weight gain and obesity. We studi ed the familial correlation of both 24-h respiratory quotient (RQ), an index of the ratio of fat to carbohydrate oxidation, and the possible influence of dietary macronutrient composition expressed by the food quotient (FQ), i.e. the theoretical RQ produced by the diet. We measur ed the habitual FQ of the 7 days diet by weighed food records, followe d by measurement of 24-h RQ in respiration chambers in 71 healthy Cauc asian siblings hom 31 families. After adjustment for age, gender, and 24-h energy balance, 24-h RQ correlated in families as indicated by an intraclass correlation coefficient (r(i)) of 0.31 (P = 0.03). FQ, adj usted for age and gender, was also a familial trait for the two days i mmediately preceding diet (r(i) = 0.32, P < 0.01). The familial effect on 24-h RQ, adjusted for age, gender, and 24-h energy balance, remain ed after adjustment for the FQ of the two days preceding diet (r(i) = 0.27, P < 0.05) and was reduced but not abolished after further adjust ment for fasting plasma insulin plus free fatty acids (r(i) = 0.24, P < 0.09). By a correlation analysis aimed at separating familial and in dividual nonfamilial factors influencing both 24-h RQ and FQ, we found a great but insignificant familial (eta(F) = 0.49, P < 0.18) and a so mewhat lower, but significant individual nonfamilial correlation (eta( NF) = 0.35, P < 0.03). We conclude that substrate oxidation rates meas ured by RQ exhibit familial correlation after proper adjustment for co nfounders Such as energy balance, gender, and age, and that this effec t could not be fully explained by preceding diet composition, fasting plasma insulin, and free fatty acids. Further RQ and the habitual diet ary composition shared familial and nonfamilial factors.