Relation of birth weight to lean and fat thigh tissue in young men.

BACKGROUND: Birth weight is positively associated with body mass index (BMI, kg/m2) in later life, but is inversely associated with cardiovascular risk. To understand this paradox, we examined the relationships between birth weight, adult BMI, and estimations of lean and fat tissue in young men.
METHODS: From 192 applicants for military service (ages 17-22 y, mean BMI 23.2 kg/m2) with known birth weights we measured the circumference and anterior skinfold thickness at midthigh to estimate thigh muscle+bone area and subcutaneous fat area. Linear regression models including birth weight as the independent variable were adjusted for race and adult height.
RESULTS: BMI was linearly associated with birth weight (standardized regression coefficient, [SRC]=+0.27; P=0.0004), as was the thigh muscle+bone area (SRC=+0.22; P=0.0029), but not the thigh subcutaneous fat area (SRC=+0.13; P=0.086). The BMI-birth weight association was reduced by 68% when the regression model was further adjusted for thigh muscle+bone area. Separate adjustment for thigh subcutaneous fat, however, reduced the BMI-birth weight association by only 30%. Waist circumference was also associated with birth weight (SRC=+0.24; P=0.0014), sagittal abdominal diameter was weakly associated (SRC=+0.17; P=0.028), but waist/thigh ratio and abdominal diameter index were not associated with birth weight.
INTERPRETATION: The larger BMI associated with higher birth weight may reflect increments in lean tissue more than increments in fat. Birth weight's influence on lean tissue is observed in the thigh and, among fit young men, perhaps at the waist. Increased muscularity in young men may partly explain the cardiovascular benefit in middle age ascribed to higher birth weight.