Body Size At Birth And Later Body Composition

People who had high birth weight or were heavy during the first year after birth are at increased risk of obesity in later life (6,7). The effect, however, is small. In a study of military conscripts in Denmark, for example, mean body mass index rose from 22.7 kg/m2 in men whose birth weights were <2500 g to 24.9 kg/m2 in men whose birth weights were >4500 g (8).

People who had low birth weight tend to accumulate fat on the trunk and abdomen, a pattern of adiposity found in the insulin resistance syndrome in which central obesity, impaired glucose tolerance, hypertension, and altered blood lipid concentrations occur in the same patient (9). This disorder is associated with an increased risk of coronary heart disease (10). In two studies in the United Kingdom men who had low birth weight had high ratios of waist-to-hip circumference after allowing for adult body mass index (Table 1) (11). This association with low birth weight has been replicated in a study of men in Sweden (12). In the Swedish study birth weight was also associated with truncal fat, as measured by a high ratio of subscapular to triceps skinfold thickness. After allowing for current body mass index truncal fat increased by 0.30 standard deviations with each kilogram decrease in birth weight.

In one of the U.K. studies placental weight was also available (Table 1). Its association with the waist-hip ratio was in the opposite direction to birth weight so that the highest ratios of waist-to-hip circumference were in men who had had a high placental weight in relation to birth weight. High placental weight to birth weight ratios are a known marker of fetal hypoxia or undernutrition (13-15). The association with placental weight has not been examined in other studies, and may be inconstant because placental hypertrophy is only one

Table 1 Simultaneous Effects, Regression Coefficients (95% CI) of Birth Weight, Placental Weight, and Adult Body Mass Index on Waist-Hip Ratio (%) in Two Samples of Men in the United Kingdom

Preston (men aged 51)

Hertfordshire (men aged 64)

Adult body mass index (kg/m2) Birth weight (lbs) Placental weight (lbs)

of the fetus's responses to undernutrition. It depends on the timing of undernutrition and the mother's nutritional state (14).

Low birth-weight has been shown to be associated with truncal fat in young people. This association occurs across the normal range of birth weight. Among 30-year-old Mexican and non-Hispanic Americans low birth weight was associated with truncal fat but not with a high waist-to-hip ratio (16). Among English girls aged 14-16 years, those who were smallest at birth but fattest as teenagers also had the highest ratio of sub-scapular to triceps skinfold (17). Among children aged 7-12 years in Philadelphia, there was a similar association between low birth weight and truncal fat deposition with partial correlations between birth weight and the ratio of subscapular to triceps skin-folds of ~0.20, after allowing for body mass index (18). In 8-year-old Indian children the subscapular-to-triceps skinfold ratio fell from 84.8 to 77.7 across what is the normal range of birth weight in India, from 2.0 kgs or less to 3.25 kgs. Above 3.25 pounds the ratio rose to 82.2, which may reflect macrosomia at birth and persisting overweight in the offspring of mothers with gestational diabetes or impaired glucose tolerance (19). No association between birth weight and truncal fat was found in a comparison between 15-year-olds who had low birthweight, <5.5 lbs, and controls within the normal birth weight range (20). This could reflect aspects of body composition specific to babies weighing <5.5 lbs, since as adolescents these low-birth weight babies were markedly thinner than the controls.

High waist-hip ratio and high truncal fat may reflect different aspects of body composition, hormonal status, and metabolism (12). In the Swedish study low birth weight predicted a smaller hip but not a larger waist and therefore did not reflect abdominal obesity. A study of young American men applying for military service led to the same conclusion (21). Birth weight was linearly associated with body mass index and with the area of thigh muscle but not with subcutaneous fat on the thigh. The association between birth weight and body mass index may therefore have reflected an association with lean tissue rather than fat. Waist-hip ratio and truncal fat also have different metabolic associations. Waist-hip ratio is more strongly associated with serum triglyceride and HDL cholesterol concentrations than is the sub-scapular-triceps skinfold ratio (22). The associations between low birth weight and these two measures of body composition are likely to reflect influences in the intrauterine environment that are either different or act at different times.

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