Heights and weights of a large sample of subjects studied longitudinally from ages 3 and 13 years were used to calculate relative weight, using a Body Mass Index score (BMI) which estimates adiposity. Males and females differed significantly in BMI scores only at ages 3 and 13 years. The correlations between BMI scores at all ages were positive and significant. The subjects were divided at each age into a 'light', an 'average' and a 'heavy' group based on the position of their BMI score relative to the 25th and 75th percentiles. Subjects were 'tracked' from ages 3, 7 and 11 years to determine whether they had remained in the same BMI group by age 13 years relative to their peers. Slightly fewer than half of the 3 year old subjects but the majority of 7 and 11 year old subjects remained in the same relative weight group by age 13 years. Only 1% of 7 and 11 year old subjects in the top and the bottom quartiles for BMI scores shifted from one extreme group to the other. Relative weight at 3, 7 and 11 years was more persistent for subjects with extreme bodyweights than subjects with bodyweights in the middle range. Consistent with the pattern of correlations, the tracking pattern for all 3 groups showed that subjects' BMI scores remained more stable as the subjects grew older.
Fertility is dependent on sexual maturity, which is associated with a number of factors, including body mass. In this study the Body Mass Index (BMI) was used to estimate body fat in a large sample of 13-year-old New Zealand girls. Nearly half of the subjects (46%) had experienced the menarche. Expected gain in body mass was calculated from growth rates at earlier ages and used to examine whether deviation from the expected gain was associated with the onset of the menarche. In general, the results show an association between the menarche and gain in body mass. However, many girls who failed to achieve their expected gain had experienced the menarche (18%), indicating that the relationship between body weight and the menarche may not be causal or is mediated by other factors.
Body Mass Index (BMI) scores were calculated for two racially similar samples, one from New Zealand and the other from the United States, at ages 3, 5, 7 and 13 years. At ages 3, 7 and 13 years, the New Zealand subjects had higher BMI scores than their American peers. Chi-square analyses showed that a disproportionately large number of American subjects had scores in the bottom 20th percentile for their age and sex than expected at each of the four ages. Similarly, a disproportionately larger number of New Zealand subjects had BMI scores in the uppermost 20th percentile at ages 3 and 13 years. It is hypothesized that the difference in BMI scores for the two samples, which have previously been shown to have similar median heights and weights, reflects a difference in degree of body fat.
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