medwireNews: Research suggests that the timing of puberty affects the accuracy and interpretation of body mass index (BMI) z-scores for judging adiposity in both girls and boys.

Specifically, BMI z-scores overestimated the adiposity of earlier-maturing children relative to those who entered puberty later, report Kaspar Sørensen and Anders Juul, from Copenhagen University Hospital in Denmark.

The researchers studied data from 486 girls and 316 boys, aged between 8.5 and 16.5 years, who were participants of the Copenhagen Puberty Study.

The girls were divided into early, average and late maturing groups according to their age, breast stage and age at menarche. BMI z-score was significantly higher among the 179 early-matured girls than the 158 later maturers, by 0.91 standard deviations. And 16.4% versus 6.3% were classified as overweight or obese.

Yet the proportion of girls classed as overweight or obese based on their percentage body fat measured using bioelectric impedance analysis was similar between the early and late puberty groups, at 8.6% and 5.1%, respectively.

The researchers explain that a higher BMI for a given age is to be expected among early-maturing children, because of earlier pubertal growth spurt causing accumulation of both lean and fat body mass. “Thus, higher BMI [does] not reflect higher adiposity per se.”

Indeed, the girls’ BMI z-scores were significantly and inversely associated with age at Tanner breast stage 2, even after accounting for percentage body fat.

The results were similar for boys, with BMI z-scores for a given testicular volume being a significant 0.75 standard deviations higher in the 87 early-maturing boys than in the 131 late-maturing boys. But again the proportion of boys classed as overweight or obese based on percentage body fat was similar in the early and late puberty groups, at 7.5% and 9.1%, respectively.

“Our result[s] indicate that adjustment of BMI for stage of maturation rather than age will improve the accuracy in the evaluation of adiposity between early and late maturing children during the pubertal period”, writes the team in the European Journal of Endocrinology.

They say: “The lack of acknowledgement of this maturational effect on BMI may at least to some extent have overestimated BMI in early pubertal children reported in most previous published population-based cohort studies.”

By Eleanor McDermid, Senior medwireNews Reporter

Eur J Endocrinol 2015; Advance online publication

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