Weight-to-length ratio at birth predicts obesity

NEW YORK (Reuters Health) - Measurement of birth weight compared with length is a better predictor than birth weight of how fat a child will become, UK researchers report.

The measurement, known as the ponderal index, is calculated by dividing weight in kilograms by length in centimeters cubed. It is similar to the better-known body mass index (BMI, weight in kilograms divided by height in meters squared) but is thought to provide a better gauge of weight in relation to height and body composition.

While birth weight has been linked with obesity risk in both children and adults, Dr. Imogen S. Rogers of the University of Bristol and colleagues note, information is lacking on how birth weight affects a person's percentage of fat and lean mass.

To investigate, the researchers used a technique called dual-energy X-ray absorptiometry to precisely measure lean body mass and body fat percentage in more than 6,000 boys and girls who were between 9 and 10 yeas old.

While both fat mass and lean mass rose with birth weight, there was no relationship between birth weight and the ratio of fat to lean mass, the researchers found. However, the higher a child's ponderal index at birth, the greater the ratio of fat to lean mass at age 9 or 10. Higher ponderal index was also associated with a higher percentage of body fat.

Children who were longer at birth had lower percentages of body fat as children, as well as less fat in relation to lean body mass.

Rogers and her team conclude that the well-established association between greater weight and birth and higher future BMI is due to increases in both fat and lean body mass.

They conclude that ponderal index at birth is a better predictor of future obesity than is birth weight.

SOURCE: American Journal of Clinical Nutrition, October 2006.