Obese Kids' Intake Much Higher Than Thought, Model Shows

Norra MacReady

July 29, 2013

A new mathematical model calculates, with unprecedented accuracy, the caloric excess that produces childhood obesity and the changes in energy balance necessary to achieve a healthier weight.

This is "the first mathematical model of childhood energy balance and bodyweight dynamics that accounts for healthy growth and development of obesity and makes quantitative predictions about weight management interventions," the model's developers, led by Kevin D. Hall, PhD, from the National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, Maryland, write in an article published online July 30 in the Lancet Diabetes & Endocrinology.

The model "allows us to accurately predict how a child's energy intake affects his or her likelihood of becoming overweight or obese," Dr. Hall said in a journal press release.

One of the model's most interesting and unexpected predictions was "that obese children might outgrow obesity if a successful weight-maintenance intervention is instituted during a period of rapid growth. In this situation, the model predicted that the strong drive to accrete fat-free mass translates to a substantial loss of body fat despite no change in bodyweight, which would be very difficult to achieve in adults," the authors state.

The model is based on the energy balance principle, which states that weight changes result from an imbalance between calories consumed and energy expended. It accounts for the metabolic adaptations that occur when body weight changes and distinguishes between the energy needs of lean and adipose tissue. The model also accounts for sex-specific differences in metabolism and growth rates. To allow for normal growth-related energy requirements between the ages of 5 and 18 years, the authors assumed a gradual increase of roughly 1200 kcal/day for boys and 900 kcal/day for girls.

When tested against existing data, the authors found that the model accurately reflected cross-sectional data from 292 healthy white children, as well as data from black and Hispanic children.

In the model, the development of childhood obesity was simulated by gradually increasing caloric intake from age 5 years while keeping all other parameters compatible with healthy growth. According to this model, obese boys consumed a mean of 750 kcal/day more than healthy-weight boys between 5 and 11 years of age. For obese girls, the mean daily caloric excess was 850 compared with their normal-weight peers. By the end of this period, the mean daily caloric imbalances were 1100 and 1300 for boys and girls, respectively, compared with healthy-weight children.

The model has yielded predictions of energy consumption among obese children that are much higher than those derived from standard calculations. For example, for a girl who was in the 50th percentile for weight at age 5 years and is 10 kg overweight by age 10 years, standard methods would estimate that her excess consumption would have been about 40 kcal/day during that 5-year period. "By contrast, use of our rule implies that she is eating roughly 400 kcal per day in excess of a peer that remained at the 50th percentile from age 5–10 years," Dr. Hall and colleagues write.

Children may outgrow their obesity on a weight-maintenance regimen, according to the model, but boys will have an easier time of it than girls, the authors warn. In their simulation, girls already had 9 kg more body fat than boys by age 11 years. At this level of excess fat, some weight loss would be necessary for normalization of body composition. In addition, it is important to time any intervention for the period of maximum growth, as weight-loss interventions attempted at other times would be predicted to result in the loss of substantially less body fat.

Study limitations include the possible failure to estimate accurately for children whose rates of growth and development are significantly outside the average or for children whose weight gain followed a trajectory different than the gradual increases assumed in this model.

In an accompanying editorial, Claudio Maffeis, MD, a pediatrician and head of the clinical nutrition and obesity unit at the University of Verona, Italy, notes that the model shows that, "contrary to common perception, the energy needed to accrue body weight in excess of that gained through normal growth is higher than the energy content of the extra mass accumulated. This finding is explained by the higher energy requirement associated with increased body weight."

He notes that obese and overweight children significantly underestimate their food intake, and "the accuracy of parents' awareness of children's portion sizes and reporting of children's food intake is only moderate," which make it hard for clinicians to help children change their eating habits. He also points out that the model suggests that the best time for intervention is before puberty, especially in girls. Given the differences in body composition and caloric needs between the sexes predicted by the model, any postpubertal dietary interventions should be individually tailored to each child.

"The model can...be used to calculate the magnitude of intervention necessary to achieve a desired change in bodyweight," the authors conclude. "Policy makers and clinicians now have a quantitative technique for understanding childhood obesity and specifying energy balance benchmarks for interventions to address the childhood obesity epidemic."

Dr. Hall reports a US patent application assigned to the National Institutes of Health related to the use of mathematical models of human metabolism for bodyweight management. The other authors and Dr. Maffeis have disclosed no relevant financial relationships.

Lancet Diabetes Endocrinol. Published online July 30, 2013.

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