Research Report: Evidence for Energy Conservation During Pubertal Growth
Charlotte Goddard
Tuesday, October 25, 2016
Children are particularly susceptible to weight gain at two stages - in infancy, which is probably attributable to diet and lifestyle choices made by parents, and again in puberty. A research team led by the University of Exeter Medical School undertook an investigation into the factors driving weight gain in adolescence.
Authors Mohammod Mostazir, Alison Jeffery, Jo Hosking, Brad Metcalf, Linda Voss, Terence Wilkin
Published by International Journal of Obesity, September 2016
SUMMARY
The team analysed data gathered from 279 schoolchildren in the Earlybird study, based in Plymouth, which took place between 2000 and 2012. The children were assessed every six months between the ages of five and 16. Blood samples were taken to assess metabolic health. Children's size, body composition - the proportion of fat, water, bone and muscle - metabolic rate and phsyical activity, were also measured. For this paper, the team tracked the children between the ages of seven and 16.
Human bodies burn calories voluntarily, through physical activity, and involuntarily, through bodily functions such as keeping the blood warm and internal organs working. To find out how quickly the children were burning calories involuntarily, they rested under special sealed hoods and their oxygen consumption was measured over a period of time. Because burning calories uses up a fixed amount of oxygen, the researchers were able to calculate their calorie use from the amount of oxygen consumed. Physical activity was measured through the use of accelerometers, which were worn on an elastic belt around the child's waist and set to run continuously for seven days.
The researchers found in young children the number of calories burned involuntarily rose with their body size. But from the age of 10, girls and boys experience a rapid drop in the number of calories burned. The fall was more pronounced in girls than boys. Researchers found this surprising as the number would be expected to rise with the growth spurt around adolescence, since growing burns up calories. Those aged 15 were found to use 400 to 500 fewer calories while at rest per day compared to when they were 10 years old - a fall of a quarter. It would take an hour of Zumba to burn 500 calories, which is about the amount contained in a Big Mac. However, by the time the young people reached the age of 16, their involuntary calorie expenditure began to climb again.
The study also found teenagers exercise less during puberty, adding to the calorie excess. This exercise drop is particularly obvious in girls, whose activity level was found to drop by about a third between the ages of seven and 16.
The team investigated different hormones, but none was able to explain the fall in resting energy expenditure. They do not know what mechanisms are responsible for turning down the energy expenditure of metabolically active cells.
The researchers speculate that humans may have evolved to preserve calories during adolescence to ensure there are enough to support changes in the body during puberty. However, in a culture where food is in abundance, the drop in energy expended leads to excessive weight gain.
IMPLICATIONS FOR PRACTICE
Childhood obesity and associated diabetes are two of the greatest health challenges of our time with a third of English schoolchildren found to be overweight or obese at 10 or 11. Strategies to prevent obesity, which is more prevalent among adolescents, should take note of this particularly vulnerable period, the researchers conclude.
FURTHER READING
Childhood Obesity: Evidence for Distinct Early and Late Environmental Determinants - A 12-Year Longitudinal Cohort Study (EarlyBird 62), Mohammod Mostazir and others, International Journal of Obesity, July 2015. An earlier study that found children tend to gain weight in early childhood and at puberty.
Exploring the Adolescent Fall in Physical Activity: A 10-Year Cohort Study (EarlyBird 41), Brad Metcalf and others, Medicine & Science in Sports and Exercise, October 2015. An examination of trends in physical activity suggesting inactivity in adolescence may be partly under biological control.
A Longitudinal Study of Resting Energy Expenditure Relative to Body Composition During Puberty in African American and White Children, Min Sun and others, The American Journal of Clinical Nutrition, February 2001. A US study that found differences in the resting energy expenditure of white and African American children.