"Exercise 'doesn't prevent childhood obesity'”, read the headline in The Daily Telegraph . The newspaper reported that a study in 300 children suggested that “the obesity epidemic amongst children is caused more by what they eat than lack of exercise”.
These are not the facts as they were reported by the scientific study that gave rise to the story.
The study sought to test whether the government recommendation that children do at least one hour’s exercise a day had a measurable impact on their health. Specifically, it looked at whether the recommended amount of exercise reduced the children's BMI and other measures of obesity.
What the study found was that a large proportion of children do not do an hours exercise a day (only 11% of girls met the target). In addition, even the children who met the target did not show a change in their BMI, although blood tests showed them to be healthier than the more sedentary children.
The study's authors concluded that “in children, physical activity above the Government-recommended [guidelines] is associated with a progressive improvement in metabolic health, but not with a change in BMI or fatness”. They say that girls do less physical activity than boys, and this may mean that girls need to be encouraged to increase their activity, or that the recommendations may need to be adjusted for girls.
Where did the story come from?
Dr Brad Metcalf and colleagues from the Peninsula Medical School in Plymouth carried out the research. The study was funded by Diabetes UK, Bright Futures Trust, Smith’s Charity, Child Growth Foundation, Diabetes Foundation, Beatrice Laing Trust, Abbott, Astra-Zeneca, GSK, Ipsen, and Roche. The funding sources had no input into the design, analysis, interpretation or writing of the study. The study was published in the peer-reviewed medical journal: Archives of Disease in Childhood .
What kind of scientific study was this?
In this prospective cohort study, the researchers looked at the effect of physical activity on weight, body fat, and metabolic health in children who had not yet reached puberty. They focused on the minimum level of exertion for children as recommended by the Government. They report that these recommended levels, which are similar in the US and UK, state that “all young people should participate in physical activity of at least moderate intensity for one hour or more a day”. This is equivalent to three or more METs (metabolic equivalent of thermogeneis). METs are the internationally used unit of energy expenditure, and one MET corresponds to a person’s energy expenditure at rest. Three METs are equivalent to walking at about 4km per hour.
The researchers recruited 307 five-year-olds from 54 randomly selected schools in Plymouth. The children’s physical activity was measured using special lightweight monitors called accelerometers, which the children wore around their waists for at least nine hours a day for a seven-day period. The accelerometer measured the intensity, duration and time of the day the activity occurred. These figures were adjusted for the season in which the readings were taken. Using the data from the accelerometer, the researchers calculated how much time the children spent in at least moderate physical activity (three METS or higher) on average each day.
The children’s weight and height were measured and used to calculate their BMI. The researchers also measured two indicators of the children’s body fat (skin fold thickness at a number of points on the body, and waist measurement). They also measured indicators of the children’s metabolic health, including blood pressure, cholesterol and fat levels in the blood, and their resistance to the hormone insulin (indicating blood sugar balance in the body). The indicators of metabolic health were examined individually, and also pooled into a composite metabolic score.
All of the measurements were taken annually, from age five to eight years. The researchers used statistical methods to look at whether the amount of time the children spent doing moderate physical activity changed over the three years. They then looked at whether the children’s activity levels during the study were related to changes in their body mass, body fat or metabolic health between ages five and eight years. These analyses took into account the children’s age, years to follow-up, measurements at baseline, and socioeconomic status. When looking at the effect of activity on metabolic outcomes, the analyses were also adjusted for changes in body mass and fat.
What were the results of the study?
In their analyses, the researchers included the 212 children (69% of those recruited) who had complete metabolic data at ages five and eight years, and who had provided at least 20 days of physical activity data. At the start of the study, boys had an average weight of 19.5kg and BMI of 16.3, and girls had an average weight of 20kg and BMI of 16.2. The average amount of physical activity among the children did not change significantly over the three years of the study. Boys spent on average 57 minutes a day in at least moderate physical activity, and girls spent on average 45 minutes a day in at least moderate physical activity (ranges not reported). Among the boys, 42% (47 out of 113) met the Government’s suggested levels of physical activity, compared to only 11% (11 out of 99) of the girls.
There was no relationship in either girls or boys between the numbers of minutes spent in at least moderate physical activity and changes in measures of body fat or body mass over the study period. For example, during the study, average BMI increased by 0.5 in boys who were in the top 50% of physical activity, compared with an increase of 0.6 in boys who were in the bottom 50% of physical activity. Girls in both the top and bottom 50% of physical activity had an average increase in their BMI of one. However, children with higher levels of physical activity had better improvements in their metabolic health (as indicated by their composite metabolic health score) over time.
What interpretations did the researchers draw from these results?
The researchers concluded that “in children, physical activity above the Government-recommended intensity of three METs is associated with a progressive improvement in metabolic health, but not with a change in BMI or fatness”. They say that girls do less physical activity than boys, and this may mean that girls need to be encouraged to increase their activity, or that the recommendations may need to be adjusted for girls.
What does the NHS Knowledge Service make of this study?
This study looks at the effects of average physical activity between the ages of five and eight years on children’s body mass, body fat and metabolic health. There are a number of issues to consider when interpreting these results:
- It is unclear how many, if any, of the children in this study would be considered overweight or obese either at the start or end of the study, and therefore whether it would have been healthier for them to lose weight or reduce their BMI. Children will naturally increase in BMI as they grow, and childhood BMI may not accurately predict BMI as an adult, as many children change diet and activity patterns as they grow older.
- Body mass and fat depend on the balance of calorie intake and calorie use in physical activity, as well as genetic factors. The study did not assess how many calories or what types of food the children were eating. If children who engaged in more physical activity consumed more calories than children who engaged in less physical activity, the two factors would balance each other out, leading to similar changes in body mass and fat in the two groups.
- This was a relatively small study carried out in one region, and on children who were predominantly caucasian. The findings may not apply to populations of children from different regions or other ethnic backgrounds.
- Although the accelerometer provides an objective measure of children’s activity, it is possible that the week’s recording may not have reflected the children’s typical activity levels.
- The news reports that BMI may not be an accurate measure of the success of exercise targets and that “blood testing may be the only way to measure exercise benefits” is misleading. The value of screening BMI levels in children in efforts to target obesity has frequently been questioned, and this study contributes to this debate. Although there was some link between the measures of metabolism used in this study and activity levels, this does not mean to say that routine blood testing would be the answer to monitoring weight or activity in children.
The results of this study should not be taken to mean that physical activity has little effect on children’s health. Physical activity is essential to maintain health and not just to lose weight or fat. Children should be encouraged to engage in and enjoy physical activity as this will hold them in good stead when they become adults.
Sir Muir Gray adds...
All children need more exercise, some also need a change in diet.