Thursday July 8 2010
The study recruited 307 healthy children at school entry age
“Lack of exercise is not to blame for the childhood obesity crisis,” The Daily Telegraph reported. The newspaper explained that researchers have found children become less physically active because of obesity and are not necessarily obese because they are inactive.
The debate about whether overeating or inactivity is the greater cause of childhood obesity hinges on which comes first. This good quality study followed 200 seven-year-olds for three years, and found that 10% more body fat at age seven leads to four minutes less moderate or vigorous exercise each day by age 10.
These results imply that focusing on diet rather than exercise may be the best approach for addressing the growing childhood obesity problem. However, it is still not clear which intervention is the most effective, and both a balanced diet and more exercise are probably needed. Physical exercise has many health benefits and the study should not be interpreted to mean that it has no benefit for children.
Where did the story come from?
The study was carried out by researchers from the Peninsula Medical School in Plymouth and the University of Plymouth in the UK.
It was funded by several organisations including the Bright Futures Trust, Diabetes UK, Smith’s Charity, the Child Growth Foundation, the Diabetes Foundation, the Beatrice Laing Trust, and the pharmaceutical companies Abbott, Astra-Zeneca, GSK, Ipsen and Roche.
The study was published in the peer-reviewed medical journal Archives of Disease in Childhood.
The BBC also covered this research, focusing on the debate about causes of obesity and emphasising that ‘there is no suggestion that exercise is not good for children’.
What kind of research was this?
The aim of this study was to see if inactivity is the cause of fatness or fatness the cause of inactivity. The researchers wanted to test the widely believed assumption that childhood obesity is caused by physical inactivity. They claim that public health and school-based interventions, designed to make children more active, rarely succeed in reducing obesity.
The study was a cohort design, which was appropriate for answering this sort of question. A particular strength is that it was prospective, and first recruited children, took a range of measurements and then followed them up over time to answer this question of causality.
This research is part of a large ongoing study called the EarlyBird study, running in Primary schools in Plymouth since 2000. Findings from EarlyBird are generally respected within the scientific community and it has produced several reports so far. More are expected and the results that have a longer follow-up will be of particular interest.
The researchers say previous studies testing the link have usually been cross-sectional (that is, they have taken a ‘snapshot’ and looked at associations between factors at only one point in time). As such, they cannot be used to imply cause and effect.
What did the research involve?
EarlyBird began in 2000 when 307 healthy children were recruited at school entry (aged five years) from Plymouth primary schools. The pupils were selected so that the overall group was similar in demographic and socio-economic mix to the city as a whole and to other cities in the UK.
Just over half of the children selected were boys (55%) and 98% were Caucasian (white). The researchers excluded children with illness such as diabetes, any condition or disease likely to affect growth and those with physical disability or taking steroids. The final population consisted of 202 children from 40 schools, a quarter of whom (25%) were overweight or obese.
All participants had a range of physical activity and body fat measurements taken every year from 2000. This particular research covers four of these annual time points from age seven (when body fat measurements were first measured objectively) to age 10.
Physical activity was recorded using an accelerometer (an electronic device that records movement and therefore indicates levels of physical activity). This was worn for seven consecutive days (five school days and two weekend days) at each annual time point, and only recordings that captured at least five days (including one weekend day) were used. Parents were asked about any period of inactivity when the child had taken the device off so that average readings could be used to fill the gap. These devices have been well validated in other studies to reliably measure physical activity and its intensity.
The researchers analysed two measurements of activity: the total volume of activity and the time spent at moderate and vigorous intensities. Body fat as a percentage of total weight was scanned using a dual energy X-ray absorptiometry scanner. Measurements of body mass index (BMI, kilogram per meter squared) and waist circumference were also recorded. Measurements were made at four annual time points when the children were 7, 8, 9 and 10 years old.
The researchers analysed the data appropriately using linear regression modelling, a statistical technique that is able to quantify the association between variables, and adjusted the results for age and sex.
What were the basic results?
It was found that measurements of the children’s body fat could predict changes in physical activity over the three years of follow-up. Conversely, activity levels did not predict subsequent changes in body fat percentage over the same follow-up period.
Using the model to estimate the strength of an effect the researchers say that for each 10% increase in body fat at age seven, there was a predicted reduction in the daily moderate and vigorous intensity activity of about four minutes a day from age 7 to 10 years (r=−0.17, p=0.02).
In contrast, more activity at age seven years did not predict a relative decrease in the percentage of body fat between 7 and 10 years (r=−0.01, p=0.8).
How did the researchers interpret the results?
The researchers say that physical inactivity appears to be the result of fatness rather than its cause. They go on to say that the ‘reverse causality’ may explain why attempts to tackle childhood obesity by promoting physical activity have been largely unsuccessful.
This research comes from a well designed UK cohort study, and sheds some light on the question of whether physical inactivity or fatness come first in overweight children.
However, physical activity, diet, energy intake and weight are interrelated in a complex way. Although physical inactivity may be the result of increased body fat to some extent, physical activity will have other benefits for children’s fitness, health and enjoyment of life. A combination of a healthy balanced diet and increased physical activity are likely to remain the best way for children to maintain a healthy weight and a focus on the overall benefits of activity is important.
The study has several strengths:
- the three-year interval between measurements allows the researchers to infer the direction of causality: that fatness comes before physical activity
- The measurements of body fat and physical activity were obtained using an accelerometer and a scan for body fatness. These are objective techniques and increase the reliability of these findings.
The researchers also mention a potential limitation in that they did not directly measure energy consumption. Saying this is usually an unreliably measured in this age group, they say they were unable to exclude the possibility that this was a confounder. This means that it could possibly be an underlying explanation for both activity levels and body fat.
This research may help prioritise which interventions are likely to be useful for children and in which order they should be tried.