Most of the studies included in the review were based in high-income countries such as the USA and Europe, although 14 were in middle-income countries. Just over half of the studies focused on 6 to 12-year olds, a quarter on 0 to 5-year olds and the rest were on teenagers (13 to 18-year olds). The studies were carried out in different settings including home, pre-school or school.
Comparing children’s weight is tricky because children are growing all the time. One way of assessing children’s weight is to calculate a score based on their height and weight (this is called the body mass index or BMI), then comparing this to average scores for that country. This is called the zBMI score. We found 61 studies that reported zBMI scores, involving over 60,000 children.
Children aged between birth and five or six to 12 who were given an intervention to change both their diet and physical activity reduced their zBMI score. But interventions which focussed only on changing diet or on changing physical activity didn’t make a lot of difference to zBMI. The reviewers found fewer studies for children aged 13 to 18 years and from these they found physical activity interventions might reduce zBMI but certainty for this evidence is low.
The team looked to see if the strategies to change diet or increase physical activity were likely to work fairly for all children, for example girls and boys, children from wealthy or less wealthy backgrounds, and children from different racial backgrounds. In the studies where this was reported, there was no indication that the strategies increased inequalities. However, not many studies reported on this so question can’t be answered decisively.
They also looked to see if children were harmed by any of the strategies, for example by having injuries, losing too much weight or developing damaging views about themselves and their weight. Again, not many studies reported this, but in those that did none reported any harms to the children who had changed their diet or physical activity levels.
Strategies for changing both diet and physical activity for children to help prevent overweight or obesity are effective in making modest reductions in zBMI score in children up to the age of five and in children aged between six and 12. This is useful information for parents and children. It is also useful for governments trying to tackle a growing trend of childhood obesity. We found less evidence for adolescents and young people aged 13 to 18 strategies to increase their physical activity may be useful to reduce their zBMI score.
Because of the volume of studies on this topic with more than 4,000 papers published every year, Cochrane will be splitting this review into three reviews based on age ranges in the future.