Children are fat or thin not because of the physical activity they do but because their bodies are so programmed. According to a study conducted by a team at Peninsula Medical School in Plymouth on 300 school children which found that the amount of exercise a child does is not correlated with their Body Mass Index. BMI is a calculation of a person’s height and weight used as a measure to see if they are healthy or not.

The findings published online in the Archives of Disease in Childhood tracked the long-term health of 300 children from 54 different schools around Plymouth all five years old, showed that obesity among children is more due to their diet than exercise. Prof Terry Wilkin, co-author of the study said that the research has shown that there is a ten fold difference in the amount of daily activity that children do, and that children who are very active can still have a high BMI.

"Each child is programmed to a particular level of activity whatever you do to them they will modify their activity to remain at that level, some will always do more and some will always do less.

Some children do meet the physical activity guidelines and some don't but BMI will not tell you which ones do and which ones don't. But it is worth doing exercise because there is clearly a benefit in doing so, it is just that the BMI does not reflect that,’ said Prof Wilkin.

Finding obese children will be a help in monitoring them later in life as if they remain overweight in their adult years they could be at a greater risk of heart disease, stroke and cancer.

"We are getting away from the notion that we are a nation of couch potatoes and that is why we are fat, it may well be more to do with diet as that has markedly changed over time,” said Prof Wilkin.