More than a third of overweight or obese teenagers surveyed by researchers did not see themselves as being too heavy and think their weight is about right, according to a study published Thursday in the International Journal of Obesity.

Researchers from University College London (UCL) used data from around 5,000 teenagers aged 13 to 15 years old who were asked about their weight and if they thought they were too heavy, about right or too light.

Researchers then checked their answers against their Body Mass Index (BMI) to see whether the reality matched the teenagers' perceptions of themselves.

The results showed that almost 73 percent of the teenagers had a BMI within the normal-weight range, and 20 percent had a BMI in the overweight category and seven percent were categorised as obese. Of the latter two categories, around 40 percent thought they were about the right weight.

Carrying excess weight increases the risk of up to 10 different types of cancer, including cancers of the breast and bowel. It's estimated that in the UK around 18,000 cases of cancer each year in people of all ages are related to excess weight.
"We need to find effective ways of helping too-heavy teenagers slim down and maintain a healthier weight, and it's vitally important that we find out whether it helps if they are more aware of their weight status. There are no easy answers," said Professor Jane Wardle, from UCL.