A third of teenage girls admit they are on a diet or have recently tried to lose weight, according to a survey today which also reveals the ignorance of children of both sexes about what they need to eat to be healthy.

Questioned about their average daily calorie intake when dieting, nearly half of girls aged 13 to 18 said they consume less than 1,200 calories a day - 40% less than the recommended daily amount (1,978) for their age.

The survey by supermarket chain Sainsbury's reveals that although only 14% of teenage boys admitted to dieting, a quarter of these consume less than 800 calories a day - only a third of their guideline daily amount.

In addition, more than half of all teenagers wrongly estimated the guideline daily amount of calories for an adult woman (2,000) and man (2,500).

Asked what makes up a healthy diet, 98% of teenagers believe they know and 76% say they consider their diet to be healthy. However, the vast majority (85%) of young teenagers are not eating the government recommended intake of fruit and vegetables.

The findings of the Sainsbury's youth diet survey will be published in full at a seminar on Thursday, which is being held to examine some of the key issues around diet, exercise and food for young people.

Sainsbury's nutritionist, Charlotte Parker said: "These results show that teenagers struggle to understand what constitutes a balanced diet. When this knowledge is combined with the issue of childhood obesity in the UK, it is clear that we need to help both teenagers and their families."