It has been estimated that, by 2012, there will be 1m obese children in England.
Being overweight or seriously underweight as a teenager curbs life expectancy as much as smoking 10 cigarettes a day, a study suggests.
Swedish researchers followed 46,000 men from the age of 18 for 38 years.
Being obese or smoking more than 10 a day doubled the premature death risk, the British Medical Journal reported.
Being overweight, seriously underweight or smoking 10 or less raised it by 30% - and interestingly the fat non-smoker ran the same risk as the fat smoker.
Those who were underweight - with a Body Mass Index (BMI) of less than 18.5 - had no increased risk of dying early, regardless of whether they smoked or not.
But the team from the Karolinska Institute in Sweden found that those who were seriously underweight - with a BMI of 17 or less - ran the same risk as those who were overweight.
They looked at 45,920 men born between 1949 and 1951 who were military conscripts. In this period as few as 3% of Swedish men were exempt from military service.
Nearly 3,000 of the participants died - with the incidence of death lowest in the normal weight category.
"Compared to normal weight adolescents, being overweight at the age of 18 increased the risk of premature death by just over a third, while being obese more than doubled the risk," the team, led by Dr Martin Neovius, wrote.
And this risk increased regardless of smoking status, they added.
Dr Ian Campbell, of the charity Weight Concern, said the findings were "very interesting".
BODY MASS INDEX
Calculated by dividing weight in kilograms by height in metres squared
Normal: 18.5 - 24.9
Overweight: 25 - 29.9
Obese: Above 30
"When patients who are overweight and smoke ask what they should work on first, we've always said smoking as the wisdom has always been that this poses more health risks - yet this study suggests otherwise."
Betty McBride, of the British Heart Foundation, said: "The government need to bring the same level of sustained focus to tackling the obesity crisis it has previously brought to smoking."
"The number of young people who are overweight and obese is growing. Without tackling this now we risk the next generation growing up with more health problems than their parents."