Obese teenagers run the same risk of premature death in adult hood as people who smoke more than 10 cigarettes a day, according to a new study.
The British Medical Journal study also found that people who are overweight have the same risk as less heavy smokers.
To reach the conclusion, Dr Martin Neovius at Karolinska Institute in Sweden, analysed the cause of death of over 45,000 men who underwent mandatory military conscription tests in Sweden.
All participants had their body mass index (BMI) measured and reported their smoking status at the age of 18 and were tracked down for an average of 38 years.
Overall, the researchers assessed 1.7 million person-years of follow-up in relation to the health and mortality of all the participants.
During the follow-up period 2,897 subjects died, the incidence of death was lowest for people with normal weight and highest in obese subjects.
When matched to normal weight adolescents, being overweight at 18 raised the risk of premature death by just over a third, while being obese more than doubled the risk, the study found.
Being underweight carried no increased risk, irrespective of smoking status. However, being seriously underweight (a body mass index of less than 17) carried the same risk of premature death as being overweight.
Early death was also linked to the number of cigarettes participants smoked per day. The risk gradually increased the more participants smoked, with heavy smokers at more than double the risk of premature death compared to non-smokers.
But, interestingly, when the effects of weight and smoking were combined, the researchers found no significant change in the results.