LONDON (Reuters) - People who were obese at the age of 18 are twice as likely to die prematurely compared to those who were normal-weight teenagers, Swedish researchers said on Wednesday.

They also found that men who had been overweight at 18 were one-third more likely to die prematurely compared to their normal-weight peers.

The study of 45,920 men over an average 38 years underlines the dangers of being overweight and the need to tackle a growing obesity epidemic.

About 400 million people around the world are classified by the World Health Organization as obese, including 20 million children under the age of 5.

"Obesity and overweight were as hazardous as heavy and light smoking," Martin Neovius of the Karolinska Institute in Sweden and his colleagues wrote in the British Medical Journal.

"The obesity pandemic seems to affect children and adolescents more than adults."

Neovius and his team analyzed the cause of death of more than 45,000 men who had mandatory military conscription tests in Sweden at the age of 18.

All had their body mass index (BMI) measured and were asked whether they smoked. For the 3,000 who died during the 38-year follow up, the incidence of death was highest among the obese.

Heavy smokers, considered those who puff on 10 or more cigarettes daily, were twice as likely to have died prematurely compared to non-smokers, the researchers said.

Since 1969, the number of overweight men in Sweden has tripled and those who are obese has risen five-fold, the researchers noted. This highlights an urgent need for public health programmes to deal with the problem, they said.

"The global obesity epidemic and smoking among adolescents remain important targets for intensified public health initiatives," Neovius and colleagues wrote.

(Reporting by Michael Kahn, Editing by Maggie Fox and Katie Nguyen)

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