Eat less and exercise more - it has long been the mantra for shedding those extra pounds. But now it seems that all the hard work may have been in vain.

Scientists say there are ten other reasons why people just keep getting bigger. They claim that sleeping habits, central heating, medicines and even some pollutants can play a role in weight gain.

Traditionally, health practitioners have focused on diet and exercise - and a large dose of willpower - to treat the problem.

However the scientists from top U.S. and Canadian universities say that these other factors must be considered.

Lack of sleep could be partly to blame. In recent years, the average night's sleep has dropped from nine hours to just seven.

Sleep deprivation alters levels of the hormones that regulate food intake and body fat and increase hunger and appetite.

If it is too hot or too cold, we burn calories to cool down or heat up. But if the temperature is just right, the calories may be turned into body fat instead.

Those who take beta-blockers to control their blood pressure can often put on 3lb. Similarly, studies have shown that going on the Pill can add 11lb to a woman's weight.

Mother Nature may also be to blame, with our body shape being partly inherited. The overweight are also more likely to settle with partners of a similar size, and their children are more likely to be obese.

And the use of pollutants is on the rise, say the researchers. The manmade chemicals in pesticides and plastics can interfere with our hormones and lead to weight gain.

Your mother's age and your weight at birth are also important, with older women more likely to have obese children and underweight babies having a bigger chance of being obese in later life.

We also tend to put on weight as we get older. Finally, giving up smoking can also help pile on the pounds.

The scientists from Yale, Cornell and Johns Hopkins said there was only 'circumstantial' evidence supporting poor diet and lack of exercise as the main causes of obesity.

Ten other factors behind the flab:

Sleep - Too little increases appetite
Medicine - Many modern medicines lead to us putting on weight

Mother's age - Older women are more likely to have obese children

Choice of partner - Overweight people tend to pair with each other - and have overweight children

Heating and air conditioning - 'Comfortable' temperatures stop us from burning off fat

Genes - Body shape may be partly inherited

Giving up smoking - Nicotine suppresses the appetite

Manmade chemicals - Some chemicals interfere with hormones that keep fat levels low

Weight at birth - Underweight babies have a bigger risk of obesity

Getting older - Weight tends to increase with age
 

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