Ask anyone why there is an obesity epidemic and they will tell you that it’s all down to eating too much and burning too few calories. That is undoubtedly true – you cannot get round the first law of thermodynamics. It’s also true that we live in an “obesogenic environment”: calorific food is plentiful and cheap and our lifestyles are increasingly sedentary. Most of us have to make an effort not to get fat.

That explanation appeals to common sense and has dominated efforts to get to the root of the obesity epidemic and reverse it. Yet obesity researchers are increasingly dissatisfied with it. Many now believe that something else must have changed in our environment to precipitate such dramatic rises in obesity over the past 40 years or so. Nobody is saying that the “big two” – reduced physical activity and increased availability of food – are not important contributors to the epidemic, but they cannot explain it all (see “Why the ‘Big Two’ just will not do”, below).

Earlier this year a review paper by 20 obesity experts set out the 10 most plausible alternative explanations for the epidemic (International Journal of Obesity, DOI: 10.1038/sj.ijo.0803326). Here they are.

1 Not enough sleep

It is widely believed that sleep is for the brain, not the body. Could a shortage of shut-eye also be helping to make us fat?

Several large epidemiological studies suggest there may be a link. People who sleep less than 7 hours a night tend to have a higher body mass index (BMI) than people who sleep more, according to data gathered by the US National Health …