Obesity Epidemic Ask anyone why there is an obesity epidemic and they will say that it's all down to eating too much and burning too few calories. That is undoubtedly true. But it's also true that we live in an 'obesogenic (肥胖基因的) environment': calorific food is plentiful and cheap and our lifestyles are increasingly sedentary. Now, obesity researchers are increasingly dissatisfied with such explanations. They believe that something else must have changed in our environment to cause 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. Earlier this year a review paper by 20 obesity experts set out the 9 most plausible alternative explanations for the epidemic. Here they are. 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 and Nutrition Examination Survey. Similarly, the US Nurses Health Study found that those who slept an average of 5 hours a night gained more weight during the study period than those who slept 6 hours, who in turn gained more than those who slept 7. It's well known that obesity impairs sleep, so perhaps people get fat first and sleep less afterwards. But the nurses' study suggests that it can work in the other direction too: sleep loss may cause weight gain. One factor that could be at work here is the way sleep deprivation alters metabolism (新陈代谢). Leptin, the hormone that signals satiety (过饱), falls while ghrelin, which signals hunger, rises -- and this boosts appetite. Climate Control We humans, like all warm-blooded animals, can keep our core body temperatures pretty much constant regardless of what's going on in the world around us. We do this by altering our metabolic rate, shivering or sweating. Keeping warm and staying cool take energy. There's no denying that surrounding temperatures have changed in the past few decades. In the US, the changes have been at the other end of the thermometer as the proportion of homes with air conditioning rose from 23 to 47 per cent between 1978 and 1997. In the southern states -- where obesity rates tend to be highest -- the number of houses with air con has shot up to 70 per cent from 37 per cent in 1978. Could air conditioning in summer and heating in winter really make a difference to our weight? Sadly, there is some evidence that it does -- at least with regard to heating. Less Smoking Bad news: smokers really do tend to be thinner than the rest of us, and quitting really does pack on the pounds, though no one is sure why. It probably has something to do with the fact that nicotine is an appetite suppressant and appears to up your metabolic rate. Katherine Flegal and colleagues at the US National Center for Health Statistics in Hyattsville, Maryland, have calculated that people kicking the habit have been responsible for a small but significant portion of the US epidemic of fatness. From data collected around 1991 by the US National Health and Nutrition Examination Survey, they worked out that people who had quit in the previous decade were-much more likely to be overweight than smokers and people who had never smoked. Among men, for example, nearly half of quitters were Overweight compared with 37 per cent of nonsmokers and only 28 per cent of smokers. Prenatal Effects Your chances of becoming fat may be set, at least in part, before you are even born. Children of