Regular exercise and a balanced diet are, of course, the cornerstones of achieving a healthy weight. Yet some obesity specialists are now convinced that the cause of the Western world's expanding waistlines is rather more complex -- and that there are additional factors to blame for unwanted weight gain. Here are some of them.
1. You need to sleep more
The link between inadequate sleep and weight has become a major area of investigation in the US in the last two years -- since scientists raised concerns that the dramatic increase in obesity over the last three decades may be linked to the reduction in sleep during that time by an average two hours.
Columbia University scientists raised the alarm with a study confirming apparently counter-intuitive claims: that people who are deprived of sleep are more likely to be obese, despite burning calories through tossing and turning. "We think it has more to do with what happens to your body when you deprive it of sleep as opposed to the amount of physical activity that you get," says lead researcher Dr James Gangwisch.
"Sleep deprivation reduces levels of leptin, the 'I'm full' hormone, and raises levels of grehlin, which stimulates appetite."
2. Side effect of medication
A common cause of unexpected weight gain is as a side effect of a prescription drug -- and it can be considerable, up to 30lb over several months. "A wide range of drugs including steroids, antidepressants, anti-psychotics and anti-epileptic drugs can make people put on weight by stimulating the appetite," explains Professor Steve Field, chair of the Royal College of General Practitioners. "Another class of offender, including high blood pressure drugs, calcium antagonists for heart disease, the contraceptive Pill and HRT, result in weight gain by causing fluid retention."
A frequent response, he says, is to panic and stop taking the medication, most dangerously without telling the doctor. "People rationalise this with the thought that they don't want to upset their doctor. The best policy with your GP is honesty. In almost every case, we can switch patients to another medication that has the same desirable effects but which will not cause weight gain and may even help you to shed a few pounds," he says.
3. You're getting older
Youth is not just gilded -- it's also thinner. As men and women age, they almost inevitably put on pounds, particularly in the form of hard-to-shed abdominal fat. This increase is largely caused by a reduction in the amount of exercise that's taken or to hormonal and metabolic changes in the ageing body, particularly in women. Whatever the cause, it's important to recognise that the ageing population, with a high percentage of baby boomers now collecting their pension, is a small but significant factor in obesity's increased prevalence.
4. You've caught the 'fat bug'
The obesity epidemic sweeping the US and making inroads over here could be caused by a virus, according to researcher, Nikhil Dhurandhar, who has successfully canvassed for funding to support a major research programme at Wayne State University in Detroit, Michigan to prove his thesis. Sceptics are unimpressed, pointing out that viruses have never been linked with a long-term disorder such as obesity and there are far more obvious explanations. However, in Bombay in the early 1980s, Dr Dhurandhar noticed that chickens killed by a flu epidemic all died plump -- rather than thin and emaciated as you might expect an infected chicken to be. He found that the chickens were infected with an adenovirus, which cause the common cold in humans. Further studies revealed that one in five obese people show signs of adenovirus infections and around 33lb heavier than people who have never been infected.
5. You don't move enough
We all burn up calories, even when we're sitting still -- but the rate varies dramatically. The big difference between individuals, according to scientists at the Mayo Clinic in Minnesota, is NEAT (non-exercise activity thermogenesis) -- or the amount of fidgeting you do. Professor James Levine, an endocrinologist, says fidgeting, above all, is the body's way of telling you to get up and move.
"Our body wants to move, but the environment suppresses it," says Professor Levine. "On a typical weekend, it's possible to use many of 40 different Neat-squelching devices: alarm clock, cellphone, BlackBerry, home computer, microwave, remote controls, electric toothbrush, snow blower, lawnmower. The result is that a desk-bound man or woman takes only 5,000 to 6,000 steps a day. That compares with about 18,000 steps a day for a man and 14,000 for a woman in an Amish community."
6. You've given up smoking
A heavy smoker burns up to 200 calories just by puffing -- while regularly inhaling nicotine can be an appetite suppressant by inhibiting the release of insulin from the pancreas. It's hardly surprising then that quitting cigarettes is a surefire way to put on weight. The average ex-smoker adds an average 5-10lb in the first few months even when their appetite remains the same. And for many ex-smokers, food becomes both a replacement for smoking as well as an emotional comfort -- resulting in an urge to reach for the chocs or the toast and peanut butter to beat cravings. The key health education message though is to do one thing at a time -- and remember that your weight can recover but your lungs cannot.
7. It's your genes
There are now at least 50 genes linked to obesity in humans, the most recent of which, known as FTO, is said to put one in six of us at up to 70pc increased risk of becoming overweight. This means we inherit a complex messaging system that links the gut, fat stores and appetite centres in the brain -- and that, to some extent, affects how full or hungry we feel, how much fat we lay down and how active we are. Research with both animals and human beings suggests that even body-mass index (BMI) may be inherited. There is also some evidence that conception may be more straightforward to both men and women with a high BMI -- resulting in more children with genes that predispose them to obesity.
8. Room temperature
Scientists have shown that mice living in cosily warm cages are fatter than those lodging in more extreme temperatures -- hot or cold. And the same may go for humans. A recent study by the American National Center for Healthy Housing found that the rise in obesity in the US correlates with the increase in central heating and air conditioning in homes and offices. It seems unlikely that this research will result in widespread abandonment of either of these moderating influences. But it does underline useful weight loss strategies, says Carole Caplin, director of the LifeSmart health and wellbeing centre in London. "When you're hot and sweating, you're eliminating fluid retention -- as well as stored toxins -- rather than burning calories," she says. "That's why a steam or sauna is both healthy and promotes weight loss. When you're uncomfortably cold, on the other hand, your body works harder, burning fat to get warm. Cycling, brisk walks or playing any sport in cold weather is a way to feel good and burn calories."