It's an unsettling fact: the number of overweight people in the world has now topped one billion, compared with 800 million undernourished people. And here in the UK obesity has risen by 400% in the last 25 years. We are, perhaps not coincidentally, also significantly less active nowadays. According to the Department of Health, our collective inactivity costs the government about £8.2bn annually.
Exercising if you are overweight or obese can be challenging on many levels, but it is perfectly possible to become fit even if your Body Mass Index puts you in the danger zone. Medically speaking, there is nothing stopping even the fattest person from embarking on an exercise regime, says Dr Simon Till, chair of the British Association of Sports Medicine and consultant physician in sports medicine at Sheffield Teaching Hospitals Trust. "Nobody is too obese to increase their activity, and anyone who raises the level of activity will benefit." As for the common fear of giving yourself a heart attack, or straining something. "There is no more risk associated with exercise for an overweight or obese person than there is for someone of a healthy weight, providing they increase their activity levels gradually," says Till.
This year the BBC followed George Ikediashi, an overweight drag queen who had topped 25 stone at his heaviest, as he trained for and completed the London Marathon. He talked about a childhood of fat jokes and bullying, and how the physical challenge helped him to see life differently - and lost eight stone on the way.
Personal trainer Matthew Robinson, who hit 14 stone aged 14, can relate to being the fat kid: "I was miserable. I used to get my mum to pick me up on school swimming days, saying I had a dentist appointment so I wouldn't have to face the jokes. The doctor told my parents I'd be in a really bad way by my early twenties if I didn't lose weight. Because I was so much bigger than other boys, I found I was good at rugby. Luckily, I got into sport that way. It transformed my life."
Some forms of exercise should be given a wide berth if you are carrying extra pounds. Ken Fox, professor of exercise and health sciences at Bristol University, advises avoiding high-impact sports - squash, running, kickboxing or high-impact aerobics. These could put more strain on your joints which are already carrying extra weight. But overall, says Fox, "The benefits of exercise, whatever your girth, far outweigh any small risks."
By exercising, says Fox, you will "reduce your chances of diabetes, heart disease and certain forms of cancer" whatever your size, even if you never lose weight. Even so, it can be hard to make the first step. "Very large people often cannot find exercise clothing that fits or feels comfortable," says Lisa Docherty, regional manager for the Obesity Awareness and Solutions Trust (Toast), a charity that offers weight management advice, support and information. It can also be psychologically tough to enter an arena where most other people are thin and - potentially - judgmental of anyone who is not. "The first time I set foot in a gym I was a size 24," says Docherty. "It took nerves of steel just to get through the door. I thought, 'People are going to stare.' I felt, 'What the hell am I doing here?' "
This fear, says Fox, is very common among overweight exercisers. Known by sports psychologists as "social physique anxiety", the paralysing sense of being singled out and stared at in the gym or swimming pool can be a powerful deterrent. "The reality is that the gym is a very selfish environment," says Robinson. "People are largely not bothered about anyone else's size or weight because they're too busy obsessing on their own." Even so, many of his clients shell out thousands for training in their own homes, believing themselves - somewhat ironically - to be too fat and unfit to exercise in public.
The simple answer is to forget about organised exercise if it fills you with dread and instead, integrate activity into your daily life. "Start thinking in terms of simply increasing your daily activity," says Dr Till. Ideally you want to work up to as little as three 10-minute bursts of moderate-intensity activity daily - the kind that leaves you slightly out of breath.
Walking is ideal, particularly if you are overweight. It is weight-bearing and therefore calorifically more demanding than swimming or cycling. And, fabulously, the fatter you are, the more quickly you'll see payback. "If a man weighing 160kg walks at five miles per hour he will burn twice the number of calories as a man who weighs 80kg and walks at the same pace," says Dr Till. You do not even have to power-walk ostentatiously to see results: one study this year showed that leisure walking - rambling with a friend or walking round the shops - is just as beneficial as organised walking for fitness.
Ultimately, attitudes to exercise and obesity could change as we all get fatter. In the US, the fattest nation in the world, there is now a growing demand for plus-sized personal trainers and exercise instructors. Many clients view a rotund trainer as less intimidating and more understanding than a skinny one who calls you "fatso" and puts you through a gruelling near-death experience each time you meet. At the very least, watching someone else's thighs wobble alongside your own could be mildly reassuring.
