I went for a walk the other day, and made a pig's ear of it. It was a sunny afternoon when I headed into the woods, along a clearly marked route I'd taken many times, and inky black and freezing by the time I made it back. For the last hour I'd been stumbling along with nothing to light the way but a head torch. When it pointed at the ground, I couldn't see the markers on the trees. When it pointed at the trees, I couldn't see the obstacles on the ground. If the battery gave out I reckoned I'd have to scream for help. By the time I got home I felt like throwing away my walking boots.

Most people, of course, are less stupid. But whatever your IQ, this is a tough time of year to stay in shape. The dark and the rain force you off the bike and on to the buses. The mud makes jogging an ordeal. Sleet turns every stroll into an ice-cold shower. Gyms are, as always, full of sweaty narcissists. Before long, you're sitting at home, fondling a HobNob and wondering why your trousers don't fit any more.

How much weight do we put on over the winter? First the good news. It's probably not the 5lb-10lb (2kg-4kg) you imagine. "That's an urban myth," says dietician Lucy Jones. Studies in the US – where they have Thanksgiving to contend with as well as Christmas – put the average winter weight gain at about a pound. Fewer than one in 10 people put on more than 5lb.

The bad news? "You don't lose that pound afterwards," Jones warns, "which is why people tend to gain year on year. Over 10 or 20 years that adds up to a good few stone."

The problem isn't just idleness, of course. We eat differently when winter comes, telling ourselves that more treats are allowed because it's cold outside and we need the extra fuel. Unfortunately, according to both Jones and Sian Porter, another dietician I consult in the hope that Jones might be lying, we're just kidding ourselves. They admit that Arctic explorers can get through 5,000 to 8,000 calories a day, versus the average man's 2,500. You will also burn extra calories if you're shivering, chopping wood in the snow, or doing something else physical. But, Porter points out, "If you go outside well wrapped up, stay warm and don't do any activity you may not burn off any extra energy at all."

Surely there's an upside? Isn't fat good insulation against the cold? More bad news, I'm afraid. Walruses may need a thick layer of blubber to protect them from sub-zero temperatures, but for humans surplus fat carries considerable health risks. "It might be better," Jones points out mildly, "to wear insulating layers of clothing."

So how can you avoid that winter pound? No one is suggesting you stick to salads for every meal while those around you gorge themselves on mince pies. "We don't want to be killjoys," says Porter. "If you know you're going to go out and have a great big meal, then have a smaller one before or after. It's sort of self-regulation." And when you're cooking for yourself, you don't have to avoid all hearty, enjoyable food. Porter, Jones and their colleagues at the British Dietetic Association swear by vegetable soups and stews. It's important to have a well-stocked freezer or store cupboard, Porter says: "When you come back and it's cold, all you may want to do is get the takeaway leaflet out of the drawer. But if you can throw a meal together in 10 minutes or so, you'll save money and feel better."

She also believes in removing temptation wherever possible. "If it's not in the house," she points out, "you can't eat it." She operates an "amnesty" for unsuitable treats after Christmas so the family aren't still eating Quality Street well into the new year. Her children occasionally tell her they wish she did something else for a living.

And exercise? Try to see this as a time to adapt any keep-fit efforts, rather than abandoning them. If walking or cycling are your thing, invest in decent all-weather kit. Or explore indoor options such as swimming and ice-skating. And if you're lucky enough to visit a ski resort, spend at least a few hours on the slopes rather than in the bars, and try some of the intense activities such as cross-country skiing or snow-shoeing. If you take a Thermos, you'll be able to have a hot, healthy lunch and feel like staying out longer.

Above all, don't believe that you have to end the winter in worse shape than you started it.