"I'll put on weight"

This fear is reasonable, because it's based on something true. Most people who stop smoking do put on some weight. If you've stopped before and gone up a size, nobody needs to tell you. Books that skip this, or promise you'll stay the same, aren't being honest with you.

The best figures I know of are these. In 2012 a team led by Henri-Jean Aubin pooled 62 studies that had weighed people before they stopped and at intervals afterwards, counting only those who stayed stopped. The average gain was a little over one kilogram after a month, nearly three after three months, and between four and five after a year. Most of the gain came in the first three months and then it levelled off.

The average hides a very wide spread, though, and I think the spread is the more useful finding. At twelve months, about one in six people had lost weight. Rather more than a third had gained less than five kilograms. About a third had gained between five and ten. About one in eight had gained more than ten. So gaining weight is likely, gaining a lot is unusual, and a fair number of people gain nothing. The researchers also found the figures were much the same whether people used stop smoking medicines or not, and whether or not they'd been particularly worried about their weight beforehand.

The reasons usually given are straightforward. Nicotine takes the edge off appetite, so without it you're hungrier for a while. Food tastes better once your sense of taste recovers. And hands and mouth that have been busy twenty times a day go looking for something else to do, which is often a biscuit.

There are two ways of reading that list. The common one is that smoking keeps you slim, so it has a benefit after all. I'd put it differently. Smoking has been holding your appetite slightly below where it would naturally be, and when you stop, it returns to where it would have been if you'd never smoked. The weight you arrive at is roughly your weight as a non-smoker. It's giving something back again, in a form you may not welcome. You haven't been slim because of a gift. You've been a bit under your ordinary weight because of a drug, and the drug has had other effects.

Then there's the question of what to set this against. One of the longest studies in medicine followed about 34,000 British doctors for fifty years. The men who smoked cigarettes throughout died on average about ten years younger than those who never smoked. Those who stopped at sixty gained back about three of those years. Stopping at fifty gained about six, stopping at forty about nine, and stopping at thirty about ten. I promised not to lecture you about health and I'll keep this to one paragraph. But when people weigh four or five kilograms against stopping, this is what's on the other side of the scale. I don't know of any finding that puts a few kilograms anywhere near it. If your weight worries you for medical reasons, such as diabetes, that's a good question for your GP, and the answer will almost certainly still be to stop.

On the practical side, I'd suggest three things, and they're my own view and not a tested programme.

Don't start a diet on the same day you stop smoking. The first few weeks ask enough of you, and strict dieting while you're irritable and hungry sets up a choice between the two projects. People tend to resolve it by going back to cigarettes, which don't solve the weight question in the long run either. Give the smoking a month or two, then look at food if you still want to.

Expect the hunger and plan something for it. If you know that the mid-morning urge will send you to the vending machine, bring something you'd rather eat. The NHS suggests keeping your mouth busy with things like sugar-free gum or chopped vegetables, which sounds joyless but does work for some people.

And use the fact that moving gets easier. The NHS says that within days or weeks of stopping you start to breathe more easily and have more energy, so stairs and hills feel different. A brisk ten-minute walk is also one of the better responses to an urge. You don't have to take up running. But if moving about gets more pleasant, you'll probably do more of it, and that can take care of some of the extra.