You eat the same foods in the same quantities as you did 15 years ago, your daily routine has not changed and you do exactly the same amount of exercise. So how come you weigh a stone more now than you did then? And why is it that all that fat seems to have settled on your belly?

It is a sad fact of life that you do not have to let yourself go to acquire that middle age spread – just keep doing the things you always did and it will magically appear.

Addic Brillon, an Israeli-born dietician at Wythenshawe hospital, Manchester, explains how, for women, going through the menopause can change both weight and body shape.

He says: "As we age we lose muscle mass. The unfortunate part of that is that muscle burns far more calories than fat, so your calorie requirements are lower as you get older. Women require 50 calories less per day for every five years after the age of 30. So by the age of 50 you will require 200 fewer calories less per day than you did when you were 30. Women can gain 10 to 15 pounds during this time without eating more."

This is not the only thing women need to worry about. Not only can fat levels increase but its distribution will also change. Says Brillon: "When oestrogen levels start to fall in the menopause, fat is distributed differently. Before the menopause it is distributed around the hips, buttocks and arms. After the menopause it accumulates in the abdominal area. In some cases a woman's weight won't change but her shape will."

After the menopause, fat settles around the stomach

If this was not bad enough, there is also stress to deal with. Middle age can be a very stressful time. And the stress hormone, cortisol, which is released in large amounts when your teenage daughter goes off the rails, when you encounter problems with your marriage or your boss and cannot pay the mortgage, also has the effect of encouraging fat to accumulate around your middle. Men are not immune to this process either. Although there is no sudden menopause to deal with, as testosterone levels decline, men lose muscle mass, their metabolism slows and they can develop a pot belly too.

It is not just that the fat around your belly looks unsightly, there are also consequences for health, says Brillon. "There are two types of fat. The first is the called subcutaneous - this is the pinchable fat around your middle. Then there is the visceral fat between your organs. This is the important fat. People with fat around their bellies tend to have more visceral fat and this makes them more prone to heart disease, high cholesterol, high blood pressure, stroke and various cancers. Having liposuction will do nothing to remove the dangerous visceral fat."

So getting rid of that belly is not just a cosmetic exercise – it can save you from illness. Fortunately, there is nothing inevitable about that advancing paunch – there is plenty you can do about it through diet and exercise.

For example, if your body naturally burns fewer calories, you can encourage it to burn more. And the muscle that has wasted away can be replaced by resistance training, or load bearing exercise - weights, press-ups, sit-ups etc - and cardiovascular exercise will burn even more fat and keep your heart healthy as well.

Brillon says: "Exercise for at least 30 minutes a day. You need to take at least 10,000 steps, which is quite substantial. This would mean walking for at least half an hour a day at a good pace.

But walking won't increase your muscle mass. You can do this by weight training. You don't need to be a body builder, just to do enough to restore some of that muscle you used to have in your 30s."

The other crucial area is diet. Women in particular,should forget regimes which restrict carbohydrates, says Brillon. "There are no quick fixes. Women's serotonin levels are lower than men's. On a low carb diet, serotonin levels drop, causing women to suffer from lethargy and depression. So eat a healthy diet - concentrate on unrefined carbohydrates like brown rice, brown pasta, wholemeal bread, and cut refined white pasta, rice and refined sugar. Unrefined food gives you a constant level of blood glucose, meaning a reduced tendency to snack.

"Eat lean protein, have fish twice a week and throw out your frying pan. Health problems start when carbohydrates touch fat. All the worst junk foods - chips, crisps, biscuits, cakes and doughnuts are a combination of refined carbs and fats. High fat dairy products are also a problem because they are one of the most potent causes of high cholesterol and middle age is a source of problems for cholesterol. Cut alcohol too. It contains empty calories with little or no nutritional benefit."

Unfortunately, there is no magic bullet for getting rid of the middle age spread but the good news is that only small modifications in lifestyle can have pronounced effects - and it is never to late to start. With good diet and exercise the extra stone that took 15 years to develop around your middle could be gone in seven weeks - just when you are getting ready for your summer holiday.