Belly fat after 40: same plate, bigger waist.

Monday morning, in front of the wardrobe. The suit trousers do up, but your belt has moved on a notch. The scales say much the same as five years ago. Your waist doesn't.

'But I eat the same as ever,' men and women past forty tell me. Often it's true. What's changed is the body that plate lands in, and the day around it.

How to spot it

There are two kinds of fat. The one under your skin you can pinch. The deep one sits behind the muscle, around your liver and pancreas, and works like a gland, making substances that keep inflammation simmering, above all in your arteries.

• Your belt tells you more than the scales. At the same weight, a bigger waist means higher risk, even if you're slim.
• Your belly is firm and round. There's little to pinch, because the fat is underneath.
• Your arms and legs get thinner. Muscle goes down while your waist goes up.
• You're sleepy after lunch, then want something sweet. Your blood sugar is see-sawing.

Why it happens anyway

The engine has shrunk. Muscle is your engine size: the more you have, the more you burn even at rest. Leave it unused and you lose a little every year. The same plate goes into a smaller engine, and the surplus gets stored.

Your hormones have shifted. It starts early and speeds up after fifty: the hormones that keep you lean drop, while cortisol and insulin climb. Cortisol eats into muscle; insulin sends fat to your abdomen. In women, oestrogen keeps fat on hips and thighs. At the menopause it moves to the belly.

Stress stays switched on. Under pressure, cortisol raises your blood sugar and insulin files it away round your waist. And deep fat has an enzyme that reactivates cortisol on the spot. More belly, more cortisol. More cortisol, more belly.

Short nights and the evening drink. One bad night is enough to make you respond worse to insulin and crave carbs the next day. Alcohol raises cortisol, and the fat settles mostly round your waist.

Where to start

Measure your waist. Tape measure, in the morning, level with your navel, without pulling your stomach in. Once a month. Above 94 cm for men and 80 cm for women, the risk starts to rise.

Put your plate in order, especially at dinner. Vegetables and protein first, carbs last, in a normal portion: sugar arrives more slowly and you need less insulin. Keep regular times for meals and sleep. Skipping dinner to compensate makes blood sugar and insulin worse.

Walk after meals. Ten to fifteen minutes. Working muscles take up the sugar that's just arrived, leaving less to store.

Hold on to your muscle. Train for strength every week, so the engine stays big. And get up often. You move less sitting down than you do chewing gum, and an hour at the gym doesn't cancel out a day in a chair.

Swap the wind-down drink. Monday to Friday, no alcohol. But that glass is how you shake off the day: just drop it and it comes back. Instead, as you shut the laptop, take two breaths in through your nose, the second short, then a long breath out through your mouth. One to three times.

Start with the tape measure tomorrow. Then pick one habit and keep it for a month. The belly arrived slowly, one habit at a time. It leaves the same way.