Does intermittent fasting work? Start with twelve hours overnight.

A colleague has lost weight and told you the secret in the lift: intermittent fasting. You half caught how it works, and on Monday you start. No breakfast, two coffees, and you last until half twelve.

Then at lunch you eat twice as much, and by four you're at the vending machine. It's the version I see most in the people I work with. Intermittent fasting does work, under the right conditions.

The signs of a fast made up as you go

• Coffee in place of breakfast. One, then another.
• The blowout when you eat again. You eat fast and struggle to stop.
• The cupboard at night. You graze after dinner.
• Lighter sleep. You go to bed hungry and toss and turn.

Does intermittent fasting work for weight loss?

Yes, when it gets you eating less. Most of the result comes from that. With the same calories and protein, the edge over three regular meals is small.

The clock changes, the food stays. You eat inside a window of hours, and for the rest of the day insulin stays low, so your body tends to draw more on its reserves. The benefits of intermittent fasting depend on what you eat in the window.

How many hours. Twelve hours between dinner and breakfast is enough for your body to start adapting, and it suits every day. Sixteen, known as 16:8 fasting, does more for insulin sensitivity. Daily for months, it can slow your metabolism.

Timing counts too. In a five-week comparison of men with borderline blood sugar, eating between seven and three gave lower insulin and less appetite than running on until seven in the evening, at the same weight. So I bring dinner forward and keep breakfast.

Who it helps, who it wears down. It helps if you snack at night and like one simple rule. It wears you down if you sleep badly, live under pressure or already eat too little. Once fasting turns into an endurance test, cortisol can rise and the edge is gone.

Where to start: twelve hours overnight

Close the kitchen for twelve hours. Say dinner finished by seven and breakfast from seven, just water in between. That also gives you the three hours between dinner and bed. For breakfast, eggs, Greek yoghurt and nuts, with your first coffee.

Sixteen hours, if twelve suits you. Once or twice a week. Pick the day in advance, one without weights. The best follows a planned big meal, like Sunday lunch: Monday you eat from midday, vegetables and protein first. Never as payback for a blowout. Until then water, tea or a black coffee. Just the one. Coffee after coffee on an empty stomach can push cortisol up, and hunger lands all at once. Skip zero-sugar drinks, as the sweet taste may still nudge insulin.

Full protein, fewer meals. If you train or keep active, 1.6 to 2.2 grams per kilo a day, 20 to 40 per meal. In a short window, two meals rarely get you there, and after fifty your muscle can pay for it, so add a third meal with protein and keep weights for the hours when you eat. The sums are in the protein article.

Sixteen hours can wait. Start tonight with the kitchen closed, and in a few days look at how you sleep.