Skipping lunch: you pay for it by evening.

The meeting runs past half twelve, then a call drags on. By two you decide to skip lunch, and part of you is pleased: one less meal might even help you lose weight. A coffee, and on you go.

You aren't hungry. If anything, you feel sharper. The bill comes later, in two instalments: one at half three, one at home in the evening. I see it often in the people I work with, and hardly anyone traces it back to lunch.

What happens when you skip lunch

• At one, the hunger has gone. Looks like good news. It doesn't last.
• By half three, you crash. Foggy head and a short temper. The craving for something sweet walks you to the vending machine.
• You pick while you cook. Bread, cheese, whatever's there. You've eaten before you sit down.
• At dinner you struggle to stop. You have seconds, and after dinner you're back at the cupboard.

Does skipping lunch help you lose weight?

No, not when it happens at random, on days when time runs out. A planned fast at set times can work; a lunch missed two days in five unsettles your blood sugar and gives worse results than eating regularly.

One hormone lowers blood sugar. Five raise it. For your body, running out of blood sugar is the real emergency. When lunch doesn't arrive, your body calls on the hormones that push it back up, including adrenaline and cortisol, the stress hormones. That's why you feel alert and not hungry at one.

Cortisol and your waistline. People who often skip meals make more of it, and cortisol encourages fat around the middle. If it's still high in the evening, sleep can suffer. So is skipping lunch bad for you? As a habit, yes.

Hunger builds up. Ghrelin, the appetite hormone, rises on an empty stomach and only drops once it fills. When you haven't eaten, your brain listens to it more, which is partly why you pick and have seconds. It also primes your fat tissue to store.

The timing of lunch matters most. Among 420 people on a diet, those who ate lunch early lost more weight than those who ate late, and their insulin worked better. The timing of breakfast and dinner made no difference.

The back-up lunch

Keep a stash at work. You need a plan B that fits in a drawer and takes five minutes to eat: tinned tuna, a jar of chickpeas, nuts. In the fridge, if there is one, Greek yoghurt and cherry tomatoes. Buy it on Mondays, so you decide before you're hungry.

Put it together like this. A protein, a vegetable, a handful of nuts. Tuna, chickpeas and cherry tomatoes; or Greek yoghurt, walnuts and fruit. Protein and fibre let your blood sugar rise slowly and keep you full for hours: at half three you're still on form.

In the evening, slow down. Before you cook, a glass of water and something raw to chew: fennel, carrot. Then sit down and start with vegetables and protein, so your stomach fills before you reach for seconds.

The back-up lunch is nothing to look at, and it works. Do the shop for the drawer today. The next packed day is already in the diary.