Lose fat without losing muscle: speed matters.

Six weeks of strict dieting, seven kilos down. You wanted to lose fat without losing muscle, yet your jacket hangs off your shoulders and your belt is on almost the same hole.

'I look deflated.' 'My muscles have shrunk.' I often hear this after fast weight loss. Some of those kilos weren't fat.

Am I losing fat or muscle?

Look at three things together: how fast the weight falls, your waist and your strength. A smaller waist and the same weights mean you're losing mostly fat. These signs mean you're losing muscle mass too.

• Your strength drops. On a diet it's normal to tire on the last few reps. The warning sign is the weight you lift going down week by week.
• Your clothes. Loose at the shoulders, arms and thighs, much the same at the waist.
• The mirror. You look smaller but no firmer. You've lost tone and the belly is still there.
• Tiredness. You often take longer to recover between sessions.

Why losing weight fast costs you muscle

When you eat less than you burn, your body takes the difference from its reserves, fat or muscle. The split depends largely on the pace.

Fat has a speed limit. Your body can burn an estimated half a kilo a week, a kilo at most. If the scales show more, the rest is usually water and muscle.

The scales reward lost muscle. A kilo of fat holds roughly seven thousand calories. A kilo of muscle, mostly water, about eight hundred. The same cut costs you about a hundred grams of fat or nearly a kilo of muscle. That's why crash diets seem to work.

How do you lose fat without losing muscle?

Lose weight slowly, do strength training and eat protein at every meal. Drop one of the three and muscle tends to go.

Aim for 0.4 to 0.8 per cent of your weight a week. At 75 kilos that's 300 to 600 grams. The first weeks go faster as you shed water, so go by the monthly average. With a lot to lose you can start faster, as more fat means less risk to your muscle. Near your target, slow down.

Cut a little. At most a fifth of what you need to maintain your weight. Better still, take half from your plate and half from moving more, so you eat more and your muscle is better fed.

Keep the weight on the bar. Two strength sessions a week, as in the article on having no time to exercise. On a diet it's tempting to go lighter and do more reps. Try to lift what you lifted before, because the load tells your body the muscle is needed.

Protein at every meal. 1.6 to 2.2 grams per kilo of body weight a day, 20 to 40 per meal. The sums are in the article on protein after 40.

Sleep regular hours. Comparing two groups on a diet, those who slept little in the week and caught up at the weekend lost the same weight as the others but far more muscle and less fat. Nobody lifted weights, which probably widens the gap.

Going slower costs a few extra weeks. In return, you're more likely to reach your target with your strength intact.