Body fat

Body fat isn't a flaw – it's vital: it stores energy, cushions organs, insulates against the cold and produces hormones such as leptin, which regulate hunger and metabolism. Everyone needs a certain amount – around 10 to 13% in women and 2 to 5% in men. Women naturally carry more because pregnancy and breastfeeding require reserves.

Depending on age, around 20 to 32% in women and 10 to 22% in men is considered healthy; athletes are below this range. More important than the exact number is where the fat is stored: fat directly under the skin (subcutaneous) is metabolically fairly quiet, whereas fat between the abdominal organs (visceral) is highly active and linked to type 2 diabetes and cardiovascular risks. A large waist on an otherwise slim figure therefore matters more than a few percentage points on the scales.

There are several ways to measure body fat: body fat scales (bioelectrical impedance: a weak current you can't feel measures the body's resistance) are convenient, but their readings vary by several percentage points depending on how hydrated you are. Skinfold measurement with callipers takes practice. The US Navy formula estimates body fat from neck, waist and – for women – hip circumference plus height, and is surprisingly useful at home. The most accurate method is a DXA scan at a clinic – an X-ray measurement with very low radiation. Whichever method you use: same conditions, same time of day – then it's the trend that counts.

When you're losing weight, the goal is to lose fat and keep muscle. Enough protein, strength training and a moderate deficit make sure that what comes off on the scales is mainly fat. In children and teenagers, body fat percentage changes naturally as they grow – girls quite normally gain body fat during puberty. Percentages from adult tables aren't suitable for them.