Body composition
Body composition separates fat mass from fat-free mass – and explains why two people with the same BMI can look so different.
Ausführlich erklärt
Your weight doesn't tell you what you're made of. Body composition breaks it down into fat mass and fat-free mass – muscle, bone, organs and water. Two people weighing 75 kg with a BMI of 24 can be completely different: one with 22% body fat and strong muscles, the other with 32% and little muscle. For health and shape, body composition matters more than the number on the scales.
That's why the goal when losing weight isn't “weigh less” but “lose fat, keep muscle”. If you lose weight with strength training and enough protein, you lose almost nothing but fat; if you starve yourself and stay inactive, you lose a considerable amount of muscle – and later wonder why you're weaker and burning fewer calories. Sometimes the scales don't move at all while you're losing fat and building muscle. Then your tape measure and your clothes tell the real story.
Here's how you can measure it: body fat scales use bioelectrical impedance – a weak current flows through the body, and because water and muscle conduct it well but fat conducts it poorly, the scales estimate your body fat percentage. That's quick but sensitive: how much you've drunk, exercise, meals and the time of day can shift the result by several percentage points. Only use them under the same conditions and watch the trend over weeks. More accurate options are skinfold measurement (with practice), circumference formulas such as the US Navy's and – the most accurate method – a DXA scan at a clinic (an X-ray measurement with very low radiation). For everyday purposes, your waist circumference, photos taken four weeks apart and your strength in training are often all you need.
Body composition changes naturally over a lifetime: during puberty, boys build a lot of muscle mass, while girls quite normally gain more fat tissue. From the age of 30, you lose muscle unless you train, and in older age body fat percentage rises even if your weight stays the same. Strength training is the most effective way to influence this curve at any age.
For children: body fat scales aren't calibrated for them and aren't necessary – growth is assessed with percentiles, not percentages.