Underweight

The WHO defines underweight in adults as a BMI below 18.5 – for children and teenagers, the weight curve counts instead: below the 10th percentile for their age and sex. That may come as a surprise on a platform about losing weight, but it's important: if you're underweight, you don't need a calorie deficit but the opposite.

There are many causes, and most are harmless: a naturally slight build, a fast metabolism, lots of sport, or a growth spurt in teenagers where height races ahead of weight. Things to take seriously are unintended weight loss, persistent loss of appetite, digestive problems or an overactive thyroid – and always the question of whether an eating disorder could be behind the low weight. A visit to your GP or paediatrician will clarify this without any drama; if you're worried about an eating disorder, the helpline of the German Federal Institute for Public Health (BIÖG, formerly BZgA) on 0221 892031 can also help.

Why underweight matters: when energy and nutrients are lacking, the body breaks down muscle and bone mass, the immune system becomes more vulnerable, girls and women may stop having periods, and children grow more slowly. Healthy weight gain therefore doesn't mean “anything greasy” but nutrient-rich, energy-dense foods: nuts, nut butter, avocado, olive oil, oats, full-fat dairy products, pulses, eggs, fish – and regular meals plus two snacks.

A surplus of 300 to 500 kcal a day above your needs lets weight rise slowly and steadily – mathematically, about one to two kilos a month. Strength training ensures that more of it becomes muscle rather than just fat. Endurance sport can stay, but in moderation.

For parents: children who are slim but growing steadily along their percentile, who are lively and eat well, are usually just slim. Comments about their bodies (“eat a bit more”) don't help; shared, relaxed mealtimes do. If you're worried, the growth chart at the paediatrician's is what counts, not comparisons with other children.