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A person is significantly underweight if he or she is more than 10 per cent below their optimal body weight. It must be stressed that this optimal weight varies from individual to individual (see ideal weight).

Some people are constitutionally thin and their optimum weight will be lower than those who are more comfortable, fit, and healthy at a higher weight. Some athletes, such as long-distance runners, are thin by nature and suffer no ill-effects from being thin. However, such is the pressure for success in sport, that many athletes are unnaturally thin by design rather than by nature. Many exercisers, particularly young female runners and gymnasts, take great pains to keep their weight low. There is no doubt that their high power to weight ratio gives them a competitive advantage, but this may be at a cost to health if taken to extremes.

An obsessive desire for thinness can lead to eating disorders. A survey of more than 4000 recreational runners found that 8 per cent of the men and 24 per cent of the women had attitudes to food similar to those of people suffering anorexia or bulimia nervosa. Many of the women had irregular periods (see amenorrhoea) and, although weight-bearing exercise offers partial protection, some may be in danger of suffering brittle bone disease in later life (see osteoporosis). In addition, low food intake may result in malnourishment and all its attendant problems, including greater risk of sports injuries. Extreme underweight is often associated with an insufficient intake of the vitamins and minerals required to maintain health.

If you are underweight, your first priority should be to ensure that you are obtaining adequate nutrients. If you wish to increase your weight to its ideal level, you should do this gradually. Aim to gain about 1 pound (0.5 kg) per week by increasing your intake of lean meat, complex carbohydrates, and low-fat dairy products. These foods, combined with a well-devised exercise programme, will help you gain muscle, not fat.

Subjects: Medicine and Health.

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