# 3.1 The body mass index

The body mass index (BMI) links weight and height to determine whether someone is an ideal weight, overweight or obese. This measurement is now used by the World Health Organization to make global comparisons about levels of over- and under-nutrition for people in different countries (Webb, 2008). The table below shows how different BMI scores relate to weight classifications, and you will see that the distinction between ‘overweight’ and ‘obese’ is actually a very precise difference in the BMI. ‘Overweight’ is classified in cases where the BMI is in the range 2530, and ‘obese’ is classified for BMIs above 30. Obesity becomes ‘severe’ at a BMI above 40.

## World Health Organization BMI classifications

BMI value: weight (kg)/height (m2) | Classification
--- | ---
Under 18.5 | Underweight
18.5–25 | Ideal range
25–30 | Overweight
Over 30 | Obese
Over 40 | Severely obese

BMI measurements need to be applied carefully, however. The same volume of muscle tissue is heavier than that of fat, so an athlete with well-developed muscles and little excess body fat is likely to be heavier than someone of the same size who has more fat and less muscle. The BMI measurement for the athlete is therefore likely to be abnormally high and would not provide a good guide to their possible health risks.

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