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Exploring family health
Exploring family health

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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 25—30, 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 18.5Underweight
18.5–25Ideal range
Over 30Obese
Over 40Severely 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.