6 Calculating body mass index
Being overweight is another risk factor for CHD, as well as a risk factor for developing Type 2 diabetes. Assessing whether somebody is overweight can be done in two ways. Simply weighing someone may not accurately determine if they are overweight. For example, two people may weigh 70 kg but one may be of an average weight whereas the other person is obese. Height is an important factor in determining if the person's weight is healthy for them. To take this into account, the relationship between weight and height is determined by calculating the body mass index (BMI).
A BMI of between 20 and 24.9 kg/m2 is desirable. A BMI of 25 kg/m2 or higher is overweight, and over 30 kg/m2 is classed as obese.
The distribution of fat on the body is also significant; carrying fat around the abdomen is associated with highest risk. This is termed ‘central obesity’ and people with this distribution are described as ‘apple-shaped’. A simple assessment for central obesity is to measure waist (or girth) circumference in centimetres using a tape measure (Figure 12c). Ideally, waist circumference in men should be less than 94 centimetres (cm), and less than 80 cm in women. (You may see other values quoted in other sources as the exact value is open to debate.) If these values are exceeded it is an indication of excessive body fat around the abdomen. A further calculation that can be made is the waist to hip ratio. The waist should be measured at the level of the umbilicus (belly button) and the hips at the widest point. The waist measurement is divided by the hip measurement. Ideally the ratio should be 1.0 or less in men or 0.8 or less in women.
Mrs Soames has a BMI of 29 kg/m2 with a waist measurement of 92 cm. Her husband also has a BMI of 29 kg/m2 with a waist measurement of 92 cm. Who has the higher risk of coronary heart disease?
Mrs Soames has the higher risk. Although both of them are overweight, Mrs Soames has central obesity, with a waist measurement higher than is ideal for a woman.