Body Mass Index Limitations
Body Mass Index (BMI) is a useful tool in monitoring someone's health status, but it has its limitations.
BMI measures total body weight, not the actual amount of fat a person is carrying. Some people are naturally stocky and have a body mass index that is in the overweight category, when in fact their weight is due to muscle mass and a heavier bone structure rather than excess fat.
It is common for athletes such as rugby players and weight lifters to have a BMI indicating they are overweight when they are not. Their body mass index is higher because of their extra muscle mass, not because of excess body fat (muscle weighs more than fat).
Conversely, some athletes will be underweight according to their BMI (such as long distance runners). This is due to low body fat and aerobic slow twitch muscle fibres, which develop naturally as a result of their particular sport.
Some people are thin-framed and their BMI will inaccurately determine them as being underweight. Those who have lost muscle mass (such as the elderly and infirm) may also appear to be underweight, when in fact it is normal to lose muscle mass and body fat in old age and as a result of poor health.
On certain occasions the way BMI is interpreted can be adjusted according to the person's lifestyle, natural build and health status.
If body mass index falls within the normal range, this does not necessarily indicate someone is in perfect health, that they are getting enough exercise or eating a healthy range of foods. They may also be carrying more body fat than is good for them.
In addition to body weight, shape is also important. A person's body mass index may be normal but their waist circumference may be too high. Being apple shaped carries more health risks (including a higher risk of heart disease) than being pear shaped - regardless of a normal BMI.
It is important to be physically active and eat healthily, while limiting salt, sugar and alcohol, and not smoking, and body mass index is not necessarily indicative of any of these things.