What is BMI and how accurate is it?

What is body mass index or BMI?

BMI is a formula that is used to categorize body weight. To calculate BMI simply divide a person’s weight in kilograms (kg) by the square of their height in meters (m) to get a result in kg/m2.

Interpreting BMI

The ideal BMI is in the range of 18.5 to 24.9.
Below 18.5 is classed as underweight.
Between 25 and 29.9 is classed as overweight.
Between 30 and 39.99 is classed as obese.

Being underweight or overweight or obese has significant health. Low BMI risks include nutritional deficiencies, weakened immunity, fertility and osteoporosis. High BMI risks include cardiovascular disease, diabetes, joint problems and cancers.

Merits of BMI

It is a quick calculation
For children and young people between 2 and 18 years old, it is possible to use modified formulas taking age and gender also into account.
High body fat will impact BMI and can indicate potential health risks.

Inaccuracies and limitations of BMI

It does not truly diagnose the amount of body fat
It may not reflect the current health of a person. Research has shown that a subset of overweight/obese individuals actually have normal cardiometabolic measures (including blood pressure, blood glucose, cholesterol) and a subset of normal BMI individuals have unhealthy cardiometabolic measures.
Muscular individuals may have a higher BMI. BMI does not distinguish between body fat and muscle mass. The same volume of muscle compared to fat weighs more. Healthy athletes may be in the overweight range for example. Men also tend to have a greater percentage of muscle mass than women.
Older individuals may have a lower BMI. This is because with age we tend to lose muscle and bone mass. An individual could be of a normal range while they may actually be overweight.
The BMI calculation is based on Caucasians. There may be further inaccuracies for other ethnicities. For example, Asians have four percent higher total body fat compared to white Europeans of the same BMI.
BMI should not be used if you are pregnant. Your GP or midwife should be contacted if there is any concern.

Alternatives to BMI

Waist circumference - Abdominal fat is correlated with diseases usually because it is associated with high physical inactivity and high blood pressure.
Weight/height to the power of 2.5 - This may be more accurate a calculation as BMI also makes short people seem thinner than they are and tall people seem fatter than they are. Plotting normal weights of people against their height it can be seen that the power of 2.5 is more accurate.
Waist-to-height ratio - There is also evidence that this measure is better. A ratio of waist circumference to less than half your height is thought to be healthier.

Summary

BMI is useful as a quick tool to indicate if you have increased health risks. However, it is not always accurate in all circumstances. If you have any concerns regarding your weight consult your doctor. They will be able to advise you and order investigations if relevant.