Body Mass Index is not an infallible measure

Body mass index (BMI) is a measure of body weight and is widely used to calculate the prevalence of obesity in the population. According to this criterion obesity levels are high and rising, which is alarming in view of the associated risks of ill-health, eg heart disease and diabetes. However, BMI is a very crude index when applied to an individual as opposed to a population. We need to become more discriminating in our use of BMI and more conscious of the health benefits of exercise. Jane Brody reviews recent insights into BMI in her New York Times blog.

BMI is a ratio between a person’s weight and height. It is calculated by dividing weight in Kg by height in meters squared. Those with a BMI below 18.5 are classified as underweight, 18.5 to 24.9 are normal, 25 to 29.9 are overweight, 30 to 34.9 are grade 1 obese , 35 to 39.9 are grade 2 obese, 40 and above are grade 3 obese. I have a BMI of 24.5.

Classified on BMI, 26 per cent of men and 21 per cent of women in Ireland are obese. Nineteen per cent of 9-year-olds are overweight and 7 per cent are obese. By 2025, it is predicted that 50 per cent of men and 33 per cent of women will be obese.

The BMI index was devised in 1835 by a Belgian statistician. BMI was intended to measure weight distribution in large populations but it has been gradually adopted by physicians as a convenient way to assess an individual patient’s weight because weight and height are so easily measured. However, at best BMI is a crude measure that misses many people with excess body fat. Someone who had a normal BMI could still have too much internal fat and be prone to obesity-related illness. Body weight is made up of bone, muscle and water, as well as fat. Arnold Schwarzenegger’s BMI was in the obese range when he was Mr Universe, yet he was not fat.

Also, BMI doesn’t take distribution of body fat into account. Excess fat on the hips, bottom and thighs is metabolically inert and is not linked to chronic disease. On the other hand, abdominal fat is metabolically active, interacting with other body organs and increasing the risk of diabetes, high blood pressure, cardiovascular disease and erectile dysfunction, among other problems. The distinction between metabolically active and metabolically inert is analogous to the difference between a new TV set still packed in a cardboard box and a TV set plugged into a socket, switched on and tuned into a program.

Assessing individual fatness A more accurate way than BMI to assess individual fatness is to take two measurements with a tape measure – around the waist and around the hips. Divide the waist by the hip measurement. A ratio above 0.9 for men or 0.85 for women flags abdominal obesity and increased health risk.

Probably the single most useful thing anybody can do to protect their health is to get aerobically fit.. Being fit can improve your health at any weight, reducing the risk of developing heart disease, diabetes or blood pressure. Overweight (as defined by BMI) and fit beats thin but unfit.

A recent widely reported meta-analysis covering almost three million people and 270,000 deaths (KM Flegal and others, Journal of the American Medical Association, Vol. 309, pp 71 – 82, 2012), found that those who were overweight and those with grade 1 obesity (as defined by BMI) were not at greater risk of death from all causes than those in the normal BMI range. The study did not control for diet, smoking or socio-economic status, but it nevertheless illustrates the dull predictive power of BMI.

Another recent study (JE Winter and others, American Journal of Clinical Nutrition, Volume 99, pp75 – 890, 2014) of older people (65 and older) showed that being overweight did not increase mortality, but the risk was increased for those with BMIs below 23.

So, the message is control your weight and get fit. By all means calculate your BMI but pay more attention to the ratio of your waist to hip circumference. Be conscious of abdominal fat. How do you get fit? Exercise 30-45 minutes per day, five days a week. A brisk three-mile/ 4.8km walk done in about 45 minutes each day would keep you fit. If you are unfit to begin with you need to gradually work up to this pace.

How do you keep your weight down? This is achieved through diet and exercise. I will repeat my mantra on diet – eat a wide variety of whole foods in moderation, mostly plants.

[ understandingscience.ucc.ie ]