Doctors weigh in on importance of BMI in health outcomes

We hear a lot about the worldwide epidemic of obesity and consequent ill-health effects – diabetes, heart disease, high cholesterol and more. A “normal” weight is seen as essential for good health and this has spawned a weight-reduction industry.

However, the relationship between body weight and health is complex and experience has shown that all our intense focusing on weight loss isn't generally making people healthier. Some physicians are now moving from a "weight-centric" model to one that prescribes healthier behaviours regardless of whether such behaviours reduce body weight. This approach is reviewed by Virginia Sole-Smith in Scientific American (July 2020).

Let me emphasise that I write this article as background information for your general consideration. Anyone concerned about body weight should discuss the matter with their physician before deciding what action to take.

Early in the 20th century life-insurance companies collected data that correlated shorter lifespan with higher body weight. In the 1970s, research implicated dietary fat in heart disease and the modern body-mass-index (BMI) system was introduced. BMI measures body fat based on weight and height and is widely used to categorise health by weight.

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BMI is your weight in kilograms divided by the square of your height in metres. A BMI of less than 18.5 is underweight; 18.5 to 24.9 is normal; 25 to 29.9 is overweight; 30 to 34.9 is obese 1; 35 to 39.9 is obese 2; 40-plus is obese 3. This classification system is probably too unforgiving in categorising people as overweight and mildly obese. Despite my delightfully slim profile, my BMI is 25.8, near the bottom of the overweight range!

BMI is easily measured, which encourages its widespread use, but it is not a very reliable measure of body fat. Arnold Schwarzenegger had a BMI of 31 (obese 1) at the top of his bodybuilding career despite having very little body fat – his BMI reflected his copious dense musculature.

Waist-to-hip ratio

A better measure of fatness is waist-to-hip ratio (WHR) which measures your abdominal fat. Measure your circumference, while exhaling, by circling a tape measure around your waist at belly-button level. Take a hip measurement at the largest circumference around your buttocks. Excess fat is indicated by WHR greater than 0.8 for women and greater than 0.9 for men.

Excessive abdominal fat is classed as particularly undesirable because it is close to vital organ blood supplies into which it releases easily. It correlates with the negative consequences associated with obesity, eg diabetes and high blood pressure.

Aerobic exercise is a particularly powerful ameliorator of the ill-effects of obesity – 'fat but fit beats lean but unfit'

Apart from at its highest and the lowest levels, BMI is a poor predictor of mortality and research cited by Sole-Smith shows no increased mortality risk for people in the overweight and lower obesity range compared with people whose BMIs are in the normal range.

Much research demonstrates that people’s behaviour has a bigger impact on mortality than body size does. In a large study researchers tracked BMI and four lifestyle habits of people over a 14-year period. They found that, regardless of BMI class, people lived longer when they practised one or more of four healthy habits – didn’t smoke, moderate alcohol intake, ate five or more servings of fruit/vegetables daily and exercised 12 or more times per month. The more healthy habits practised the bigger the effect.

Aerobic exercise is a particularly powerful ameliorator of the ill-effects of obesity – “fat but fit beats lean but unfit”. Indeed, taking plenty of aerobic exercise is probably the single most effective practice any of us could take up to improve our general health.

Obesity is undoubtedly a negative health factor, correlating with a wide range of ill-health conditions. And body weights at either extreme of the BMI scale are serious conditions requiring urgent medical intervention. But the general relationship between weight and health is complex and obesity is only one factor, albeit a significant one, of many factors underlying the root causes of disease at the intersection of genetics, lifestyle habits and the environment.

Sole-Smith warns against a “weight-centric” approach that focuses intensely on weight loss to address health problems. This approach can cause more harm than help by focusing primarily on weight loss before prescribing evidence-based treatments for specific conditions, for example diabetes.

Some/many doctors are now persuaded to move from the weight-centric model to focus on inculcating healthy behaviours. Finally, the recent book Modern Culture and Well-Being, edited by Catherine Conlon (Veritas Publications), has interesting insights on diet, weight and exercise.

William Reville is an emeritus professor of biochemistry at UCC