HEALTH

BMI tells you little about your health

Relying only on BMI as a proxy for health contributes to and exacerbates weight stigmatisation.

In Summary

• Since the pandemic erupted, numerous studies have reported that a majority of the sickest Covid-19 patients have been people with obesity.

• New studies also show even individuals who are overweight are at higher risk.

A man's waistline is measured
A man's waistline is measured
Image: COURTESY

Your BMI is 29.7, indicating your weight is in the overweight category, inching toward obesity.

For your height (5 ft 8, inches) a normal weight range would be between 55 and 72.5kg. I am sure this is familiar. But what really does it mean?

Few single anthropometric measures mean so much in health and morbidity than Body Mass Index or BMI. BMI is the ratio of your weight and height in meters squared. The function used to derive BMI was devised in 1830 by Belgian polymath, Adolphe Quetelet to describe the average man.

It was in an article published in 1972 that American physiologist Ancel Keys gave Quetelet’s formulation a name, BMI, along with the evidence that underpins its current application. Keys and his co-workers demonstrated that BMI was a parsimonious and accurate predictor of body fat. Since 1972, BMI has gained predominant usage in the quantitative study of body mass and obesity, in health and morbidity.

The simple ratio of weight (kg) divided by height (m) squared produces for categories of BMI — underweight; normal; overweight and; obese. Generally, higher BMI; overweight or obese categories are associated with a higher risk of Type 2 diabetes or heart disease.

Since the pandemic erupted, numerous studies have reported that a majority of the sickest Covid-19 patients have been people with obesity. New studies also show even individuals who are overweight are at higher risk. Moreover, obesity is associated with impaired immunity, chronic inflammation and blood that is prone to clot, all of which can lead to complicated illness from Covid-19.  

Although the use of BMI remains hugely popular, its limitations have come into sharp focus. BMI cannot untangle what proportion of an individual’s weight is attributable to bone mass, muscle or body fat. You can imagine how awfully obese spectacularly muscular athletes are. Moreover, the proportion and distribution of bone, muscle and fat changes with as one ages. There have been calls for a “new BMI”.

A study conducted in 2016 revealed sobering misclassification of metabolic health among individuals categorized as obese, overweight and normal. The study revealed over 30 per cent of “normal” weight individuals were metabolically unhealthy. They had high levels of blood sugar, high levels of body fat, high cholesterol, high glucose and high blood pressure. Conversely, 29 per cent of individuals classified as obese were metabolically healthy.

Clinical decisions based solely on BMI may be misdirected. There are more accurate markers of health than BMI, such as waist circumference, or body fat percentage or the ability of the respiratory system to supply oxygen to skeletal muscles, or their combination. Simple things like how you feel when you walk up a flight of stairs and the measure of the circumference of your waist are more reliably diagnostic of your health than BMI.

Relying only on BMI as a proxy for health contributes to and exacerbates weight stigmatisation. This is particularly unfair given that health insurance providers demonstrate high levels of anti-fat bias. Doctors should stick with blood markers.

Alex O. Awiti is Vice Provost at Aga Khan University. The views expressed are the writer’s