Body Mass Index, an outdated health calculator

For decades, people have been told that the Body Mass Index (BMI) is the most effective technique to measure a person’s body fat. But that’s not true! BMI can be a helpful health measure for large groups of people, but it won’t give an accurate individual health index.

Introduced in the 1830s by a Belgian statistician who wanted to describe the ‘average man’ quantitatively, the calculation was named the Body Mass Index and popularised in the 1970s by the Minnesota physiologist Ancel Keys.

Dr Keys wanted to prove to life insurance companies that estimating people’s body fat and their risk of dying cannot be determined by comparing their weights with the average weights of others of the same height, age, and gender. Through an extensive study, he and his colleagues showed that the BMI was a more accurate — and far simpler — predictor of body fat. This became the most popular technique to measure body fat. But this technique is quite deceiving.

How is BMI calculated? Take your weight (kg) and divide it by the square of your height (m). The result is meant to divide your body weight into four main categories - underweight (BMI less than 18.5 kg/m2), normal weight (18.5 to 22.9kg/m2), overweight (23 to 24.9kg/m2), and obese (25kg/m2 or more). People around the world, thin and overweight alike, have struggled to achieve the ‘perfect’ BMI.

So, the question is does it accurately describe individual health? BMI can’t tell what percentage of a person’s weight is from their fat, muscle or bone. The way BMI is calculated, it leaves no room for the relative proportions of bone, muscle and fat in the body. This explains why muscular athletes often have high BMIs despite having little body fat.

And as people age, it’s common to lose muscle and bone mass but gain abdominal fat, a change in body composition that would be concerning for health but might go unnoticed if it didn’t change a person’s BMI. The measure also does a poor job of predicting a person’s metabolic health.

For example, in 2016, a study was conducted in the US to understand the relation between BMI and metabolic health. This study found that measuring a person’s BMI does not consider the metabolic health of the individual.

The study including 40,000 adults in the US was found to be ‘overweight’ since researchers calculated their BMIs, keeping their insulin resistance, markers of inflammation and blood pressure, cholesterol and glucose levels in mind. A quarter of those classified as obese were metabolically healthy by these measures. However, 31% of people with a ‘normal’ body mass were metabolically unhealthy.

If BMI isn’t accurate, then what is? Don’t be entangled in the fatphobia concept. Rather than calculating body fats, people should check their blood glucose levels, triglyceride and blood pressure to ensure a healthy lifestyle and body. Rather than focusing on your weight, give importance to how you feel while carrying out physical activities.

If you know anyone struggling with body image issues, reach out to them and make them understand that having a healthy lifestyle is all that matters. BMI does not determine anyone’s physical health. It is a healthy lifestyle that will determine your overall health and wellbeing.

(The writers are a clinical nutritionist and a general physician respectively, at a hospital in Mumbai)