Should We Stop Giving So Much Weight to BMI

Are you a healthy weight? To get an answer, you’re likely to measure your body mass index or BMI. But how helpful is this?

Your health can be gauged by a simple formula we are told. Divide your weight in kilograms by your height in metres squared (kg/m2) and you have your BMI.

This can be then used by your doctor, nutritionist, or your fitness instructor to assess if your weight is ‘normal’ or ‘healthy’ for your height or whether you are hovering close to being classified as ‘overweight’, ‘obese’ or ‘underweight’.

BMI is used around the world to measure obesity and give an estimate of our overall disease risk. There is an assumption that a normal BMI equates to good health, while scores in the ranges above or below suggest your health is at risk.


Is there a link between BMI and health?

BMI does have value. Being quick, simple and cheap, it can quickly identify people who may be at risk of conditions such as type 2 diabetes or heart disease and be a useful starting point for further investigations.

But many health professionals, backed by a growing body of evidence, are now questioning the reliability of BMI as a marker of health.

A 2016 study published in the International Journal of Obesity looked at BMI as an indicator of heart health. The research team checked individual BMI results against other indicators, including cholesterol levels, blood pressure and insulin resistance. What they found was surprising:

  • 30 per cent of people in the normal BMI range were at above-average risk of heart disease.
  • 48 per cent of ‘overweight’ and 29 per cent of ‘obese’ people were heart-healthy.

What else can’t BMI tell you?

  • The amount of fat on your body

In adults who have stopped growing, an increase in BMI is usually caused by an increase in body fat, but there are many exceptions to this.

“Having a high ratio of muscle to fat is liable to put you into the ‘overweight’ category,” says Canadian dietician Abby Langer. “Muscle weighs more than fat, and if you’re solid and muscular – think athletes or weightlifters – BMI won’t recognise that; it will just categorise you as overweight when you’re not.”

Body composition, including your per cent body fat or muscle mass, can also vary by race and ethnic group.

  • Where that fat is stored

Where you store your fat is critical to your health. Body fat stored around the abdomen (an ‘apple’ shape) is more dangerous than fat on your hips or thighs, but BMI won’t tell you where your fat is located. The ‘apple’ shape is associated with an increased risk of heart disease, stroke and type 2 diabetes, and you can have a BMI in the ‘normal’ range but carry risky amounts of belly fat.

  • Your individual risk

Your risk of disease doesn’t automatically increase with weight. The normal ranges don’t work so well for predicting health risks in older adults, who don’t appear to have a greater risk of death when their BMI is in the ‘overweight’ category.


What are the alternatives?

Your waist circumference is a better predictor of health risk than BMI, because it can indicate how much fat is stored around your abdomen, where it is potentially more dangerous.

Both waist circumference and BMI can be good starting points, but other measurements will give a fuller picture of health risk. Lifestyle factors such as smoking, activity levels, diet and stress, along with blood pressure, blood sugar and cholesterol levels should all be considered.