Introduced in the 19th century by Lambert Adolphe Jacques Quetelet, the formula was used to give a quick and easy way to measure the degree of obesity within the population.
The formula is calculated using weight and height measurements as follows: weight in kg/(height in meters)2.
These results are then classified as follows:
<18.5 = underweight, 18.5-25 = heathy weight 25-30 = overweight and >30 = obese.
While BMI can be a handy, quick way to spot if someone if potentially over or underweight and at risk of poor health, there are concerns about the accuracy of this system.
The main downfall is that BMI doesn’t distinguish between tissue type - bone is more dense than muscle and twice as dense as fat, so a person with strong bones, good muscle tone and low fat will have a high BMI. These people could therefore be classified as overweight or obese, despite having a healthy level of body fat. This has led to many body builders or extremely fit rugby players being classified as obese!
BMI also does not account for distribution of body fat - and it’s well known that people who carry more weight around the middle are at a much higher risk of heart disease and type 2 diabetes than people who hold fat around the thighs and lower body. This could lead to someone with a healthy BMI yet unhealthy level of abdominal fat being classified as low risk.
Why is it still used?
Although BMI has its downfalls, it can still be a useful tool. Garry Jennings from the Heart Foundation believes the BMI can be used in conjunction with other health measurements to provide a better understanding of an individual’s health. Unfortunately there is no single measure that can provide a detailed picture of health status, therefore it is not the right time to ditch the BMI!
To complement the BMI, a body fat percentage test and/or waist circumference measurement are valuable tests to complete. The body fat percentage test can determine the tissue type breakdown and whether the individual is within the recommended healthy range. Monitoring waist circumference is a quick and easy way to determine risk of the conditions that are associated with being overweight - men should aim for a waist circumference of less than 94cm and for women it’s less than 80cm. It’s also important to have regular checks of blood pressure and cholesterol levels.
If you’re trying to lose weight, some people find that it is helpful to weigh yourself regularly, yet don’t forget that if you’re exercising and gaining muscle, you may not see dramatic weight loss - yet you’ll be benefiting on the inside.
Written by Ruth Tongue