Body mass index (BMI) – what are the pros and cons?
Body mass index (BMI) is a useful screening tool for obesity and being overweight, but it has some limitations (see below). It is thought to reflect body fat (either too much or too little).
How is BMI calculated?
BMI = your weight (in kg) divided by the square of your height (in cm) = weight/height2.
This is a good calculator.
With BMI, what is normal, or shows you are overweight or obese?
Body mass index (BMI) – for adults:
- Underweight: <18.5
- Healthy weight: 18.5–25
- Overweight: 25-30
- Obese: ≥ 30
- Severely obese: ≥ 40
Note: different thresholds apply to certain ethnic backgrounds.
So, what are the pros and cons of the BMI measurement?
Pros
BMI is cheap and easy to calculate.
It’s a good indicator of the risk of co-morbid diseases associated with higher body fat, such as ischaemic heart disease (IHD), type 2 diabetes, and high blood pressure (hypertension).
Cons
BMI is not a direct measure of body fat, and it doesn’t account for body fat distribution.
BMI may overestimate body fat in people with a muscular build, and underestimate it in older people or those who have lost muscle.
BMI’s association with health risk varies by age, sex, and ethnicity. It also doesn’t assess the presence of other conditions or functionality.
Summary
We have described the pros and cons of the BMI measurement. We hope it has been helpful.
Historical note
BMI is not a new concept. It was invented by Adolphe Quetelet (1796 -1884). He was a Belgian astronomer, mathematician, statistician, and sociologist, who devised the basis of the BMI between 1830 and 1850; as he developed what he called ‘social physics’.
