How accurate is the BMI?
You probably know what the Body Mass Index (BMI) is and may have used it in the past to assess whether you’re overweight, or not. But is it really accurate?
The BMI (Body Mass Index) is widely used as a tool to assess if a person is overweight, or not. However, some people have a normal BMI and yet their waist circumference measurement is above the normal, healthy range; others are fit and healthy and yet their BMI exceeds 25, the upper level of normal.
How should we use this tool and is it really accurate?
What is the BMI?
The BMI defines the level of adiposity (fat) in the body according to the relationship of weight to height. The standard formula is as follows:
Weight in kilograms divided by height in metres squared (height in metres times height in metres). For example: To calculate the BMI of a woman who is 1.6m tall and weighs 59kg, you would use the following equation: 59 ÷ (1.6 x 1.6) = 59 ÷ 2.56 = 23,04. In other words, this woman has a BMI value of 23, which falls nicely within the normal range.
The different ranges for the BMI are divided into:
- Underweight: less than 18.5
- Normal: 18.5 to 24.9
- Overweight: 25.0 to 29.9
- Obesity, Class I: 30.0 to 34.9
- Obesity, Class II: 35.0 to 39.9
- Extreme Obesity, Class III: more than 40.0
Why use the BMI?
In Krause’s Food & the Nutrition Care Process, a much-respected nutrition resource used by dieticians worldwide, the authors admit that it’s possible to be overweight according to BMI standards, but not be overfat or obese. It’s also possible to have excessive fatness, and yet not be overweight.
This is perhaps why people often challenge the usefulness of the BMI as an accurate measure of fatness. The BMI is most useful to assess fatness when a variety of other measurements cannot be done too.
These measurements, which would typically be done by a dietician, fitness instructor or researcher, include:
- Determination of the waist circumference.
- Use of a formula (e.g. the Deurenberg formula) to calculate percentage body fat.
- Use of devices to measure bioelectrical impedance (this method is popular in gyms).
- Whole-body underwater weighing (usually only used in scientific studies).
- Use of radioactive markers to determine body composition (also usually only used in scientific studies).
Determining your BMI is thus a rapid, inexpensive way of determining whether you’re overweight or underweight, and has a reported accuracy of 88% (this is regarded as more than adequate).
When should the BMI not be used?
The BMI shouldn’t be used for athletes, or pregnant and lactating women, because it will probablyoverestimate body fatness. In older people who have lost a lot of muscle tissue (sarcopenia) or people who have wasting diseases, the BMI will underestimate body fat – other measures such as skinfold thickness readings should rather be used. The same applies to eating disorders associated with wasting such as anorexia nervosa.
Experts also discourage the use of the BMI to assess the fatness of people in Obesity Class III, which is described as “extreme” (values exceeding 40).
In general, the BMI isn’t used for children under the age of 18, where age-specific tables are more accurate.
In all cases where the BMI should rather not be used, further measurements should be made by trained and experienced evaluators, e.g. a dietician, biokineticist or medical doctor.
A combination of BMI and waist circumference, or skinfold measurement, or fat percentage calculations using the Deurenberg formula, should be able to determine if a person is overweight or obese, and if he or she is at risk of developing diseases of lifestyle.
An honest look in the mirror may also be valuable – both for people who are overweight or obese, and those who have anorexia. If you can’t see yourself or your child realistically, consult a dietician or your medical doctor to help you assess your weight.
- Mahan K L et al (2021). Krause’s Food & the Nutrition Care Process. Elsevier Publishers, USA.
- Formula Medical (2015). Formula for Life.