If you have children and remember following their growth on a chart when they were young, or even if you’re well-acquainted with dieting and weight loss, then you’ll have probably heard of BMI. But what is it and why do we use it?

BMI stands for body mass index, and it’s a measure that relates your weight, or more accurately, your mass in kilograms, to your height in metres. The equation is simply:

mass/height²

This measurement was first developed by the Belgian-French astronomer, mathematician, statistician and sociologist Lambert Adolphe Jacques Quetelet. Perhaps you may have noticed that ‘doctor’ or ‘medic’ does not feature among his many professions.

It should come as less of a surprise then, when I say that BMI was not intended to be used by individuals, i.e. you and me, to determine whether or not we’re in good health.

BMI (or the Quetelet Index as it was referred to in the 1800s before Ancel Keys coined the term ‘BMI’ in 1972,) was intended by Quetelet to be a statistical tool to characterise the ‘normal man’ on a whole-population level (Eknoyan, 2007). And it works well in these contexts.

However, while on the individual level it can give an indication of whether one is healthy or at risk of chronic disease, it is not the most accurate measurement out there. It is fascinating to me then, that decades later, BMI is still one of the most widely-used tools to assess our health, despite other measures being just as simple to calculate, and likely more accurate.

Why is BMI Useful?

Let’s start here. Using BMI has some benefits, and I would be remiss if I did not mention them.

1. It is very easy and cost-effective to measure and calculate.
2. It is good for population-level studies, and as it is a well-established measure, there is a lot of research on BMI and health.
3. It can give a rough indication of health and disease risk levels, making it a useful screening tool.
4. The cut-off for a healthy weight is well-known (between 18.5 and 24.9 km/m²), making it easy for individuals to track their own weight and approximate health at home.

What are the problems with BMI?

Despite the benefits, it has its limitations.

1. It cannot distinguish between lean mass and body fat. A higher body mass, is not always bad and it does not always mean that one needs to lose body fat. Rugby players for instance, are generally lean and carry a lot of muscle mass, but their BMI might indicate that they are obese.
2. It cannot tell us where body fat is distributed. In this case, even if someone’s BMI is high because they have an increased amount of body fat, the measurement still cannot tell us where that fat is stored. As I discuss in more detail in this article, where our body fat is stored (around our internal organs or beneath our skin) is more important for our health than just how much body fat we have.
3. It is based on European populations, meaning it is actually less accurate for other ethnicities. For instance, in the case of some Asian populations, the upper cut-off for a healthy BMI is actually 22.9 kg/m². This is because Asian populations tend to have a higher body fat percentage than Western populations at the same BMI.

So is there anything we can do about this?

What can we use instead of BMI?

I’m glad you asked because recently, scientists have been trying to identify other, more accurate measures of body fat levels and distribution for health. A couple of measures that are easy to take and have been found to be clinically relevant are:

1. Waist-to-hip ratio. This is the ratio of the circumference of your waist to the circumference of your hips. Generally, to have a healthy body composition, this should be less than 0.8–0.85 for women and less than 0.9–0.95 for men (Qiao & Nyamdorj, 2009).
2. Waist-to-height ratio. This is the ratio of the circumference of your waist to your height. In general, people with a healthy body composition have a waist circumference that is less than half their height, i.e. the ratio is less than 0.5 (Gibson & Ashwell, 2020).

I also highlight these in a previous post on adiposity, which you can read here.

Why are these measures better?

Storing body fat in your abdomen and around your waist has been found to be more dangerous than storing it around your hips, glutes and thighs. Therefore, as these measures specifically assess body fat distribution and how body fat around the waist compares to that in other parts of the body, it can give a better indication of whether one is at greater risk of cardiometabolic diseases than BMI can, with its inability to distinguish between fat mass and fat-free mass.

Conclusion

In conclusion, BMI is a widely-used measure clinically and a lot of research has made use of it too. Because it has been so widely-adopted, it continues to be used, despite its glaring limitations. Now that we know this, we can try to use other measures to assess our own health and body fat distribution, which are more accurate for us on an individual basis. In this way, we can hopefully get a better indication of our health status and what we can do to improve our body composition and fitness, in a more bespoke way.

References

Eknoyan, G. (2007). Adolphe Quetelet (1796 1874) the average man and indices of obesity. Nephrology Dialysis Transplantation, 23(1), 47–51. https://doi.org/10.1093/ndt/gfm517

Gibson, S., & Ashwell, M. (2020). A simple cut-off for waist-to-height ratio (0·5) can act as an indicator for cardiometabolic risk: recent data from adults in the Health Survey for England. British Journal of Nutrition, 123(6), 681–690. https://doi.org/10.1017/S0007114519003301

Qiao, Q., & Nyamdorj, R. (2009). The optimal cutoff values and their performance of waist circumference and waist-to-hip ratio for diagnosing type II diabetes. European Journal of Clinical Nutrition, 64(1), 23–29. https://doi.org/10.1038/ejcn.2009.92