How concerned should you be about your BMI?

I consider myself a healthy person. I eat lots of fruit and veg, obsess over fibre, climb mountains twice a week and try to squeeze in a lunchtime jog on the days I don’t. But when I recently calculated my body mass index (BMI), which involves dividing a person’s weight by their height, I got a surprise: I’m overweight.

For many of us, such news can ring alarm bells. Especially for anyone like me who has a history – in my early teens – of being unhealthily concerned with where the dial sits on a scale. But how worried should we really be about our BMI?

First, you should know that the measure was never intended to assess a person’s health. It wasn’t even created by a doctor or anyone with any kind of medical training, but rather a 19th century mathematician named Adolphe Quetelet who wanted to track the height and weight of entire populations. No offense to number gurus, but that’s not exactly an auspicious origin story for a health calculator.

If we fast forward to the 1970s, the measure started to gain popularity as a quick and cheap way to measure body fat levels and obesity levels: all it takes is a tape measure and some weights, after all. Then, in 1997, the World Health Organization—illuminated by BMI’s simplicity—rolled it out as a health assessment tool.

In the following years, the measurement was deeply embedded in the healthcare system. It now provides access to dozens of therapies, from knee surgery to GLP1 weight loss drugs and fertility treatments. Generally, if someone’s BMI falls below 18.5, they are classified as underweight; if it is 25 to 29.9, they are overweight, while numbers above 30 mark obesity. The reasoning goes that limiting treatments to certain BMI brackets can reduce safety risks and maximize success rates.

But there is a big problem. BMI does not differentiate between bone, muscle and fat. This means that someone who is super muscular with little body fat can end up in the category of overweight – or even obese – despite being physically fit and strong.

I used to have a “healthy” BMI, but (to my delight) rock climbing has given me actual, tangible arm muscles for the first time in my life – and I suspect this is partly what has pushed me into the overweight bracket.

In other cases, people with a “healthy” BMI may lack body fat to the point where they stop menstruating, which can cause problems such as brittle bones, heart attacks and infertility. It’s obviously not okay.

And that’s not all. BMI does not take into account where fat is stored in the body. But we know that the fat around our abdominal organs, known as visceral fat, increases the risk of many conditions – such as heart disease, high blood pressure and type 2 diabetes – significantly more than what is found in the arms, bottom and thighs.

I’m not saying that BMI is completely useless. In some cases, it really is a sign that someone will benefit from medical help to help them gain or lose body fat.

But we now have other, much better, ways of measuring body fat levels. For example, a landmark study found that the waist-to-hip ratio, which involves dividing the circumference of the waist by the circumference of the hips, trumps BMI in predicting someone’s risk of heart attack. Another showed that it was a better predictor of mortality.

There is also the weight-adjusted waist index, where you divide your waist circumference by the square root of your body weight. Like the waist-to-hip ratio, this puts more weight on the more harmful visceral fat and has been shown to improve BMI.

Another option is the body roundness index (BRI), which uses height, waist circumference and weight data to measure body shape. Studies have shown that the BRI predicts total and visceral fat levels better than BMI, waist or hip measurements alone. We also have tools that zap the body with low-voltage electricity, which indicates where fat is spread around the body.

So if you are concerned about your weight, these options are much more informative than calculating your BMI. But personally, I think that instead of worrying too much about specific numbers, most of us are better off focusing on healthy lifestyle habits like eating lots of fruits and vegetables, maintaining social connections, getting enough sleep, and participating in regular exercise. That’s definitely what I’m going to do!