Healthy Body

Why Your BMI Score Isn’t Always The Best Measure of Health

It’s the measurement that determines whether you’re a healthy weight, but could there be flaws with the formula? H&W investigates

Body mass index (BMI) has been ingrained in our health system for decades, used as an accepted way of informing an individual of their size and weight, or more specifically, the weight of an average man when it was first created. It takes into account your body composition (muscle-to-fat ratio) and your height, but what it doesn’t recognise is your age, race and gender, so should we start using a more accurate method to calculate our health and call time on BMI? Our experts weigh in…

History in the making

First created back in the 1800s, BMI became a tool to measure a person’s body mass, or in other words, to find out whether they’re a healthy weight, but it wasn’t until the early 1970s that this method began to catch on with medical professionals. “BMI was designed to provide a simple numeric measure and the formula assumes an average body composition (the ratio between muscle, fat and bone tissues),” says Professor Francesco Rubino, consultant bariatric and metabolic surgeon at London Bridge Hospital* ( “[This means] BMI is proportional to fat mass, as long as the contribution of other tissues to body composition is within average, and so your score can be influenced by higher than average levels of muscle or bone tissues, not just fat mass.” But it’s because of this straightforward formula that experts are questioning whether it’s a reliable measure of health almost 40 years on.

Off the charts

According to the NHS, a BMI between 18.5 and 24.9 is considered healthy; anything above that indicates that you’re overweight or obese. However, this can be misleading for people who have a higher muscle-to-fat ratio, such as those with a lean physique, as personal trainer Tommy Owens (, explains. “BMI determines how much stress will be placed on your vital organs. For example, if you’re drastically over a normal range, that results in a greater workload for organs overall, whether that’s fat tissue or muscle tissue.” So, why is this important? “People view a lean, muscular physique as healthy, but if a bodybuilder was to enter a competition this way, they would be facing the opposition in arguably their worst state of health, due to the stress factors of becoming so lean and maintaining a muscular frame along the way.”

One size doesn’t fit all

The fact that you could be classed as overweight when you may in fact be fit and healthy has been evidenced too. A study by UCLA found that millions of people who had “overweight” and “obese” BMI results were in fact perfectly healthy, and 30 percent of people with “healthy” BMIs were in fact not at all, based on their other health data. “BMI assumes one size will fit all when this simply isn’t the case,” says leading registered nutritionist and author, Rhiannon Lambert ( “Two people of the same age, weight and height may have the same BMI, yet one of them could be very active with a low percentage of body fat and a high percentage of muscle mass, and the other sedentary with a high percentage of body fat and a low percentage of muscle mass.” This begs the question: can you still be fit as well as ‘fat’? Tommy believes that it’s possible to increase your fitness if you’re overweight, but you still have risks to your health to consider in the long run. “Being obese myself (on the [BMI] scale), and having worked with many clients who have also been classed as obese, but carry less than 15 percent body fat, a waist-to-hip ratio scan will give you the snapshot of how things are looking from an easy health marker to assess.”

A misleading measure

Professor Francesco is also in favour of utilising another method for measuring the health of an individual, as he believes BMI doesn’t give the full picture and could cause mental health problems after receiving a potentially inaccurate result. “Unfortunately, BMI is now misconceived as a diagnostic tool when this was never its intended use. For this reason, an ‘abnormal’ BMI may feel as receiving the news of having been diagnosed with a disease when there may not be disease yet. There is also a significant psychological impact of receiving an overweight result due to widespread weight-related stigma. Body weight regulation is very complex and depends on several biological mechanisms that can be modified by environmental factors, only few of which we can control with actions at individual level.” So, is there a place for BMI? Nutritional therapist Ian Marber ( believes it’s a good starting point. “We can trust [BMI] as much as we know whether or not we’re active or inactive, and so knowing one’s BMI is useful to a point, as long as we bear in mind it’s no more than a rough guide.”

The happy medium

So we needn’t do away with BMI altogether, as Rhiannon points to another case for it. “There are some cases where BMI can be useful, namely when assessing underweight people who are trying to recover from eating disorders, although I think BMI is more often a problem than a reliable measure of health.” And it’s this measurement of health that should still remain simple, Ian stresses. “Measure out a piece of string to match your height, cut it exactly in half and try to circle your waist with one of the halves. If you get it around your midriff easily, then this suggests that the amount of visceral fat that you have is acceptable. However, if you struggle to make the two ends of the string meet, this probably means that the level of visceral fat is too high. It’s important that health messaging is simple, and to an extent BMI is just that.”

Health & Wellbeing