BMI has long been used as the gold standard for categorising someone’s weight. Research uses it, gyms use it, healthcare services use it. But there are some pretty significant flaws in the use of BMI calculations that raise the question of whether it’s really suitable. Is it time for BMI to be scrapped in favour of an alternative method?
What Is BMI?
BMI stands for Body Mass Index. It’s a measurement that uses a person’s height and weight to calculate a numerical result, which is then categorised. In a nutshell, the result is labelled as being healthy or unhealthy due to being under or over weight. A “healthy” BMI result is between 18.5 and 24.9. The categories are based on averages and ideals because there’s no scientifically objective value for a weight that equals good health.
BMI is often used by medical professionals to quickly assess whether someone is a healthy weight or not for their age and height. While some doctors may take into account limited variables that affect weight, the basic result is often taken at face value.
How Do You Measure Your Body Mass Index?
To calculate BMI, divide the weight in kilograms by the height in metres squared. This needn’t be done by hand as there are online BMI calculators to quickly work this out for you and show where the result falls on the weight continuum. Some home bathroom scales or scales at chemists and gyms can also calculate BMI.
Most calculators will want the following information: Height, weight, age, sex, ethnic group, and potentially activity level.
You can find your BMI using the NHS BMI Calculator here.
There are 4 basic ranges that show if the result is underweight, healthy, overweight or obese :
- Below 18.5 : Underweight Range
- Between 18.5 – 24.9 : Healthy Range
- Between 25 – 29.9 : Overweight Range
- 30 and over : Obese Range
What Are The Implications Of Its Use?
BMI is used as a tool and means of categorising, judging, inferring and making decisions on behalf of individuals and patients. For patients, BMI is what is used to tell you “if you’re a healthy weight”. If you’re classed as not being at a healthy weight, BMI is used as a starting point for further advice and action.
BMI can feature in research, and that research can go on to dictate clinical practice. It can be featured in giving risk estimates to individuals, including school children, regarding the dangers of obesity if their BMI puts them in the overweight category. It can be a factor in decisions about whether someone is eligible for healthcare services or treatments.
It is also used as a method for assessing the general population via surveys and medical records to see the proportion of those who are overweight as part of the “fight against obesity”.
You’ll notice that the focus is often on obesity and the underweight category is often sidelined.
There’s an assumption made that a “healthy” or “normal” weight and BMI equals overall good health generally. That’s not always the case. Overall health is a complex picture that includes things we cannot control like our genetics. It also includes personal choices and lifestyles, which can be independent of weight.
For instance, one individual may live a healthy lifestyle, have less health complaints, and be physically very fit but have a higher BMI and is thus classed as being less healthy. This is in comparison to someone who is slimmer and within a “healthy” BMI range but who is chronically ill and with various health complaints, or who has a less healthy diet and lifestyle that actually puts them at far greater risk of further health problems. The outcome is that the first person may be cautioned about their weight without due cause and the second person can have potential concerns overlooked on the basis that they look “fine” and have an average BMI.
So if so many things are heavily reliant on BMI, and this method of calculation is crucial to research underpinning certain elements of healthcare systems, we need to be confident in the accuracy of it. If BMI turns out to be flawed and inaccurate, everything that follows on from that – everything that uses BMI in some way – will likewise be flawed.
The Flaws & Limitations Of BMI : What It Doesn’t Tell You
BMI calculations are very basic. It allows for a speedy quantitative result that conveniently fits into a graph or a tick box, but life isn’t that simple and neither is “health”.
Here’s a look at just a few limitations and complications that suggest BMI can provide an incomplete or inaccurate picture :
- Waist size is often thought to be an indicator of health, with more fat being carried around the stomach related to a higher risk of conditions like type 2 diabetes, stroke and heart disease. A person can have a healthy BMI that suggests they’re a normal, healthy weight, but their hip to stomach ratio or waist measurement can suggest otherwise.
- BMI cannot tell the difference between excess muscle, fat or bone.
- Your BMI can tell you if you’re carrying too much weight, but it cannot tell if you’re carrying too much fat.
- The adult BMI does not take into account muscle mass. This means that very muscular adults and athletes may be classed “overweight” or “obese” even though their body fat is low. Meanwhile, adults who lose muscle as they get older or through chronic illness may fall into the “healthy weight” range even though they may be carrying excess fat. Someone could also be classed as being underweight when lack of muscle mass isn’t taken into consideration.
- Alongside muscle mass is bone density as a personal variable. The higher the density the heavier the skeleton will be. Conversely, those with osteoporosis will have less bone weight than someone with average density for their age.
- Another individual variable would be missing body parts. Having an appendage amputated or the large bowel removed, for instance, would reduce a person’s weight.
- Pregnancy will also affect a woman’s BMI result. Your BMI will go up as your weight increases. You should use your pre-pregnancy weight when calculating your BMI.
- Weight is not a measure of health in and of itself. A heavier individual could be far healthier because of their diet and lifestyle than a lighter individual, but based solely on BMI they’re classed as having a problem and of being at higher risk of conditions like diabetes.
BMI is a relatively straightforward and expedient way of assessing someone’s weight. The fact that it doesn’t give a full or reliable picture of the health of someone’s weight is what puts a question mark over the consistent reliance on BMI.
Calls For BMI To Be Scrapped
MPs have recently criticised the use of BMI in determining the healthiness of someone’s weight. They’ve called for the measurement index to be scrapped, and furthermore called the government’s obesity tactics “dangerous”.
There’s a concern that the UK is turning into a “nanny state” and I personally think that many strategies employed, such as shrinking chocolate bars and ready meal sizes or adding sugar taxes, make the situation worse. The more miserable people become, the greater the likelihood of reaching for junk food or overeating. The more focus there is on fat, calories and health foods, the greater the likelihood of eating disorders, obsessions and anorexia. All too often, strategies are more about political statements, box ticking exercises and money rather than anything to do with public health or common sense.
The NHS significantly integrates BMI for children and adults within healthcare. It brands adults as needing to lose weight or put on weight on the basis of their weight and this BMI calculation alone. The suggestion is that a person isn’t healthy and the weight isn’t healthy because it doesn’t fall within a certain range. Schools have been increasingly called out for deeming some children as being “overweight”, while they appear to be a normal size and are otherwise healthy.
Parents are understandably outraged and there is real concern that body dysmorphia, calorie counting, self-consciousness about weight and eating disorders will all proliferate in any age group as a result of BMI reliance and the focus on weight and obesity.
One suggested alternative to the body mass index is the waist to hip ratio to determine fat distribution. The waist-to-hip ratio is calculated by dividing the circumference of your waist by the circumference of your hips. A high risk level of visceral fat, which can increase the risk of type 2 diabetes, falls within a different range for men and women : For women it’s a ratio of 0.85 or more, and for men it’s a ratio of 0.9 or more. But even this doesn’t tell you the whole story of health and weight. Nor does it put the focus on the underlying issues around being under or overweight, which is not just about access to fast food and chocolate. Until strategies tackle those underlying issues and dispel stigma around eating disorders and “fat”, the situation will remain unhealthy and ill-advised.
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It seems it may be time to ditch the convenience of limited and potentially harmful quantitative data for a more equitable and realistic alternative. Weight and healthiness do not necessarily go hand in hand and the issues are far from black and white. It’s all very well to put a tick in a box or berate those who don’t fit within the ideal category, but if it’s not accurate, not useful and not fair, then perhaps it’s time for a change.
Do you think BMI is an unfair or unhelpful way to categorise weight & health?