Should you be using BMI?
BMI or Body Mass Index is a quick calculation to tell you whether you’re a ‘healthy’ weight for your height. It's used to determine risk of diseases like heart disease, diabetes, and stroke. The calculation is your weight divided by your height squared: weight (in kg) / height2 (in m).
The BMI categories are:
- Underweight <18.5kg/m2
- Healthy weight: 18.5 – 24.9kg/m2
- Overweight: 25.0 – 29.9kg/m2
- Obese: 30.0 – 39.9kg/m2
- Morbidly obese: >40kg/m2
BMI is a flawed measurement:
BMI is quick and easy to use, but that doesn’t mean it’s the
best option. There are a few reasons for this.
1. It doesn’t show body composition:
BMI can’t tell you what percentage of your body weight fat
mass is, or what’s muscle mass.
A classic example of this is a body builder or rugby player. They will have a larger body weight and so BMI would classify them as overweight or obese. Even though their body is mainly muscle, and they’re very active.
It also doesn’t consider the fact that people can be the same BMI but be different weights and so have different levels of body fat versus muscle mass.
Measuring body composition could be a better way to
determine health risks.
When using body fat % to determine obesity, > 32% body
fat in females and > 25% body fat in males would be classed as obese.
Research has shown that BMI is a poor indicator of body fat % and fails to
accurately classify obesity, particularly in males and older adults.
It also doesn’t consider that people could be in the
underweight or healthy weight BMI category but have a higher body fat %. This
is particularly in older people as we lose muscle mass (and so lose weight) as
we age. So, the BMI result is skewed.
2. BMI doesn’t tell you where your body fat is located:
Fat distribution also has an impact on disease risk. You may have heard of ‘apple’ or ‘pear’ body shapes.
An apple body shape is where most
of your body fat is above your waistline, and you have a lot of fat stored
around your stomach. A pear body shape is where most of your fat is stored
below your waist, so you may have wide hips and large thighs.
An ‘apple’ body shape or storing more fat around your
abdomen is linked with an increase of heart disease, insulin resistance, and
high blood pressure. But as BMI doesn’t distinguish body shape it can’t tell
you the whole picture.
3. It doesn’t consider other aspects of health:
As BMI just focuses on weight, it doesn’t consider any of
the healthy behaviours a person does. In my earlier post about diet culture I wrote about how weight doesn’t show you
the whole picture.
There are so many things that affect and impact your health
like genetics, environment, sleep, stress, activity, and diet. Just looking at
your weight and BMI isn’t enough to determine how healthy you are.
You can be an overweight BMI but do all the ‘right’ things
like exercise regularly, or eat healthily, or get 8 hours of sleep each night. You
can also be a normal BMI but have unhealthy behaviours like smoking, or not do
any physical activity. Focusing on weight alone dismisses all the behaviours a
person does.
BMI is heavily used in healthcare, but it wasn’t actually
created for this purpose.
4. It wasn’t intended to be used for health:
BMI was created by a Belgian mathematician and astronomer
called Adolphe Quetelet. He wanted to quantify ‘the average man’ and looked at
population’s weight and height to determine this. As a maths guy, he found the
BMI formula presented an easy way to do this.
There is no scientific basis for the calculation. Weight
divided by height squared doesn’t really mean much. As I’ve said earlier,
weight alone doesn’t show body composition or fat distribution. And this is a
more useful way to look at someone’s health, so the BMI calculation isn’t that
helpful.
Also, the BMI cut off categories were based on white males
and so this doesn’t represent everyone. People from an African or Asian
backgrounds are more likely to have a higher percentage body fat at the same
BMI of people from a Caucasian background. This is why you may have seen that
the overweight BMI category for people from African/Asian backgrounds starts at
23kg/m2.
As you can see, there are lots of flaws with BMI. But what
could be used instead?
What you can measure instead of BMI:
BMI is weight focused, but there are other things you can
look at too.
Body measurements:
Taking body measurements can be a useful to identify fat
distribution and assess health risks.
Waist circumference is a great measurement to see if you’re carrying fat around your stomach. This is more useful than BMI as it actually looks at where the fat is located. The general rule is that if you’re at increased risk of disease if your waist measurement is more than the following:
- 94cm (37 inches) or more for men
- 80cm (31.5 inches) or more for women
Looking at people’s waist to hip ratio has also been shown
to be a better marker for risk of heart attacks, compared to BMI. Waist to hip
ratio is dividing the measurement of your hips by the measurement of your
waist.
A low risk waist to hip ratio is:
- 0.95 or less in men
- 0.80 or less for women
Waist to hip ratio can help you understand more about your
body shape and health risks linked with it, which BMI can’t.
Lifestyle factors:
If you want to take the focus away from your body, you could ask yourself some lifestyle questions instead. These can offer a great insight into your health, and they can also be measured regularly.
- Are you getting 7-9 hours of sleep every night?
- Are you getting regular physical activity? (around 150 minutes of activity a week.)
- Are you stressed? If so, how are you managing this?
- Are you having less than 14 units of alcohol a week?
- Are you getting your 5 portions of fruit and veg a day?
These questions are just examples of other areas you could look at that aren’t weight based. BMI doesn’t consider any of them.
Key points:
- BMI is a quick calculation used to tell if you’re a ‘healthy’ weight.
- BMI doesn’t take into account body composition (% body fat vs muscle) or the location of fat in the body.
- BMI wasn’t created to be used for health.
- BMI doesn’t consider other factors that affect health and weight like genetics, stress, or physical activity.
- Taking body measurements like waist circumference can be a better indicator of disease risk.
I hope you enjoyed this blog post! I’d love to hear your
thoughts in the comments below.
Bye for now! 👋
References:
https://www.nhs.uk/common-health-questions/lifestyle/what-is-the-body-mass-index-bmi/
https://www.nhs.uk/live-well/healthy-weight/bmi-calculator/
https://www.nature.com/articles/ijo200811
https://www.thelancet.com/journals/lancet/article/PIIS0140-6736(06)69222-2/fulltext
https://pubmed.ncbi.nlm.nih.gov/15652713/
https://pubmed.ncbi.nlm.nih.gov/28564650/
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5644968/
https://www.hindawi.com/journals/jobe/2013/790654/
https://www.diabetes.co.uk/waist-to-hip-ratio-calculator.html
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