What is BMI, and is it accurate?

Your BMI, short for body mass index, looks at your height and weight and tells us whether you’re likely underweight, normal, overweight, or obese. But is it a good measurement, or are there better ones out there?

This article looks at:

• What is my BMI?
• Why does my BMI matter?
• Where does the BMI calculation come from?
• Does BMI cause a massive problem?
• What’s a better measurement than BMI?

What is my BMI?
 

Let’s start by calculating your BMI. Just put in your height and weight in the calculator below:

Supplied by BMI Calculator UK

The number you get classifies your weight range:

• Less than 18.5: underweight
• Between 18.5 and 24.9: healthy weight
• Between 25 and 29.9: overweight
• More than 30: obese

Why does my BMI matter?
 

Being under or overweight is associated with diabetes, fertility problems, high blood pressure, heart attacks, strokes, and arthritis. An out-of-range result indicates that some adjustments may be needed to look after your long-term health.

Where does the BMI calculation come from?
 

It may surprise you to know that the BMI calculation was invented in 1835 (yes, almost 200 years ago) by Lambert Adolphe Jacques Quetelet. He was a Belgian astronomer, who liked to measure things.

Quetelet was interested in the social average concept and wanted to determine the characteristics of an average man.  Over time his index turned into the BMI, and we began to associate it with health.

Besides being slightly outdated, the problem with the BMI is that it’s a crude measurement. It simply looks at your height and your weight and throws out an extremely generalised number. It doesn’t consider that muscle is denser than fat – for example, Dwayne ‘The Rock’ Johnson would be obese by BMI standards, and it’s not great at adjusting for age or ethnicity, though some tables try to account for this.

In short, BMI is good for looking at populations but not so good on a personal level.

Does BMI cause a massive problem?
 

For the average person, BMI gives a good idea as to whether they are overweight or not.

However, now we can do things better with far more tailored measurements. Not only are these more useful for sporty people and those with muscle mass, but they’re also more useful for the rest of the population.

So, what's a better measurement than BMI?
 

We’ve got five alternative measurements that might give you a bigger picture than just BMI.

1) Waist circumference

Fat distribution is important. Visceral fat, which accumulates around the belly, is more of a risk factor for disease than fat that accumulates in the legs and bum.

If you are from Afro-Caribbean, South Asian or Japanese origins, visceral tummy fat is a greater risk factor for cardiovascular disease.

Place a tape measure around your waist, halfway between the bottom of your ribs and the top of your hips, usually simply above your belly button. Pull it tight, but don’t dig it in, and don’t suck it in! Breathe out and then measure it.

2) Relative Fat Mass (RFM)

So, you have your waist circumference. Now let’s use it to work out your relative fat mass.

The RFM estimates your body fat based on the ratio of your height to your weight. This measure has recently been validated as a better measure than BMI and has cut down on false classifications of obesity for both men and women. It correlates well with more high-tech DEXA and impedance scans.

The RFM is gender-specific, and you can work out your RFM on the MDApp website.

Authors of the recent paper propose cut-offs for an obesity diagnosis as 40% for women and 30% for men, though you can also look at the cut-offs established by the American Council on Exercise:

3) Bioelectrical impedance analysis (BIA)

You may have seen BIA scanners at your gym, and some people may even have a set at home.

Simply put, the scanners send a small electrical current that runs in all directions through your body. Fat resists the electrical charge, whereas tissues like muscle are more conductive. Some calculations are made, and the machine estimates your fat mass, muscle mass and often a lot more as well. 

Although this is a high-tech measurement tool, it’s still not perfect.

BIA can be affected by:

• The contact between your skin and the metal (if you are sweating it may affect the measurement; as sweat is conductive)
• Recent eating (best to fast overnight before measuring) and drinking
• Exercise (no heavy exercise in the hours preceding a scan)
• Certain medical conditions – especially those with heart, liver or kidney issues and cold temperatures. 

There is ongoing research into ethnic variation, body type and menstrual cycle/menopause factors. 

Overall, this is a good measurement tool if you have access to it. The BIAs in the gym tend to be great at letting you visually track and trend your results over time, which is something we love and make sure we do here at Medichecks, as well with blood test results.

4)  Hydrostatic weighing

Hydrostatic weighing is a gold-standard technique that involves weighing someone underwater. Hydrostatic weighing is complicated, and we don’t expect anyone to do it… but it’s here because it’s cool to know about.

Different tissues displace water differently –fat is buoyant, whereas muscle tissue becomes denser underwater. You can compare your landmass to your underwater mass and get a very accurate body composition.

5) Blood tests

Blood tests are the best way to let you know how your body is regulating itself. They track and trend well over time and, most importantly, they are tailored to you (if you test with Medichecks at least).

You may have a larger waist, and we know visceral fat is an issue. But looking at your cholesterol and sugar levels can help you understand that risk more comprehensively.

If you want to lose weight, you may see changes in your bloodwork before seeing changes in the mirror. Your lipid and fatty liver markers may improve.

Similarly, if you’re using weight loss injection medications, it’s a good idea to get regular bloodwork. This way, you can make sure your cholesterol is improving, your organs are functioning well and look out for any nutritional deficiencies that may occur from changes to your diet.

Many people find getting insights into their biomarkers extremely motivating, proving all their hard work is making a difference.

Equally, if you are athletic, your blood tests can tell you if you are optimised for performance.

Ultimately, we want to create the most holistic picture of you possible. That involves knowing your blood so that you know how your organ systems are functioning, and whether there are any obvious risks to work on.

You'll also know your composition, from your waist measurement through to your impedance scores. These measurements can be tracked over time to demonstrate how your hard work is paying off or whether further adjustments are necessary.

Summary

BMI is alright, but it's far from perfect. Nowadays, there are better ways to measure yourself both on the inside and the outside.

Knowing yourself inside out will help you make the right decisions in your life to ensure you stay as healthy as possible for as long as possible.

References

1. John D Sorkin, BMI, age, and mortality: the slaying of a beautiful hypothesis by an ugly fact, The American Journal of Clinical Nutrition, Volume 99, Issue 4, April 2014, Pages 759–760, 
2. Measuring your Waist: https://www.bhf.org.uk/informationsupport/heart-matters-magazine/medical/measuring-your-waist
3. Woolcott, O.O., Bergman, R.N. Relative fat mass (RFM) as a new estimator of whole-body fat percentage ─ A cross-sectional study in American adult individuals. Sci Rep 8, 10980 (2018). 
4. Nuttall, Frank Q. MD, PhD Body Mass Index, Nutrition Today: May/June 2015 - Volume 50 - Issue 3 - p 117-128
    
5. Dehghan M, Merchant AT. Is bioelectrical impedance accurate for use in large epidemiological studies?. Nutr J. 2008;7:26. Published 2008 Sep 9. https://doi:10.1186/1475-2891-7-26