A Healthy Curiosity: Fat measures - BMI vs. body fat

I have a friend who is a competitive rower. She is tall and toned and has barely an ounce of visible fat on her fantastic physique. Yet this friend, whose body would be the envy of any woman, is also officially plump. When she calculates her Body Mass Index, or BMI, the most widely used measure of relative fatness, it comes out at a whopping 27 placing her firmly in the overweight category. She is not alone. From Olympians to the likes of Brad Pitt who was considered ‘overweight’ during his Fight Club prime, the BMI calculation can deliver a damning blow. What is going on?

What is the BMI?

Firstly, the BMI is a rudimentary formula that has somehow exceeded all expectation when it comes to its own popularity. Devised by the Belgian statistician Adolphe Quetelet, it has been used to define weight for over a century. It’s been adopted by hospitals and doctors' surgeries, insurance companies, university researchers, drugs companies and slimming clubs; and many professions require prospective employees to have a BMI that does not exceed healthy recommendations. Part of its appeal is that it’s so simple to work out: weight in kilograms is divided by height in metres squared - someone with a BMI of less than 18.5 is considered underweight, between 18.5 and 24.9 is "normal", 25 to 29.9 is "overweight" and 30 or greater is clinically obese.

So how is BMI is flawed?

It’s simple, but the downfall of BMI is that it does not take into account either gender or body composition - whether or not excess weight is fat or muscle - which is why fit people like my rower friend often find themselves in a less desirable category of the BMI rating system. It can also produce skewed results in studies. In one study that looked at the accuracy and usefulness of the BMI, scientists at the renowned Mayo Clinic reviewed data from 40 studies involving 250,000 people with heart disease. They found that while severely obese patients had a higher risk of death, overweight people had fewer heart problems than those with a normal BMI. “I really don’t like the BMI for these reasons,” says Debbie Georgiou, a former Olympic gymnast who is now a leading trainer. “Even an experienced fit athlete can have a BMI that is classed as overweight and on the flip side, an individual with a low BMI does not necessarily have a desirable toned and healthy physique.”

What’s the alternative?

If BMI is flawed, how else should we assess our fatness? Matt Roberts, the celebrity trainer, says body fat measurements are “probably the best way to evaluate most people”. If you have ever joined a gym, you will be familiar with the skin caliper tests that pinch your fat, and they can give an accurate reading if done by someone experienced. “Fat caliper tests are notoriously difficult if someone is very overweight, and tests are also regularly skewed by things such as thicker skin on males’ backs, for example,” Roberts says. “You need someone who knows what they are doing and who will take a few tests to gain an average.” Body-fat scales provide an estimate of your body fat percentage and work by sending a very small current through the body when you stand on the scales, but their accuracy varies. Once you have your body fat percentage, you can use the figure to determine whether you need to shift any weight. These figures are a useful guideline for women:

Essential fat

10-12%

Athletes

14-20%

Fitness

21-24%

Normal

25-31%

Obese

32% plus

Is a tape measure better?

There are other options, the most popular of which is the waist circumference test, which measures abdominal obesity (the fat that settles riskily around your stomach). It's considered a step up from BMI when it comes to measuring someone's risk for illnesses like heart disease and diabetes because it is a direct measure of the part of the body that tends to accumulate fat. Having a waistband of more than 88cm (35in) in women and 102cm (40in) in men indicates the highest risk of cardiovascular and metabolic disease. There is an increased risk of the diseases for women with measurements of more than 80cm (32in) and men whose measurement is over 94cm (37in).

However, an emerging new favourite is the ‘half waist: height’ ration, says Jon Denoris, who trains elite athletes and several celebrities. “I prefer this newer measure which has some research from Canada to back it,” Denoris says. “The idea is to measure your waist circumference and make sure that figure is always equal to or less than your height. So, if you are 180cm tall your waist should be less than 90cm. It really works.”

A pinch of salt

Roberts says no measurement of fatness and leanness is entirely foolproof. “I would include waist and hip measurements along with body fat assessments,” he says. “And if you are being told in two out of three of the tests that you are overweight, then you probably need to do something about it. Think of the figures as a benchmark from which to measure change, rather than just absolute numbers.”