Fat or Fiction? Why You Should Be Wary Of Your BMI

Forgive the personal nature of this question, but how fat are you? Are you running to lose weight? Or have you come to Runner's World because you feel a little on the large side and don't know how to start?

Unless you've spent the past few years under a duvet, you've probably heard about the 'obesity crisis'. More than two-thirds of Brits are either overweight or obese, according to the government. But how do they know this?

They rely on the Body Mass Index (BMI). It's a well-known formula for calculating your 'healthy' weight, which the government uses in surveys and the World Health Organisation uses for measuring global obesity. But this so-called 'crisis', where we keep being told that we are fat and useless, could be a self-fulfilling prophecy, in that it discourages people from pulling on some running shoes and doing something about it.

And that's wrong, because the first thing you should know is that the BMI is ancient, and largely unscientific. It was devised as long ago as 1869 by Adolphe Quetelet, a Belgian astronomer who decided that, for adults, weight should be proportional to the square of the height. That's how he devised the formula we still use today: your weight in kilograms divided by your height in metres squared. In broad terms, if your score is greater than 25, you are overweight. If your score is 30 or more, you are obese.

Yet although the formula is proportional, it is much less accurate than it was 140 years ago. We are simply very different shapes and weights to mid-19th-century people.

Most of the world's top runners have healthy BMIs, but they are athletes with personal coaches and nutritionists. Sprinters, for example, are bigger because they are built for power and speed over endurance – but this highlights the problem with the BMI. Recently crowned European 60m indoor champion Dwain Chambers is technically overweight, despite kicking the steroids, and so was former world-record holder Maurice Greene when he won the Olympic 100m in 2004. His successor, Usain Bolt, is not, but that is because he's 200ft tall and runs on wheels.

How elite athletes' BMIs compare

Paula Radcliffe
Height 5'7"
Weight (kg) 54
BMI 18.0 (Underweight)

Haile Gebrselassie
Height 5'4"
Weight (kg) 56
BMI 20.5 (Normal Weight)

Usain Bolt
Height 6'5"
Weight (kg) 88
BMI 22.3 (Normal Weight)

Mark Lewis-Francis
Height 6'
Weight (kg) 86
BMI 25.1 (Overweight)

Dwain Chambers
Height 5'9"
Weight (kg) 83
BMI 25.6 (Overweight)

OK, he doesn't really, and he's actually 6ft 5in (196cm), and 13.8 stone (88kg), with a healthy BMI of 22.3. But that's in large part why his success at last year's Olympics confounded seasoned experts and coaches, because he's far less muscular than the top sprinters of the past 20 years. This highlights the BMI's greatest flaw, particularly for runners. It cannot differentiate between muscle and fat. It doesn't account for body type or bone density. It pays no attention to your age, and cannot possibly take into your account your cardiovascular efficiency, stride length or genetic ability to run.

So while the BMI is a handy guide for nagging government ministers, it's oversimplistic and, frankly, out of date.

Scientists from the Mayo Clinic in Rochester, USA, are also questioning the accuracy of the BMI. Looking at data from 40 studies involving 250,000 people with heart disease, they found that overweight people had fewer heart problems than those with a normal BMI.

The good news is that not all government departments are hung up on the BMI. Sports minister Gerry Sutcliffe says, "The Department Of Health uses the BMI, but we tend to measure participation in sport. We use the 3 x 30 minutes a week measure." Which is to say that being active for 30 minutes, three times a week is healthy for your heart and weight.

Noel Pollock, sport and exercise medicine physician for UK Athletics, goes one step further, but broadly makes the same point. "The NHS chief medical officer's advice is 30 minutes of moderate-intensity exercise five times per week. It's a good initial goal. It can be done in several 10-minute bouts because this is as effective as one longer bout of exercise."

So ignore the doom-mongers. "Running is a sport that anyone can take up," says Sutcliffe. "Events like the London Marathon prove this as thousands of people of all ages and abilities cross the finish line every year. Running is fun, convenient and, best of all, it's free."

There is another way

It's called the waist-hip ratio, and as a measure of obesity it's far more useful to you as runner. All you have to do is divide your waist in centimetres by your hips in centimetres – Australian researchers claim it is a more accurate indicator of obesity and, more importantly, your health, than the BMI.

During an 11-year study of 9,000 adults, they found that men whose waist-hip ratio was 1.0 or more were more likely to suffer from the cardiovascular problems associated with obesity. We don't want you to keel over, so this is one simple test worth taking at home – all you need is a tape measure – before you see your GP, start running or, for that matter, feel any chest pain while running.

Neville Rigby, who was formerly director of policy for the International Association for the Study of Obesity, agrees that the tide has turned against the BMI. "One of the biggest problems is abdominal obesity, which the BMI doesn't take into account. Stomach fat, leading to a big waistline, is usually the killer." In a US study that involved more than 17,000 obese Americans, the worst combination of cardiovascular risk factors – irrespective of weight – was found in people with large waists and narrow thighs.

This throws up another problem: "It's possible to be obese yet have a normal BMI," says Rigby. So a short man with a beer gut and skinny limbs might have a lower BMI even than Usain Bolt, but is in far more danger of suffering heart disease. Girth is more important than overall weight.

The crux of the matter

The waist-hip ratio is simple but also has certain flaws, especially if you have an endomorphic, round-bodied shape but low body fat. It's the same as the BMI in that it's a useful indicator, but not something you should use to demotivate you from running, so long as you have no underlying health problems.

"We want to get two million people more active through sport and physical activity by 2012," says Sutcliffe. "We're giving almost £500 million to sports governing bodies over the next four years through Sport England. For this investment we want to see returns, and sports governing bodies will now be accountable for driving forward participation in their sport, including running, through UK Athletics."

Don't give up

So you see, the point we are trying to make is that health is actually more important than weight. You can still run, whatever your body shape, whether it's for fun or, if you're genetically predisposed, to a higher level than maybe you thought possible. Don't jump into it (you're meant to put one foot in front of the other, for starters) – but read our beginners' advice (right) and don't let your weight hold you back.

"You can start slowly by walking and building up to jogging," says Dr Jonathan Williams. "If you do Pilates and swimming as well you'll reduce the risk of injury. Finally, don't be self-conscious. Set a goal for yourself and don't compare yourself to anyone else."

So while the BMI is a useful guide, and can give you a wake-up call if you are genuinely obese, don't obsess about it – or let it stop you from putting one foot in front of the other.

How To Run Safely

Tips from Dr Noel Pollock, sport and exercise medicine physician for UK Athletics and EIS London medical officer

  • See your GP first. You should get your health checked out before you start running, just to make sure there are no underlying health problems.
  • Start slowly, with short distances, and only do what you can. If you have to stop running and walk for a bit, that’s fine.
  • Rest for 48 hours between each run while you get started. That doesn’t mean lying down the whole time – just make sure you give your muscles, lungs and heart time to recover.
  • Build up gradually. Don’t try to do much too soon, because you risk burning out, injuring yourself or simply getting frustrated. Don’t give up if you don’t see instant results. You won’t. But you will feel and look better within weeks.
  • If you do get an injury, see a physiotherapist if it doesn’t settle quickly or appears to be getting worse with each run.
  • Get the right kit – go to a good running shop and get your running gait assessed so you buy the right type of running shoes for you. And buy breathable fabrics. Just don’t go for skin-tight gear if you’re self-conscious.