Growing evidence is causing some experts to rethink the way they assess a healthy body. We investigate why they’re questioning body mass index (BMI) and want a better measure.
Ever used your BMI score as an excuse to eat that extra piece of cake? If you’re within the healthy weight range, you have nothing to worry about, right? Wrong.
Whether you’re at the bottom, middle or top end of your ‘healthy’ BMI scale, you might actually be more at risk of weight-related diseases than women with a so-called ‘unhealthy’ BMI score.
And you could be well on your way to joining a new classification of people in the UK dubbed the invisible obese.
‘Take a fit, active woman with minimal body fat – she could be misdiagnosed as overweight’ according to her BMI, purely because her weight is predominantly made up of muscle, which is heavier than fat,’ says Dr David Haslam, clinical director of the National Obesity Forum
‘In the same way, women with skinny arms and legs, but podgy tummies, who might conform to a healthy weight statistically, are actually far more at risk of obesity-related illnesses, yet they’re likely to slip through the net. These are the invisible obese.’
With almost a quarter of all UK adults now classed as obese according to the BMI, and another 12 million well on their way, you’d be forgiven for thinking that, by maintaining a healthy size and weight, you’re not at risk. But you don’t need to be a size 20 and live on junk food to qualify.
In fact, convenience-food diets and inactive lifestyles aside, one of the main risks of becoming ‘invisibly obese’ is whether you judge your health according to your BMI score.
The hidden dangers of body fat
‘BMI is fine for scientific analysis, but it’s utterly useless for assessing an individual’s health,’ says Dr Haslam.
‘Measuring your waist circumference and waist-to-hip ratio is far more effective. But, at the moment, we’re stuck with BMI because so many studies and medical evidence are based on it.’
In fact, the snappy equation that’s supposed to tell you in an instant whether you need to start tightening your belt can lull you into a false sense of security by ignoring the most dangerous health risk of all – body fat.
‘The problem with concentrating on BMI is that it focuses your mind on weight, not body fat,’ says Dr Adam Carey, head of nutrition for the British Olympic Association.
‘I once treated a 24-year-old model whose BMI was within the normal range, yet she had 32 per cent body fat: the healthy range for a woman her age is about 17 to 24 per cent.’
A large-scale study by the Centers For Disease Control and the National Cancer Institute in the US found that being overweight (having a BMI of 25 to 29.9) was not linked to increased death rates from cancer or heart disease.
And a study of 250,000 people with heart disease by the Mayo Clinic in the US found that patients identified as overweight according to BMI standards actually survived longer than normal-weight patients.
The fact that BMI fails to distinguish not only your body fat, but where you carry it is another hidden danger for the invisible obese – where weight accumulates on your body is key.
Fat that accumulates around your waist – known as abdominal or visceral fat – is far more unhealthy than fat on your hips, as it puts you at a greater risk of cardiovascular problems.
‘As a consequence of this, we may be seriously underestimating our obesity statistics,’ says Dr Haslam.
‘As it stands, based on BMI scores, it’s believed that less than 24 per cent of people in the UK are obese, whereas figures from studies based on waist measurements are closer to 30 per cent.’
Fat on the inside
And don’t be fooled if you fit easily into your size 10 jeans – women who look slim could be even more at risk.
A recent study by Imperial College London found that it’s possible to store large amounts of fat around your internal organs, a concept known as TOFI (thin on the outside, fat on the inside).
High levels of internal or visceral fat (as opposed to subcutaneous fat, which is found just below the skin) are linked to Type 2 diabetes, coronary heart disease and cancers of the breast, oesophagus and colon.
Lead researcher of the Imperial College London study Professor Jimmy Bell says: ‘People have become obsessed with dieting, but we’ve found that people who do this without exercise may be storing fat in the wrong places. It’s a metabolic time-bomb waiting to go off.’
Fat inside your body is more metabolically active, which means it can get into your blood and cause fatty deposits along your artery walls.
So what can you do? Get your measuring tape out, for starters.
‘Since BMI doesn’t tell you where your fat is distributed, it would be far more effective to measure abdominal body fat simply by taking waist measurements,’ says Dr David Ashton, medical director of Healthier Weight Centres (www.healthierweight.co.uk).
‘The more you have around your waist, the more you’re likely to have around your internal organs. A waist measurement of 32in or above for a woman (and 37in or above for a man) shows you are at higher risk.
'Even if your waist measurements stay within the healthy range, you should still have regular blood pressure and cholesterol checks, because high readings could be indicators of internal fat.’
What can I do now?
Here are some alternative methods of measuring your body fat levels.
- Scan it: a dual energy X-ray absorptiometry (DEXA) scan is mainly used for bone mass, but it can also show fat and lean tissue. It costs about £100 at selected private clinics.
- Pinch it: the skinfold measurement, based on how much fat you can grip with callipers at sites on your body. Callipers cost from £15.99.
- Zap it: bioelectrical impedance analysis (BIA) scales send a low electric current through your body. This encounters resistance with fluids in fat, which the BIA measures to compute body fat. Available from private clinics or buy your own monitor for about £35.
- Shop it: 1000 Healthy Weight Check machines have been set up at Boots stores in the UK. For 70p they measure your bodyfat percentage and weight.
Next, look to your mum. Studies show that your birth weight and your mum’s age when she gave birth are important factors, because older women are more likely to have babies who will grow up to become obese, while underweight babies are also at higher risk.
By discovering whether you’re predisposed to obesity, you can begin to make the necessary lifestyle changes to safeguard your future.
Research from the University Of Rochester School Of Medicine, US, found that the average woman loses half a pound of muscle every year between the ages of 20 and 50, while at the same time, most adults gain 1½lb of fat.
But you can help reverse this. In fact, it’s relatively simple to disperse internal fat with regular exercise.
The Imperial College London study found women could reduce their internal body fat by as much as 60 per cent by performing a simple one-hour cardiovascular routine three times a week for six months.
But cardiovascular exercise alone isn’t enough.
‘I never use BMI to assess my clients’ health.
'I use body-fat callipers and then encourage them to do resistance exercise, such as weight-training, as this helps build muscle mass and keeps body fat down,’ says Peter Rodwell, a fitness instructor who runs www.playtrainer.com, a fitness programme you can download onto your phone or MP3 player.
‘You really need to do half an hour’s weight training three times a week to keep your muscle mass at an optimum level,’ he says.
If you step up your weight-training and you begin to reduce your waist size, don’t be alarmed if your BMI increases.
BMI’s failure to differentiate between muscle and fat often means that those with leaner, more muscular bodies, such as athletes, register as overweight according to the BMI.
The American College Of Sports Medicine, which is considered the world authority on fitness, has long since denounced BMI due to its misclassification of athletes.
The next big thing
Soon, you could walk into your GP’s surgery and have a full-body scan in just seven seconds.
The body volume index scanner (BVI) measures your weight by taking into account:
- your shape
- muscle mass
- where the majority of your body fat is stored
- waist measurements.
This is uploaded onto a computer.
Experts can then distinguish between your body fat and muscle, section by section - from the width of your calves to the thickness of your neck.
A two-year trial has been carried out at Heartland Hospital in Birmingham.
For example, a female sprinter, such as Christine Ohuruogu, who is without doubt extremely fit, healthy and with limited body fat, could measure equally as ‘healthy’ as someone who weighs the same amount but does no exercise and has a much higher proportion of fat.
‘I was horrified when I calculated my BMI,’ says Sarah Donohue, 36, a professional powerboat racer and fitness and endurance athlete, who was also a stuntwoman in the Bond movie The World Is Not Enough.
‘I follow a balanced diet, do weight-training and cardiovascular exercise at least four times a week and generally keep fit through walking and running.
'I carry about 14 per cent body fat – which is way below average – yet my BMI is 26, which makes me overweight!’
But you can rest assured that the BMI score aside, exercise will help you achieve a healthier body.
‘Whatever method of exercise you choose to increase your fitness, your body fat will be reduced and the dangerous fat around your organs is the first to disappear,’ says Dr Haslam.
‘It’s really encouraging that you can actually do something about this. Plus, studies show that within half an hour of initiating exercise, your body’s metabolism improves, and insulin resistance improves almost instantly.
'Obviously, you need to keep exercising regularly to maintain this improvement, but the signs are promising.’