What is BMI?
Doctors, nurses and other healthcare professionals around the world use BMI to determine whether a person is overweight or clinically obese. The latest statistics, published in the Health Survey of England 2004, show that almost a quarter of adults are obese.
This means they have a greater risk of developing heart disease, diabetes, stroke and certain cancers (breast and colon).
Other health problems related to having too much fat around the body include a greater chance of developing osteoarthritis (wear and tear) of the joints, and emotional problems such as low self-esteem and depression.
A significant drawback with BMI is that it doesn't take into account a person's body fat content, which is an indicator of the risk of future health problems.
Adults and BMI
Your BMI is calculated by dividing your weight in kilograms by your height in metres squared. You can estimate your BMI using our calculator. The result you get is then classified into the following groups
Recommended BMI ChartUnderweight / BMI less than 18.5
Ideal / BMI 18.5-25
Overweight / BMI 25-30
Obese - should lose weight / BMI 30-40
Very obese - lose weight now / BMI greater than 40
A BMI measurement is not as accurate if you're an athlete or very muscular (muscle weighs more than fat) as this can push you into a higher BMI category even if you have a healthy level of body fat. It's also not accurate for women who are pregnant or breastfeeding, or people who are frail.
Children and BMI
The number of clinically obese children has also increased sharply in recent years, and there are fears that if present trends continue obesity-related diseases, such as diabetes, will occur at a much younger age than at present.
A child's BMI is calculated using the same method as for adults - weight in kilograms divided by height in metres squared. But adult BMI figures must not be used to determine whether a child is overweight or obese. Specific age-adjusted charts are needed. An example can be found at the Centres for Disease Control and Prevention website .
Waist circumference
Waist circumference is now believed to be a much more accurate measure of future health risk than BMI alone. Carrying too much fat around your middle is associated with an increased risk of developing heart disease and diabetes.
It's also associated with a greater risk of high cholesterol levels which, in turn, increase the risk of heart attacks and strokes.
A waist circumference greater than 80cm (32in) for women and 94cm (37in) for men increases the risk of cardiovascular diseases, for example, heart attacks and stroke, and diabetes.
The greatest risk is for women with a waist measurement of more than 88cm (35in) and men with a waist measurement of more than 102cm (40in).
To measure your waist size
Place a tape measure around the narrowest point of your waist (between your lower ribs and your hips), breathe out and measure the circumference
Apples and pears
Most of us store body fat in one of two distinct ways - around our hips and thighs, or around our middle. Those who store fat around the middle are often known as having an 'apple shape', while those who store fat around the hips and thighs are known as having a 'pear shape'. The shape of your body is directly linked to your risk of poor health.
If you're an apple shape, friends and family may say you've got a bit of a tummy or a beer belly. Medically, this is known as central obesity.
This means waist circumference is taking on a more important role in determining future health outcomes
Over the past few years, scientific research has demonstrated that carrying extra weight around the middle puts a person's health at greater risk than carrying extra weight around the hips or thighs. This means waist circumference is taking on a more important role in determining future health outcomes
The bottom line? Excess fat in the abdominal region puts you at a greater risk of developing serious illnesses such as heart disease and diabetes than people with excess fat in the hips and thighs.
Waist-hip ratio
Recently, it's been suggested that waist-hip ratio, which measures the proportion of fat stored on your body around your waist and hips, is the best predictor of a person's risk of a heart attack, making it a more accurate measure than BMI.
Measure your waist-hip ratio while standing relaxed and naked. Measure your waist at its narrowest point. This is usually around your navel. Next, measure your hips at their widest point. Most often this is around the buttocks.
It's important not to pull the tape tight when doing either of these measurements - let the tape rest on your skin.
Finally, divide your waist measurement by your hip measurement.The figure you get from this calculation is your waist-hip ratio. For example, if your waist is 85cm (33in) and your hips are 100cm (39in), your waist-hip ratio is 0.85.
If you're a man and your ratio is more than 1.0, or a woman and your waist-hip ratio is more than 0.8, it means you're an apple shape and at greater risk of health problems.
Any future for BMI?
The emerging research suggesting that BMI is no longer the best predictor of future health problems doesn't mean you should ignore your BMI. It still has some value as a rough screening tool. However, it's become clear that fat deposited around the waist is a more important risk marker.
Don't forget, however, that abdominal fat is only one risk factor. Smoking, drinking too much alcohol, a lack of exercise, an unhealthy diet and stress are other important illness risk factors that also need to be addressed.
This article was published in October 2006.