Keisha Brown, Health Writer

Scientists have known for years that excess body fat isn't good for you. More recently, however, studies have begun to show that while obesity isn't good, carrying excess body fat, specifically around your belly, is particularly harmful.

Traditionally obesity was determined by how much a person weighed, or more accurately by taking the body mass index (BMI). The BMI is a mathematical formula which calculates your body size while trying to account for bone mass.

what is body mass index?

The BMI is a person's weight in kilograms divided by their height in square metres. BMI doesn't take into account muscle mass, and as a result, some athletes with low body fat and a high percentage of muscle mass would be incorrectly calculated as obese. This is one of the drawbacks of using the BMI.

Recently, in the medical journal Lancet, doctors concluded that a person's waist measurement is a more accurate forecaster of heart attacks than the BMI.

The National Institutes of Health has set a waist measurement cut-off point at about 35 inches for women and 40 inches for men - regardless of weight - as an unhealthy sign of excess fat, the dangerous hidden fat that surrounds vital organs inside the body.

Another measurement that is slightly more targeted to belly fat is the waist-to-hip ratio, which reflects the concentration of fat around your belly as opposed to around your hips or thighs. Scientists have found that heart attack sufferers have similar BMIs, but higher waist-to-hip ratios than those who have never had a heart attack.

Measuring Your Waist

The waist-to-hip ratio compares the measurement of the narrowest part of your waist to the broadest section of your hips. Your waist measurement should be taken in the spot that falls between the ribcage and the hipbone as viewed from the front.

Your hip measurement is truest if you turn sideways to the mirror and make sure you incorporate your derrière in the measurement. Now, divide your waist measurement by your hip measurement. For example, a woman with a 30-inch waist and 37-inch hips has a waist-hip ratio of 0.81.

According to the Centers for Disease Control and Prevention, a healthy waist-to-hip ratio for women should not exceed 0.8.

You don't have to be overweight to have a lot of fat inside your body. Scary, isn't it? It's hard to imagine, but one can be thin and fat at the same time. This understanding is relatively new, and describes individuals who are thin on the outside but have excess fat on the inside.

producing 'fat maps'

Jimmy Bell, PhD, a professor of molecular imaging at Imperial College, London, has shown that this is possible. Bell and his team have been using MRI machines to scan nearly 800 people in an effort to produce what they call 'fat maps'. He found that about 45 per cent of the thin women and 65 per cent of the slim men, he tested, carried excess fat.

Now that you are armed with this information, what should you do? Find out your waist-to-hip ratio. If you don't fall within the healthy range, try to reduce your weight slowly with aerobic exercise, such as walking and a healthy diet. Your waist will thank you for it.