HEALTHNET

Can BMI help reflect your body fat?

Two independent studies found that even modest increase in BMI can increase mortality...

(Posted on 28 December 2006)

On 19 August, researchers from the Mayo Clinic College of Medicine published a leading article in The Lancet, an internally well-known medical journal. They found that among patients with heart disease, death was actually less likely if a person was overweight, as defined by the Body Mass Index, or BMI.

What is BMI?

The BMI is calculated by dividing the weight in kilograms by the square of the height in meters (BMI=W/H2). According to guidelines from the National Institute of Health in America, an adult with a BMI between 25.0 and 29.9 is overweight, and someone with a BMI of 30 or more is obese. (WHO suggested that for Asian adults, a person with a BMI between 23 and 24.9 is classified as overweight and the one with a BMI of 25 or above is obese. However, more evidence is needed.)

BMI could tell the potential risk of death

Within hardly a week, research papers appear in the New England Journal of Medicine (NEJM) that showed otherwise. Two independent studies found that even modest increase in BMI can increase mortality.

One of the studies involved nearly half a million Americans. Data showed that being overweight (defined as BMI between 25.0 and 29.9) in midlife increased the risk of death by 20% to 40% compared with people of a normal weight. Obese people (with a BMI of 30.0 or more) had nearly double or triple the risk of death.

The other study involving more than a million Koreans also concluded that both overweight and obese patients had higher rates of death. The risk of death from atherosclerotic heart disease or cancer was consistently higher among subjects with a higher BMI.

How does obesity relate to BMI?

The insurance industry has decades of actuarial data to show that there is excess mortality in the obese. The study published in The Lancet does not provide new information, but is a "meta-analysis" that uses statistical method to gather data and draw useful implications from previously published studies.

"Rather than proving that obesity is harmless, our data suggest that alternative methods might be needed to better characterize individuals who truly have excess fat," the authors wrote.

As a statistical index of human physique derived over 150 years ago, the BMI has long been regarded as a simple figure that strongly correlates with the amount of fat in the body, but it is not a direct measurement of fat. Theoretically a person's body-fat percentage would be a more useful tool to quantify obesity. But body fat is notoriously difficult to measure accurately.

That's one reason why the BMI has been used widely in the past despite complaints from stocky muscular athletes that they are denied of insurance for "obesity" - BMI cannot distinguish their muscle mass from body fat.

Pros and cons of using BMI

Even when BMI predicts obesity, it cannot tell how the fat is distributed. Fat stored in the hips and thighs might not be as harmful as fat crouching onto internal organs in the abdominal cavity. In an article published last year again in The Lancet, authors suggested replacing the BMI with the waist-to-hip ratio as a marker for obesity and its inherent risk.

To measure the waist-to-hip ratio, all doctors need is a measuring tape, and divide the circumference of the waist by that of hips. A healthy waist-to-hip ratio is 0.8 for women, and 0.95 for men.

Should BMI "be left aside as a clinical and epidemiological measure of cardiovascular risk", as suggested by one author in The Lancet.

Reliability of self-reported height and weight were often in doubt but studies have shown that both of them are reliably remembered by patients even after decades. Based on this evidence, the American study in the NEJM calculated BMI based on patients' self-reported height and weight.

Source: Quality HealthCare