BMI is based on weight and height, not on body fat.

When it comes to defining what body weight is considered healthy, one type of measurement does not fit all, some scientists say.

Body mass index is the standard metric for determining who is normal-weight, overweight and obese, but BMI is not an accurate measure of fat, and doesn't explain the causes of poor health, scientists argue in an editorial today (Aug. 22) in the journal Science.

Obesity can be a major risk for diabetes, heart disease and death, yet paradoxically, some studies suggest being overweight can improve survival of chronic diseases.

"Most studies depend on BMI, and we know it's not a very accurate measure," said Dr. Rexford Ahima, a medical professor at the University of Pennsylvania, in Philadelphia, and co-author of the editorial. [8 Reasons Our Waistlines Are Expanding]

A person's BMI is calculated as her weight in kilograms divided by her height in meters, squared. A BMI of 18.5 to 24.9 is considered "normal," a BMI of 25 to 29.9 is "overweight," and a BMI greater than 30 is "obese."

People with BMIs higher than 30 are at an increased risk of dying from heart disease, diabetes, cancer and other diseases, many studies have shown. But several recent studies suggest that in some cases, a high BMI could actually protect a person from dying of heart failure, kidney failure and other chronic diseases.

When someone has a chronic illness, having more fat could possibly provide additional energy reserves. And in some cases, a low BMI may be a result of a person having an illness.

The health risks and benefits of obesity have generated "a lot of sniping back and forth between different groups of researchers," Ahima told LiveScience.

The problem stems from the fact that BMI is an inaccurate measure of health, Ahima said.

For one thing, BMI doesn't take into account fat, and it doesn't indicate where fat is distributed on the body. Belly fat (fat around the abdominal organs) increases the risk of diabetes, heart disease and death, whereas peripheral fat (fat beneath the skin elsewhere in the body) may be more innocuous, studies suggest. BMI also fails to account for differences in race, gender and age.

So why is the measurement so widely used? "Because it's simple," Ahima said, adding that it's easy to weigh people and measure their height. For most people, BMI provides a "reasonable measure" of body fat, but is not accurate for athletes (who weigh more because of muscle) or older people who have lost height, he said.

Other methods of measuring body fat also have their pros and cons. CT and MRI scans can accurately measure body fat, but are usually very expensive. DEXA scans, normally used to measure bone density, can also be used to measure body fat, but are costly as well. Measuring levels of the hormone leptin can also be an indicator of body fat.

There's no single number that can represent a healthy weight, Ahima said. It depends on starting weight, genetics and gender, among other factors.

Obesity is a complex disorder — it's not just having too much fat, he said. It's important to consider fat in relation to the amount of muscle.

And scientists need to start looking more closely at cause-and-effect in body fat and disease, Ahima said. "What is it about being obese that makes one unhealthy or healthy? We need to understand the molecular mechanisms."

Follow Tanya Lewis on Twitter and Google+. Follow us @livescience, Facebook & Google+. Original article on LiveScience.

Editor's note: This article was updated at 9:20 p.m. on Aug. 23 to give the correct BMI range for being overweight.