BMI, the body mass index is a simple measure that has long been used to assess people's risks for poor health and shortened life expectancy.

But researchers at the University of Pennsylvania caution that the measure is not as powerful and accurate as previously assumed, and doesn't offer a comprehensive picture of a person's health and disease risks.

You can find out your BMI in seconds, if you know your height and weight and have a handy chart. If you want to do the math yourself, take your weight in kilograms, then take your height in meters, squared. Divide your weight by the squared height, and voila, that's your body mass index.

A person with a BMI over 30 is considered obese and therefore generally assumed to be at risk for illnesses such as heart disease or diabetes. Penn Medicine professor Rex Ahima says this quick and simple tool works on many levels,  but it's limited. "It doesn't even distinguish between men and women, also the distribution of body fat," said Ahima.

For example, athletes can get tagged with high BMI's because muscle is heavier than fat and BMI says nothing about where the fat is located on the body. Belly fat has been found to be more closely associated with various diseases than other fat zones on the body.

Ahima and several colleagues just published a paper in the journal Science on the use of BMI in medical practice and research. They argue that the BMI measure should only be seen as one of several risk factors. "It's not enough to just take a patient and say 'oh, your BMI today is 28,'" explained Ahima. "It's important over time to see what the trends are. Did it increase, did it decrease, then in relation to what?"

Ahima says in addition to tracking weight over time, other factors play a big role in predicting health outcomes for patients.

"The waist circumference, the person's family history for the disease in question, the level of fitness of the person, and very importantly, the trend in the BMI, how much of a change has occurred over time."