Some experts use a number called the BMI (Body Mass Index) to determine whether a person is at a healthy weight to prevent excess fat and reduce the risk of disease. The question is, should you be more concerned with BMI or with your body fat percentage if you are looking for the best health? By looking at how these two numbers are determined, we can figure out which one you should be most concerned with.

BMI is a relationship between your height and weight. To calculate it, you need to know your weight in kilograms and your height in meters. Once you know those two things, you divide your weight by the square of your height. The result is a single number known as your BMI.

Research has shown that a BMI of greater than 25 tends to be associated with excess body fat and an increased risk of disease. It is easy to find your height and weight, and from that calculate your Body Mass Index. Concentrating on getting this number under 25 seems to be a good way to get fit. But there's a problem.

BMI only takes into account your height and weight. It says nothing about your body composition. It knows nothing about how muscular you are or the size of your frame. If you are an average person with an average build, BMI is a pretty useful number. If you aren't average, it is misleading.

Many top athletes have large frames and are heavily muscled, with little body fat. Thanks to the weight of all their muscle, they often have a Body Mass Index of over 25. This number indicates that they are likely fat and at increased risk of disease, even though they are actually some of the fittest and healthiest people on the face of the Earth. Clearly, there are some problems with blindly relying on Body Mass Index.

Body fat percentage is harder to determine than BMI. The index just requires you to plug height and weight into an equation, while there is no 100% accurate way to measure body fat. Still, any of the techniques for measuring the Body Fat Percentage are attempts to directly measure the thing we're most concerned about.

Remember the top athletes we talked about earlier? They might have a BMI that says they are overweight, but any decent measurement of their percentage of body fat will correctly show them to have very low body fat and be very fit. Clearly, body fat percentage, while harder to measure exactly, is the more important of the two numbers for maintaining a lean body and minimizing your risk of disease.