BMI, Body Fat & Health Risks

You may have heard of the body mass index, but what does your BMI number mean?
By Martica Heaner, M.A., M.Ed., for MSN Health & Fitness

BMI is a formula that factors in height and weight to produce a number designed to estimate the presence of excess body fat. Compared to going by body weight alone, a BMI measurement is a better assessment of fatness since it takes height into account. For example, only knowing a person weighs 160 pounds isn’t enough information to assess whether they carry too much fat. Factoring in a person’s height helps put their weight into perspective: Someone who is 5-foot-10 and 160 pounds may not be over-fat, while someone who is 5-foot-1 and 160 pounds is more likely to carry excess fat.

BMI and health risks
Higher BMI numbers are associated with increased risks of disease and death, from conditions such as cardiovascular disease, type 2 diabetes and some cancers. Research has found that the lowest and highest BMIs are associated with highest risks. So BMI numbers are grouped into categories meant to reflect the degree of risk a person faces. Those people with the lowest risks of disease seem to fall in the 18.5 to 24.9 BMI range, so they are considered to be “normal.”

A BMI of 25 seems to be the threshold where disease risk significantly increases, and a BMI of 30 confers even greater health risks. So these ranges have been separated into “overweight” and “obese” categories.

Extremely high BMIs are linked to even greater risks of certain diseases. An “underweight” category is included because being overly thin is also associated with increased health risks. For example, people in the underweight category may have anorexia nervosa, cancer (which is associated with weight loss), or be smokers (who tend to be thinner.)

Are you really fat or overweight if your BMI is 25 or above?
A BMI of 30 or above likely means that you are over-fat. But a 25+ BMI merely indicates that you are heavy, and while a heavier person is usually over-fat, this is not always the case.

People who are heavy may be that way because they are highly muscular. They may have a very low percentage of body fat, despite weighing more than expected on a scale. So their BMI number might unreliably suggest they have more body fat than they do. Athletic people, for instance, often have higher BMIs—but since they’re fit and lean, they are not necessarily at increased risk of certain diseases simply because they have a higher BMI.

On the other hand, older people may have more body fat and less muscle, but their BMI number may be on the low end of the scale, suggesting that they have less body fat than they do.

Very short people (under 5 feet) may also have high BMI numbers that do not reflect their degree of fatness. People who are ill or on medications that cause abnormal amounts of edema, or swelling in the body, may weigh more from excess fluid accumulation; a potentially higher BMI number may not reflect the absence or presence of body fat.

BMI is a useful tool when working with research data to estimate the prevalence of overweight and obesity and related diseases risks in thousands of people. It is also a useful way to monitor weight changes over time.

But since it does not directly measure body fat, or where body fat is distributed, it may not be the best method of judging individual levels of fatness and how it relates to health risks. That’s why other factors, such as waist size, are taken into account when assessing a person’s overall health risks.

How can you find out how fat you are?
Body fatness needs to be measured in a lab setting. Some lab equipment such as underwater scales, dual-energy X-ray absorptiometry and other scanning methods such as the Bod Pod measure it directly. These methods may be impractical and too expensive for most people. Less reliable methods (but still more accurate than BMI) include skin fold testing and a commercial body fat scale. You may be able to be tested with these methods at a local health club.

Since fat in the abdominal area is considered to carry the greatest health risks, waist measurements are considered to be a valid way to assess risk associated with excess fat, and to monitor changes in belly fat over time.