What's a body mass index, and is it important?

Dr. Catherine Carrigan

Each month physicians from ProMedica and Mercy Health Partners will write columns about weight loss and fitness.

Congratulations to all the participants in this year's Million Pound Challenge! You are completing the first part of your lifelong journey to a healthier you.

As a physician, I help my patients learn to live healthier lives, and they share their questions and concerns with me. They often ask about body mass index (BMI): They want to know what it is, and if it matters. Body mass index does matter, and there is a lot to it.

Most people know their height and weight, or at least can make a good guess at it. What does that information tell us? For years doctors have used height and weight values to determine what defines "low" weight, "normal" weight, "overweight," and "obese."

People think it is as simple as looking in the mirror to determine whether you are under or overweight, but the degree to which a person's weight compares to his or her height is valuable information for medical providers. A person's excess weight can affect his or her likelihood of developing serious chronic medical conditions such as diabetes, high blood pressure, and arthritis.

To better describe the degree of excess weight and obesity, BMI was developed. By definition, BMI is the value assigned to people based on their weight versus the square of their height. It is a mathematical formula defined in kilograms/meters squared. To determine your BMI, use the following formula: weight in pounds divided by height in inches squared times 704 (example: the BMI of a 150-pound woman who is 5 feet, 8 inches tall would be: 150 divided by 4,624 (her height in inches squared) multiplied by 704 equals 22.8kg/m2).

BMI values are now widely used to describe what is considered under or overweight. A BMI of less than 18.5 is considered underweight. Values from 18.5 to 24.9 are considered "normal" weight. BMI of 25-29.9 is "overweight," and a BMI of 30-39.9 is considered obese. A BMI greater than 40 is considered "morbid" or extreme obesity.

Knowing BMI is important because research has shown that as BMI increases, so do risks for health conditions and the sickness and death that result. For example, the risk of diabetes begins to rise with a BMI of 25, which is just outside the "normal" weight range.

However, BMI does have its limitations. While BMI allows for a single value to describe the degree of overweight in an individual, it does not make a statement about body composition. Heavy, lean bodybuilders can have high

BMIs, even in the obese range, but their weight is high for their height due to a high percentage of muscle mass, not fat mass. For the general population, however, the excess weight on a person's frame is from excess fat mass. Over the general population, BMI is correlated with most other measures of excess body fat.

Different BMI standards apply to young children, however, as older children and teens become more overweight, comparisons can be made with adult BMI values and obesity. Also, healthy BMI can vary by ethnic group. For example, in Asian individuals BMI's over 23 are considered overweight.

BMI is just one measure of a person's overall degree of health risk, but it does allow for health care professionals, researchers, and other interested parties to make generalized estimates of the degree of obesity present in an individual or a population group. It can help guide decisions for weight management treatment (people with a BMI greater than 40 generally are considered candidates for bariatric surgery.) Also, it can help individuals track their weight loss progress and set body weight goals that are likely to place them at a healthy weight. People who are interested in finding their BMI and what implications it has for their health should visit their family doctor for more information.

Dr. Catherine Carrigan is medical director of Mercy Weight Management as well as a family medicine physician with Mercy Family Physicians in Perrysburg.