As a registered dietitian, I don’t have a problem with the Body Mass Index (BMI.) This is probably because my expectations are entirely in line.
I understand that it is only one of several nutritional status assessment tools. It is an inexpensive and easy way to administer and uncover possible health problems. No screening tool is perfect, however. There are always false negatives and false positives. The BMI was actually designed for population studies, not for diagnosing individuals.
Body Mass Index is a proxy measure of a person’s “degree of adiposity,” or fatness. It is calculated as weight in kilograms divided by height in meters squared. The concept was devised by scientists in the 1800s, but it did not become an international standard for measuring obesity until the 1980s. In the late 1990s, it received popular attention when the government made it part of healthy eating and exercise initiatives.
For practical use, BMI is displayed as a “BMI chart” with weight on one axis and height on the other. A BMI of 18.5 to 25 corresponds to the “healthy weight” range; BMI less than 18.5 indicates underweight; BMI between 25 – 29.9 is the overweight range. Obesity starts when BMI is 30, and it is considered “extreme” when BMI is 40 or higher.
Until 1998, the healthy weight threshold was BMI 27.8, but the National Institutes of Health lowered it to 25 to match international guidelines.
A BMI in the obesity range is almost always associated with medical problems including diabetes, sleep apnea, gallbladder disease, gout, cardiac conditions, and other medical problems, but not so for a BMI in the overweight range. For many people, especially black women, the overweight range can be perfectly healthy. That is also often the case when extra fat is stored around the hips, thighs, butt, back, and back of the arms. The BMI is not a valid measurement for bodybuilders, many athletes, and pregnant women because their extra weight is from muscle and the products of conception, not from fat. The BMI is not for children and adolescents because they have different standards.
Most people do belong in the healthy weight range because the range affords enough latitude to take in a variety of body types. For instance, the healthy weight range spans from 112 to 150 pounds for a 5 foot 5 inch tall woman. Naturally muscular people with dense bones belong at the top of the range, whereas small-boned willowy types are found at the other end.
It is always important to measure waist circumference along with the BMI. When the waist is large, whether or not weight is high, there is a greater risk of disease. The BMI is not a test of body composition, the amount of tissue, fat, and water that makes up body weight. When the BMI score is ambiguous, it is a good idea to measure body composition along with tests that measure overall fitness as well.
If you’re curious what your BMI is, use the calculator from our healthy resources center to determine if you’re in a healthy range. In addition to your BMI, it will also determine your estimated Basal Metabolic Rate (BMR) and your Ideal Body Weight (IBW).
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