It’s easy to hop on the bathroom scale and declare that your weight is just right, but your body mass index (BMI) can more closely gauge whether or not you are in a healthy weight range. BMI is a number, based on your height and weight, that is an indicator of body fatness. Though it doesn’t measure the exact amount of fat you carry—only a DEXA scan can do that precisely, though calipers (the tool used to pinch skin folds and measure thickness) can come close—research shows that BMI does provide a useful estimate. Doctors can use BMI as a good starting point for flagging those who may be at risk for many weight-related health concerns. (Measures of waist circumference, waist-to-hip ratio and body composition are also useful for this.)
Your BMI score can’t be used to diagnose you, but your doctor may turn to it to decide if additional assessments, tests or recommendations may be necessary. Here, a look at what the range of possible scores mean:
|18.5 to 24.9||Normal Weight|
|25 to 29.9||Overweight|
|30 or higher||Obese|
Unfortunately, “two thirds of Americans now meet the criteria for being overweight or obese,” says Cindy Geyer, M.D., Medical Director at Canyon Ranch in Lenox, Mass.
How to Calculate Your BMI
If you’re eager to find out what your BMI is, you can do some simple math to find out right now. Take your weight and multiply it by 703. Then divide that number by your height in inches, squared.
Let’s calculate the BMI for a 200-pound, 5-foot 5-inch tall woman as an example:
200 x 703 = 140,600
140,600 / (65 x 65) = 33
This woman has a BMI of 33 and is considered obese.
Why High BMI Is a Health Risk
Those with a BMI of 25 or higher are at high risk for heart disease, diabetes and more, because of the impact excessive body fat can have on the body’s organs, cells and functions:
Cancer: Obesity is linked with an increased risk of certain types of cancer, including cancer of the esophagus, pancreas, colon, rectum, breast, endometrium (uterine lining), kidney, thyroid and gallbladder. Several factors are in play:
- Fat tissue produces excess estrogen, which has been associated with the risk of breast, endometrial and some other cancers.
- Fat cells also produce hormones called adipokines, which can stimulate growth of cells—some of which may be cancerous.
- Fat cells are also thought to affect other tumor growth regulators in the body.
- Increased levels of insulin may promote the development of certain tumors.
Why Low BMI Is a Health Risk
While not as widespread of a health issue as a high BMI, a very low BMI puts you at an increased risk for some conditions:
Anemia: People with a low BMI often do not eat a well-balanced diet, and anemia—low red blood cell count—can result, particularly when folic acid and iron levels aren’t adequate. Since your red blood cells supply oxygen to the tissues of your body, too few can result in fatigue and, in severe cases, heart arrhythmia.
Weakened Immune System: Good nutrition fuels the cells that respond to infection and illness. Being severely underweight due to a low-calorie diet that’s lacking in essential nutrients puts you at increased risk for sickness, since you aren’t reaping the immune system-boosting benefits of a well-balanced diet.