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Why Your Body Mass Index Matters

The number on your scale doesn’t tell the whole story about your weight…or your health
Written by 
Canyon Ranch Staff
Canyon Ranch Reviewer: 
Updated on: 
December 2, 2013

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:

Below 18.5 Underweight
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.

More: Aiming For Your Healthy Weight

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:

Heart Disease: The more fat pushes on your organs, the more work it takes to move blood around the body. When your BMI is 27 or greater, or you have excess abdominal fat (a waist measurement of 35 inches or greater in women, or 40 inches or greater in men), you’re more likely to develop high blood pressure, increased blood lipid levels and coronary heart disease. In addition to your BMI and waist measurement, your doctor may compare your hip and waist circumferences, which is called your waist-to-hip ratio, as another tool for determining whether you are carrying too much belly fat.
Type 2 Diabetes: After a meal, food is broken down into a sugar called glucose. This triggers the pancreas to produce insulin, which allows that glucose into cells throughout the body, where it’s used as energy. In some overweight and obese people, this process doesn’t work efficiently and glucose isn’t well-absorbed. Instead, it builds up in the bloodstream, which can damage nerves and blood vessels, leading to complications such as heart disease, stroke, blindness, kidney disease, nerve problems, gum infections and amputation.

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:

Osteoporosis: Underweight people put less pressure on their skeletons, which might sound like a good thing, but actually means less bone strengthening from daily activities.

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.
Reference(s) 
American Heart Association
Centers for Disease Control and Prevention
Mayo Clinic
National Cancer Institute
United States Department of Health and Human Services