The body mass index, or BMI, is commonly used in doctors' offices as a way to estimate your body fat level. It provides a quick and easy way to evaluate obesity trends in the general population, but the equation can overestimate or underestimate body fat levels in many people. Medical professionals use it as one of many screening tools, such as cholesterol checks and family history questionnaires, to evaluate your risk of chronic disease related to your weight. But your doctor cannot rely on BMI alone for diagnosis of whether you're overweight or obese and the health risks posed by these conditions.
What Is BMI?
Your BMI is equal to your weight in kilograms divided by your height in meters squared. The equation using American measurements is: BMI = weight / (height x height) x 703. For many online calculators, you enter your weight and height, and the calculations are done for you.
BMI roughly correlates with more accurate measures of body fat, such as underwater weighing and measurements of skin thickness with calipers. A BMI of 18.5 to 24.9 is considered normal. Below 18.5 is underweight, from 25 to 29.9 is overweight and 30 or higher is obese. BMI seems to most accurately determine fatness for people who register a high BMI.
BMI is useful as a way to evaluate the rate of being overweight or obese in the general population. It's easy, convenient and inexpensive, and doesn't require any specific training to take the measurements. But BMI only provides a rough estimation of your body fat because it doesn't involve any direct measures of your tissue.
BMI Mistakes Muscle for Fat
BMI uses your weight in the formula but doesn't distinguish if that weight comes from an abundance of fat or from lean tissue. Athletes and gym enthusiasts who carry a great deal of muscle may seem heavy for their height or overall size, but that's because muscle is denser than fat. These highly muscular folk may have a high BMI but not have too much fat.
Your health care provider can easily see with a physical evaluation and lifestyle questions that your high BMI is due to muscularity rather than fat. Further evaluations, such as blood pressure checks and cholesterol screenings, may still be performed to rule out any underlying health issues.
BMI Can Underestimate Fat
Because BMI does not directly measure fat, it can miscategorize people as healthy who have a normal weight for their height, when they're actually carrying too much fatty tissue. A man with 20 percent or higher fat and a woman with 30 percent or higher, but both at normal weight, can be at the same risk of chronic disease as a person who looks obviously overweight.
Sedentary people and older adults are at particular risk of this condition, called normal-weight obesity. If you don't exercise, you lose valuable muscle mass and accumulate excess fat -- even if you don't rank high on height-weight charts. Older adults naturally lose muscle mass as they age, along with some bone density. For this reason, health providers often run lifestyle screenings, family history questionnaires and annual blood tests in addition to BMI calculations on all patients. This helps rule out normal-weight obesity in otherwise seemingly healthy patients.
A normal BMI is only one factor in your overall picture of health. If you smoke, eat a nutritionally poor diet that contains a lot of sugar and saturated fat, or sit the majority of your day, you may still be at risk of health problems.
BMI May Not Reflect Positive Change
BMI is a broad number that doesn't accurately reflect changes in behavior, which could be improving your health. People with a high BMI who are physically active are at lower risk for many health problems than people with a high BMI who are sedentary. For example, physical activity correlates with reduced risk of coronary heart disease and early death, regardless of your weight.
People who adopt a healthier lifestyle by exercising more and choosing healthier foods over junk food may not lose weight if they haven't reduced their calories significantly. They are healthier, but BMI doesn't change because their weight has remained stable. If they rely on BMI as the only marker of their health, their new habits don't seem to be doing much good.
Even if you lose weight, your BMI may not change noticeably. The Centers for Disease Control and Prevention notes that losing 5 to 10 percent of your weight can lead to positive benefits, such as decreased blood pressure and cholesterol. In a 200-pound person, this is a loss of 10 to 20 pounds. Losing the weight may not move your BMI to a normal range, however. For example, a 5-foot-11-inch person must weigh between 136 and 178 pounds to register a normal BMI. If he started out at 200 pounds and lost 10 to 20 pounds, he may have improved his health, but he still falls into an overweight BMI range. Although a bit frustrating, the change still has positive benefits.
Weight Distribution and BMI
Healthier habits often also change the distribution of your weight, even if weight loss isn't showing up on the scale. You may lose some visceral -- or belly -- fat, which is inflammatory and increases your risk of disease. Exercise in particular helps you lose this fat. BMI can't tell that you've reduced a wide waist circumference and added muscle, creating a healthier body composition. It may just show an unchanged ratio of height and weight, putting you in an overweight category.
Your waist size may be a better marker of your health status because it indicates where you store fat. Use a measuring tape to measure around your waist just below your belly button. A waist wider than 40 inches on a man or 35 inches on a woman can be dangerous.