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Is BMI an accurate measure of obesity?

Neal D. Barnard, MD
Psychiatry
Body mass index is a measurement of your weight, adjusted for your height; if you're taller, then carrying more weight is normal. In this video, Neal Barnard, MD, explains what BMI indicates, and why it can be a deceiving measure of overall health.
Eric Beard
Sports Medicine
Body Mass Index (BMI) testing is a method of estimating a person's body fat levels based upon a person's weight and height.

BMI is a number calculated from the relationship between a person's weight and height. A BMI number or "score" places individuals in weight categories. These categories can range from severely underweight to morbidly obese.

Having a low BMI can place someone at risk for the following health conditions and diseases; osteoporosis, impaired immune system function, amenorrhea, cardiac abnormalities and more. Obesity and morbidly obesity, or having a high BMI score are risk factors for;  blood (fat) lipid abnormalities, cancer, including cancer of the uterus, cervix, ovaries, breast, colon, rectum and prostate, depression, gallbladder disease, gynecological problems, such as infertility and irregular periods, heart disease, high blood pressure, metabolic syndrome, nonalcoholic fatty liver disease, osteoarthritis, skin problems, such as intertrigo and impaired wound healing, sleep apnea, stroke, type 2 diabetes according to the Mayo Clinic.

Below 18.5 = Underweight
18.5 – 24.9 = Normal
25.0 – 29.9 = Overweight
30.0 and Above = Obese
BMI is considered an overall indicator of ideal body weight. BMI indicates a healthy range of body weight based on height. BMI does not include other factors that indicate a person's health such as body fat percentage versus lean muscle mass, eating habits and exercise.
BMI does not calculate body fat percentage; it merely determines whether your weight is in a healthy range based on your height. Larger BMI values are associated with an increased risk for many health complications. 
Merle Myerson, MD
Cardiology (Cardiovascular Disease)
Cardiovascular specialist Dr. Merle Myerson discusses body mass index (BMI) and what it determines. Watch Dr. Myerson's video for important tips and information about heart health.
Marjorie Nolan Cohn
Nutrition & Dietetics
For adults, overweight and obesity ranges are determined by using weight and height to calculate a number called the "body mass index" (BMI). BMI is used because, for most people, it correlates with their amount of body fat.

It is important to remember that although BMI correlates with the amount of body fat, BMI does not directly measure body fat. As a result, some people, such as athletes, may have a BMI that identifies them as overweight even though they do not have excess body fat. For more information about BMI, visit Body Mass Index.
Body mass index (BMI) is a calculated ratio between your height and weight. This formula is a quick measure of body fat that fits most of the population. However, muscular athletes often find themselves on the overweight or obese portion of the BMI chart because it does not take into consideration the actual percentage of body fat. Muscle is more dense than fat, but the chart does not differentiate where the weight is coming from.
 
For example, a defensive back who is six feet tall and 220 pounds with 5% body fat is labeled as borderline obese on the BMI chart. For athletes, a more accurate measure of body fat is done with a skinfold caliper or underwater weighing.

(This answer provided for NATA by the Appalachian State University Athletic Training Education Program.)
Body mass index (BMI) is not always an accurate measure of obesity. BMI tends to overestimate body fat in people who are very muscular and underestimate body fat in people who are highly sedentary. People who are very muscular, particularly men, may have a BMI higher than the normal range of 20 to 25 and still appear to have a normal weight and be healthy.

BMI also doesn't show where the body fat is located. Abdominal fat carries the greatest health risk.

Check your waist circumference to help determine if you have weight-related health risks. If it is greater than 35 inches in women and 40 inches in men, you have a higher risk for certain disease, such as diabetes, hypertension, and heart disease.

Weight Watchers can help you lose weight and keep it off. Learn more about Weight Watchers and how to join.
Some people who have an "overweight" BMI may have a normal amount of body fat with a large muscle mass, and others who have a "normal" BMI may have excess fat and reduced muscle mass. However, on average, these statistics hold true and are a warning sign to all Americans who consume more food than necessary.
Body mass index (BMI) is based on a formula that uses both weight and height to estimate associated health risks. It was invented between 1830 and 1850. Due to its ease of use, it is the most widely used diagnostic tool to identify weight problems within a population, whether individuals are underweight, overweight or obese. Though it does not actually measure body fat percentage, your BMI is a general measurement that can help guide you in determining a diet and exercise program.

However, the body fat percentage you get from a body fat scale or hand-held body fat device can vary depending on many factors. How warm or cold your body temperature is at the time you take the test, how hydrated you are when the test is taken. In fact, it's difficult to get an accurate body fat measurement from most commercially available body fat analyzers. A hydrostatic body fat test is the best and most accurate way to have our body fat taken. A hydrostatic body fat test is where you are submerged in water. This kind of  testing is a more exact measurement because bone, muscle and connective tissue, known as lean mass, sinks while body fat floats, giving you a more accurate measurement.

Body Mass Index is accurate most of the time. However, it may overestimate or underestimate body fat. Common examples:

  • BMI doesn't distinguish between muscle mass and body fat. Muscle mass weighs more than fat, which is why many pro football players have been labeled "obese" because of their high BMI. In fact, they have a low percentage of body fat.
  • BMI is not always accurate in the elderly, who often have less muscle and bone mass. Their BMI might lie within a normal range, but they could still be overweight.
  • BMI may relate differently to some ethnic groups. For instance, Asians may be at risk for health problems at a lower BMI than Caucasians.

The chance for error means BMI should be only one of many gauges used to assess a person's weight status and health. The National Institutes of Health recommends three factors to doctors in assessing whether their patients are overweight:

  1. BMI.
  2. Waist circumference-a measurement of abdominal fat.
  3. Risk factors for obesity-associated diseases, including high blood pressure, high LDL ("bad") cholesterol, low HDL ("good") cholesterol, smoking and high blood sugar.

Many health experts say body fat percentage is a better indicator of weight status than BMI. But body fat can be difficult, and sometimes costly, to measure.

Continue Learning about Body Mass Index (BMI)

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Important: This content reflects information from various individuals and organizations and may offer alternative or opposing points of view. It should not be used for medical advice, diagnosis or treatment. As always, you should consult with your healthcare provider about your specific health needs.