The Pros and Cons of Using Body Mass Index (BMI)

Anytime you go to the doctor, one of the first things you do is to step on the scale. Your body weight and height is then calculated to determine your Body Mass Index (BMI), which has long been considered the gold standard in determining if you are at a healthy weight or if you may be at risk for potential health problems.

BMI is an inexpensive and easy-to-perform method of screening to determine excess weight that may lead to health problems. However, many doctors and scientists have questioned the usefulness of the BMI and its accuracy in determining overall health risks.

According to Robert H. Shmerling, MD, senior faculty editor at Harvard Health Publishing, BMI is still a useful starting point in predicting a person’s risk for certain health conditions that are more likely to occur when a person is overweight or obese. But it’s also important to know there are limitations to BMI.

How is BMI used?
BMI is used as a screening tool to identify possible weight problems in adults. However, to determine if excess weight is a health risk, a doctor would need to use the BMI along with further testing such as skinfold-thickness measurements, evaluations of diet, physical activity, family medical history, and other health screenings.

How is BMI calculated?
BMI is calculated (using a mathematical formula) by dividing an individual’s weight in pounds by height in inches squared and multiplying by a conversion factor of 703.

Example: Weight = 150 lbs, Height = 5’5” (65″)
Calculation: [150 ÷ (65)²] x 703 = BMI = 24.96

There are many on-line calculators that will calculate BMI by simply plugging in your height and weight.

What do the BMI numbers mean?
For adults 20 years old and older, BMI is explained using standard weight status categories that are the same for all ages and for both men and women.

The standard weight categories associated with BMI ranges for adults: a normal BMI is between18.5 and 25; a BMI between 25 and 30 is considered overweight; and a BMI over 30 is considered obese. A person is considered underweight if the BMI is less than 18.5.

Someone with a BMI of 26-27 is about 20 percent overweight, which is generally believed to carry moderate health risks. The higher the BMI, the greater the risk of developing additional health problems.

For adults, what are the health consequences of being overweight or obese? 
Overweight and obese individuals are at increased risk for many diseases and health conditions, including the following:

  • High blood pressure
  • High cholesterol
  • Type 2 diabetes
  • Heart disease
  • Stroke
  • Gallbladder disease
  • Osteoarthritis
  • Sleep apnea and respiratory problems
  • Some cancers (endometrial, breast, and colon)

What are the limitations of using BMI?
BMI on its own is not a measurement of health or a measurement of a physiological state (such as taking your blood pressure) that shows the presence or absence of disease.

There are people who have a high BMI who are healthy, and there are those with a normal BMI that are unhealthy. For example, someone with a low BMI who smokes and has a strong family risk of heart disease or cancer may have a higher risk of early death than someone with a higher BMI who does not smoke and is physically fit.

Just as blood pressure or cholesterol numbers should not be used as a single measure of overall health, neither should BMI be used solely to identify cardiovascular disease or other illnesses. BMI is just part of a bigger picture your doctor can use when assessing your health and disease risk.

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