Is Body Mass Index a Good Measure of Health and Fitness?

Updated on May 28, 2018
Mike Esco profile image

Dr. Mike Esco has a Ph.D. in exercise physiology and has worked with numerous weight loss clients during his 20 year fitness career.

What is BMI?

When people think of body composition, thoughts of fat come to mind. However, fat is only one part. Body composition is actually the proportion of all the tissues that make-up your body. So it includes body fat, as well as muscle mass, body water, and bone.

There are many ways to measure body composition. Body mass index, or BMI, is the simplest. It is calculated from a person's height and weight. It is not a direct measure of body fat like the more sophisticated techniques of dual-energy x-ray absorptiometry, underwater weighing, bioelectrical impedance, or skinfold thickness. However, because it is strongly correlated to cardiovascular and metabolic diseases, and it is easy to calculate, it is the most widely used measure of body composition.

High BMI is linked to a number of chronic diseases including, but not limited too:

  • Cardiovascular disease
  • Some forms of cancer
  • Obesity
  • Type 2 diabetes
  • Stroke
  • Sleep problems
  • Poor quality of life
  • Depression
  • Arthritis

BMI is certainly not the only thing related to the above mentioned health problems. To specifically determine a person's risk of chronic disease, especially heart disease, BMI should be considered in addition to:

  • Family history
  • Age
  • Cholesterol
  • Blood pressure
  • Blood sugar
  • Smoking history
  • Physical activity status


Calculate your BMI

There are basically two ways to calculate your BMI, depending on whether you are accustomed to imperial or metric units. First you must measure your weight and height.

Use the following steps for imperial units:

  1. Multiply your weight in pounds by 703
  2. Then, divide that number by your height in inches
  3. Then, divide that number by your height in inches again

Here is an example for someone that weighs 175 pounds and is 5’11” (71 inches) tall.

Step 1: 175 x 703 = 123025

Step 2: 130055 / 71 = 1732.75

Step 3: 1831.76 / 71 = 24.41


Use the following steps for metric units:

  1. Square your height in meters (m2)
  2. Then, divide your weight in kilograms (kg) by your height m2

Here is an example for someone that weighs 79.5 kilograms and is 1.803 meters tall.

Step 1: 1.803 x 1.803 = 3.25

Step 2: 79.5 / 3.25 = 24.46


This person’s BMI is about 24.4 kg/m2. If you refer to the classification mentioned below, this person is considered to be in the “healthy weight” range.

BMI is a ratio of height-to-weight

Source

A person can be classified into one of the weight categories in this Table based on their BMI.

BMI Level
Classification
Under 18.5
Underweight
18.5 to 24.9
Healthy Weight
25 to 29.9
Overweight
Above 30
Obese
The categories in this table are based on the World Health Organization standards

Measure your height

A stadiometer is a tool used to measure height. If you don't have access to one, standup straight against the wall. Have someone else put a mark on the wall at the top of your head. Then measure the distance between the floor and the mark.
A stadiometer is a tool used to measure height. If you don't have access to one, standup straight against the wall. Have someone else put a mark on the wall at the top of your head. Then measure the distance between the floor and the mark. | Source

Limitations of BMI

In general BMI is a good measure of body composition, but it is not without limitations. Here are a few:

  • A major disadvantage of using BMI is that it does not account of body fat and lean mass. This is not good for someone who is overly muscular as BMI may classify them as obese when they are actually lean. They may just weigh a lot because of their muscularity. So, BMI is not an accurate body composition method for many athletes and people who exercise a lot.
  • BMI does not take into account a specific region. This is a problem if a person distributes body fat in a particular area. For example, if BMI is less than 30 but someone's waist circumference is large they can still be considered obese. The reason for this is because fat storage in the abdominal (or visceral) region is the main concern for developing chronic diseases. So, a person can actually be considered "healthy" by BMI standards, but still be considered obese if their waist circumference is too large. Please see the section below for more information about the importance of waist size.
  • It is difficult to use BMI for classifying weight standards for anyone under the age of 20 years. Instead of the BMI categories mentioned in this article, BMI for children are based on percentile rankings. For more information related to BMI and kids, please see the article here from the Centers of Disease Control.
  • BMI may not be very useful for tracking changes in body composition as we get older. Muscle mass tends to decrease with aging, especially if a person does not exercise. This problem is referred to as sarcopenia. The loss of muscle tissue is often replaced with unhealthy fat. This can result in no change in body weight and BMI even though body fat percentage increases.

Be sure to measure your waist circumference

Source

BMI and Waist Circumference

In a recent study published in the Journal of Cardiovascular Echogradiograhy, measuring waist circumference and BMI was more predictive of cardiovascular disease markers than just measuring BMI alone (Antonini-Canterin et al., 2018).

Thus, it may be best to calculate your BMI in addition to measuring your waist size.

Waist circumference is one of the most powerful body composition measures, as it directly relates to body fat that is stored deep within your abdominal cavity. This type of fat storage is referred to as “visceral adipose tissue” and is strongly related to chronic diseases of vital organs. In fact, a person is placed at an elevated risk of heart disease if they are a man with a waist circumference of greater than 40 inches (101 cm) or a woman with a waist circumference of greater than 35 inches (89 cm).

The waist circumference measure itself is not the same as the waist size of your pants. To measure it, you must use a flexible, non-elastic tape measure. Stand up straight with your feet together. Place the tape measure horizontal around the top portion of your hipbone which is usually directly below the belly button. The tape measure should be placed firmly over your skin, but not too tightly that it compresses your waist. Be sure that the tape is not too tight and that it does not shift up or down. This may result in inaccurate measures. Having someone else take the measure may be best.

If a large waist circumference is coupled with a high BMI, then there is an even greater risk of developing heart disease at an early age. Therefore, it is very important to have a BMI of under 30 (preferably under 25) and a waist circumference of less than 40 inches for men or 35 inches for women.

What to do if your BMI is high

If your BMI is higher than you hope, do not be discouraged. This may be a great time to strive for a healthier weight. It is very important that you consult with a professional, such as a physician, exercise physiologist, or registered dietician. Such individuals may have a more accurate method of acquiring body fat percentage, which is a more important indicator of health status than BMI.

It is extremely important to eat reasonably healthy, find an exercise program that you enjoy, and stay physically active throughout the day. These three things are vital for striving for and maintaining a healthy body composition.

BMI and Body Fat Percentage?

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This content is accurate and true to the best of the author’s knowledge and does not substitute for diagnosis, prognosis, treatment, prescription, and/or dietary advice from a licensed health professional. Drugs, supplements, and natural remedies may have dangerous side effects. If pregnant or nursing, consult with a qualified provider on an individual basis. Seek immediate help if you are experiencing a medical emergency.

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