Body mass index (BMI) is not a reliable indicator of overweight and obesity, in spite of what the National Heart Lung and Blood Institute says. Body fat percentage is the key information that you should have for assessing your weight and health risks. Here is why.
Calculating Body Mass Index
You can look up your BMI based on your height and weight at the bottom of this post. That would be a lot easier than calculating it yourself. Nevertheless, to understand why this index is unreliable, it is instructive to first take a look at how the BMI is calculated.
The formula for body mass index (BMI), with an example for the same person in English and metric units, is:
English Units: BMI = Weight (lb) x 703 / Height (in) x Height (in)
Example: 195 lbs x 703 / 73 inches x 73 inches = 25.7
Metric Units: BMI = Weight (kg) / Height (m) x Height (m)
Example: (86.6 kg / 1.85 m x 1.85 m) = 25.3
(The small difference in these results, starting with the same height and weight, are due to rounding errors in converting from English to metric units.)
The NHLBI says that the following are meaningful BMIs for adults between the ages of 18 and 65 years:
Weaknesses of the BMI
We can all probably agree that overweight and obesity are a global epidemic. Obesity alone is a significant risk factor for dying from a heart attack, regardless of whether other known risk factors are present. Obesity is also believed to contribute to other serious health conditions including high blood pressure, high cholesterol, Type II diabetes, gallbladder disease, osteoarthritis, sleep apnea, and respiratory problems, as well as endometrial, breast, prostate, and colon cancers. That is a good slew of individual issues that I may pick apart in another post (e.g., the myth of high cholesterol). For now, let’s just focus on BMI vs. body fat, which is what we want to know in the first place.
In particular, let’s take a look at the comment on this page of the NHLBI: “BMI is a useful indicator of overweight and obesity.”
This and other pages at the NHLBI site quickly point out variables that the BMI fails to account for:
- BMI varies by sex, race, and age
- At the same BMI, women tend to have more body fat than men
- At the same BMI, older people, on average, tend to have more body fat than younger adults
- Highly trained athletes may have a high BMI because of increased muscularity rather than increased body fatness
Here is an interesting contradiction: I have a close friend who is 64 years old and has maintained a weight of about 205 lbs for the past 20 years. His height 20 years ago was 5′ 10″ (70 inches), which calculates to a BMI of 29.4. Recently, due to normal bone loss, he is closer to 5′ 8″ (68 inches) which puts his BMI now at 31.2.
He went from ‘Overweight’ to ‘Obese’ with no change in weight, just a shrinking height that often occurs as we age. By the way, he has always been muscular, so his BMI in the past was already incorrect in placing him in the ‘Overweight’ category in the first place.
Height and Weight Do Not Equal Body Fat Percentage
What the BMI does is dance around the core issue: body fat percentage. Knowing how overweight you might be, even if the BMI fits your body type, gender, age, etc., is irrelevant. The key is body fat percentage. The body mass index may correlate with body fat. However, it is a disservice to you to accept advice about BMI when these days it is so easy obtain your body fat percentage directly.
I’ve posted the concepts that make more sense in this regard, and what to do about getting the right information for tracking your body fat, here:
By the way, measuring your body fat percentage, instead of relying on your body mass index, is easier than ever. Many large retail stores, local and online, offer bathroom scales that have bioelectrical impedance devices built in so you can read your body fat percentage at the same time you weigh yourself. They are usually inexpensive, in the 40 dollar range.
Use the body mass index as a first approximation of where you are physically. However, most certainly get a bioelectrical impedance monitor, handheld or in a bathroom scale, to keep track of what is really important. You may be surprised, if you by chance take some bad advice to diet by cutting out too many calories, by seeing your weight and BMI drop, while your body fat percentage stays the same or increases.
Here are the promised BMI lookup tables. Just take your BMI with a grain of salt, so to speak.
Good luck with your Body Mass Index,