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Why The BMI is Useless for Assessing Body Composition: More Bad Than Good
10/30/2012 3:22:04 PM
Domenic Gareri / ShutterstockYou probably know that the Body Mass Index or BMI is an inaccurate body composition assessment for muscular population such as athletes. With this knowledge, you might write the BMI off as irrelevant for yourself, or your clients if you are a coach, and think little more of it. But, did you know that it is an inaccurate measurement for a number of other populations as well?

Yep, it’s true. Not only is BMI a poor predictor of body fatness, it is an imprecise measurement for predicting health risk. For example, in a review of patients with heart disease, BMI didn’t predict mortality risk at all. Individuals with the lowest BMI (low to normal) had the highest mortality risk, whereas overweight participants had better survival rates!

What about when you read scientific studies that use BMI as the primary body composition measurement? Do you value the results and conclusions that use BMI? In the circumstance of weight loss due to training, diet, or a combination, BMI is very misleading.

For example, one study that compared weight loss in overweight men on a diet, half of which were on a strength training program as well, showed that the training-diet group only produced a small advantage in decreasing BMI over the diet with no training. However, body fat statistics showed that the training-diet group lost an average 11.2 kg of fat and only half a kg of lean mass compared to the diet-only group that lost 5.2 kg of fat and 4 kg of lean mass—big difference on body composition!

If only BMI had been used without skinfold measurements, radically incorrect interpretations about the benefit of exercise on fat loss might have been made. It is the widespread use of the BMI that has produced some of the misperceptions about the best methods for improving body composition.

The BMI Defined
The BMI is defined as weight in kg divided by height in meters squared. It is the body composition assessment that is widely used to define obesity by health and scientific agencies including the Center for Disease Control and the U.S. Department of Health and Human Services. Proponents of the BMI suggest the benefits (little equipment required, no tester expertise needed, it is inexpensive) outweigh the errors.

But since the BMI is inaccurate, being cheap and easy doesn’t really matter. Critics of the BMI question why health professionals have chosen to ignore or at least tolerate the limitations of the BMI. Wouldn’t it be better to invest training time and money into an effective assessment tool, especially since body composition is a widespread problem in the increasingly overweight U.S.?

Uselessness of the BMI
A  2004 study that compared eight methods of measuring body composition in body builders showed that the BMI was the least reliable method and produced significant errors in predicting body fat. Study authors write, “Bias is largest with the BMI estimate, which is evidently not the method to determine body fat in body builders and most other strength trained athletes.”

Researchers of a study from 2005 agreed. They compared BMI with the Jackson Pollock skinfold assessment in college athletes and found that “BMI frequently classified muscular individuals who did not have high skinfold measurements as overweight or obese.” 

Interestingly, the CDC even highlights the uselessness of a simple equation for assessing body composition for different populations, writing on their web site, “the correlation between the BMI number and body fatness is fairly strong; however the correlation varies by sex, race, and age.”

Indeed, a study from the University of Miami shows how BMI is ineffective for female college athletes of different races. Average body fat percentage was found to be very different among the racial groups, but the average BMI was nearly equal for all three groups. For example, African American athletes had the highest average BMI of all at 23.8, but they had the lowest average body fat percentage at 24.1. White athletes tied for the lowest BMI at 22.8 with Hispanic American athletes, but had highest average body fat percentage at 27.2. Hispanic American athletes had a body fat percentage of 26.3 percent.

Negative Effects of BMI On Body Image
More concerning than simply being a useless and annoying measurement, this study found that, regardless of race or body fat percentage, the athletes with higher BMIs were preoccupied with their weights, wanted to lose weight, and exercised in order to do so.

It is alarming that a measurement like the BMI that doesn’t accurately reflect body fat, body composition, or performance is creating weight concern and preoccupation in athletes, particularly female athletes who are at risk for disordered eating. In fact, studies show that between 25 and 75 percent of college female athletes suffer from disordered eating patterns and dissatisfaction with their bodies. Body dissatisfaction affects male athletes as well, though it does not appear to be as widespread.

BMI Inaccurate for Other Populations As Well
A large-scale study young athletes ranging in age from 11 to 18 years old shows that it is not only very muscular individuals that are being classified as overweight by BMI. Nearly 34,000 student athletes in middle and high school in Georgia and Alabama were tested for BMI and body fat using a skinfold assessment. Results showed that BMI was highly inaccurate, with 62 percent of the students being falsely classified as obese by BMI.

One might assume that BMI could be used for people who actually are obese as measured by a 12-site skinfold assessment. Not so! A study of people who underwent bariatric surgery and lost significant body weight showed that there was a standard error ranging from 4 to 8 percent in measuring actual body composition changes using BMI. Researchers conclude BMI can not be used to assess body fat changes in the obese.

BMI Doesn’t Predict Health Risk Either
A recent review found that BMI doesn’t work for predicting heart attack risk or mortality in patients with cardiovascular disease either. In fact, results found that a low BMI was strongly associated with an increased risk of total mortality and other cardiovascular events. Overweight patients were consistently associated with a better survival and lower cardiovascular events. Obesity was associated with a higher total mortality only in patients with heart disease. Severe obesity was linked to much higher cardiovascular disease but not with total mortality risk.

It’s not that having more body fat or being obese is actually healthier. This study is a classic example of the ineffectiveness of BMI. First, as we know body fatness increases heart disease risk because it causes inflammation, however, since BMI doesn’t adequately reflect adiposity, the BMI classifications in this study didn’t reflect the amount of dangerous body that patients had.

Second, a lower BMI suggests a lower muscle mass, or sarcopenia. People with sarcopenia, especially overweight individuals with heart disease, have significantly reduced mobility and restricted exercise capacity. Researchers suggest that sarcopenia is a risk for mortality and may be just as dangerous as excess body fat.

Third, a side effect of the inept BMI measurement is that people with a low BMI who are sedentary, or have poor diet, low muscle mass, or low bone mass, are less likely to be targeted for treatment or intervention by their doctors or public health programs. Low or normal BMI is giving a false security to people who are at high risk of disease.

The Solution: Skinfold Assessments
A 12- or 14-site skinfold method remains the best simple, affordable, and quick assessment for body composition. It requires extensive practice, but for any trainer, nutritionist, scientist, or  physician, it is worth the investment for accurate body composition data. To accurately council and coach for fat loss and health, we must have a reliable method of tracking athletes or clients and providing feedback. Not only will we see better results, but this is one step toward preventing further body image problems that can lead to poor performance and health.
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