Why BMI matters–Commentary from my soapbox.

My Health & Fitness column in the Aiken Standard this week is about body mass index (BMI), the most commonly used measure of body fatness and health. It was prompted by a recent article in the journal Science that raised familiar questions about whether or not BMI is a valid indicator of obesity and health.

This lead to reports in the popular media about BMI, like this one in Time magazine, the tone of which suggests that doctors and researchers who use BMI don’t know what they are doing. Since I use BMI in research and in practice, I  felt I should address both the article and share some other important points and additional thoughts about BMI.

1. BMI is so widely used because it is simple to measure – it only requires knowing height and weight – and because it is a good indicator of body weight and health risks. That said, it’s not perfect (see below). You can calculate your BMI here.

2. One of the biggest criticisms of BMI is that it can’t distinguish between fat and non-fat tissue, meaning that a person could have a high BMI but not be overly fat. The elevated BMI could be due to excess muscle, as is common in athletes. This is true: BMI doesn’t differentiate between muscle and fat. In fact, it isn’t even a measure of body fat. But in most cases, people who have a high BMI do have a high percent fat.

Don’t believe me? Then try what I call he Wal-Mart test. Go to Wal-Mart someday and measure the BMI and percent fat of everyone who comes through the door. What you will no doubt find is that the majority of the people who have a high BMI are overfat and relatively few have a high BMI due to excess muscle. This is why BMI works so well on a population level.

3. Even though BMI may misclassify some people as overfat when they aren’t, there is a simple way to avoid this: measure their waist circumference. People who have a high BMI due to excess muscle will have a narrow waist (think body builders) and those who are overfat will have a high waist circumference (think Homer Simpson). The point is that it is easy not to misclassify someone if they are in front of you.

4. Another criticism is that it is possible to have a normal BMI but have a high percent fat. This is something that we have seen in our lab and it almost always occurs in people who are thin but unfit–they don’t exercise.  The problem is that a normal BMI indicates good health, but if a person has a lot of fat they really aren’t very healthy.

This is also a valid criticism. And it is especially true if the excess fat is visceral fat, which is stored deep in the abdomen and is associated with higher risks of diabetes, hypertension, and heart disease. But there are other indicators of health risk beyond body fatness, including blood pressure, blood glucose, and blood cholesterol. BMI should be used in conjunction with these other factors to assess overall health.

5. Research also suggests that being overweight or obese according to BMI isn’t necessarily unhealthy and that having a normal BMI isn’t always better. But that is only part of the story. The part that gets left out is physical fitness. It’s true…research shows that the risk of death is lower among people who are overweight or obese but have a high level of physical fitness compared to normal weight individuals who are unfit.

This suggests that regular exercise (to become and stay fit) is at least as important as body weight in determining the risk of death. No question this is true!

However, this doesn’t mean that people who are “fit and fat” are necessarily healthier. The research shows a lower risk of death during a specific follow-up period, but doesn’t make claims about the health during that time. It is possible, even likely, that obesity is associated with some impairment of health—back pain, for example—that normal weight subjects might not experience. Exercise can reduce those problems, but there are likely still some reasons why losing some weight might benefit even those obese individuals who are fit.

Furthermore, I wonder how many obese people are fit enough to have a lower risk of death. I suspect that number is low. Maybe we should add a fitness test to the Wal-Mart test to find out.

6. These problems aren’t a failing of BMI itself, it is the fact that BMI isn’t meant to be used alone. BMI is just one measure of health. When used in combination with others, like waist circumference and physical fitness, BMI is a much better predictor of current and future health problems.

Using BMI alone and complaining that it doesn’t accurately assess health is inappropriate.  Your doctor wouldn’t measure just your blood pressure, find hat it is normal, and proclaim you healthy, right? The same is true for BMI.

With that said, I will step off my soapbox. At least for now.

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