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Ep. 64 – Body Mass Index

BMI, or Body Mass Index - Is it a good way to set a weight loss goal? Some people don’t think so. On this episode of the Straight Talk on Health podcast, Dr. Chet Zelasko checks out what’s being said and gives his take on the subject

Welcome to Straight Talk on Health. I’m your host Dr. Chet Zelasko. Straight Talk on Health is a joint production with WGVU in Grand Rapids MI. I examine the world of health. Nutrition. Exercise. Diet. Supplementation. If there’s something new, I look at the science behind them, and let you know whether it’s real or not. You can check out other things that I do on my website Drchet.com and sign up for my free emails.

Here we go again. The Body Mass Index BMI for short, as a measure of fitness and fatness is coming into question again. A weight loss expert from Australia has suggested that the BMI, as a measure of fatness, is simply not accurate. I don't know that I'm going to be able to settle this question once and for all, but I'm going to try to put things in perspective. I'm going to do that by explaining what BMI is, how it was developed, and how it was originally intended to be used. Then I'm going to go into what appears to be the real issue: how the measurement makes people feel about themselves.

Let's begin with what the weight loss expert from Australia actually said, paraphrased to save time without changing intent: while he does suggest that BMI does a reasonable job when looking at large groups, it shouldn't be used to set a healthy weight loss goal. The reasons are that it fails to consider body fat percentage and how that body fat is distributed. In addition to that, he suggests that it doesn't account for differences in body composition based on gender, ethnicity, and age. There's a whole lot there to deal with so let me address things one step at a time. It may help clarify things for you and keep it in perspective.

Let's start with a little history on BMI. Almost 200 years ago, a mathematician wanted to find a way to categorize the average man in physical terms. Lambert Adolphe Jacques Quetelet took measurements of thousands of men in the hopes of being able to obtain the characteristics of the average man. In fact, for the longest time it was called the Quetelet index.

In my research for this, I happened upon an article which claimed that Quetelet was racist in the way he obtained data. He only included European men in his calculations. He did not consider women, races, or ethnicities. I can understand how someone in 2024 might write a headline disparaging the BMI as being old and racist. But that doesn't consider what he saw in Belgium 200 years ago. Also, as a product of the patriarchal era of those times, the other specific groups would never have been considered. He was a statistician. What he wanted to do, was to simply calculate a number indicative of the “average” man. That's what statisticians do; find averages.

When you look at the actual formula, it is a measure of surface area. It is body weight in kilograms divided by the height in meters squared. The greater the surface area, as it turns out, the fatter a person is for any given height or weight. It is not a measurement of percentage of body fat. But it is a basis to examine an individual further to see if there are factors that can confound the BMI for any specific individual.

Currently, waist circumference is being added to BMI to get an additional measure of fatness. The reasoning is that fat that accumulates around the waist can be more indicative of issues such as type 2 diabetes, blood pressure, and cardiovascular disease. I don't disagree. However, having done thousands of measurements over the years, I don't see how it would really impact the categorization of obesity. Sure, it could split hairs between whether someone is classified overweight or obese but in general, I don't think it makes a difference with one exception I'll get to in a little bit.

The next factor is the distribution of fat in the body. The example that’s always used is a bodybuilder. Someone who accumulates a large amount of muscle and thus may qualify as being categorized as obese or even morbidly obese in certain cases. While it may be true that excessive muscle might skew a calculation, that really fits into the category of diagnosing the individual in a medical setting. It is an exception but rare in today’s age

The next categories would be gender, ethnicity, and age. I think BMI stands the test of time for both gender and ethnicity. There may be cultures that are extraordinarily tall or extraordinarily short. But I have yet to see a case where the current measure of BMI wouldn't apply to other races. The one category that does have an impact on BMI is age.

The reason that BMI may cease to be accurate would be an extraordinary loss of muscle mass in the extremities. That’s entirely possible due to sarcopenia, the loss of muscle generally associated with aging. The classic example would be an MRI imaging of the thigh of a 25 year old woman to compare with a 75 year old woman with identical mid-thigh circumferences. In the case of the 25 year old, there is very little marbling within the muscle and the muscle extends almost to the skin. In the example of the 75 year old woman, the image shows very little muscle with a lot of marbling, and most of the thigh is fat. If that occurs in all the extremities, that would mean that people could be obese even though the BMI puts them in the normal weight category. Therefore, age can be a factor in the accuracy of BMI.

The real issue with BMI occurs with the categorization of an individual as obese. Overweight—not so much but people, especially women, hate that word obese. In the commentary by our esteemed obesity researcher, as well as others who’ve commented on his article, suggest that all that women will do is condemn themselves. They provide examples of such responses.

But I have a question. What if they took a valid and reliable assessment of body fat such as a DEXA exam and it came back with a body fat of 42%? In that men are supposed to be between 5-18% fat and women between 20 and 25% body fat, how is that going to make anyone feel any better? The test outcome is that someone is carrying too much fat and that may carry health risks. Believe me, as someone who has had a weight problem their entire life, I’m sympathetic. It still doesn’t change the results and the health implications. Too fat is too fat.

The weight loss expert has written several books about a stepwise approach to weight loss. Lose 5-10% of your body weight and stop for a while to enjoy the health benefits such as lower BP, lower HbA1c, lower LDL cholesterol and more. I think that’s a great approach and one that I’ve recommended to people as well. While the author suggests that the health benefits are the goal, at some point, the body will adjust to the new weight. What happens then? Will all those metrics of health stay at those levels or will begin to rise again? We don’t know because it’s never been studied. Another possibility is that maybe even with regaining weight as many people do, the metrics will stay in the healthy zones. Again, we assume a lot but we just don’t know.

The BMI is as good a way to set a target weight loss goal as any other. The numbers don’t change whether you feel good about them or you don’t. They just are. Many commenters talked about “being impossible to lose that last 10 pounds.” What I’ll tell you is if you’re that close to your weight loss goal and you can’t lose any more? You’re there. Now stay there and enjoy the health benefits because you’re done. That’s my opinion and I’m out of time. Until next time, this is Dr. Chet Zelasko saying health is a choice. Choose wisely today and every day.

Dr. Chet Zelasko is a scientist, speaker, and author. Dr. Chet has a Ph.D. and MA in Exercise Physiology and Health Education from Michigan State University and a BS in Physical Education from Canisius College. He’s certified by the American College of Sports Medicine as a Health and Fitness Specialist, belongs to the American Society of Nutrition, and has conducted research and been published in peer-reviewed journals. You can find him online at drchet.com.
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