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Why BMI Is Not a Great Indicator of Body Composition

Why BMI Is Not a Great Indicator of Body Composition

Story at-a-glance -

  • BMI, which gauges weight in relation to height, may underestimate obesity rates and misclassify up to one-quarter of men and nearly half of women
  • One of the primary reasons why BMI is a flawed measurement tool is that it uses weight as a measure of risk, when it is actually a high percentage of body fat that makes a person obese
  • Many experts are now leaning toward body fat percentage as the most accurate measure of obesity
  • Waist size is another simple and free measurement that can be used as a superior measurement to BMI
  • You can also measure your body fat percentage simply by using skinfold calipers or a digital bathroom scale that uses bioelectrical impedance analysis (BIA) to measure your body fat

By Dr. Mercola

If you'd like to know how much body fat you have, and whether or not your levels put you into a weight category that might lead to health problems, most public health agencies, and therefore most physicians, promote the use of the Body Mass Index (BMI).

The U.S. Centers for Disease Control and Prevention (CDC) call it a "reliable indicator of body fatness" for most people, but new research confirms that the method, which gauges weight in relation to height, is actually seriously flawed.

BMI is Not a Reliable Method for Diagnosing Overweight or Obesity

According to CDC estimates, which are based on BMIs, the United States has an obesity rate of 20 percent, with certain states approaching 30 percent. Researchers from New York University compared a DXA (duel-energy x-ray absorptiometry) scan, which is a FAR more accurate method to assess body fat percentage, to BMI and found the CDC estimates likely seriously underestimate the rate of obesity in the U.S.

They found:

  • BMI characterized 26% of the subjects as obese, while DXA indicated that 64% of them were obese
  • 39% of the subjects were classified as non-obese by BMI, but were found to be obese by DXA
  • BMI misclassified 25% men and 48% women

According to lead author Dr. Eric Braverman, president of the nonprofit Path Foundation in New York City:

"Based on BMI, about one-third of Americans are considered obese, but when other methods of measuring obesity are used, that number may be closer to 60%."1

One of the primary reasons why BMI is a flawed measurement tool is that it uses weight as a measure of risk, when it is actually a high percentage of body fat that makes a person obese. Your weight takes into account your bone structure, for instance, so a big-boned person may weigh more, but that certainly doesn't mean they have more body fat. BMI also tells you nothing about where fat is located in your body, and it appears that the location of the fat, particularly if it's around your stomach, is more important than the absolute amount of fat when it comes to measuring certain health risks, especially heart disease.

Improving BMI Accuracy

The researchers noted a strong relationship between increased leptin and increased body fat, and recommended that "clinicians can use leptin-revised levels to enhance the accuracy of BMI estimates of percentage body fat when DXA is unavailable." The hormones your fat cells produce impact how much you eat and how much fat you burn. One of these hormones is leptin, and leptin sends signals that reduce hunger, increase fat burning and reduce fat storage. That is, if your cells are communicating properly and can "hear" this message.

In my mind this may work but is not nearly as practical or certainly as inexpensive as measuring a person's body fat through a variety of techniques that are discussed below. My favorite is waist size because it is a simple easily reproduced measurement that is virtually free. The only requirement is a tape measure and if you don't already have one we carry one in our store that is specifically designed for measuring waist size easily.

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What's One of the Most Accurate Measures of Obesity?

The other tool, which many experts are now leaning toward as the most accurate measure of obesity, is body fat percentage. As it sounds, this is simply the percentage of fat your body contains, and it can be a powerful indicator of your health. Too much body fat is linked to chronic health problems like high blood pressure, high cholesterol, heart disease, diabetes, and cancer. Too little body fat is also problematic and can cause your body to enter a catabolic state, where muscle protein is used as fuel. A general guideline from the American Council on Exercise is as follows:2

Classification Women (% fat) Men (% fat)
Essential Fat 10-13% 2-5%
Athletes 14-20% 6-13%
Fitness 21-24% 14-17%
Acceptable 25-31% 18-24%
Obese >32% >25%


I would encourage you to not make the mistake that I did as I was seeking to get healthy. More is not necessarily better. What does that mean here? One might think that getting to be highly athletic and "super fit" and have really low body fat percentage would be a sign of excellent health, but the reality is that it isn't.

It would certainly allow you to more effectively compete at whatever athletic endeavor you choose but the down side is that it would lower your fertility level. This is typically a good indicator of how healthy you are. You see you can't optimize for health and competitive fitness, you can only have one or the other. So if you are striving for health it would be wise to strive for the fitness levels.

How to Measure Your Body Fat

Body fat calipers are one of the most trusted and most accurate ways to measure body fat when used correctly. A body fat, or skinfold, caliper is a lightweight, hand-held device that quickly and easily measures the thickness of a fold of your skin with its underlying layer of fat. Taken at three very specific locations on your body, these readings can help you estimate the total percent of body fat within your entire body. (Skinfold measurement is the method most widely used by fitness trainers.)

For even greater accuracy, you can resort to hydrostatic weighing, where you get weighed under water. This measures the density of your body, which is then used to calculate how much body fat you have.

Another technique that is gaining support by medical and fitness experts is the bioelectrical impedance analysis (BIA). To measure body impedance, an electrical signal is passed through your body. Impedance is greatest in fat tissue, which contains low amounts of water, while fat-free mass, which contains up to 75 percent water, allows the signal to pass through fairly unimpeded. This measurement, along with other factors such as your height, weight, and body type, is then used to calculate your percentage of body fat, fat-free mass and other body composition values.

There are now bathroom scales that use this technology. I picked one up from Eat Smart3 that I have been using for the last four months. I find it's a simple way to monitor my body fat percentage. It may overestimate a bit as it has me at 13.5 percent when I tested between 11 and 12 percent using other methods, but it is very accurate in measuring day-to-day variability, and can be an excellent and inexpensive way to monitor your progress on optimizing your body fat so it is in line with your health goals.

A DEXA scan is probably one of the most accurate ways to measure body fat but it is difficult to find a center that uses them to measure body fat percentage. There are many DEXA scans out there but most are used to measure bone density.

Additionally, a DEXA scan is an X-ray, which will expose you to radiation, and is far more expensive than all of the other methods discussed. For a fraction of the price of one DEXA scan you can pick up an Eat Smart scale, or for even less a set of skinfold calipers, and use them daily to monitor your progress. Remember that it is FAR better to monitor your body fat percentage than it is your total weight, as your body fat percentage is what dictates metabolic health or dysfunction -- not your total weight.

What About Measuring Your Waist Size?

This is my favorite method for indirectly measuring body fat. This is another simple tool you can use to gain even more insight into your health, as waist size provides a far more accurate benchmark for predicting your risk of death from a heart attack and from other causes than does BMI. Determining your waist size is easy. With a tape measure, measure the distance around the smallest area of your abdomen, below your rib cage and above your belly button. The following is a general guide for healthy waist circumference:

  • Men: 37 to 40 inches is overweight; greater than 40 inches is obese
  • Women: 31.5 to 34.6 inches is overweight; greater than 34.6 inches is obese

The reason why this is a better indicator of heart disease risk is because your waist size is related to the type of fat that is stored around your waistline, called "visceral fat" or "belly fat." This type of fat is strongly linked to type 2 diabetes, heart disease, stroke, and other chronic diseases. It is thought that visceral fat is related to the release of proteins and hormones that cause inflammation, which can in turn damage your arteries and affect how you metabolize sugars and fats.

An expanded waistline is associated with insulin resistance, high blood pressure, lipid imbalance, cardiovascular disease, thickening of the walls of your heart, and even increased risk for developing Alzheimer's disease decades later.

What to Do if Your Body Fat Percentage is Too High

For the majority of people, severely restricting carbohydrates such as sugars, fructose, and grains in your diet will be the key to weight loss. Refined carbohydrates like breakfast cereals, bagels, waffles, pretzels, and most other processed foods quickly break down to sugar, increase your insulin levels, and cause insulin resistance, which is the number one underlying factor of nearly every chronic disease and condition known to man, including weight gain.

As you cut these dietary villains from your meals, you need to replace them with healthy substitutes like vegetables and healthy fats (including natural saturated fats!). I've detailed a step-by-step guide to this type of healthy eating program in my comprehensive nutrition plan, and I urge you to consult this guide if you are trying to lose weight.

Additionally, a growing body of evidence shows that certain forms of fasting are particularly effective for losing weight. One of the mechanisms that makes fasting so effective for weight loss is the fact that it provokes the secretion of human growth hormone (HGH), which is a fat-burning hormone. It also plays an important role in muscle building. Fasting also increases catecholamines, which increases resting energy expenditure, while decreasing insulin levels, which allows stored fat to be burned for fuel.

Together, these and other factors will turn you into an effective fat-burning machine. Hence, if like many tens of millions of people, your goal is to shed excess fat, fasting can be both effective and beneficial for improving many disease markers. The type of fast you choose appears to be less important, so pick whichever one fits your lifestyle, schedule, and temperament the best.

The variations include:

  1. Scheduled Eating, or intermittent fasting. In essence this fitness-enhancing strategy looks at the timing of meals, as well as when NOT to eat. The longest time you'll ever abstain from food is 36 hours, although 14-18 hours is more common. You can also opt to simply delay eating. For example, skipping breakfast may be just the thing to kick-start you off a plateau in your fitness routine. For more details, please see this more in-depth article on intermittent fasting.
  2. LeanGains (a fasting protocol by Martin Berkhan4 )—A daily 14-16 hour fast, during which time you consume nothing, with the exception of non-caloric fluids. Sleeping time is included in this time-frame, leaving an 8-10 hour window during the day when you're allowed to eat.

    This protocol is designed with regular exercise in mind, with specific nutrient ratios for workout days and rest days, and is geared for those who want to shed excess fat and gain muscle mass. Hence, it's best suited for those who are actually exercising and lifting weights each week and can tolerate working out in a fasted state.
  3. Eat Stop Eat (created by Brad Pilon5 )—In this protocol, you fast for a full 24 hours once or twice a week. Your fast should be broken with a regular-sized meal (i.e. avoid gorging when coming off your fast), and you can maintain a regular exercise program without any special diet recommendations for workout days.

    Fasting for 24 hours can be tough for some people, but eating a high-fat, low-carb diet can make 24-hour fasting easier, as a higher fat diet will tend to normalize your hunger hormones and provide improved satiety for longer periods of time.
  4. The Warrior Diet (by Ori Hofmekler)—This is another protocol designed to improve your fitness by exercising in a fasted state. I've interviewed Ori and posted detailed articles on this in the past. His plan calls for 20 hours of fasting, and four hours of "feasting." You exercise during the day in a fasted state. Raw vegetables are allowed during your fast, but no protein, which is reserved for "feasting" or post-exercise recovery meals.

    To learn more about the Warrior Diet, please see this previous interview with Ori.
  5. Alternate Day Fasting—This fasting protocol is exactly as it sounds: one day off, one day on. When you include sleeping time, the fast can end up being as long as 32-36 hours. This may be the most difficult of all types of fasting, as it will require you to go to bed with an empty stomach a few times a week. It's definitely not for everyone.

You can also simply let your hunger guide you and skip meals if you're not hungry. While this should work really well for those who are otherwise healthy and are not struggling with food cravings, it may not work if you're constantly craving food. Food cravings is a sign that you're not providing your body with proper nutrients in the appropriate ratios, so following your hunger in this case could be staggeringly counterproductive.

Fasting is not something you should undertake willy-nilly. You need to pay careful attention to your body, your energy levels, and how it makes you feel in general—especially if you're diabetic, hypoglycemic, or pregnant, categories for which fasting is not appropriate.

The foods you choose to eat, and how much you eat, will be the driving force behind successfully achieving your weight loss goals -- even more so than exercise. But exercise is still important for weight loss and optimal health. The key to boosting weight loss and getting the most out of your exercise routine is to make sure to incorporate high-intensity, short-burst-type exercises, such as my Peak Fitness Program, two to three times per week. Several studies have confirmed that exercising in shorter bursts with rest periods in between burns more fat than exercising continuously for an entire session.