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Body Comp, BMI and BIA 


What do you weigh? If you’re like most Americans, you know because you just checked it sometime in the past few days. Do you know your body composition, or your body mass index (BMI)? Chances are, you’re not sure. Your doctor may have told you what your BMI was the last time you met with him or her, but you didn’t pay much attention. If you do know your body composition, it’s likely because you had it measured by a health and fitness professional who spent some time explaining where you’re at and what it means.

Body Comp vs. BMI

Body mass index has been used for identifying whether an individual is overweight since the 1800s. It became popular in the United States in 1980. BMI is intended to identify whether someone’s weight is appropriate for his or her height. It’s calculated by taking your weight in kilograms divided by your height in meters squared.

Across a large population, using some kind of height/weight measurement, which is what BMI is, can do a reasonable job of estimating if a population is becoming more overweight. Most of the statistics we read about Americans continuing to become more overweight are based on BMI data. While BMI allows for researchers to get a general idea of where a population’s weight is at, it is less valuable at an individual level. In a clinical setting, many physicians rely on BMI with their patients. However, without understanding what makes up an individual’s body weight, it is difficult to coach someone about whether they need to gain or lose body fat.

Skinny Fat

One scenario where BMI is not valuable is for those who are often called “skinny fat.” I often saw this in people who followed very low-calorie diets to lose weight. Though they lost weight according to the scale, they often lost significant amounts of lean mass, or muscle tissue, along with losing some body fat. As a result, their body fat percentage would still be quite high, even though they looked thin in street clothes. Their BMI might suggest they are at an appropriate weight, but a check of their body fat percentage would say otherwise. It’s important for skinny-fat individuals to adopt a solid resistance training program, get plenty of protein in their diet, and focus on quality foods rather than counting calories.

Muscle Bound

At the other end of the spectrum from the skinny-fat individual, you’ll find the muscle-bound athlete. In this case, individuals carry a significant amount of muscle tissue, yet have very little body fat. Think of the sprinters you see in the Summer Olympics. According to BMI standards, these athletes are considered obese, yet no one would tell them to lose weight. Although they have high BMIs, their body fat levels are clearly below what would be considered risky. BMI to this group is also irrelevant.

The average, out-of-shape, sedentary individual who has a high BMI also has a high body fat percentage, but it isn’t always the case. Even though BMI can work to provide insight on a large population of average people, on an individual basis, it’s really quite useless.

Body Fat Percentage

The real risk for an individual is how much body fat he or she has. Though many people think of their fat tissue as little storage places for excess calories, fat tissue is actually an endocrine organ, meaning it secretes hormones of its own. The more fat one has, the more of these unhealthy hormones may be secreted. Visceral fat, or belly fat, is the worst, and is becoming more and more common in both men and women. Belly fat is difficult to measure with skin fold calipers because the fat tissue is found under the abdominal muscles. If you have a lot of belly fat, you may have a hard time “pinching an inch” from your stomach, yet your stomach is quite distended.

In the past, at Life Time, we used calipers for estimating body composition. Caliper measurement is quick, easy and cost-effective. Calipers are relatively inexpensive — but they’re not the most accurate. Other types of body composition measurement include hydrostatic weighing, DXA scans and Bod Pods. After long consideration, we decided to move to using a new form of bioelectrical impedance with a system called the Biospace InBody 520.

What is bioelectrical impedance?

Bioelectrical impedance (BIA) is not new. This method for measuring BIA was discovered in the 1960s, though early equipment proved to be inaccurate. BIA is a form of body composition measurement in which a very small current is sent through your hands and feet. Water, fat, muscle and other tissues create varying resistance to the electrical impulse, which the InBody uses to determine the amounts of fat, muscle and water found in an individual’s body. The InBody is very accurate, with a 98% correlation to DXA, considered to be the gold standard in body composition measurement. A DXA machine is very large and extremely expensive, and usually found only in hospitals and research labs, so the InBody is a great alternative. The InBody design is quite different than most other BIA units, so if you talk with someone who says they do BIA analysis, be sure to check on whether the unit they use is the InBody or something else. If you want to learn more about the technical aspects of the equipment, you can read more here.

Losing the Right Kind of Weight

If you’re fortunate enough to be a member at Life Time, you can schedule time with a Health & Fitness Professional to have your body composition measured. It’s free. But once you know the number, what do you do with it? Here are a few principles for healthy body fat loss that you should remember:

1. Fat loss is good, muscle loss is bad.

Low-calorie diets often result in a loss of lean body mass, or muscle, along with some fat loss. Ideally, a routine should be designed to maintain, or even increase, lean body mass while lowering body fat levels. Resistance training and some cardiovascular exercise help, but nutrition is critical. To maintain muscle while losing fat, focus on eating protein with each meal and getting most of your carbohydrates through non-starchy vegetables. Higher-protein, moderate- or reduced-carbohydrate, and moderate-fat diets consistently outperform low-fat diets for modifying body composition.

2. Focus on how you look, not what you weigh.

It’s easy to get hung up on the scale. Kick the habit of weighing in each day and step on the scale just once a week, ideally at the same time of day each time. We’re often told people shouldn’t lose more than 1 or 2 pounds of weight per week, but time and again we find that when people clean up their diet and start exercising right, they may lose much more than that. For those with 50 pounds or more to lose, don’t be surprised if much more weight comes off in the beginning. If you’re doing things right, most of it will be fat and water and you’ll maintain most of your muscle tissue. Check the mirror and your belt to measure your success more often than you check the scale.

3. Fat doesn’t just look bad, it IS bad.

Extra fat isn’t just an issue of aesthetics. It really can impact your long-term health. Fat releases many inflammatory compounds into the body. If you haven’t convinced yourself or a close family memberabout how important it is to get body fat levels down to a healthy range, now is the time to make a change.

The table below shows the gender and age-specific ranges for body composition. If you haven’t had yours checked in a while, stop by the Fitness Services desk at your local Life Time. If you’re not a member or if you go somewhere else, your club will likely offer some form of body composition measurement, though it might not be the InBody BIA.


*The scoring above (0, -8, -16) is part of the new myHealthScore Assessment soon to be availalbe at Life Time's throughout the country. Being in the yellow is an 8 point deduction and red is a 16 point deduction for the myHealthScore.

Ask questions and post comments below.

Written By Tom Nikkola - Director of Nutrition and Weight Management

This article is not intended for the treatment or prevention of disease, nor as a substitute for medical treatment, nor as an alternative to medical advice. Use of recommendations in this and other articles is at the choice and risk of the reader.

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Reader Comments (9)

What's the significance of the 0, -8, and -16 in the table headings?

July 22, 2012 | Unregistered CommenterCarlos

@Carlos: Great question. I just added some text in the article to explain it. The scoring above (0, -8, -16) is part of the new myHealthScore Assessment soon to be availalbe at Life Time's throughout the country. Being in the yellow is an 8 point deduction and red is a 16 point deduction for the myHealthScore.

July 22, 2012 | Unregistered CommenterTom Nikkola

Good question, Carlos.
Additionally, is the "Body Composition" number the same thing as body fat percentage? I didn't see a clear definition for "body composition". It sounds like it may be a series of numbers (i.e. muscle mass, fat, bone mass, water). However, given the context, it appears that its just body fat percentage. Also, the chart shows numbers that look like they may be body fat percentages. Yet this wasn't clearly spelled out.

July 22, 2012 | Unregistered CommenterPatrick

@Patrick: Correct, the numbers are for body fat percentage

July 22, 2012 | Unregistered CommenterTom Nikkola

Why are the fat percentages different for different ages? why as you get older is it ok to have more fat? Is it because you have less muscle mass as you get older? Is it possible to have a high muscle mass for women after menapause?

July 22, 2012 | Unregistered CommenterPat

I second Pat's questions and look forward to more information about this subject.

July 23, 2012 | Unregistered CommenterAnne

Great article. Wow- fat cells secrete bad stuff- I did not know that. Also- belly fat is not pinchable- did not know that either. Thanks!

July 23, 2012 | Unregistered CommenterPhil B.

So if im 5'9, 224lbs & 30% body fat working out 6 days a week 3 days are 60min cardio the other 3 days are 60 min full body workouts how long (estimate) would it take me 2 get my body fat down to the healthier 18%?

July 24, 2012 | Unregistered CommenterLogan

@ Logan: You left out the most important parts of the equation: What are you eating? What are your stress levels? How much quality sleep are you getting?

If all of these variables are optimally controlled for your unique physiology, you can expect between 1 & 2% body fat loss per month (maybe more, but evidence shows it's not likely to be faster than this). Give yourself 6 months, minimum.

I hope this helps!

July 24, 2012 | Unregistered CommenterPaul Kriegler, RD/LD

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