Body Fat Percentage

Not a day goes by (it seems) that we aren't hit with statistics telling us how "fat" Americans have become.

What do the experts mean when they say that someone is "overweight" or "obese?" Why is it bad?

How do you determine if you (or your spouse/children) are "obese?"

What is your "ideal" weight?

What is the best way to lose weight?

This series of articles will answer these questions and give you practical advice with a minimal amount of "jargon."

Before we can discuss finding your "ideal weight" and offer suggestions on weight loss, we need to nail down exactly what we are talking about.

"Underweight/Overweight"

Once upon a time (1), an insurance actuary figured out that men and women who met certain height and weight standards lived longer. If an individual's weight was lower than the "desirable weight" for their height, they were considered "underweight." If an individual's weight was higher than the "desirable weight" for their height, they were considered "overweight."

The height/weight tables were designed purely for insurance purposes. This is neither "good" or "bad", just a fact. Many other factors contribute to an individual's life expectancy, but height and weight remain (even after all these years) good indicators.

(Of course, genetics trumps everything else - if your parents and grandparents lived to be 100 years old, you have a better chance of living to be 100 years old than the person whose parents and grandparents died of heart attacks when they were 50 years old...)

The problem with height and weight charts is that they do not allow for differences in "physical fitness level."

Examples:

#1

Imagine two people who both weigh 300 pounds: one person works in an office and doesn't "workout", the other person is a defensive lineman playing in the NFL. Are they both "unhealthy?" Maybe, but probably not - the inactive office worker is probably much worse off than the football player.

#2

Imagine two people who are the same height and weight. Does this mean that they are equally "healthy"? Again, we don't know. It is possible that one person is much less "healthy" than the other.

To determine the difference between these individuals, we need to understand "body composition."

Body composition

We can divide the human body into two general components: adipose tissue ("fat" plus the water and small amount of protein associated with fat), and "lean body mass" (consisting of muscle tissue, connective tissue, bones, organs, and "blood").

Fat has important functions in the human body. The most obvious is as a source of energy, but fat also serves to cushion internal organs and is essential for many other purposes at the cellular level. However, having too much fat ("overfat") has negative effects on an individual's health. Being "overfat" is referred to as "obesity."

Negative effects of obesity: (2)

- increased risk high blood pressure

- increased risk type II diabetes

- increased risk of heart attack

- increased risk of certain type of cancer

Too little body fat is also a problem. Your body needs a certain amount of fat to function properly. Having too little fat is a condition often associated with eating disorders, exercise addiction, and diseases like cystic fibrosis (3).

The commonly used term "body fat percentage" allows us to discuss the different levels of "fatness." Body fat percentage can be expressed as a simple equation:

Body fat % = weight of fat / total body weight

Examples:

Remember our two 300 pound individuals from above? If we were to measure their body fat percentage, we might find that the office worker is "overfat" (body fat percentage > 24) and that the NFL defensive lineman is around "average" (body fat percentage 15%)

We can't even guess about our two individuals with identical height and weight. Which is why it is beneficial to determinate body fat percentage.

Now that we understand the difference between "overweight" and "overfat" we can discuss different methods of determining body composition.

Do you need to lose weight?

How much weight should you lose?

Are your exercise and nutrition programs doing what you want?

Determining your body fat percentage allows you to intelligently answer these questions.

Now that we understand the "what and why" - we will discuss the "how."

Determining Body Fat Percentage

The most accurate way to determine how much of an individual's body weight consists of fat is to perform an autopsy. The fat can then be put on scale and accurately weighed.

Considering the obvious drawbacks of the "autopsy method" it is fortunate that many other methods are available to determine body fat percentage.

Okay, it was a bad joke - but it illustrates the fact that all of these methods are "estimating" to one degree or another.

While discussing the various methods we will distinguish between "accurate" and "reproducible" estimates. An "accurate" determination of body fat percentage (getting the "real" numbers) is not as important as a "reproducible" determination (being able to get the same result each time - and therefore measure the change between testing periods).

What we want is a method that requires no special equipment, no special training, and will give us a reproducible number that can be used to measure change.

Common methods of determining body fat percentage:

Hydrostatic Weighing

For years the "gold standard" has been hydrostatic (underwater) weighing. Since fat "floats", weighing an individual underwater can be used to determine body density. Body density is then plugged into an equation to get body fat percentage.

Although this method is highly reproducible, it is impractical for 99% of the population simply because the equipment is available. Your local college/university may have the specialized equipment required to perform hydrostatic weighing, but most health clubs/fitness centers don't.

Skinfold Measurement

The amount of fat under the skin (subcutaneous fat) is directly related to total body fat. Therefore, taking "skinfold measurements" at various sites can be used to determine body fat percentage.

The skinfold measurements can then be plugged into "population specific" (age and sex) equations to get body fat percentage.

While the "skinfold calipers" can easily be purchased, there is a certain amount of training/experience required to get "reproducible" results. (The quality of the calipers will also impact the results/)

Bioelectrical Impedance Method

With bioelectrical impedance analysis (BIA) a low-level electrical current is run through the body. Since fat is a poor electrical conductor, the current's resistance can be used to determine the "total body water." Body fat percentage can then be determined based on the "total body water."

The readily available "body fat scales" are using a form of BIA. These scales are the classic example of giving easily "reproducible" results but not particularly "accurate" results.

This doesn't mean that "body fat scales" are useless. What becomes most important (since we are most concerned with tracking change) is testing under identical circumstances.

Remember that BIA is measuring "total body water" - so an individual's hydration level will change the reading. Don't eat or drink anything within 4 hours of testing (but don't be dehydrated either).

For example:

You decide to start an exercise program wanting to lose body fat. You purchase a body fat scale, and measure yourself (and you even write the numbers down somewhere so you don't forget).

A week later you measure yourself again, but this time you drink a gallon of water before stepping on the scale.

There resulting number will tell you something about the sensitivity of your scale, but nothing useful about your body fat percentage.

(By the way, weighing yourself weekly is a good idea. Daily weighing will just show you the normal fluctuations of your weight - not whether you are gaining or losing weight.)

Air Displacement Plethysmography

Air displacement plethysmography (ADP) estimates body fat by measuring air displacement. ADP is similar in concept to hydrostatic weighing.

The individual sits in a closed chamber. Body volume is then determined by changes in pressure.

Obviously special equipment is required for ADP. If you happen to have access to a facility with the equipment, then the results are highly reproducible.

Anthropometric Methods

Anthropometry is the $20 word that boils down to "measuring the size and proportion of the human body."(4)

The most famous "anthropometric method" is the body mass index (BMI). In my opinion, BMI is just a height and weight chart converted to a single number (which makes it inappropriate for athletes and is at best a rough estimate of body fat percentage). With that in mind, BMI is easy to calculate and highly reproducible.

Calculate BMI by dividing body weight in kilograms by the square of height in meters. (A body mass index "calculator" is available at http://www.fulleffort.com/ )

Another "anthropometric method" is to simply "take your measurements" with a measuring tape. A measuring tape made of flexible material that won't stretch will give the most reproducible results. Take as many or few measurements as you like. An individual can easily take waist and hip measurements (the important parts are to take the measurements at the same locations each time, and write the numbers down to track the change).

Conclusion

The "best" (or at least the most "practical" for the average person) method to estimate your body fat percentage is either a BIA or simple Anthropometric method.

More important than the resulting number, is striving for reproducible testing, and then tracking the change in the number. Your body fat percentage is a useless number until it is compared to another number.

Next article - Determining Desired Weight

References:

1. Harrison GG, Height-weight tables. Ann Intern Med 1985; 103:489-94.

2. Heyward, VH, Applied Body Composition Assessment,1996, Human Kinetics, 2

3. Heyward, VH, Applied Body Composition Assessment,1996, Human Kinetics, 2-3

4. Heyward, VH, Applied Body Composition Assessment,1996, Human Kinetics, 66