body fat measurement

Methods of Body Fat Measurement

The purpose of measuring body fatness is to give one an idea of how much of the body consists of fat mass (FM), and how much consists of fat free mass (FFM). Here are five ways to measure.

Body fat is expressed as a percentage of total body mass. For example, a 150 lb. woman who is 35% fat will have 52.5 lbs. of fat and 97.5 lbs. of FFM. When it comes to tracking progress of a diet and/or exercise program, knowing body fat percentage is infinitely more useful than simply measuring body weight.

Day to day fluctuations in weight are caused by variations in hydration status, gut residue, hormonal fluctuations, etc., and standard scales are not sensitive enough to track day to day changes in body fatness. Similarly, increasing dietary protein and adding exercise may cause increases in skeletal muscle mass, which can offset any decreases in fat mass, resulting in no change on the scale.

Furthermore, an increase in muscle mass that occurs faster than a decrease in fat mass will cause body weight to increase, leading one to mistakenly believe that their dieting strategy is ineffective.

Knowing body composition is critical if one is get accurate insight into their eating and exercise strategy. Finding an accurate method of body fat measurement can be confusing and difficult, so we’ll present the most common methods here.

Skinfold Measurement

body fat measurement
Measuring body fat using skinfolds is based on the theory that a certain amount of fat is stored under the skin (subcutaneously), and by measuring the thickness of folds of skin at specific sites on the body, we can mathematically estimate how much fat there is in the total body.

While this can be accurate in some people some of the time, there are several problems with this technique. First, the accuracy of the test is highly dependent on the skill of the person conducting the test. They must pinch the skin at the correct site, and they must pinch just enough skin to grab the fat underneath, and not the deeper tissue.

The equations used in this method also assume a certain amount of fat is stored internally, but this amount varies greatly between individuals. The device used to take the measurements is also important, as cheap, readily available calipers will not provide accurate or consistent readings.

Bioelectrical Impedance Analysis (BIA)

BIA devices often look very much like normal bathroom scales, but there are several more high-tech options that operate on the same principle. BIA works by sending a small electrical charge throughout the body, and then measuring the rate at which that charge travels from one point on the body to another.

Fat is a poor conductor of electricity, and slows down the rate at which the charge travels through the body. Muscle, on the other hand, has a high water content, which allows for the rapid transmission of electricity through the body.

The machine calculates these rates of transmission to estimate the amount of fat and muscle. As you might expect, because the amount of water in the body is highly variable, BIA is subject to a significant degree of variability.

To get the most accurate readings from BIA, one should ensure that they are well hydrated and all conditions should be kept as similar as possible from test to test. Machines that have more “points of contact” (hands and feet) will be far more accurate than devices that are only hand held, or that only send charges through the feet.

Hydrostatic Weighing

Until recently, hydrostatic weighing was considered the “gold standard” for body fat measurement. This method calculates body density by measuring the body mass and then comparing that mass to the weight of the body while completely submerged.

Because fat is buoyant, the more fat someone has, the less they’ll weigh underwater. This allows for the calculation of body volume, which is used to calculate body density. Once density is known, body fat percentage can be calculated via the Siri Equation: % Body Fat = (495 / Body Density) – 450.

Hydrostatic weighing is time consuming, inconvenient, and it is difficult to find a place to have it done.

Some Universities will do hydrostatic weighing for the public, and mobile hydrostatic weighing trucks are available in some areas of the country.

Air Displacement Plethysmography (BodPod)

The BodPod is a device that operates similarly to hydrostatic weighing, except that instead of measuring the amount of water displaced by the body, it measures the amount of air displaced by the body.

The subject sits in an egg-shaped enclosure, and the displaced air is measured. Once volume is known, body density can be calculated, and body fat is calculated mathematically, as is the case with hydrostatic weighing. The BodPod is highly accurate and comparable to the values obtained with hydrostatic weighing.

This technique requires the subject to wear a tight-fitting swimsuit or spandex, as well as a swim cap to smooth out the head. The process is somewhat inconvenient, but not as much as underwater weighing. BodPod testing is more readily available, and can be found in some gyms, many universities, and there are also mobile units.

Dual Energy X-ray Absorptiometry (DXA)

body fat measurement
This technique is considered by many to be the new gold standard for body composition testing. Originally designed as a means of measuring bone density, DXA is quickly becoming the preferred way to measure body fat, mainly due to its ease of use, convenience, and highly consistent results.

The test involves lying down on a device similar to an open-air MRI. A scanning arm slowly passes over the body and collects low energy x-rays that shoot up through the body from below. The x-rays absorb at different rates through different tissues, giving an graphic representation and mathematical calculation of fat in the body.

The advantages of DXA are that it can measure where the actual fat is located in the body, not just how much you have. This is especially advantageous when tracking changes in visceral fat, which is the unhealthy fat around the organs.

DXA also provides information on bone health. The test requires no special preparation, and normal street clothes can be worn during the test.

Disadvantages are that the machines can be difficult to find, but more commercial testing sites are becoming available (e.g. www.liveleanrx.com ). The test also exposes the subject to a small amount of radiation, but this is less than the amount of background radiation that one receives from the natural environment in a typical day.

While all body fat measurements have their advantages and disadvantages, the most important thing to remember when tracking body composition is to use the same method of testing from test to test, and ensure that all test conditions are kept as consistent as possible.

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Todd Miller

Dr. Todd Miller is an Associate Professor of Exercise & Nutrition Sciences at the George Washington University Milken Institute School of Public Health. He is an expert at designing eating strategies and training programs that are specifically tailored to a person’s metabolism, and is currently studying the role that strength training plays in weight management. He is a Certified Strength and Conditioning Specialist, a Tactical Strength and Conditioning Facilitator, and Fellow of the National Strength & Conditioning Association. Learn More

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