Comparing Methods of Body Composition Analysis

An analysis of your body composition can provide valuable information about your health. Various methods of body composition analysis (BCA) assess the content of fat, muscle, and bone in a person’s body. Many modern scales and even wearable devices provide body composition data, including estimates of the amounts of fat, muscle, bone, and water present.

Often, the goal of BCA is to determine whether a person has too much or too little fat in the body, putting them at risk for various conditions like cardiovascular issues, type 2 diabetes, bone loss, and heightened susceptibility to infection. BCA is also used to track the effects of weight loss and athletic training programs. In this article, we’ll examine various methods of BCA, their accuracy, and how they can support various health goals.

How is body composition assessed?

Several different methods can provide varying levels of insight into an individual’s body composition. Examples include body mass index, abdominal circumference, bioelectrical impedance analysis (BIA), water or air displacement, and dual-energy X-ray absorptiometry (DEXA). The type and accuracy of information provided by these methods varies considerably.

BMI

Body mass index (BMI) is commonly used in doctors’ offices to give providers and their patients a general indication of whether a person is under or overweight. BMI is a ratio of weight to height, calculated by dividing a person’s weight in kilograms by the square of their height in meters. A person with a BMI of less than 18.5 is considered underweight, while a BMI of 25 or more indicates overweight, and BMI of 30 or more suggests obesity.

Because it does not give information about muscle mass, fat content, or fat distribution, BMI is best used as a screening tool. If a person’s BMI falls outside the “healthy” range, it doesn’t necessarily mean that their body composition is unhealthy, but it may prompt further investigation. Because muscle is denser than fat, people with more muscle mass will have higher BMI, while those with low muscle mass will fall lower on this scale. As a result, healthy individuals can be flagged as overweight (or even obese), and some people with unhealthy amounts of fat may nonetheless fall into the “healthy” range.

Abdominal Circumference

Another method of screening for unhealthy levels of body fat is measuring abdominal circumference. This method attempts to zero in on visceral fat, which is stored around the internal organs. Unlike subcutaneous fat, which is stored just beneath the skin, visceral fat has been linked to increased risk of problems like cardiovascular disease, fatty liver disease, and type 2 diabetes. When using abdominal circumference to assess body fat, it is often expressed as a ratio to either a person’s height or their hip measurement. Abdominal circumference is considered excessive if

• It is more than 35 inches for women or 40 inches for men
• It is greater than half a person’s height
• When divided by the hip measurement, it is more than .85 in women or .9 in men

While abdominal circumference measurements can provide more insight than BMI alone, these methods do not differentiate between visceral fat, subcutaneous fat, and muscle, all of which exist around the abdomen.

Hydrostatic Weighing

Hydrostatic weighing is a more precise method of measuring overall body fat. Before advanced imaging techniques were used for BCA, hydrostatic weighing was considered the most accurate method available. The procedure involves weighing a person on land, weighing them again when completely submerged in water, and observing the amount of water their body displaces while submerged. These measurements are used to determine the body’s density, which, in turn, is used to estimate overall body fat. While this is far more accurate than relying on BMI or abdominal circumference techniques, it does not provide any information about visceral vs. subcutaneous fat.

The “Bod Pod”

Similar to hydrostatic weighing, a device known as the Bod Pod uses air displacement rather than water displacement to determine body density. Because it is a similar method, air displacement shares the limitations of water displacement methods of assessing body composition. A study examining the accuracy of Bod Pod measurements found the device to be most accurate when estimating the body fat of people who fall into the healthy BMI range and significantly overestimated body fat in underweight individuals.

Bioelectrical Impedance

Bioelectrical impedance analysis (BIA) is the method that enables devices like body fat scales, wearable fitness trackers, and other tools to deliver body composition information when in contact with the body. The device emits an electrical current and calculates estimates for a variety of measurements based on how the electrical current moves through the body and the individual’s age, height, and sex. These measurements may include not only body fat percentage but also muscle mass, bone density, water weight, and more.

While BIA is widely available, inexpensive to perform, and delivers more information than the methods described above, it is not capable of a high degree of accuracy. Studies have shown significant errors in the technology’s ability to determine individual body composition. BIA devices tend to underestimate body fat in obese subjects and overestimate fat content in lean bodies. Due to the high error rates of BIA when calculating individual body composition, the information it provides is of limited use.

DEXA

DEXA is the current gold standard of body composition analysis. This method uses low-dose X-ray imaging to generate a detailed image of the body, revealing areas of fat, bone, and muscle. With these images, it is possible to determine not only the proportions of these body components but also pinpoint their location. As a result, DEXA is the only one of these BCA technologies that can accurately differentiate between subcutaneous and visceral fat, providing important insight into body composition-related health risks. In addition to informing weight loss regimens, DEXA BCA can be a useful tool for guiding athletic training and tracking injury recovery.

When considering DEXA BCA, should I be concerned about X-ray exposure?

A DEXA scan uses a miniscule amount of radiation to create images of the body, making it one of the safest X-ray procedures you could undergo. The human body receives the amount of radiation involved in a DEXA BCA scan in a single day of exposure to the natural environment from sources like sunlight and naturally occurring radon.

What should I expect during a DEXA scan?

Dual-energy X-ray absorptiometry is a quick, simple, non-invasive procedure. You simply lie face up on the exam table, and the machine scans your body over the course of just a few minutes. The technologist will be able to discuss your results with you immediately and will also send a report to your physician.

At Iowa Radiology, we provide DEXA BCA without the need for a physician’s referral. A single scan costs just $49, and a series of three can be purchased for $139.

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Resources

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Centers for Disease Control and Prevention. Assessing Your Weight. CDC.gov. Reviewed June 3, 2022. Accessed February 29, 2024. https://www.cdc.gov/healthyweight/assessing/.

Harvard School of Public Health. Waist Size Matters. Published January 26, 2011. Harvard.edu. Accessed February 29, 2024. https://www.hsph.harvard.edu/obesity-prevention-source/obesity-definition/abdominal-obesity/.

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