The Science Behind Body Fat Percentage: What It Means and How to Measure It

Shannon Miller

March 29, 2024

Body fat percentage is a significant metric to consider when assessing overall health and fitness. While many individuals focus solely on weight, understanding body fat percentage provides a more accurate picture of body composition and can help tailor fitness and dietary goals more effectively.

Fat plays a crucial role in the body, from energy storage to insulation. It is a “metabolic” tissue, meaning that fat is important in the production and secretion of hormones. While it serves many life-supporting functions, excess body fat can lead to negative health outcomes. There are two main types of body fat, subcutaneous fat (fat stored under the skin) and visceral fat (fat stored around internal organs). Excess body fat, in particular, visceral fat, is typically associated with an increased risk of health conditions including heart disease, type 2 diabetes, and certain cancers. There are several factors that can affect body fat levels, including diet, activity level, hormone imbalances, genetics, and age.

What is Body Fat Percentage?

Body fat percentage refers to the proportion of fat mass to total body mass. Unlike body weight alone, which doesn’t distinguish between fat, muscle, and other tissues, body fat percentage represents the amount of fat relative to the total mass of the body. It is often difficult to assess body fat percentage through observation and close to impossible to gauge using a traditional body weight scale. In fact, individuals with a healthy weight can have an unhealthy body fat percentage if their ratio of fat to lean mass is too high. The opposite can be true as well: an individual with a high total weight can have a healthy body fat percentage if they have a significant amount of lean muscle mass relative to body fat.

How is Body Fat Percentage Measured?

The only way to assess body fat percentage is through an accurate test. There are several methods to estimate body fat percentage, each with varying degrees of accuracy and convenience.

Calipers: This involves using skinfold calipers to measure the thickness of skinfolds in specific places on the body. These measurements are then used to calculate body fat percentage using standardized equations based on age, gender, and the sum of skinfold measurements. Caliper testing is one of the first methods used to estimate body fat percentage and an often not be as precise or accurate as more advanced techniques.

Bioelectrical Impedance Analysis (BIA): BIA devices send a low-level electrical current through the body and measure the resistance encountered by the current. Since fat tissue conducts electricity differently than lean tissue, BIA devices can estimate body fat percentage based on the body’s impedance to the electrical current.

Hydrostatic Weighing: Also known as underwater weighing, this method involves submerging the body in water and measuring the displaced water volume. Since fat is less dense than water and muscle is denser, body fat percentage can be calculated based on the differences in buoyancy. This method requires an individual to submerge completely under water. several variables can be sources of error within a hydrostatic test, such as an individual not expelling all of the air out of their lungs, or if the test administrator is not accurate with calculations.

Air Displacement Plethysmography (ADP): ADP, commonly known as BodPod, measures body composition by determining body volume using air displacement. Like hydrostatic weighing, it calculates body fat percentage based on body density. A BodPod is a chamber of air an individual is placed in to calculate body fat percentage. Results from a BodPod analysis are based on a series one-size-fits-all calculations, which can run a risk for a higher margin of error.

Dual-Energy X-ray Absorptiometry (DEXA): DEXA scans use low-powered X-rays to measure bone density, lean mass, and fat mass. A DEXA scan was originally used in healthcare setting to assess bone health, however, they also provide the most accurate measurements of body fat percentage available, in addition to other significant markers of health. A DEXA scan has a very low error rate of 1-2%, compared to 5-15% for most methods. This is due to the advanced X-ray technology that leaves little guesswork and no extra calculations that may be impacted by human error. The DEXA scan can also differentiate between visceral fat and subcutaneous fat, and provide a more accurate breakdown of body composition measurements among different regions of the body.

From a scientific perspective, body fat is broken down through a process called lipolysis, which breaks down triglycerides into molecules that can be used by the body for energy. When compared to energy from carbohydrates and proteins, triglycerides from fat provide twice as much energy per unit mass. However, fat is typically stored before they are used for energy, whereas carbohydrates are more metabolically accessible and more readily available for energy needs. This evolutionary adaptation was designed through nature to ensure survival when food sources were unavailable for long periods of time.

Body fat percentage is a key indicator of overall health and fitness, offering valuable insights into body composition beyond just weight. For those aiming to improve fitness or change their body composition, monitoring body fat percentage provides a more accurate measure of progress than weight alone. Increasing muscle mass while reducing body fat percentage is often the goal for improving overall health. By understanding the science behind body fat percentage and utilizing accurate measurement methods, individuals can better track their progress, set realistic fitness goals, and make informed decisions to improve their health and well-being.

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