Many people start the new year by focusing on their health, and a trip to the gym often includes a body composition test to assess their physical condition.

Body composition analyzers commonly found in gyms and health centers are often called “InBody,” but this is a brand name, much like Kleenex or Band-Aid. The technical term for this test is Bioelectrical Impedance Analysis (BIA), which measures body composition using electrical resistance.

BIA sends low-level electrical currents through the body at different frequencies and pathways to assess bioelectrical impedance in various tissues.

The human body contains minerals, fluids, and other components that vary in distribution depending on tissue type. Because of these differences, electrical currents pass through tissues at varying levels. Muscle, which holds more water, conducts electricity more easily, while fat, which contains less water, offers greater resistance.

Professional-grade BIA devices use eight electrodes—two for each limb—to send electrical currents through multiple pathways, providing detailed regional measurements. Home scales and smartwatches typically have only four footplate electrodes, estimate body composition by measuring only certain areas, and then extrapolating the total values.

The eight-electrode system is the most accurate BIA method. However, results can be affected by hydration levels and electrolyte balance, leading to potential discrepancies.

For example, increased muscle water content may be misinterpreted as muscle gain after a workout. Sitting for an extended period before testing may cause fluid to accumulate in the lower body, affecting regional readings. Maintaining consistent testing conditions can help track body composition changes over time despite these limitations.

More precise body composition analysis methods exist but often require specialized equipment and are more expensive.

Underwater weighing, also known as hydrostatic weighing, is one such method. This technique measures weight on land and then underwater, accounting for lung residual volume to determine body density and fat percentage. Due to its accuracy, hydrostatic weighing has long been used to validate indirect methods like BIA.

However, because this method requires full submersion and specialized equipment, it is not widely accessible. As an alternative, Dual-Energy X-ray Absorptiometry (DEXA) has become a standard for body composition analysis.

DEXA uses two energy levels of X-rays. Higher-energy X-rays penetrate deeper, while lower-energy X-rays are absorbed differently by various tissues. By analyzing these differences, DEXA provides precise fat mass, lean mass, and bone density measurements.

More recently, 3D body scanning technology has been introduced. This method utilizes cameras and infrared sensors to create a digital body model. It provides body composition measurements and external features such as circumference and posture analysis. However, it cannot measure internal fat distribution, such as visceral fat.

Many people start the new year by focusing on their health, and a trip to the gym often in …

Body Composition Analysis: What You Should Know About BIA