Bioelectrical Impedance Analysis (BIA) vs. Dexa: Which is better?
March 21, 2025
BODY COMPOSITION ASSESSMENT is extremely important because BMI does not adequately assess MUSCLE MASS and BODY FAT PERCENTAGE.
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Bioelectrical impedance analysis (BIA) is a method of quantifying body composition that involves passing a very light electrical current through the body.
The examination is based on the principle that impedance or resistance to an electrical current is related to the volume and length of the conductor, in this case, the human body. Hoffler demonstrated that the amount of body water is directly proportional to the volume and length of the body and inversely related to electrical resistance in humans. Thus, body water content and fat-free mass can be predicted, and considering mineral content as a constant related to height, the fat percentage can be inferred.
Impedance can be calculated by measuring current and voltage according to Ohm's Law (V=RxI). Since water is considered the only component of our body with electrical conductivity, when current passes through the body, water impedance can be measured. With this impedance value, the total body water volume is calculated using appropriate formulas and algorithms, which in turn leads to the calculation of lean mass (muscles and bones) and fat mass (body fat).
The obtained impedance result allows for the calculation of the exact volume of water in the body, thus accurately computing the amount of lean and fat mass contained in the body. It is important to emphasize that the higher an individual's body fat percentage, the greater the difficulty for the electrical current to pass through the body.
The use of dual-energy X-ray absorptiometry (DEXA) for body composition assessment allows for the measurement of bone mass as well as body fat content and lean mass. The confirmation of the excellent accuracy and small precision error of the examination, through comparative studies with chemical analysis of animal carcasses, has made it a reference for studying body composition in humans.
Whole-body DEXA densitometry is the only method that directly assesses all body compartments (bone mass, muscle mass and water, fat mass) without inferring data from the measurement of only one compartment. In DEXA body composition analysis, body water is incorporated into the lean mass compartment (muscles), not affecting the measurement of fat or bone tissue content.
Other non-invasive methods for body composition assessment (neutron activation analysis, underwater weighing, labeled water, total body potassium, etc.) require more than one of these examinations to complement data, as they assess only one body compartment, inferring others. Furthermore, they are laborious, expensive, and not available in clinical practice, being restricted to research laboratories.
There are numerous clinical applications for body composition studies, particularly in Sports Medicine, Obesity, Anorexia, motor rehabilitation for neuromuscular diseases, and nutritional assessments. Other applications include monitoring expected changes in lean and fat mass in patients using growth hormone, corticosteroids, sex steroids, and transplant recipients on immunosuppressants.
The examination, being non-invasive, is very simple for the patient, requires no preparation, and lasts 10-15 minutes. The radiation is extremely low, similar to the ambient radiation we receive on a sunny day, and can be repeated as many times as necessary. The examination is particularly useful in children due to its practicality and provides an excellent assessment of bone mass acquisition.
The coefficients of variation (CV%) of the parameters evaluated in body composition are comparable to those obtained in lumbar spine and femur densitometry examinations, around 1 to 2%, offering good reproducibility of results. Only fat mass shows a slightly higher CV%, around 3-4%, probably related to the heterogeneity of soft tissues in obese patients.
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