Pathophysiology
P-II-27. Body composition monitor (bioimpedance spectroscopy)
バイオインピーダンス分光法による体組成計
Principle of Bioelectrical Impedance Analysis (BIA)
- Method for estimating body composition, especially body fat + muscle mass
- A weak electric current flows through the body; voltage is measured to calculate impedance (resistance)
- Most body water is stored in muscle → more muscular person → more body water → lower impedance
Parameters Determined
- BMI / body weight / WHR
- FM — fat mass (kg)
- PBF — percentage of body fat (%)
- FFM — fat-free mass (kg) (FM + FFM = body weight)
- VFA — visceral fat area (optimal < 100 cm²)
- SMM — skeletal muscle mass (kg)
- TBW — total body water
- BMR — basal metabolic rate
- Protein + mineral content
一問一答
▶Why does a more muscular person have lower impedance?
Muscle stores most of the body water, and water conducts current well, so higher muscle/water content lowers impedance.
▶What is the principle of bioelectrical impedance analysis (BIA)?
A weak electric current is passed through the body and the resulting voltage/impedance is measured to estimate body composition.
▶What body compartments does BIA mainly estimate?
Body fat mass and muscle (fat-free) mass, along with body water.
▶What does FFM stand for, and how does it relate to body weight?
Fat-free mass; body weight = fat mass (FM) + fat-free mass (FFM).
▶What does PBF mean in body composition analysis?
Percentage of body fat (fat mass expressed as a percentage of total body weight).
▶What is visceral fat area (VFA), and what is its optimal value?
The cross-sectional area of abdominal visceral fat; optimal is less than 100 cm².
▶What does SMM measure in BIA?
Skeletal muscle mass in kilograms.
▶What does TBW represent in body composition analysis?
Total body water.
▶What does BMR mean, and why is it reported by BIA devices?
Basal metabolic rate — the energy used at rest; it helps guide nutritional and weight management.
▶Why is body water the key conductor measured in BIA?
Electrolyte-rich body water conducts the current, so its amount (mostly in muscle) determines the measured impedance.
▶How does fat tissue affect impedance compared with muscle?
Fat contains little water and conducts poorly, so a higher fat proportion increases impedance.
▶Why is a high visceral fat area clinically concerning?
Excess visceral fat is associated with increased metabolic and cardiovascular disease risk.
▶Besides fat and muscle, what other components can a BIA device estimate?
Protein and mineral content of the body.
▶Why does hydration status affect BIA accuracy?
Because BIA estimates composition from body water; over- or dehydration alters impedance and distorts the results.
▶How does BIA improve on simple BMI assessment?
It separately quantifies fat, muscle, and water rather than just relating weight to height.
▶What does FM stand for in body composition analysis?
Fat mass, expressed in kilograms.
▶Why is BIA considered a practical clinical tool?
It is quick, non-invasive, and provides multiple body-composition parameters from a simple impedance measurement.
▶Which standard anthropometric indices can a BIA device also report?
BMI, body weight, and waist–hip ratio (WHR).
▶Why must fat mass and fat-free mass add up to total body weight?
Together they represent all body tissue, so by definition FM + FFM equals total body weight.
▶What does 'impedance' mean in the context of BIA?
The body's resistance to the flow of the applied electrical current.