What Do the Different BIA Results Mean?
Guest writer: Rob van Berkel, Research dietitian and writer on nutrition and health
What Do the Different BIA Results Mean?
Most people have a clear idea of what body weight represents. Modern BIA equipment goes further and displays a range of results. These data can be useful, but what exactly do they mean?
Classification of Body Composition
Body weight can be classified at different levels:
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Atomic level (carbon, oxygen, nitrogen, etc.)
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Molecular level (fat, protein, water, etc.)
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Cellular level (intra- and extracellular cell mass)
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Functional level (adipose tissue, skeletal muscle, bone tissue, etc.)
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Anatomical level (trunk, arms, legs, etc.)
The most commonly used classification is the molecular level, which can be subdivided into multiple compartments (see Figure 1). The most widely used models are based on two, three, or four compartments (Prado et al., 2025). These compartments can be estimated using bioelectrical impedance analysis (BIA).
Figure 1: Overview of the multi-component model of body composition (Tanita, 2025a).
What Does BIA Measure? (Resistance and Reactance)
BIA works by passing a very weak alternating electrical current through the body via electrodes. This allows resistance and reactance to be measured, from which impedance and phase angle can be calculated.
Reactance reflects the ability of cell membranes to temporarily store electrical charge, creating a delay in current flow. Importantly, body fluids conduct electrical current well and therefore provide little resistance.
Figure 2: Visual representation of impedance and phase angle based on resistance and reactance (Tanita, 2025a).
Muscle tissue contains a high proportion of water (approximately 73%) and therefore offers low resistance, whereas fat tissue contains relatively little water and provides higher resistance. These properties allow estimation of body composition, such as body fat percentage, muscle mass, and hydration status.
Modern devices may also use:
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Multiple frequencies to differentiate intracellular and extracellular water
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Segmental analysis to assess individual body parts
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Proprietary algorithms based on large reference databases with DXA comparisons
BIA Results
Depending on the device, different parameters may appear on the display. These results help analyse body composition and guide lifestyle advice.
Total Body Water (kg)
The total mass consisting of body fluids. Used to determine hydration status and fat-free mass.
Low frequencies (< 50 kHz) measure only extracellular water; higher frequencies measure both extracellular and intracellular water.
Figure 3: Difference between low and high frequency in measuring extra- and intracellular fluid (Tanita, 2025a).
Total Body Water (%)
The percentage of body weight consisting of water.
Body Fat (kg)
Total body weight minus fat-free mass.
Body Fat (%)
Percentage of body weight consisting of fat. Reference tables indicate healthy ranges. Fat distribution measurements are also recommended.
Figure 4: Overview of what is generally considered a healthy and less healthy body fat percentage for adults (Gallaghar et al., 2000; Tanita, 2025b).
Segmental Body Fat (%)
Fat percentage per body segment, providing insight into regional fat distribution and progress during interventions.
Figure 5: Example of a segmental fat analysis (Tanita, 2025b).
Visceral Fat (kg)
Fat located in the abdominal cavity around internal organs and associated with increased health risks. Estimates are based on empirical regression models and may also be expressed as an index. Even individuals with low total body fat may have elevated visceral fat.
Figure 6: Example of a visceral fat index (Tanita, 2025c).
Fat-Free Mass (kg)
Total body weight minus fat mass, calculated primarily from body water. Includes muscles, bones, organs, proteins, minerals, and fluids.
Muscle Mass (kg)
Weight of skeletal, cardiac, and smooth muscle combined, including their water content. Low values may indicate muscle loss, malnutrition, or inactivity.
Segmental Muscle Mass (kg)
Muscle mass per body region, providing insight into muscle distribution and training progress.
Figure 7: Example of a segmental muscle analysis (Tanita, 2025d).
Bone Mass (kg)
Estimated mass of bone minerals. Provides insight into skeletal health and long-term trends.
Figure 8: Estimated bone mass by sex and body weight (Tanita 2025e).
Phase Angle
The angle derived from the ratio between reactance and resistance. A higher phase angle (typically 5–7°) indicates good cellular integrity and membrane function.
A lower phase angle (< 5°) is often associated with malnutrition, disease, mortality risk, and reduced muscle quality. Across the lifespan, phase angle follows a characteristic pattern: low in infancy, increasing through adolescence, stable in adulthood, and declining in older age.
Basal Metabolic Rate
Basal metabolic rate (BMR) is the energy required at complete rest over 24 hours to maintain vital functions. Expressed in kcal/day and influenced by body weight, fat-free mass, age, sex, hormones, and temperature. BMR accounts for approximately 60–70% of total daily energy expenditure.
Points of Attention
For reliable BIA measurements:
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Ensure proper measurement technique
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Ensure adequate hydration
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Use appropriate algorithms for the individual
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Avoid heavy exercise beforehand
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Avoid alcohol or caffeine beforehand
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Measure fasted or 2–3 hours after meals
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Measure in the same posture
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Measure at the same time of day
Conclusion
BIA enables more detailed analysis of body composition than a scale and tape measure alone. Understanding the different outputs is essential for providing effective lifestyle advice tailored to clients’ goals.
References
Abdollahpour N, Emadzadeh M, Shahri AMM, Ferns GA, Ghayour-Mobarhan M, Seifi N. Association of Phase Angle Determined by Bioelectrical Impedance Analysis with Cardiovascular Risk Factors: A Systematic Review and Meta-analysis of Observational Studies. Nutr Rev. 2025;83(11):2084-2103.
Aragon AA, Schoenfeld BJ, Wildman R, et al. International society of sports nutrition position stand: diets and body composition. J Int Soc Sports Nutr. 2017;14:16. Published 2017 Jun 14.
Costa Pereira JPD, Rebouças AS, Prado CM, et al. Phase angle as a marker of muscle quality: A systematic review and meta-analysis. Clin Nutr. 2024;43(12):308-326.
Gallagher D, Heymsfield SB, Heo M, Jebb SA, Murgatroyd PR, Sakamoto Y. Healthy percentage body fat ranges: an approach for developing guidelines based on body mass index. Am J Clin Nutr. 2000;72(3):694-701.
Garlini LM, Alves FD, Ceretta LB, Perry IS, Souza GC, Clausell NO. Phase angle and mortality: a systematic review. Eur J Clin Nutr. 2019;73(4):495-508.
Mattiello R, Amaral MA, Mundstock E, Ziegelmann PK. Reference values for the phase angle of the electrical bioimpedance: Systematic review and meta-analysis involving more than 250,000 subjects. Clin Nutr. 2020;39(5):1411-1417.
Prado CM, Purcell SA, Alish C, Pereira SL, Deutz NE, Heyland DK, Goodpaster BH, Tappenden KA, Heymsfield SB. Implications of low muscle mass across the continuum of care: a narrative review. Ann Med. 2018 Dec;50(8):675-693.
Prado CM, Gonzalez MC, Norman K, et al. Methodological standards for body composition-an expert-endorsed guide for research and clinical applications: levels, models, and terminology. Am J Clin Nutr. 2025;122(2):384-391.
Tanita, 2025a https://tanita.eu/bioelectrical-impedance-analysis
Tanita, 2025b https://tanita.eu/understanding-your-measurements/body-fat-percentage
Tanita, 2025c https://tanita.eu/understanding-your-measurements/visceral-fat
Tanita, 2025d https://tanita.eu/understanding-your-measurements/muscle-mass
Tanita, 2025e https://tanita.eu/understanding-your-measurements/bone-mass
Wang Y, Luo D, Liu J, Song Y, Jiang B, Jiang H. Low skeletal muscle mass index and all-cause mortality risk in adults: A systematic review and meta-analysis of prospective cohort studies. PLoS One. 2023 Jun 7;18(6):e0286745.
Wang Z, et al. Japanese-American Differences in Visceral Adiposity and a Simplified Estimation Method for Visceral Adipose Tissue. North American Association for the Study of Obesity. Annual Meeting. Abstract 518-P. 2004.
Xia XX, Li CX, Xue XX, Chen YJ, He F, Guo HR. Association between phase angle and all-cause mortality in adults aged 18-49 years: NHANES 1999-2004. Sci Rep. 2025;15(1):2785. Published 2025 Jan 22.
