Bioelectric impedance analysis, also known as BIA, is a quick and painless way to measure a person’s body composition. The BIA can measure and track changes in the amounts of body fluids, fat, bone mineral content, and lean body mass, which includes your muscles and organs.
The way it is done is by placing one skin electrode each on the wrist, third finger, ankle, and near the second toe. The machine is then turned on, and an electrical current goes through body and measures some values, including phase angle, resistance, and reactance.
The phase angle, which is recognized by the World Health Organization (WHO) as a general marker for tissue health. According to the National Health and Nutrition Examination Survey (NHANES), typical phase angle numbers are 4-9 with the higher number meaning the healthier the tissue, and therefore the healthier the person. The phase angle is calculated by taking the arctangent of the reactance (body’s opposition to changes in the flow of electrical current) over the resistance (the opposition to the flow of an electrical current).
When determining the body composition of an individual, other factors including height, weight, gender, and age are needed to help formulate the body composition report. The way reports are created are based off of algorithms, which were generated by testing 10,000+ people and comparing their BIA readings to dual energy X-ray absorptiometry (DEXA) scans, which is the gold standard of body composition analysis. When comparing the accuracy of the BIA values to the DEXA scans, the BIA readings are 97.5% reliable. The 2.5% discrepancy came from test subjects that were short and had considerably more amounts adipose tissue than the average person.
There are a lot of different outcomes a person can have with a BIA, but I would say the most common issue for people is dehydration. Men are supposed to have 60% of their body water, while women are supposed to have 55% of their body water (due to musculature since muscles carry more water than fat). People may be drinking enough water throughout the day, however, they may not be retaining that water proportionately inside and outside the cell. The way the body absorbs water (both through the digestive tract and kidneys) is by of a nutritional gradient (mainly salt/trace minerals like sodium). Using an example, dirt is able to hold onto more water than sand, because of all the nutrients that are found within the dirt (and sand is void of nutrients).
So even if a person is drinking enough water throughout the day, they are wetting their body rather than hydrating their body.
One possibility for sub-optimal hydration levels is that a person may not be getting enough nutrients throughout the day, and if they are, they are not absorbing and assimilating those nutrients. People who exercise excessively may be excreting more salt than they intake, which is another possibility (but less common).
Kidneys and Adrenal Glands
Another possibility deals a person’s kidneys and adrenal glands. The adrenal gland has an inner (medulla) and outer (cortex) part. The adrenal medulla is responsible for catecholamine production (epinephrine and norepinephrine) for your sympathetic nervous system response, while the adrenal cortex is responsible for making hormones in your body. The adrenal cortex has three layers, and makes its own hormones in the body. The outer most layer is the zona glomerulosa, which makes mineralocorticoid (aldosterone) which helps with water retention. The middle layer is the zona fasciculata which makes glucocorticoids (cortisol AKA the stress hormone), and the inner layer is the zona reticularis, which makes particularly DHEA, the precursor to sex hormones. So essentially what happens is that a person is pumping out more than desirable amounts of cortisol, and that shunts production of the other hormones, thus making less than desirable amounts of aldosterone and sex hormones. The shunting of aldosterone can lead to electrolyte imbalances, which in turn causes water retention imbalances.
There is a lot of other information that can be determined from the BIA, including where the water in your body is distributed. Water distribution shift (from inside to the outside of a cell) is the first sign of physiological dysfunction.
If you plan on changing your weight (either increase or decrease), we highly advise you get a body composition reading to make sure you are changing the correct and desired tissue that is needing to be changed. For example, if you are looking to lose fat and you are losing weight, there could be a situation where you are losing muscle mass instead of fat. Losing muscle mass is the last thing you want to do, because the amount of muscle mass is the number one indicator for longevity.
Dr. Eric, Johnson, Doctor of Chiropractic, Madison Wi
Bucholz AC, McGillivray CF, and Pencharz PB. Differences in resting metabolic rate between paraplegic and able-bodied subjects are explained by difference in body composition. Am J Clin Nutr. 2003 Feb; 77(2): 371-378. https://www.ncbi.nlm.nih.gov/pubmed/12540396
Chumlea WC, Guo SS, Kuczmarski RJ, Flegal KM, Johnson CL, Heymsfield SB, Lukaski HC, Friedl K, and Hubbard VS. Body composition estimates from NHANES-III bioelectrical impedance data. Int J Obes. 2002 Dec; 26(12): 1596-1609.
Costanzo LS. Physiology, 5th edition. Philadelphia, PA. Saunders Elsevier. 2014.
Gallagher D, Heymsfield SB, Heo M, Jebb SA, Murgatroyd PR, and Sakamoto Y. Healthy percentage body fat ranges: an approach for developing guidelines base on body mass index. Am J Clin Nutr. 2000 Sep; 72(3): 694-701. https://www.ncbi.nlm.nih.gov/pubmed/10966886
Giuseppe Sergi, Bussolotta M, Perini P, Calliari I, Ceccon A, Scaferla F, Bressan M, Moshini G, and Enzi G. Accuracy of bioelectric impedance analysis in estimation of extracellular space in healthy subjects and in fluid retention states. Ann Nutr Metab. 1994; 38(3): 158-165.
Halliday D, Resnick R, and Walker J. Fundamentals of physics: extended, with modern physics, 4th Ed. New York [etc.]: John Wiley & Sons, cop. 1993.
Kim J, Wang Z, Heymsfield SB, Baumgartner RN, and Gallagher D. Total-body skeletabl muscle mass: estimation by a new dual-energy X-ray absorptiometry method. Am J Clin Nutr. 2002 Aug; 76(2): 378-383.
McCarthy HD, Cole TJ, Fry T, Jebb SA, and Prentice AM. Body fat reference curves for children. Int J Obes (Lond). 2006 Apr; 30: 598-602.
Shumei SS, Chumlea WC, Heymsfield SB, Lukaski HC, Scholler D, Friedl K, Kuczmarksi RJ, Flegal KM, Johnson CL, and Hubbard VS. Development of bioelectric impedance analysis prediction equations for body composition with the use of a multicomponent model for use in epidemiologic surveys. Am J Clin Nut. 2003 Feb; 77(2): 331-340.