Using BIA to estimate body fat (CES2015)

The International Consumer Electronics Show or CES is the premiere electronics and technology trade show. It is held in Las Vegas in early January and boasts the largest and most glamorous array of tech companies showing off their wares including Sony, Dell, HP, Samsung, etc. Health technology is a rapidly growing segment of the show, and this year I went to CES 2015 to check out some of the new devices in this space (and gamble a little bit).

There were many interesting products and demonstrations. I plan to make two or three blog posts on what I learned. First up was a device that uses a somewhat older technology, BIA, to estimate body fat. However, adoption of this type of device seems to be increasing despite being around for awhile.

According to Wikipedia, Bioelectrical Impedance Analysis or BIA "determines the electrical impedance, or opposition to the flow of an electric current through body tissues which can then be used to calculate an estimate of total body water (TBW). TBW can be used to estimate fat-free body mass and, by difference with body weight, body fat." Let's break it down.

There are two basic equations (link). First, your total body mass consists of the fat-free mass (FFM) and the fat mass (Fat) i.e. conservation of mass:

(1) body mass = FFM + Fat

Second, there is an interesting relationship between the amount of fat-free mass (FFM) and the total body water (TBW) i.e. the total water content of your body. It has been found in a number of mammals that the amount of fat-free mass is proportional to the TBW. This relationship arises from the fact that FFM contains significantly more water than fat, and that the more fat in an individual, the less hydrated is that fat so that the total amount of water in fatty tissue is relatively constant despite variations in the amount of fat. The proportionality fraction is approximately 0.73 so that TBW ~ 0.73 x FFM. Thus combining this equation with Eq (1), we obtain an estimate of fat based on TBW:

(2) Fat = body mass – TBW/0.73

This is where BIA comes into play; it can be used to estimate TBW. As described in the Wikipedia excerpt above, it uses current (at different frequencies) to calculate the impedance in a biological object. If the body is modeled as a cylinder filled with water as well as nonconducting matter (e.g. lipid molecules), then there is a relationship between the volume of body water V, impedance Z, and the length of the body L. The impedance is a measure of "resistance" to an electrical current. The water (e.g. blood) contains various ions (electrolytes) that allow it to conduct electrical current and so the amount of water is inversely related to impedance resulting in the following equation:

(3) V = k * L²/Z

where k is some proportionality constant. A BIA instrument measures Z by placing electrodes near the top (e.g. outstretched hands) and bottom (e.g. feet) of the body and then running a small current through the body.

At CES2015, I encountered a sales representative for the company InBody, which manufactures a BIA machine for mainly medical (hospital) customers advertising it as an "analyzer of body composition." She convinced me to give the InBody a try (Figure 1).

Figure 1. The InBody body composition analyzer (right) and personal analysis sheet (left).

I took off my shoes and socks and cleaned the palms and soles of my feet. After giving information about my height and age, I gripped the electrode handles with my hands and placed my feet on to a second set of electrodes which is on a platform that also measured my weight. It is important to maintain a proper posture, and the whole BIA procedure (measuring impedance) took several minutes and I didn't feel a thing.

Figure 2. A subject whose body composition is being measured by InBody; it was not painful in any way.

After I stepped off the machine and put my shoes and socks back on, InBody printed out my analysis sheet (Figure 3). My BMI (21.2) was calculated from my weight (143.4 lb, measured) and height (I provided). InBody measured my total body water (86.4 lbs), my lean body mass (fat-free mass, FFM = 117.5 which is approximately 86.4/0.73), my body fat mass (26 = 143.4 - 117.5) and even my skeletal muscle mass (66.1). I believe that the skeletal muscle mass was estimated using a technique related to BIA called Electrical Impedance Myography.

Figure 3. My InBody body composition analysis sheet.

My percent body fat is 26/143.4 = 18.1%. For comparison I looked up a chart from Wikipedia which shows body fat percentage in men and women for different body categories.

Figure 4. Body fat percentages in women and men for 5 body description categories.

So how accurate is BIA when measuring your FFM and percent body fat? According to Wikipedia,

"BIA is considered reasonably accurate for measuring groups, or for tracking body composition in an individual over a period of time, but is not considered sufficiently accurate for recording of single measurements of individuals.

In particular your state of hydration, and whether you have eaten recently and body position which is why the device ensures a relatively fixed position of standing with arms out."

In other words, there is some variability in individual measurements because of differences in hydration state, body position, and whether you have recently eaten. If these variables are carefully controlled then the result is more accurate.  The reason for these fluctuations is that "in BIA, current travels the path of least resistance, and thus any factors that alter the current path will cause variability in the data. For example, the expansion of large vessels (e.g., veins) with increasing hydration will offer a low-resistance path, and thus distorting the resulting data. In addition, changes in abdominal contents will similarly alter the data. Body position can also have substantial effects, with joint position contributing to variations in the data."

In summary, although the technology is not new, BIA offers a convenient and potentially accurate method for measuring body fat content. Improvements in convenience, portability, and price may allow manufacturers to sell their products not only in the medical market, but also in the consumer market. In a future post I will describe a more portable device that uses both BIA and EIM.