Bioelectrical Impedance Analysis (BIA) Body Fat Testing

How Body Fat Scales Work

man pinching stomach fat
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Bioelectrical impedance analysis (BIA) is a method of measuring body fat percent by sending a low-level electrical current through the body. It is one simple method of determining your body composition and body fat percentage. BIA is used in hand-held devices or digital body fat scales you can use at home.

How Bioelectrical Impedance Analysis Works

A BIA device needs two points of body contact. This can be both of your feet, as with a body fat scale, or your two hands with a hand-held device. Some devices use both hand grips and foot contact. A very low-level, safe electrical current is sent through metal electrodes that are in contact with your skin. The current travels through your body and returns to the electrodes, where the strength of the returning signal is measured.

Electrical current travels at a different rate through the various body tissues. The current passes easily through muscle tissue (which contains a large amount of fluid), but it travels slowly as it passes through fat tissue. The resistance encountered as the current goes through the fat tissue is called bioelectrical impedance. The rate at which the electrical current travels through your body is used to estimate the amount of fat-free mass you have. It does this by also taking in consideration how tall you are, your gender, and weight measurements. These calculations determine your body fat percentage.

Although the current is safe for most people, BIA devices should not be used by anyone with an electronic medical implant, such as a heart pacemaker. Manufacturers also recommend their devices not be used by pregnant women.

Types of BIA Devices

There are a variety of BIA devices available, and all use the same basic principles to determine body fat. Some of the more popular brands include Omron, Tanita, Fitbit, and HealthTools EatSmart. Some body fat scales transmit their data to mobile apps so you can track any changes easily.

Some devices use two or more frequencies to measure the impedance and advanced algorithms to determine body fat and water content. Advanced designs use both hand and foot contacts and segmental analysis of the body. This is believed to be more accurate as the hand-held models mostly detect the composition of the arms and upper chest, while scales with only foot contact mostly measure the composition of the lower body.

Are Measurements Accurate?

For the most part, bioelectrical impedance analysis has proven to be a fairly accurate method for estimating body fat in a laboratory setting. However, the scales that are in your bathroom are usually not what’s being used in research studies. Accuracy of the measurement has a lot to do with the quality of the product as well as other factors.

The readings can be affected by hydration levels, food intake, skin temperature, and other factors, like weight. Ironically, there is some proof that BIA isn't accurate in obese people as well as very athletic ones. Apparently, when your body composition leans one way or another, it may throw off BIA results.

What you have to remember is that these are estimates, not absolutes—especially if you are using a BIA scale. Even though the actual number may not be perfect, it can still enable you to track changes to your body composition with diet or exercise.

The key to getting a BIA scale to work for you is to follow the directions and take the reading under similar conditions each time. If you track your readings, then you can see if what you are doing is making a difference toward achieving your body composition goals.

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