Bioelectrical Impedance Analysis (BIA)

Bioelectrical impedance analysis (BIA) is a method for measuring body composition based on the rate at which an electrical current travels through the body. Body fat (adipose tissue) causes greater resistance (impedance) than fat-free mass and slows the rate at which the current travels. BIA scales estimate body fat percent using bioelectrical impedance analysis.

You've probably seen body fat scales on store shelves or online that use bioelectrical impedance analysis. Since the scales can be expensive, you've probably wondered what is bioelectrical impedance analysis and is it worth paying for?

BIA Definition

While the term "bioelectrical impedance analysis" sounds fairly complicated, BIA devices use straightforward technology. BIA measures the rate at which a painless low-level electrical current travels through your body. Based on that rate, a calculation is used to estimate fat-free mass. The device then uses other data such as your height, your gender, and weight measurements to determine your body fat percent.

Different Types of BIA Devices

There are different types of BIA devices, but each device requires two points of contact. On a handheld device the two points are your two hands (called hand-hand BIA). On a typical BIA scale, the two points of contact are your two feet (called foot-foot bioelectrical impedance analysis). This means that when you use the device, you place each foot on a pad and the current travels through your body between your feet. There are also hand-to-foot BIA devices, as well.

There are many brands that make different types of BIA scales (also called bioimpedance scales), including Omron, Tanita, Fitbit, Polar, and Homedics. Many of the newer models link with a smartphone app so you can track your progress over time.

The price of your BIA scale will depend on how sophisticated the product is. Some scales use more than one frequency and more advanced algorithms to provide a result. And some provide segmental fat analysis—meaning that you can get body fat measurements for each leg, each arm, and the belly.

Some say that segmental fat analysis (using hand-foot BIA) is more accurate because hand-hand devices primarily measure the upper body, while foot-foot scales primarily measure the lower body.

Some studies published in 2015 showed that bioelectrical impedance analysis is a fairly accurate method for estimating body fat. But these research studies generally do not test the scales you find in the store. And experts generally agree that the accuracy of the measurement depends, in part, on the quality of the device.

In addition, there are other factors that may affect a reading when you use a BIA scale. 

• Body weight. Bioelectrical impedance analysis may be less accurate in obese people.
• Hydration level. Dehydration may cause fat-free mass (muscle and bone) to be underestimated.
• Recent exercise activity. High-intensity exercise may affect the accuracy of BIA readings.
• Training load. Some scales have a special setting for athletes who train more often. The settings are intended to increase accuracy.
• Recent food or drink intake. The results of a 2015 study suggest that BIA is more accurate after overnight fasting.

Some researchers in 2008 also say that ethnicity and environmental factors (like skin temperature) can affect the accuracy of BIA measurements.

Even if you get an accurate reading on a bioimpedance scale, the number represents an estimate of your total body fat percentage. Bioelectrical impedance analysis does not provide an exact measurement of your total body fat. Most scales also cannot tell you where fat is located on your body.

So is it worth it to buy a scale that uses bioelectrical impedance? For many people, it might be.

Additionally, if you use a fitness tracker by brands like Fitbit, you can get a scale to pair with the device and track all of your body metrics in one place. The Fitbit Aria 2 wi-fi smart scale that measures weight, body fat, and body mass index. The numbers seamlessly sync with the Fitbit dashboard so you can see how the numbers develop over time along with daily activity and diet changes. 

A Word From Verywell

Since many BIA scales offer several features for a reasonable cost and because it is a quick and easy way to estimate body fat percent, body fat scales that use bioelectrical impedance analysis are a worthwhile investment for many consumers.