Fitness Health

Are Bioimpedance Scales Accurate?

For most, a bathroom scale was once considered an analog relic that cluttered the floor most of the year and gave you anxiety after the holidays. Even for those that used their scales regularly, knowing your overall body weight was enough to track your diet and exercise routines. However, then came the digital scale and a plethora of information and monitoring options. In combination with advances in sports and nutritional science, knowing your body composition, especially body fat percentage, has become essential to tailoring your diet and exercise. The question is, are we getting good data from our scales, or are they misleading us?

Bioimpedance scales use a weak electric current to measure your body’s voltage to provide a bioelectrical impedance analysis (BIA). While such scales measure overall weight accurately, issues arise with data quality for another critical metric – body fat percentage (BFP). You can expect your impedance scale to report your BFP with a 10% error, and its reading depends on hydration, voiding your bladder, mealtimes, and recent exercise. However, bioimpedance scales are helpful tools for most users to see the trends in BFP and determine if their habits are meeting their health goals.

Using bioimpedance scales is a quick, easy, and inexpensive method to see how your body composition and, thus, overall health are trending over time. However, be cautious as to the inherent inaccuracies in the data collection method and use a secondary validation technique if the results are not as you would expect based on your health routine.

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How Do Impedance Scales Work?

Impedance, an electrical term, measures resistance in a circuit. When you stand on the scale, a small, harmless, weak electrical current (~50 KHz) is sent through your body – typically up one leg and down the other [1]. The time it takes for this current to make the journey is measured and converted into fat-free mass (muscle, bone, etc.) and fat mass through equations that are modified for a person’s gender and height. By simple subtraction, your body fat percentage (BFP) is calculated. 

Such readings are possible because electricity flows through water with little resistance; therefore, the more water your body has, the less resistance. Muscle is ~75% water, whereas fat is ~10%; consequently, it correlates that the lower the impedance (or, if you prefer, higher the resistance), the greater your muscle mass.

How Accurate is the Data?

Body composition is complex, with water, muscle, fat, bones, cells, organs, etc. Consequently, a simple reading of resistance cannot accurately determine the percentage of each component in your body. In fact, impedance measures the body’s density, which must be converted.

The equations that transform density to fat-free mass are not specific to each individual. For example, there is no input to account for ethnicity or body shape. Therefore, results are merely an estimate based on population averages.

A significant issue with bioimpedance scales is how they gather data. By sending the current up one leg and down the other, the body’s trunk isn’t considered. Instead, assumptions of your composition are based on that data sampling and your total weight. Therefore, if you work your legs hard at the gym, they could be more athletic than the rest of your body, meaning you might have a higher BFP than your scale reports.

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What Affects Accuracy?

Hydration is an essential factor in getting an accurate readout. If you are dehydrated, bioimpedance measurements may underestimate your muscle content. Studies have shown that dehydration can cause a deviation in the absolute value by 10 lbs, whereas drinking a large volume or measuring before urination will increase your muscle mass. 

As a user, you can help yourself get the most out of your bioimpedance scale by using your scale consistently to avoid fluctuations in readings. Measuring yourself at the same time of day, before eating, and after voiding your bladder is the best set of circumstances to obtain consistency [2]. 

Your fat-free mass will also seemingly increase after exercise, particularly after prolonged exercise (over an hour). Body temperature will also affect your readings, so try to do it before showering and in a room with a reasonably controlled climate – that is, don’t measure yourself in the sunroom!

During the day, your weight and body fat will change up to 4.2%, and that shift can be staggering for even the most casual user. However, do not confuse consistency with accuracy. Getting similar readings day after day doesn’t mean your scale is accurate; instead, it’s reliable. Some programs encourage users to weigh in each day, but this can be discouraging. The best practice is to choose a set time once a week. That way, you will see only the trend and not frightening daily/environmental fluctuations [3].

It’s important to understand that the accuracy of the BFP changes as you exercise and lose weight. Impedance scales will underestimate your body fat in the obese, but equally, they overestimate your fat percentage if you are a lean individual. If you are somewhere in the middle, it can be a guess as to which direction the scale is estimating. 

Focus on trends, not the absolute number.

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Should I Use an Impedance Scale?

With the potential inaccuracies with the readings, you might wonder if it’s worth using an impedance scale. For most people, the answer is yes! While your scale may not be accurate, the measurement trend will be. Therefore, you can track the success of your nutrition and exercise regime to meet your goals. For most, an estimate is more than enough to see if your BFP is decreasing. Do not compare BFP with other individuals unless you’re using the exact same scale to prevent being discouraged. Use bioimpedance only to monitor your progress, and you’ll likely be satisfied with your scale.

 If you are on a quest from being obese to becoming a healthy weight, then be aware of the limitations of the device you are using. Think about your resistance training and overall strength, and rationalize if the scale readings make sense. They might not. If you’ve been eating better and exercising and your fat mass and muscle mass have remained the same, you can doubt your instrument.

If you want to see your progress (and gauge how your scales are performing), get a professional to measure your BFP with fat calipers. It’s an accurate and reasonably inexpensive method of measuring BFP that can be done once in a while. 

The accuracy of impedance sales will increase with its price up to around $100. The difference is typically in the quality of the components, enabling the sending of a constant current to make the measurement and the voltage readers at the other end. Having said that, many people are happy with impedance scales around the $30 mark. I personally have the RENPHO smart scale, and it’s served me well for years (it’s frequently on sale, too!).

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Final Thoughts

Bioimpedance scales are a helpful tool in any fitness journey. However, if you have specific goals in mind, especially if you are lean and bulking up, you should be aware of inherent inaccuracies with the bioimpedance method. Physiologically, we are all different, and the equations used to convert impedance to fat-free mass (thus calculating BFP) are generalized. Consequently, measurements can be up to 10% away from the actual value. 

For most people, who use a bioimpedance scale as part of their weight loss program, the drift in data accuracy does not matter. Seeing BFP decrease as you work through your program is sufficiently motivating and a good indicator that your nutrition is creating a healthier body composition. 


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