Updated 14 April 2015

How accurate is BIA in gyms to measure body fat?

This week DietDoc discusses the pros and cons of bioelectrical impedance analysis (BIA) to determine 'fatness'.


In the first part of our discussion of methods to determining “fatness”, we considered the pros and cons of the BMI (body mass index). Last week we discussed other techniques like the waist circumference and the waist: hip ratio, as well as the new ABSI calculation and the use of the Deurenberg equation (formula).

This week we investigate the accuracy of bioelectrical impedance analysis (BIA) for measuring percentage body fat and predicting a patient’s risk of cardiovascular and other diseases of lifestyle linked to obesity.

A popular method

Bioelectrical impedance analysis is particularly popular in fitness centres and gyms. Many distressed members of the public have contacted me because their gym instructor put them on a machine or scale that produced sky-high body fat percentage readings. “What must I do? I only weigh 40kg (height 1.7m, BMI = 13,8 which is just about anorexic!) and I’ve got a body fat percentage of 22%?” In this case it is clear that something is not right at the gym: either the electrical bioimpedance analysis instrument is not working properly or the person doing the readings is not interpreting them properly!

What is BIA?

According to Willett and co-workers (2006) BIA or bioelectrical impedance analysis involves passing a weak current across the extremities (hands and feet) to measure body composition. BIA is simple, easy to use, and can in theory differentiate between fat mass and fat-free mass. As mentioned before, measures like the BMI and WC and formulas based on these measurements cannot differentiate between fat and muscle tissue in the body.

Read: Body fat tied to heart risks in normal-weight adults

From a scientific point of view, most devices used for BIA will measure the difference in impedance (the amount of opposition an electrical current encounters when it passes through a liquid or a solid) in the body quite efficiently. Lean muscle tissue has a lower impedance than fat tissue. Equations or formulas programmed into the instrument will use the readings to calculate fat mass or percentage body fat.

So although most people may find the physics behind BIA difficult to understand, they probably like the idea of stepping on a machine which calculates their body fat percentage in a flash.

Is BIA reliable?

Willett and a team of co-workers at the Harvard School of Medicine (2006) carried out a large study to answer this question because they noticed that although most of the machines used to measure BIA are able to measure differences in impedance accurately, the calculations they carry out are still based on formulas that may not be 100 percent reliable.

Willett and co-workers (2006) used more than 12,000 men and women who had taken part in the Third National Health and Nutrition Examination Survey (NHANES III) conducted in the USA to compare measurements of body fat obtained with BIA to results obtained with the calculation of BMI. The researchers also compared how accurately the BIA and the BMI are able to predict “disease or biological markers” (e.g. hypertension, increased blood sugar levels, and raised blood cholesterol, and other fat levels usually linked to obesity and other diseases of lifestyle).

Results of the study

The results of this study showed that while the BMI consistently gave good results and predicted all the biological markers, the BIA results were not as accurate, no matter which formula was used to do the calculations.

The authors concluded that “the results suggest that BIA is not superior to BMI as a predictor of overall adiposity [body fatness] in a general population”, and that “BMI is a better, cheaper, and easier measure of adiposity [body fatness] than BIA”. Other researchers have also found that bioelectrical impedance measurements are not necessarily more accurate than working out the BMI based on the height and weight of the individual. In adolescents where the use of the BMI is not recommended, studies of the BIA were also not satisfactory and tended to underestimate the body fat percentage of the young subjects.

Read: High BMI bad for brain

These studies indicate that for the general adult population, the relatively simple and inexpensive method of working out the BMI can produce results which are equal to, or even better, than the results of bioelectrical impedance analysis.

Body fat in athletes

Many strength athletes may have BMI values exceeding the upper value of the normal range of 25, because their bodies contain such large volumes of muscle tissue. The most reliable method of measuring body fat in highly trained athletes is to measure skinfold thickness and compare the average of readings taken at 6 to 8 different places on the body to various available scales like the “Oz-scale or O-scale”. Such measurements and interpretations should be done by a trained expert like your dietician or a biokineticist. 

Body fat in pregnancy and lactation

The weight and body fat of pregnant and breastfeeding mothers should be assessed by their medical doctors or dieticians and not by untrained staff. No pregnant mother should be told that she is “fat” by anyone, particularly not by people who are not properly trained, because weight loss programmes, intensive exercise regimens and the use of slimming pills and potions during pregnancy can be dangerous to both the developing baby and the mother. Moms who are gaining weight too quickly should be carefully monitored by a registered dietician who will provide an eating plan which prevents excessive weight gain, but still provides all the important nutrients that mothers and babies need to develop normally and remain healthy.

Body fat in children and teenagers

Adolescents and children should be assessed for overweight and body fat percentage by a team consisting of a medical doctor and a dietician. Considering that in 80% of cases overweight during adolescence tends to carry on into adulthood and that overweight is generally linked to diseases that cause death and disability, accurate measures of obesity in young people are important. Skinfold measurements are generally the most accurate method in everyday use when measuring overweight or obesity in youngsters.

Let your dietician or doctor do the measurements and interpret the results for your child or teenager. The dietician will also provide a diet for your child or teenager that will help the child lose weight without depressing normal growth.


There are a variety of methods available to determine how fat someone is. The most accurate, but also expensive methods involve underwater weighing or dual energy X-ray absorptiometry (DEXA). These are however rarely used except in scientific studies.

Probably the most widely used methods are calculating the BMI and measuring the waist circumference. The new ABSI calculation also appears promising.

Read: Waist size = heart health

BIA or bioelectrical impedance analysis is popular particularly at fitness clubs and gyms, but this method has its shortcomings.

Measurements of skinfold thickness are recommended for special groups such as athletes, pregnant and breastfeeding women, children and adolescents.

If in doubt about your weight or body fatness, go for an assessment by a registered dietician. Contact the Association for Dietetics in SA (ADSA) at: www. and click on “Find a Dietician” to contact a dietician in your area.

Read more:
How accurate is the BMI?
Forget BMI, these are better ways to measure your body fat
BMI can predict heart disease



- Burke L & Deakin V (2002). Clinical Sports Nutrition, 2nd Ed., The McGraw-Hill Co, Australia
- Faria FR et al (2013). Body fat equations and electrical bioimpedance values in prediction of cardiovascular risk factors in eutrophic and overweight adolescents. International Journal of Endocrinology, Vol 2013: Article ID 501638, 10 pages
- Trippo U et al (2004). Bioelectrical impedance analysis for predicting body composition. American Journal of Clinical Nutrition, 79(2):335-336
- Willett K et al (2006). Comparison of bioelectrical impedance and BMI in predicting obesity-related medical conditions. Obesity, 14(3):480-90.

Image: Fat man hiding behind scale from Shutterstock

Dr Ingrid van Heerden is a registered dietician and holds a doctoral degree in Nutrition and Biochemistry. She believes that "we are what we eat" and offers free nutrition and weight loss advice via her DietDoc service on Read more of her articles.


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