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  • Writer's pictureConrad Goodhew

Body fat % is not a measure of performance

Updated: Apr 22, 2021

Body composition is a measure of the proportion of bone, muscle, tissue and water in the body. When measuring body composition, the 2 compartments normally looked at are;

  • Fat Mass (FM)

  • Fat-free mass (FFM) - everything the body is made up of (excluding fat).

To access these components of body composition we use anthropometry. The main measurements taken are height, weight, body mass index, waist/hip/limb circumference and skinfold thickness.

Body composition is normally used in the sporting world. Athletes may have their body composition assessed for a variety of reasons which include:

  • To maintain or achieve body composition goals for their particular sport.

  • To assess progress/ effectiveness of training/diet.

  • To assess health and injury risk.

Anthropometry can be a useful tool for these reasons. However, a study looking at professional rugby players has found many experience body dissatisfactions and have reported disordered eating (1). Anthropometry becomes an issue when too much pressure is placed on an individual to reach a certain measurement without taking into consideration their discipline/ position, ethnicity and how they feel. So, let’s review different methods and how we should use this information.

Methods of assessment

There are many ways in which to measure body composition, all vary in reliability, validity, cost, and ease of use.

Dual-Energy Xray Absorptiometry (DXA) Scans:

A DXA scan works by passing two low dose x-ray beams at different energy levels through your body. This can then determine how much of your body is muscle, fat, bone or water (including where it is located). This is often used in a clinical setting to determine bone mineral density.

Although DXA scans are a popular tool to measure body composition, the technology may not be as accurate as you’d think. There is room for improvement and more research is required to determine its accuracy compared to other methods (2). Using a DXA scan is expensive, making it hard to access and can be impractical to perform for many people.

Bioelectrical Impedance Analysis (BIA)

BIA is becoming a very popular approach to measure body composition. This technique sends an electric current through your body. Fat mass creates a greater electrical resistance (impedance) than fat-free mass, which slows the movement of these currents. This will then estimate your fat-free mass. Height, weight and gender are then used to calculate body fat, and its percentage.

There are many different types of BIA with differing levels of accuracy (3). Even though BIA studies are dated, a large variation of accuracy has been shown in the current literature (up to 7.5kg in fat mass in one study and 8.4% body fat in another) (8, 9). It is important to understand that factors such as hydration, recent exercise, and gastric contents (food in your tum) at testing can all significantly impact the accuracy of these readings.

Skin Fold

Measuring body composition by skinfold measurements is the most common method in high-performance sport. Skinfold measurements are completed by pinching the skin with a pair of callipers in certain locations on the body. The measurement gained from the callipers estimates the thickness of subcutaneous fat under the skin, from here equations are used to give an estimate of body fat percentage. Although skinfold testing is inexpensive, fast, and has high reliability if the measurer is experienced, it also has downsides that need to be considered.

This method relies on the measurer remaining consistent over time, which due to human error can be difficult. However, it is achievable, if they are well trained and experienced. ISAK (The International Society of the Advancement of Kinanthropometry) has developed international standards for measuring anthropometry and is a reliable qualification [CG2] [LT3] to look out for. This makes sure that the same sites and techniques are used every time. This is crucial, as even measuring just 1 cm out from the first site gives significant differences in the body composition calculations (5).

It comes as no surprise then, that the body fat % variance can be up to 3-7% when calculating it from skinfolds (6). This could be due to the skinfold measures themselves or the variance in the calculations used, or a combination of the two. This is why practitioner training is crucial for accurate results. More on % body fat below.

Bod Pod

A bod pod is an egg-shaped device that you sit inside of. It uses air displacement plethysmograph to measure FM and FFM. This is relatively new technology with little research so we won’t be covering it in any more detail.

Under Water Weighing/ Hydrodensitometry

Underwater weight is measured while fully submerged in water and expelling all air inside of the lungs. As bone and muscle have a greater density than water and fat has a lower density than water someone with larger amounts of FFM will weigh more while in water. Underwater weight, out of water weight and the density of the water and residual volume of the lungs is used to calculate body density and estimate FM and FFM. This technique may be the most accurate, however, it is very unpractical with few places offering the service.

What method is best?

Researchers are still comparing these methods to determine which is the most accurate. It is hard to say as each comes with its pros and cons, and many factors that could be influencing the accuracy.

Should we be focused on body fat percentage?

Short answer, No. So now to explain what body fat percentage actually is. When using skinfolds, the sum of up to 8 skinfold measurements is used in specific equations to give the best estimate of your current composition (in mm). BIA and DXA scans, also use equations based on the data from the scans. Body fat % is used to understand the composition of your body to then make predictions about any changes and how this might be influencing your training and performance.

To start, body fat plays an important role in keeping our bodies functioning and well. Women, in particular, need more body fat. As a minimum, ~22% is needed to maintain reproductive ability while in mature women ~26-28% body fat is required to maintain ovulatory cycles (7). Body fat is also a key source of energy and important hormones. So, we don’t necessarily want to be encouraging the loss of body fat.

As mentioned earlier it is important to keep in mind that these calculations are estimates only. There can be a lot of variance with: a) the skinfold/body composition measurements themselves and b) the equations used. Therefore, your body fat percentage is probably not that accurate.

It can be daunting reading into how much body fat you have or being classified into a certain body type. There is too much pressure being put on athletes and now the general population to be a certain skin fold or body fat percentage. Although this is drastically changing in professional sport, it, unfortunately, is lagging with age group athletes and in the general population.

This is why I urge you not to worry about what the numbers say. So, for anyone who may be measuring someone’s body composition, it is incredibly important to ask the participant about their view of their body image and weight.

When discussing body fat percentage, it is important that is not only 1, probably inaccurate, but also 2, it could trigger or lead to other psychological issues in individuals that may already be feeling insecure.

To get your body composition measured or not?

Getting skinfolds or a scan done, can help to track changes made throughout training. These measurements can be good for creating a discussion, and possibly determine if any weight change is predominately muscle gain or if any weight lost is predominately fat loss. Many coaches and health professionals working with athletes may want to measure body comp for an indication of how their support is working for their athlete. Then, some of you just like having the numbers. That is great, but this is definitely not the only way to measure your progress.

It is important to focus on how you and your body are feeling during your training first. Have you progressed with your training? Are you hitting your performance targets? Are you getting sick or injured often? Is your menstrual cycle regular? These questions are far more important than any number.

However, if you are getting body composition measured, remember the following:

  • There is some error with whatever method of measuring you decide to use. Your results are a good guide, but they should not be taken as a be all and end-all.

  • The time of day, what you have had to eat and drink can affect your results. It is best to measure at the same time on every occasion and consider eating similar foods prior to being measured.

  • For those measuring body composition, it is important to know what the person's perception of their body is. If they have body image issues, we advise caution with the language you use.

  • Changing your diet to alter your body composition may harm your health and performance. It is important to ask yourself the above questions before doing so and work closely with your coach or Registered Nutritionist/Dietitian.

Everyone is very unique, aiming for that certain number may not actually increase your performance, be sure to check in with yourself and how you’re feeling too. This will probably improve your performance beyond any number.


1. Gibson, C., Hindle, C., McLay-Cooke, R., Slater, J., Brown, R., Smith, B., Baker, D., Healey, P., & Black, K. Body image among elite rugby union players. The Journal of Strength & Conditioning Research. 2019;33(8):2217-2222.

2. Gomes AC, et al. Body composition assessment in athletes: Comparison of a novel ultrasound technique to traditional skinfold measures and criterion DXA measure. Journal of Science and Medicine in Sport. 2020;23(11):1006-1010.

3. Demura, S & Sato, S. Comparisons of accuracy of estimating percent body fat by four bioelectrical impedance devices with different frequency and induction system of electrical current. The Journal of Sports Medicine and Physical Fitness 2015;55(1-2):68-75.

4. Moon J. Body composition in athletes and sports nutrition: an examination of the bioimpedance analysis technique. European Journal of Clinical Nutrition. 2013;67:S54-S59.

5. Hume, P. & Marfell-Jones, M. The importance of accurate site location for skinfold measurement. Journal of Sports Sciences. 2008;26(12):1333-1340.

7. Frisch, R. E. The right weight: body fat, menarche and fertility. Proceedings of the Nutrition Society. 1994;53(1):113-129.

8. Ritz, A. Salle, M. Audran and V. Rohmer. Comparison of different methods to assess body composition of weight loss in obese and diabetic patients. Diabetes Research and Clinical Practice. 2007;77:405-411.

9. Evans, M. et al. Body-composition changes with diet and exercise in obese women: A comparison of estimates from clinical methods and a 4-component model. American Journal of Clinical Nutrition, 1999;70:5-12.

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