Body Composition Basics for Endurance Athletes

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How muscle mass, bone density and fat percentage can inform your training

Healthy, fit, and fast endurance athletes still cringe with these two words- body comp. In the spectrum of society, one can count on an endurance athlete having a generally good body comp report; however, many athletes shy away from acquiring this important health information. Knowing one’s body comp, i.e., muscle mass, fat mass, water volume, and bone density can provide training insights (is muscle mass symmetric?) and serve as a good metric to know over time (muscle mass decreases with age).

Percent body fat is the traditional measure that comes with a body composition test.

Normative values for percent fat are described in the table below. While many people are aware of the increased risk of Diabetes and Cardiovascular Disease in the presence of too much fat, the endurance community may forget that a minimum level of fat is required for adequate health and proper function of hormones. Percent fat should not be interchanged with BMI or body mass index which is a metric calculated using only one’s height and weight. While percent fat is a good indicator of an individual’s health and obesity status, BMI can be significantly erroneous, particularly in athletes.

TABLE 1. Percent Body Fat (Sims, 2016)

In general, performing a body composition test annually is advantageous. Muscle mass declines with age, even in an active population, so knowing your baseline and working to improve your strength is vital for health and longevity. When endurance athletes don’t strength train, we expect to see a low fat mass coupled with below average muscle mass. Adding a strength routine into your swim/bike/run plan, not only builds strength, but will improve performance. As testing methods have improved, methods such as the DEXA and InBody, can tell you if your muscle mass is evenly distributed between left and right sides- a crucial metric for injury susceptibility. If significant changes in lean mass between sides are observed, the athlete should consider incorporating targeted strength training and/or physical therapy to counter such an imbalance.

Because even the best methods have a percent error of about 3-4%, only a change of 5% or more or a change noted when testing on the same device, should be considered a meaningful change. For best results, acute food and fluid ingestion, hydration, prior exercise, and body position, should be controlled for going into a test and kept consistent between tests. DEXA scans are considered the gold standard for body composition testing; however improved accuracy, reliability and feasibility with the InBody has made these devices more available. Contact InBody to find a testing location near you.

Heydenreich, J., Kayser, B., Schutz, Y., & Melzer, K. (2017). Total Energy Expenditure, Energy Intake, and Body Composition in Endurance Athletes Across the Training Season: A Systematic Review. Sports Medicine-Open, 3(8). doi:10.1186/s40798-017-0076-1 Ling et al. (2011). Accuracy of direct segmental multi-frequency bioimpedance analysis in the assessment of total body and segmental body composition in middle-aged adult population. Clinical Nutrition, 30. doi:10.1016/j.clnu.2011.04.001 Sims, S.T. (2016). Roar. New York: Rodale.

Michelle Slawinski is an American College of Sports Medicine Certified Exercise Physiologist and co-owner of Summit Multisport, LLC. She works as a clinical exercise physiologist performing research VO2 testing. With Summit Multisport, she provides cycling and running lactate threshold tests as well as body composition evaluations. She is an avid endurance athlete competing in marathons, triathlons, and the daily endurance event of keeping up with a toddler!