Objective: To investigate whether adipose subcutaneous tissue or skeletal muscle mass decreased during a non-stop ultra-endurance triathlon.

Design: Descriptive field study.

Setting: The Triple Iron Triathlon Germany 2006 in Lensahn: 11.6 km swimming, 540 km cycling and 126.6 km running.

Subjects: 17 male Caucasian triathletes, mean (SD) age 39.2 (7.5) years, height 178 (5) cm, body mass 80.7 (8.9) kg and body mass index (BMI) 25.4 (2.4) kg/m².

Interventions: None.

Main outcome measurements: Determination of body mass, skin-fold thicknesses, limb circumference, skeletal muscle mass and percentage body fat in order to show changes after the race.

Results: A significant decrease was shown for body mass (p<0.001), BMI (p<0.001) and calculated percentage body fat (p<0.001) whereas skeletal muscle mass did not change significantly (p>0.05). Circumferences of the thigh, upper arm and calf did not decrease significantly (p>0.05), whereas all skin-fold thicknesses decreased significantly (p<0.05), with the exception of those at the chest and thigh. A significant correlation was found between the loss of percentage body fat and the loss of body mass (p<0.01, r² = 0.55) as well as change in percentage body fat with race performance (p<0.05, r² = 0.24).

Conclusions: Ultra-endurance triathletes at the Triple Iron Triathlon Germany 2006 showed a significant decrease in body mass and percentage body fat, where decrease in percentage body fat was associated with race intensity.