An increased fluid intake leads to feet swelling in 100-km ultra-marathoners - an observational field study

In seventy-six 100-km ultra-marathoners, body mass, plasma [Na+], haematocrit and urine specific gravity were determined pre- and post-race. Fluid intake and the changes of volume of the feet were measured where the changes of volume of the feet were estimated using plethysmography.Body mass decreased by 1.8 kg (2.4%) (p < 0.0001); plasma [Na+] increased by 1.2% (p < 0.0001). Haematocrit decreased (p = 0.0005). The volume of the feet remained unchanged (p > 0.05). Plasma volume and urine specific gravity increased (p < 0.0001). Fluid intake was positively related to the change in the volume of the feet (r = 0.54, p < 0.0001) and negatively to post-race plasma [Na+] (r = -0.28, p = 0.0142). Running speed was negatively related to both fluid intake (r = -0.33, p = 0.0036) and the change in feet volume (r = -0.23, p = 0.0236). The change in the volume of the feet was negatively related to the change in plasma [Na+] (r = -0.26, p = 0.0227). The change in body mass was negatively related to both post-race plasma [Na+] (r = -0.28, p = 0.0129) and running speed (r = -0.34, p = 0.0028).An increase in feet volume after a 100-km ultra-marathon was due to an increased fluid intake.Participation in ultra-marathon running is of increasing popularity [1-5] where an ultra-marathon is a running race longer than the marathon distance of 42.195 km [5]. Within the ultra-marathons, there is a difference between single stage races [1,2,6,7] and multi-stage races [3,5], where the distance is split into daily stages. Running an ultra-marathon is associated with different problems such as a change in body mass [1,8-10], dehydration [10], a loss of skeletal muscle mass [3,7], an increase in total body water [3,4,6,11], overuse injuries of the lower limbs with especially knee injuries [5] and an impaired renal function due to exertional rhabdomyolysis [7], leading in extreme cases to a renal failure [12].Among these ultra-running associated problems, an increase in total body water has been