Background Ultramarathon races have gained popularity in recent years and test the limits of human endurance. Dietary supplements are widely used, and athletes may attempt to gain a performance advantage through replenishment of real or perceived deficiencies.

Method We conducted a prospective survey study gathering data on prevalence, types and reasons for consumption of dietary supplements among ultramarathon runners during the Al Andalus Ultimate Trail 2012. Data were obtained post race using a direct-interview technique on standardized questionnaires.

Results Thirty-two ultramarathon runners (26 male, 6 female, mean age 39.5±7.9, previous ultramarathon experience 7.7±9) enrolled in the study and completed the questionnaires. Over two-thirds of runners said they consumed sports drinks and electrolytes, and over half used carbohydrate drinks and vitamin preparations followed by herbal supplements, minerals, magnesium, amino acids, and caffeine (38%, 38%, 28%, 25% and 22%). The main reasons cited for consumption were for improvement of performance (47%), the perception of acquired deficiencies through training (34%), or being vegetarian (16%). Sixty percent of runners said that they had had recent blood tests taken at their doctors' surgeries - all being in the normal range and not showing any deficiencies. High street shops and/or the Internet were the main sources for obtaining dietary supplements (69%; 47%). Supplements were taken based mostly upon recommendations of fellow runners (50%), self initiative (34%), magazine advertisements (22%) and/or recommendations of friends (19%) and, while less likely, by the recommendation of professionals (doctors 16%, coaches 13%, physiotherapists 3% and nutritionists 3%).

Conclusion The practice of dietary supplementation is widespread among ultramarathon runners, who take a variety of supplements aimed at increasing performance or treating perceived deficiencies. Athletes should be properly educated about potential benefits and risks, especially those who are regularly tested for performance enhancing drugs, as dietary supplements can contain substances that lead to positive drug tests.