It was tested whether a marathon was completed faster by applying a scientifically based rather than a freely chosen nutritional strategy. Furthermore, gastrointestinal symptoms were evaluated. Nonelite runners performed a 10 km time trial 7 weeks before Copenhagen Marathon 2013 for estimation of running ability. Based on the time, runners were divided into two similar groups that eventually should perform the marathon by applying the two nutritional strategies. Matched pairs design was applied. Before the marathon, runners were paired based on their prerace running ability. Runners applying the freely chosen nutritional strategy (n = 14; 33.6 ± 9.6 years; 1.83 ± 0.09 m; 77.4 ± 10.6 kg; 45:40 ± 4:32 min for 10 km) could freely choose their in-race intake. Runners applying the scientifically based nutritional strategy (n = 14; 41.9 ± 7.6 years; 1.79 ± 0.11 m; 74.6 ± 14.5 kg; 45:44 ± 4:37 min) were targeting a combined in-race intake of energy gels and water, where the total intake amounted to approximately 0.750 L water, 60 g maltodextrin and glucose, 0.06 g sodium, and 0.09 g caffeine per hr. Gastrointestinal symptoms were assessed by a self-administered postrace questionnaire. Marathon time was 3:49:26 ± 0:25:05 and 3:38:31 ± 0:24:54 hr for runners applying the freely chosen and the scientifically based strategy, respectively (p = .010, effect size=-0.43). Certain runners experienced diverse serious gastrointestinal symptoms, but overall, symptoms were low and not different between groups (p > .05). In conclusion, nonelite runners completed a marathon on average 10:55 min, corresponding to 4.7%, faster by applying a scientifically based rather than a freely chosen nutritional strategy. Furthermore, average values of gastrointestinal symptoms were low and not different between groups.