We examined whether selected anthropometric and nutritional factors influenced field-based marathon running performance. An internet-based data collection tool allowed competitors in the 2009 London Marathon (n=257, mean±SD age: 39±8 years, finish time: 273.8±59.5min) to record a range of anthropometric, training and nutritional predictors. Multivariate statistical methods were used to quantify the change in running speed mediated by a unit change in each predictor via the 95% confidence interval for each covariate-controlled regression slope (B). Gender (B=1.22 to 1.95km/h), body mass index (B=0.14 to 0.27km/h), training distance (B=0.01 to 0.04km/h) and the amount of carbohydrate consumed the day before the race (B=0.08 to 0.26km/h) were significant predictors, collectively accounting for 56% of the inter-individual variability in running speed (P<0.0005). Further covariate-adjusted analysis revealed that those competitors who consumed carbohydrate the day before the race at a quantity of >7g/kg body mass had significantly faster overall race speeds (P=0.01) and maintained their running speed during the race to a greater extent than with those who consumed <7g/kg body mass (P=0.02). We conclude that, in addition to gender, body size and training, pre-race day carbohydrate intake can significantly and independently influence marathon running performance.