Carb-loading works

This one is from the "We already knew that" department: Those who had more carbs the day before a marathon ran faster.

The researchers compared 2009 London Marathon runners' carb intake and found out that those who ate more than 7 g carbs per kg of body weight (about 3.5g/pound) the day before the race ran faster and maintained their pace better.

This of course is not a proof of causality. Were the runners faster because they ate more carbs or did the faster (more experienced?) runners eat more carbs?

The study digs a bit deeper and adds gender, BMI and training distance as predictors of finish time.