I find this a curious hypothesis, to say the least.

So I'm a marathon runner (ran a 3:08 marathon this morning, actually, at age 55). The carbs I eat are neither excessive, nor low fibre.

And all the endurance athletes I know are extremely aware of their diets and the importance of eating the right mix of high quality foods.

https://www.runnersworld.com/uk/nutrition/recipes/g29413408/...