If you were running marathons two or three decades ago, choices were far more limited. In that era (starting in 1990 for me), you drank water and were appreciative of the way things had improved because of stories of the bad old days when even water was hard to come by. Clearly it is possible to run marathons drinking only water.
If your body won't tolerate food, gel or gatorade (or another energy drink), water seems to be a sensible choice. That said, the logic behind injesting easily absorbed carbohydrates seems to be solid. The body only has so much in the way of glycogen stores so additional intake is likely to stave off bonking.
Like anything else physical, practice helps. If you decide to try this modern supplementation approach, try to adapt to it in your training long runs.
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