Since you need at least 20 grams to cover 40 minutes of cycling (probably more) and these people bike all day, you have no choice but to consume the carbs for energy or risk lows. These extreme athletes get to eat a lot of carbs!
It would be interesting to hook them up to a CGM and see what their blood sugar did. I suspect a normal metabolism and a physically demanding lifestyle handles this just fine, plus most of the extra carbs are consumed during the competition. I suspect a lot of people get in trouble as their lifestyle gradually becomes more sedentary but they don't adjust their diet.
SI had an article not too long ago about D1 football lineman...who don't go onto the NFL. They get the "benefit" of training to bulk up, eating 4x fried chickens/ day and all that but then they're done and may not get a lot of support at the transition to normal lifestyle and activity level.
I'm not 100% sure if one actually needs a ton of carbs to exercise as much as one needs regular consumption of quick acting carbs during the activity? All (3) of my 1/2 marathons were around 2 hours and, while I was not "world class", in terms of the input/ output equations, I'm not sure a pro is going to gain a whole lot more "burnoff" by going twice as fast as I do? I skip dinner the night before (portapottyphobia, not "phobia" as much as gross out but I dunno the medical term...) and have had a light breakfast, generally eggs and a piece of toast, which is pretty much what I eat every day.
Here's an article about Swedish triathlete Jonas Colting who has had success with low carb high fat.The article mentions Michael Phelps specifically in regards to athletes who eat high carb having to change when their competition days are over.
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