Hi there!
When in a race - Pro Riders switch between ISO-Mix and Carb-Mix bottles - why?
Don’t they want to take as much carbs in as possible? 
Why I am asking: I am planning my nutrition for a long race (220km/3.800m elevation) and i am wondering if i should or shouldn’t put carb-mix in every bottle I will consume - or better not.
(Of course I will consume other (solid) sources of carborhydrates)
Thanks in advance! 
What if you think you’re getting dehydrated, but you’ve already taken in as many carbs as your gut can handle?
That’s why, I guess.
Sometimes you just need clean water. If I’m going out for more than 3-4 hours I make sure to have a bottle of plain water.
Maybe they are, how many carbs do they have in a bottle? Maybe they are still absorbing the carbs in the last bottle when they switch?
Because they use variety of carbs - especially gels. And gels need water (and a lot of water) to improve osmolality, gastric emptying and absorption. Without that we would have Dumoulin’s situation all the time during and after the race.