Best way to acclimate to the heat is to ride in it. Low levels of vitamin D are associated with aging, so no surprise to see that in this forum. I take 5000 IU every day, doctor's orders.
Yes, while climbing in the heat it's impossible to keep up with hydration by drinking on the bike. One has to take hydration breaks in the shade. I do some shadeless climbs, but haven't been on a waterless climb here. Sitting in the creek and drinking works, too. I use Endurolytes.
I wear sun sleeves, but not leg sleeves. I find the sun sleeves are OK on hot climbs. I think thin white technical fabric is about as good as bare dark skin for heat dissipation even in low wind conditions. I think the whole idea of "cold black" is silly.
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