Adaptation and hydration are the keys. Getting prepared for heat is not just a one-day thing, it’s a process that takes a month or more of training to acclimate your body to operate in the heat. Similar to training of your lungs and muscles to adapt to the stress of pedaling a bike, you will need to get your body adapted and ready to be physically active in the heat. You can start by setting the air conditioner in your car and home at a higher temperatures like 78-81 degrees. Sure, you’ll feel a little uncomfortable at first, but just like any other physical training, no pain no gain. After a few weeks, you might notice changes, like walking into an air conditioned store on a hot day and it feels uncomfortably cold. If overweight…well, work on losing some of the extra weight. The better shape you are in… you will find you require less water, sweat less, and will be more comfortable riding in the heat.
Know that Gatorade, soda pop, and sugary drinks just don’t quench thirst or hydrate like water with electrolytes. Your body will need to sweat way more water when physically active in the heat. As your body absorbs the water from Gatorade it leaves behind a syrupy goop of Gatorade sugar concentrate in your stomach, and excessive sugar won’t settle right in your stomach. I view Gatorade more as nutrition (calories), not hydration. Your body can only digest about 100 calories an hour (at best) so look at the Gatorade bottle and see how much that is… Over nutrition slows uptake of water and can also give you a belly ache. If you are drinking Gatorade and eating even more food, then your stomach retains extra water for digestion and this slows absorption of water. The simple carbohydrates in Gatorade are also poor nutrition for endurance… If you eat carbs I recommend fueling with complex carbohydrates (usually maltodextrin ingredient). Cytomax is one drink mix I’ve used in the past. Better still would be half a Cliff bar an hour with water and electrolytes. Just don’t mix food along with a carb drink too because it kills water absorption.
I suggest water with electrolytes. I am in very good shape and heat acclimated to about 101 degrees. As example of my routine, I begin my hydration sometimes two days before, but at the very least the night before. I drink a quart of water in the morning, noon, and at bedtime. If I know I’ll be in heat or climbing hills I’ll drink a coconut water the night before a ride (natural source of sodium/potassium). I also avoid alcohol because it dehydrates. While riding I drink plain water (adding ice is great for the heat) or water with electrolyte fizzy tablets (Nuun is good). While riding, I dring 1-3 bottles an hour (22oz each) depending on heat and exertion. I take Hammer Endurolyte capsules, 2 in the morning and about 1 every hour.