This one's a maybe. A 2011 study found wearing a cooling vest before running a 10K had no effect on core temperature or rate of perceived exertion. That said, a 2010 study found that wearing a cooling jacket for a total of 30 minutes before running on a treadmill lowered core temperature, heart rate, blood lactate, and increased the total distance run on the treadmill before exhaustion. So if it’s truly hot, and you can take it, you could try sitting around in a cooling vest before heading out. It just might help.