Your body is going to perform differently when it's 60 degrees outside than when it is 90 degrees. Although exercising in hot weather requires extra planning and caution to avoid health risks, it is possible to improve performance, enjoy your training sessions and not feel completely drained when training in the heat. There's a good chance that you are not making good decisions when training in the heat and as a result, your performance and health is being compromised. Running without a hydration belt, not planning stops accordingly on the bike, showing up to workouts dehydrated, not fueling properly (or eating enough) and not rehydrating properly after workouts will negatively impact your body's thermoregulatory process. The human body is remarkable when it comes to exericising heat, so long as you take the necessary steps to acclimate gradually, pace yourself and look for opportunities to minimize dehydration and cool the skin.
3x Author, Board Certified Sports Dietitian, Master of Science in Exercise Physiology, 2017 IM CHOO Amateur Female Champion, 19xIronman finisher including 6xIM World Championship finisher, Triathlon Coach, 30-year Vegetarian.Trimarnicoach.com