As you go from winter to spring to summer, it takes much more effort and energy to maintain a "normal" pace as the temperature rises. As a way to keep your exercising body safe in the heat, your body increases your sweat rate to remove heat from your body through evaporative cooling. As you become dehydrated (from fluid loss), blood volume decreases. In turn, more blood going to the skin means less going to your heart. To maintain cardiac output (the amount of blood pumped by the heart minute), heart rate increases in order to supply the working muscles with blood. But due to a decline in blood availability (for cooling), less blood is available to go to the working muscles. When the body can no longer take care of all of its responsibilities (remember - all of this is happening so you don't die in the heat), your body begins to shut down to keep you safe. If you ignore the signs of overheating, heat builds up in your body, your core temp increases and you are at risk for...
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