Cold weather can affect your running performance because it changes the way your body metabolizes fuel. When you run in the cold, your body uses up its glycogen stores at a much higher rate. One reason for this is shivering, which can deplete your glycogen stores five to six times faster than the normal rate. The other reason is an increased level of epinephrine (a.k.a. adrenaline) in your blood, which stimulates the breakdown of glycogen. Anyone who has ever hit “the wall” in the marathon knows what it’s like to run out of glucose and glycogen stores, and for these reasons, running in the cold can make this happen even sooner.