Don't Drink THIS Before You Exercise If you're planning to exercise later in the day, make sure you stay away from beverages containing caffeine. Drinking just two cups of coffee before exercise decreases blood flow to the heart while you're working out, and this reduction is most pronounced at high altitudes, reports Reuters of new research from University Hospital Zurich. Healthy people may be able to tolerate the reduced blood flow, but it could be especially harmful to anyone with coronary artery disease. The study: Led by Dr. Philipp A. Kaufmann, the team used young, healthy adult volunteers to test the effect caffeine had on blood flow to the heart both at rest and after exercise. They ran the tests at normal oxygen levels and then simulated the low oxygen levels that occur at high altitudes. Cardiac blood flow was measured at a baseline and then 50 minutes after the volunteers ingested 200mg of caffeine, which is about the equivalent of two cups of caffeinated coffee. The results: At normal oxygen levels, caffeine had no effect on resting myocardial blood flow, but at low-oxygen levels, resting myocardial blood flow was significantly increased by caffeine. However, during exercise, caffeine caused a significant decrease in blood flow to the heart at normal and low oxygen levels. Specifically, caffeine caused the myocardial flow reserve to drop off 22 percent at normal oxygen levels and 39 percent at low oxygen levels, reports Reuters. The takeaway: Skip the caffeine before you get on the treadmill. The study findings were published in the Journal of the American College of Cardiology. Note: I would also think that this would apply to children and young adults (the exercise that they get from all the baseball games they play, etc.) -- the caffeine in the pop that they drink.