Coffee could deliver more of a jolt than you’d expect, Swiss scientists recently determined that caffeine decreases blood flow to the heat during exercise by as much as 39 percent. Normally, as a person’s activity level increases, coronary arteries expand to let in more oxygen carrying blood. But that process was inhibited when study participants consumed 200 milligrams of caffeine – the amount in 2 cups of coffee – about an hour before working out. Although the caffeine didn’t harm the healthy study participants, lead researched Philipp Kaufmann, M.D. suggest avoiding it for 4 to 5 hours before exercise if you have a family history of heart disease.