For most people, caffeine is safe. The U.S. Dietary Guidelines for Americans recommend adults limit caffeine intake to 400 milligrams a day. An average 8-ounce cup of coffee contains 95 milligrams. You may react differently to caffeine than someone else does. How your body metabolizes caffeine depends on several things. These include your age, physical and mental health, genetics, medications you take, and whether you smoke. Caffeine stimulates your central nervous system. When it comes to heart health, coffee is good for you in some ways and bad in others. More research is necessary to determine how much is beneficial. An analysis of three studies published in Circulation: Heart Failure found drinking coffee daily may reduce your risk of heart failure. People who drank one or more cups of coffee daily had a lower risk of heart failure. But people who drank decaffeinated coffee had an increased risk. The American Heart Association says there’s “not yet enough clear evidence to recommend increasing coffee consumption to decrease risk of heart disease.” Following this analysis, a study presented at the AHA’s Scientific Sessions had different results. It tracked participants in real time using monitors for their heart rhythm, steps, and blood glucose. People who had more than one coffee drink a day had 54% more premature ventricular contractions. This type of irregular heartbeat, which feels like skipping a heartbeat, starts in your heart’s lower chambers. Having more of these irregular heartbeats can increase your risk of heart failure, the study authors noted. On the other hand, participants who consumed coffee had fewer episodes of supraventricular tachycardia. This is a rapid heart rhythm that starts in your heart’s upper chambers. Having more of these episodes increases your risk of atrial fibrillation.