A new study claims drinking coffee is a good way to have a healthy heart, and four cups a day might be best. German researchers studied caffeinated lab mice and dosed human tissues with caffeine. They found that a shot of caffeine made certain proteins inside older adult cells perform more like young cells. One of the lead researchers says drinking four to five cups of coffee a day seems to improve what he called the powerhouses of cells. He said that seems to be protective. The study is published in the journal "PLOS Biology."