In the annals of food ingredients, caffeine might just take the prize for the most medically scrutinized chemical. Linked to everything from cancer to heart disease to infertility at one time or another, the mild stimulant has followed a roller-coaster ride in scientific literature. One minute it's touted as a weight-loss aid, memory booster, and hedge against Parkinson's and Alzheimer's diseases. The next it's flogged for raising blood pressure, acting as a carcinogen, and weakening bones.

But many of the studies that suggest negative repercussions haven't panned out. Take the case of caffeine and cancer. Initial reports cited a connection between caffeine and several kinds of cancer, including digestive and bladder. Fast forward a few decades and hundreds of studies later, and the consensus has changed. "The cancer issue is pretty well settled," says Herbert Muncie Jr., M.D., chairperson of the Department of Family Medicine at the University of Maryland. "There's no proof that caffeine is linked to any kind of cancer." Ditto for caffeine's connection to ulcers, cardiac arrhythmias, blood pressure, and infertility. The issue of osteoporosis is still up in the air, but Muncie says a University of Pennsylvania study found that bone strength is not influenced by how much caffeine people consume.

Why the conflicting reports? Nutrition research normally takes lots of twists and turns as researchers uncover an association between a specific food and disease, and then try to prove a cause and effect, Muncie explains. After 30 years of intensive research, he says, no one has been able to demonstrate that moderate amounts of caffeine cause harm. What's moderate? About three cups of coffee (300 milligrams of caffeine) per day, says Muncie, who takes his with cream and sugar.