Thousands of popular headlines over the past couple of decades have touted the supposed health benefits of chocolate—particularly dark chocolate (in moderation, of course). But every single one of the major studies on which those claims are based actually failed to prove any such connection. They weren't designed to—they are observational studies, whose main purpose is to identify interesting ideas that warrant closer, more rigorous investigation without wasting too much time and energy. You can blame traffic-hungry journalists (or their editors) for the specious headlines.

Really getting to the bottom of whether or not chocolate is good for you requires what's known as a randomized, double-blind, placebo-controlled trial. This is the most scientifically rigorous type of study researchers ever conduct and it's designed to separate honest-to-goodness real evidence from wishful thinking. As it happens, just such a randomized controlled trial got underway this spring. And no, you can't volunteer for it—unless you already participated in one of two other studies.

With 18,000 expected participants, the new study is big. It has to be because no one wants to wait decades for definitive results. Because the participants are older and thus at higher risk of suffering heart attacks and strokes, investigators should be able to collect enough data to determine whether or not the intervention is worthwhile over the course of about four years. Women are being recruited from the Women's Health Initiative and male participants hail from the Vitamin D and OmegaA-3 study.

It's expensive. The budget is somewhere between $30 million and $60 million, which helps to explain why it's being sponsored by a trio of partners: the National Institutes of Health, Mars, Inc., and Pfizer. Investigators from the Brigham and Women's Hospital and the Fred Hutchinson Cancer Research Center are carrying out the actual study.

But test subjects will not be getting free samples of chocolate. Indeed, the study pills they'll be taking won't even taste like chocolate. That is because the researchers won't actually be testing chocolate. Instead, they will be studying the health benefits of certain plant-based substances called flavanols, which are found not only in chocolate but also in tea, fruits and vegetables. (There's also a section of the study that will evaluate the health benefits of multivitamins.)

Laboratory experiments suggest that the flavanols may help keep the insides of arteries nice and flexible—a characteristic that is known to protect the heart and brain over the course of a lifetime. But the process of fermenting, drying and roasting cocoa beans in order to turn them into chocolate destroys most of their original flavanol content.

Still, cocoa contains some unique combinations of flavanols that warrant a closer look, and the compounds in question have already undergone extensive safety testing. So Mars developed a proprietary process that preserves the flavanols, starting with how growers harvest the cocoa beans in the first place, says Hagen Schroeter, who is a nutrition researcher at Mars as well as the University of California, Davis.

All this helps to explain why the study, dubbed COSMOS (for Cocoa Supplement and Multivitamin Outcomes Study), is looking at flavanols derived from cocoa and not tea.

In addition to measuring the number of heart attacks, strokes and other cardiovascular ailments of its subjects, COSMOS investigators will look at whether the flavanol extracts help to lower blood sugar level or improve participant's scores on memory tests. The study will be large enough to detect a difference between the control and experimental arms of as little as 10 to 15 percent, says JoAnn Manson, chief of the Division of Preventive Medicine at Brigham and Women's and one of the COSMOS study leaders. A 25 percent difference, which Manson says is "feasible" to detect, would place flavanol's benefits for heart disease very nearly in line with that of a statin drug.

In any event, when the results are published several years from now, you can safely ignore any news items that say anything about chocolate's health benefits. If anything, it will be the flavanols—minus the extra sugar and fat that comes with chocolate—that will prove healthy. In the meantime feel free to eat chocolate (in moderation) because you like it—not because you hope it will make you live longer.