Why is the science ‘flip-flopping’ ?

here’s a nicely stated defence
– simplicity v complexity
– narrowness v completeness
– are your definitions right/are we talking about the same thing?
– value of evidence: larger, better-conducted studies are better

Why does it seem like scientists keep flip-flopping on whether coffee is bad for you or good for you?

Often people think of coffee just as a vehicle for caffeine. But it’s actually a very complex beverage with hundreds and hundreds of different compounds in it. Since coffee contains so many different compounds, drinking coffee can lead to very diverse health outcomes. It can be good for some things and bad for some things, and that’s not necessarily flip-flopping or inconsistent. Few foods are good for everything. That’s why we do studies on very specific health effects—for example, studies of how coffee affects the risk of diabetes—but we also conduct studies such as this most recent one looking at coffee consumption and mortality over a long period of time, which better reflects the overall health effect.

Coffee is also a bit more complex to study than some other food items. Drinking coffee often goes along together with cigarette smoking, and with a lifestyle that’s not very health conscious. For example, people who drink lots of coffee tend to exercise less. They are less likely to use dietary supplements, and they tend to have a less healthful diet. So in the early studies on coffee and health, it was hard to separate the effects of coffee from the effects of smoking or other lifestyle choices.

Over the several decades that coffee has been studied, there have been some reports that coffee may increase the risk of certain cancers or the risk of heart disease. But in better conducted studies, such as the one we just published—larger studies that have a lot of information about all other lifestyle factors and make a real effort to control for these lifestyle factors—we do not find many of these health effects that people were afraid of.

http://www.hsph.harvard.edu/nutritionsource/coffee/