If you’re a regular here, you’ll be familiar with our disdain for “the latest study.” Good science comes from replicating results from well-defined examinations. Sometimes media-hyped “latest studies” are so badly designed they get dismissed by national science academies, like a recent French study attacking biotech corn.

The latest “latest study” (or just “single study,” as some are quite old) hype concerns artificially sweetened diet soda. According to the Today show’s Health blog, there are seven ways diet soda is (supposedly) harming you. Apparently, the sweet-but-calorie-free stuff is responsible for everything from obesity to bad hangovers.

Meanwhile, National Public Radio’s Marketplace is hailing diet soda for reducing calorie intakes of people who choose it over the full-sugar variety.

So who is right? Take a second look at the caveat on Today’s claim that diet sodas cause metabolic syndrome: “Whether that link is attributed to an ingredient in diet soda or the drinkers’ eating habits is unclear.”

It stands to reason that diet soda drinkers might be overweight and trying to lose weight, which might confound an observational study. It is hard to make conclusions about causality from observational studies. Calories — both in from food and used in exercise — not neo-Puritan interpretations of observational studies, matter.