For years, dietary advice held that we should limit our salt, fat, and cholesterol intake. There’s a big problem with that, though: Salt, fat, and cholesterol all to varying degrees either make food taste good, or are found naturally in foods that people enjoy eating. If we’re trying to avoid them, and if the life of an ascetic doesn’t appeal to us, what shall we eat?

The answer was sugar. Sugar also tastes good. For more than a generation, Americans in particular have substituted sugar and other carbohydrates for the fats that they used to consume. They’ve also upped their calorie intake, again largely from sugar. The downsides here seemed few, because dental cavities are not life threatening, and because, although sugary foods may be low in vitamins and other nutrients, most Americans were getting those nutrients elsewhere, making deficiencies relatively rare.

Now, though, scientific opinion has shifted, prompted by two bodies of evidence. First, there has been an epidemic of obesity that seems increasingly to have been the fault of excess sugar consumption. And second, the dietary harms from salt, fat, and cholesterol seem to have been much less than initially feared. Fat does not make people fat — but sugar does, says the new thinking. Sugar also may be to blame for diabetes, heart disease, and a host of other maladies.

The role of government nutrition guidelines looms large in these matters, because these guidelines shape institutional food practices in places like schools, hospitals, and prisons, and because the guidelines shape public health campaigns for the rest of us. Has sugar been making us fat? And if so, has the federal government had a hand in it? And what’s to be done about it?

Joining us to discuss this month are noted science author Gary Taubes, whose lead essay reviews the state of the science and recaps the recent history of U.S. federal government dietary advice. Responding to him are obesity researcher Stephan Guyenet, Professor Yoni Freedhoff of the University of Ottawa, and Professor Terence Kealey of the University of Buckingham. We welcome your comments to their posts and look forward to a stimulating discussion.