Everything Causes Cancer ...

There's no cure, there's no answer.

-- Joe Jackson

The ever-reliable John Ioannidis riffs on nutritional epidemiology. I would say that if there is one reason why Jane and John Q. Public are skeptical of science it's because they keep hearing contradictory or changing advice about what to eat. 

As Ioannidis explains rather wonkily, and I will try to explain more accessibly, there are some fundamental problems with trying to sort out how diet affects health. In the first place, whatever effects there may be will take a long time to manifest. Maybe eating a lot of doughnuts will cause you to become yellow, have only four fingers, and say "D'oh" a lot, but if so it's not going to happen overnight. Following cohorts of people for 30 years or more is difficult and expensive.

Even if you do it, asking people to report on what they eat is the only practical way to measure it. You can't watch 5,000 people eat every day and record everything. And people just don't remember very well and don't report very accurately. Furthermore, what questions should you ask? Studies typically try to assess the effects of specific foods or categories of food, but any particular eating habit is likely to be associated with others. Is it really the hazelnuts (to use one of Ioannidis's examples), or do people who eat a lot of hazelnuts also tend to have other habits or circumstances relevant to health? 

Ioannidis points out (not sure where gets the info, there's no citation) that there are more than 250,000 different foods; and that even very similar foods may have different micronutrients and other chemical constituents. And our diets interact with our genetic heritage, age, and environmental circumstances and other behaviors. (E.g., more physically active people may benefit from a different diet than more sedentary people.) He doesn't want to say it, but a lot of these results are bunk.

So what should we do? I think the simple, broad advice is still best. Don't eat too many calories -- whatever you do, maintain a healthy weight. Eat a varied diet, and emphasize plant foods. (Good for the planet as well as your bod.) Minimize empty calories from sugary drinks and snacks. Whole grains, veggies and legumes are good, in part because they have a lot of fiber. But you can relax a bit about dairy and eggs, they're fine in moderation. (You may want to consider the ethical treatment of animals, however.) I'm not a real doctor, but that's my interpretation of what the most compelling science tells us.

Meanwhile, don't let the latest single study that hits your TV distract you.