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Why what we think about eating is so often wrong

The problem with diets is that they always end in disappointment. (iStock)

Here’s how public thinking on food gets shaped: Every year, researchers publish hundreds of academic studies about the health effects of various foods - chocolate, kale, red wine, anything. Those studies, in turn, become fodder for  newspaper articles, books and blog posts.

But how much of this torrent of information is worth the trouble? Surprising little, according to a number of key researchers.

In recent years, these skeptics have caused a stir by poking big holes in the nutritional science behind popular diet advice. Even the findings published in distinguished health journals have come under fire.

Collectively, their work suggests that we know far less than we think we do about what to eat.

“Is everything we eat associated with cancer?” a much noted paper in this vein asked.

Published in the American Journal of Clinical Nutrition, the paper reviewed the academic studies conducted on common cookbook ingredients. Of the 50 ingredients considered, 40 had been studied for their impact on cancer. Individually,  most of those studies found that consumption of the food was  correlated with cancer. When the research on any given ingredient was considered collectively, however, those  effects typically shrank or disappeared.

"Many single studies highlight implausibly large effects, even though evidence is weak," the authors concluded.

Indeed, much of the field appears to be beset with doubt.  And now comes the controversies sparked by the U.S. Dietary Guidelines - the governmental diet recommendations that are being updated this year. In February, we learned that the government is poised to withdraw its longstanding warning about consuming foods that are rich in cholesterol. At the same time, the government’s advice on salt, saturated fats and other foods are under attack, too.  The ongoing back-and-forth over these foods tends to buttress the skeptics' argument that public health authorities have too often issued nutrition advice before the science was settled.

With the public comment period on the Dietary Guidelines ending this week, it seemed like a good time to connect with David B. Allison, of the University of Alabama-Birmingham, one of the leading skeptics. In recent years, Allison and his colleagues have taken aim at an array of frequently dispensed nuggets of nutrition advice -- for example, that eating fruits and vegetables aids in weight loss or that skipping breakfast might cause weight gain.

More generally, Allison and others have pointed to problems in the way that nutrition research is conducted, criticizing everything from the way that food intake is measured, which is often imprecise and deeply flawed, to the inferences that scientists draw from their findings.

In conversation,Allison was exceptionally stinting in his diet advice -  unwilling to stray much beyond the fact that we need food to live, and that if we eat too much we get fat.

One final note: The professor's skepticism applies to nutritional claims made by the government as well as to those made by the food industry, and in fact, Allison and the university has received funding from both, as his disclosures show.

Nutrition science seems to be undergoing an enormous state of flux. Officials are poised to drop their longstanding warning about cholesterol in foods; the evidence for the government’s warning about salt has shifted; and scientists are still arguing over what kinds of fats are good and bad for you.

Is this normal?

Yes and no.

All scientific knowledge is provisional. That is accepted. We learn.

On the other hand, the extent to which we are changing in nutrition now seems greater than it is in some other fields, and much of what we once considered rock solid science being called into question.

[Given all this change], there are people wondering, “Hey guys, could we just hold on a second?”

We’ve put a man on the moon. We’ve mapped black holes and distant galaxies. Why is it so hard to figure out what we should eat?

There are many reasons.

For one, there is a presupposition that eating some things is better than eating other things. But is it? It’s like ESP - some people might ask ‘Why can’t you find good evidence of ESP?’  Maybe it’s because there is no ESP.

Another reason is it’s really difficult to do the kinds of experiments we’d like.

When we start to talk about how long do you live on this diet, or whether you get a cancer or stroke, that is not so easy to study in humans. You need large numbers of people to eat what you tell them for a very long period of time. Typically, you want thousands of people over a period of years. You can immediately see that if you can get them to do it at all, could you even afford to do the study? Those kinds of studies are very rare.

You’ve written about how scientists themselves distort what is known, mainly by making the evidence they have seem stronger than it really is. For example, you counted up instances in which researchers wrote that skipping breakfast “caused” weight gain, when in fact their study merely showed that skipping breakfast is associated with weight gain.

What are the motives here?

There’s more than one. One is innocent. Some people just didn’t think it through. That’s not an excuse – it’s still sloppy bad science.

Others may be well-meaning but they think they generally already know what is good and bad. They want to do as much as possible to convince everyone that what they think is ‘good’ is actually good and what they think is ‘bad’ is bad.

A third factor is a kind of moral passion or indignation.

Given, all of these ways that the science can go wrong, what do we actually know about what’s good for us?

There are a few things we are certain about. We know that you can’t live without food, and that if you eat too much, you get fat. There are certain essential nutrients - vitamins and minerals - that you need to have. You should make sure there is no lead or mercury or other toxins in your food.

After that the knowledge base gets thinner and thinner.

Maybe you shouldn’t have a diet that is extremely high in saturated fat or trans fats or sugars. Do we know this beyond any reasonable doubt? No. But we know enough to say this could be considered a prudent diet.

But that’s the way we need to tell people. We need to say, ‘We think,’ not ‘We know.’  We need to be careful about is not pretending we know more than we really do.