Food Myths: What Science Knows (and Does Not Know) about Diet and Nutrition

By Hall, Harriet | Skeptic (Altadena, CA), Fall 2014 | Go to article overview

Food Myths: What Science Knows (and Does Not Know) about Diet and Nutrition


Hall, Harriet, Skeptic (Altadena, CA)


KOALAS HAVE IT EASY. WHAT TO EAT? NO WORRIES: they eat eucalyptus leaves, period. We humans have it tougher. Ever since Eve and the apple, we have had to make decisions about what to eat. Today we are constantly bombarded with conflicting advice about food. "Eat fish because it's a great source of omega-3s." "Don't eat fish because it contains toxic mercury." (Actually both of those statements are true, so we need to quantify the actual content in specific varieties of fish and carefully consider the risk/benefit ratio.)

Fad diets and "miracle" diet supplements promise to help us lose weight effortlessly. Different diet gurus offer a bewildering array of diets that promise to keep us healthy and make us live longer: vegan, Paleo, Mediterranean, low fat, low carb, raw food, gluten-free ... the list goes on. Obviously they can't all be right. Food myths abound, often supported by the strongest of convictions and emotions. What are we to believe?

We live in the Information Age. Unfortunately, bad information comes mixed with the good. The only reliable guide to reality is science, but when it comes to food, there's a problem. It's hard to do a gold standard double-blinded randomized controlled study on diet. We could learn a lot if we could divide infants into two groups, insert feeding tubes, pour competing diets directly into their stomachs throughout their lifetimes, and see which group lived longer and had fewer illnesses. But that just isn't feasible. So we have to rely on less conclusive forms of evidence. We can compare two groups who eat different diets (such as vegans v. meat-eaters, or Mediterraneans v. Americans), but those groups almost always differ in other ways that affect results, ways that we didn't think to control for. We can ask people what they eat, but we can't trust their answers to be accurate; people tend to misremember, to misestimate portion sizes, and to misreport what they eat in the direction they think the researcher will approve of. We can tell people what to eat for a study, or even provide the food we want them to eat; but compliance is a problem, and studies are time-limited.

It's not hopeless, because we can combine less ideal types of research and reach a reasonable conclusion if the evidence from all the avenues of inquiry converges. We didn't need lifelong blinded trials to learn that smoking causes cancer: the evidence from animal studies, analysis of tobacco for carcinogens, various kinds of studies comparing smokers to nonsmokers, etc., all pointed in the same direction. If the evidence from diet studies were as coherent, as consistent, and as strong as the evidence from tobacco studies, science would have reached a consensus by now, and we would know which diet is optimal. Unfortunately, the evidence for different diets is inconsistent or lacking. If anyone claims to know how you should eat, you can pretty much guarantee they are wrong. We have hints, but we don't have a definitive answer about which diet is best, and we have pretty good evidence that some of the pronouncements on diet are just plain wrong.

What We Know Is True

Science has given us a lot of reliable information about nutrition. It took a while to realize that "our daily bread" alone wouldn't keep us healthy. As late as the 19th century we had no explanation for scurvy, beriberi, kwashiorkor, and other nutritional deficiency diseases. Vitamins were not discovered until the 1920s. Today we know that we need six categories of nutrients: proteins, fats, carbohydrates, vitamins, minerals, and water. We know that some nutrients like vitamin K are stored in body fat for future use while excess amounts of others, like vitamin C, are excreted in the urine and must be replenished more frequently. We can measure blood levels and body stores of various nutrients. We know there are 14 essential vitamins and 17 essential minerals plus a few ultra-trace minerals that the body requires in only the tiniest amounts. …

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