If you read science news regularly you may be tempted to hate the fast food industry. Restaurants like McDonald's, according to the headlines anyway, sell us fattening food, underpay their employees and damage the environment in their rush to mass produce Big Macs and chicken strips. And a recently released study may give us another reason to hate big fast food: their tasty offerings may be making our children stupid.
The idea that our diets affect our cognitive abilities has some evidence behind it. Certain foods make us fat and sick, so it wouldn't be shocking to find out that those same foods are also bad for our brains. But it's worth emphasizing that we don't know yet whether that's correct or not. Anyone who says fast food is harming kids in the classroom is contradicting the available evidence.
To begin I'd like to point out that this is an observational study, so we need to keep the usual caveats in mind as we consider the results. The researchers didn't do anything except ask fifth graders how much fast food they eat and compare those numbers with the kids test scores, so nobody knows if the diet information is accurate. That fact that the study is based on data provided by the people being studied makes it useless until the results are confirmed by some actual science. All we can know until then is that some kids who eat fast food also perform poorly in school.
In fairness, the only reasonable way to find out what people eat is to ask them. Short of putting them in a hospital ward and feeding them a strictly controlled diet, there's no other way to get the information, though every epidemiologist knows it probably isn't accurate. To combat this reliability issue, scientists will often try to control for other variables that could be responsible for the results they found, in this case the kids exercise habits, television viewing time, socioeconomic status, other food consumption and school and neighborhood characteristics.
But I can think of another variable that could jam up this study: IQ. Some individuals have been blessed genetically; they have a leg up on the rest of us in a variety of ways. Some, for example, can't get fat no matter what they eat, while others are naturally athletic. And still other people are just born smarter than the rest of us. Since highly educated people tend to eat healthier, maybe all this new study uncovered was that less intelligent people eat less healthy foods. That conclusion is only a possibility at this point, but I mention it to show that we really don't know conclusively how our diets impact our mental health.
The primary issue with this study, and fast food bashing in general, is that it asks the wrong question. When I was in the most intense phase of my diet two years back, I ate fast food multiple times a week because it was cheap. I got bored of eating hamburgers pretty quickly, but I didn't get fatter; I steadily lost weight and my cholesterol improved markedly. Despite the sensational claims made by idiotic film makers and politicians, I got healthier while eating at McDonald's because I avoided the foods that I know are unhealthy--hamburger buns, soda and french fries.
If fast food does affect our mental health as well as our metabolic health, I suspect that there are certain items on the menu that are responsible, because we have some evidence that foods like sugar and white flour can damage our cognitive abilities. But there's nothing especially harmful about a fast food item because it's fast food.
That leads us to a very important point. You've probably seen the news stories about people who go on fast food diets for a month and come off them healthier than they were before. Those examples go a long way toward demonstrating that fast food isn't the boogie man everybody says it is, but the nutritional value of fast food has been investigated in detail and there is plenty of evidence that McDonald's selection is no worse for you than the same foods purchased at a grocery store or a high-end restaurant. A milkshake is a milkshake, after all, and the sugar in it is bad for you regardless of where it comes from. Particularly striking, though, is science writer Brian Dunning's point that if you eat "four 510-calorie Big Macs a day ... you’ll lose weight ... and get more protein and vitamins than you would from most other similarly caloric diets."
Far from making kids fat, sick and stupid, fast food can actually be nutritious. People simply have to make good choices when eating, whether their food comes from a fast food joint or a grocery store. This isn't a juicy news story now that we've looked at all the facts, but the good news is that you can enjoy a Big Mac without fretting about your Alzheimer's risk.