Last week, the press covered a study published in the New England Journal of Medicine with headlines like "5 Foods That Make You Fat." An article about the same study in the LA Times claimed that Harvard University researchers declared the potato "Public Enemy No. 1 in America's battle of the bulge." These are gross misinterpretations of the science.
The study looked at what people ate over time and looked at their weight gain and loss over that time. They then looked for correlations between what was eaten and weight changes. What they found was that people who consumed the most potato chips also gained the most weight. Next on the list were potatoes, sugar-sweetened beverages, unprocessed red meats, then processed meats.
What is wrong then with claiming these foods make you fat? It is attributing cause where only a correlation was found. This is a common error, but a grievous one. In the article, the authors cautioned against making that mistake. They explicitly acknowledged that the cause could be reversed.
For example, consider the first finding, "people who ate the most potato chips gained the most weight." What the study is telling us is that people who ate the most potato chips consumed the most calories. Is it from the potato chips per se? We do not know that. The study did not report how many calories came from each food group. A reasonable alternative possibility is that people who ate the most potato chips tended to have less mindful snacking habits, perhaps consuming a variety of snacks while watching TV and not noticing portion size. Potato chips may have just been the best marker for that habit. The salt on the potato chips makes people thirsty. What were the potato chips being washed down with? I'll bet it wasn't water, but more calorie laden beverages.
Second, consider the finding, "people who ate the most potatoes gained the second greatest amount of weight." Keep in mind here that "potatoes" includes all forms of potatoes. In the US the majority of potatoes we consume are in the form of french fries, and in the home pre-formed potato products (e.g., tater tots) (Source: US Potato Board), both of which have significant added fat and calories. It is unlikely the potato per se is the culprit here. A medium (5.4 oz) baked potato has 145 Calories (source: USDA); A large (5.4 oz) order of french fries from McDonald's has 500 Calories (source: McDonald's). The reason potatoes are near the top of this list is most likely because of how we prepare them (fried), what we add to them (sour cream and butter), and what we pair them with (oversized steaks, turkey & gravy with all the fixings).
Sugar sweetened beverages probably do have the most direct cause-link to obesity. We do know that when people consume "liquid calories" they tend to consume more calories than food form. So, watching the amounts you consume does require more thoughtful intervention than with food. And we know that high consumption of sugars (all forms) delivers a high-fructose load which has a cascade of effects which may increase obesity.(1) Still, it isn't the consumption of the occasional pop that makes you fat. It is the chronic high consumption that this and other studies correlate to high obesity rates.
I hope you can see that what this study found was not obesity forming foods, but rather foods that are markers for high calories diets. Those foods are not your enemy. Rather, as with any food, you need to keep portions, variety, and overall calorie balance in mind.
The study also pointed out five foods for which people gained the least weight over time. They were yogurt, nuts, fruits, whole grains, and vegetables. Again, we need to be clear. Consuming these food won't mean you will lose weight. Rather, these foods are markers of diets which are more healthful; diets that are less likely to add weight. As a nutritionist, I highly encourage you to include these foods in your diet. But portion control, quantity, and thoughtfulness about what you add to them and pair them with are still vital lessons to avoiding weight gain.
For the record, there are other problems with the study that would prevent the thoughtful reader from putting too much stock into the findings. In addition to being cautious about assuming "link"means there is cause and effect, we always need to be skeptical about findings from any single study. These studies are useful for people like me who can put them into context. But the press does a disservice to the public with the reporting that I linked to above.
Eric Esterling is a registered dietitian (RD) with a Masters in Nutrition and a passion for dispelling bad science reporting. If you have any questions, please ask.
(1) Tappy L, Lê KAA. Metabolic effects of fructose and the worldwide increase in obesity. Physiological Reviews. 2010 Jan;90(1):23-46.