Dear Doctor: It seems like if you just wait long enough, everything about nutrition gets contradicted. So now fat is good, carbs are bad, and we don't really need more than 4 servings of vegetables a day? Really? Is everything we've been taught wrong?
Dear Reader: We hear this same frustration from many of our patients, and honestly, we're right there with you. Healthful eating is an important goal, but how are we supposed to achieve it if the rules keep changing?
When it comes to why nutrition advice keeps changing, several factors come into play: science, media and marketing.
Conclusions regarding diet and nutrition are frequently drawn from observational and epidemiological studies. Unlike in a clinical trial, where a single variable, such as a medication or a treatment approach, is scrutinized repeatedly in a randomized and rigidly controlled environment, observational and epidemiological studies draw conclusions by identifying and analyzing trends within large population groups.
Well-designed observational studies can provide valid results. But it's important to understand that the data they use include multiple variables that can't be controlled. Eggs and coffee are two culprit foods that have gone through the "it's bad"/"no, it's good" whiplash. Each of those foods was the focus as researchers combed through piles of data. However, the people in those studies were eating and drinking many more things. Add in variables like lifestyle and environment, and isolating the effect of eating any one specific food becomes challenging.
When researchers publish results that suggest a causal link between a certain food and a particular outcome, they acknowledge that it's just one single data point in an ongoing analysis. It takes time and repetition to reach a reliable conclusion. But scientific rigor pretty much flies out the window when it comes to how the emerging studies get publicized, which leads us to the second factor -- the media.
While scientists are saying, "In this one study, we see a link between eggs and heart disease," in the hands of over-eager newscasters, this is translated as "Eggs will kill you!" Meanwhile, time goes on. Many more studies are conducted. A decade or so later, armed with a wealth of data points collected over the years, the original thesis doesn't hold up. For the scientists, it's the nature of research -- precision and repetition over time. For the rest of us, it's a new -- and contradictory -- headline. "Eggs are good!"
Which brings us to marketing. Diet and nutrition are multibillion-dollar industries in the United States. That means that as food recommendations are crafted, a good bit of lobbying takes place. When new dietary guidelines were released in 2015, a number of nutrition experts, including Dr. David Heber, founding director of the Center for Human Nutrition at UCLA, bemoaned the influence of the food industry in the process.