Recently, an article was published in the Journal of the American Medicine Association titled “The Misuse of Meta-analysis in Nutrition Research.” The article discusses the problems with our current dietary evidence and how the food industry influences these studies. From the article:

“The food industry is well aware of the power of science-driven headlines and has invested in meta-analyses. In the process, nutritional science may be adversely affected.”

For those of us who like to stay aware of the most up-to-date information regarding nutrition, what does this mean? How can we carefully interpret what we are reading and draw reasonable conclusions? As a physician, analyzing journal articles is something I do on a regular basis, so here are my thoughts.

With so much nutrition advice available, how do we know what guidance to follow? In addition, in the medical and nutrition community, we seem to keep changing our minds and studies contradict one another. For example, should we be avoiding fat or carbs? Are there health benefits of cooking with coconut oil, or should we avoid this source of saturated fat as recommended by the American Heart Association?

Finding true evidence-based nutrition information can be challenging, mostly because good diet studies are difficult to perform. They are costly to do on a large scale, and the studies depend on the subjects to strictly adhere to a diet plan despite being at home with busy lifestyles. They also require participants to accurately recall and report the details of the diet each day. This leads to a problem in the medical literature called “recall bias.”

Additionally, most nutrition studies do not consider the differences in the genetic and metabolic make-up of the participants. One-diet-fits-all may not be the best approach.

The best way to do a study to prove a particular diet has better long-term outcomes is to take two large groups of people whom you think are identical, and feed each group a different diet, knowing exactly what they are eating for a long period of time. An even better approach would be to have both diets look the same, even though they are cooked differently. This way the subjects and the investigators would both be blind to which group the subjects belong to, or “double blind.”

This was accomplished in one of the most rigorous diet trials ever conducted, The National Diet-Heart Study. The researchers spent millions and ran this study for 5 years at 6 different long-term care facilities so that they could be completely aware of what the subjects were eating. One diet was cooked in vegetable oil with the thought of lowering saturated fat. The other diet was cooked in animal fat or saturated fat. But, they looked exactly the same on the trays.

What the investigators found was so surprising that they could not make sense of the results, so the study was not published for 40 years! The group with the lower saturated fat diet did have lower cholesterol, but they didn’t live longer. The opposite was true, the vegetable oil diet was associated with earlier death.

This brings up two important points.

The first is that of “surrogate results.” This occurs when a study shows that a diet lowers a lab value such a cholesterol or blood pressure but does not give us the clinically significant result we are looking for such as preventing death, heart attack or kidney failure. We have to be careful of these kinds of reports when we are reading nutrition advice.

The second is that it matters how we lower cholesterol. By substituting natural animal fats with synthetic vegetable oils, we are exposed to a chemical called linoleic acid. There is evidence that linoleic acid builds up in tissues and is related to inflammation. Exposing ourselves to this synthetic chemical at 5 times a higher rate than our ancestors, we may be doing more harm than good when lowering cholesterol.

So how should the above information affect your diet and nutrition? First, be careful how you interpret nutritional data, different sources have different biases. Second, look for foods in their most natural state, without added chemicals and preservatives. In terms of oils, extra virgin olive oil is a good choice, as it is plant-based, lower in saturated fat and typically unrefined.

References:

https://jamanetwork.com/journals/jama/article-abstract/2654401

http://www.bmj.com/content/353/bmj.i1246