A new observational study, partially funded by the International Tree Nut Council Nutrition Research & Education Foundation, claims that eating two or more servings of nuts each day may increase the likelihood of survival for patients with colon cancer. Tree nuts specifically, rather than peanuts, were found to be beneficial. But like all observational studies, this one has significant pitfalls. For one, there is no way of knowing if it is nuts only that lead to health benefits–it could be healthier eating in general. There hasn’t been a definitive mechanism discovered that links nuts to cancer survival, meaning that it can be interpreted as a correlation, but not a causation. The authors of this study also used the same cohort of patients in 2015 to claim that coffee boosted colon cancer survival rates (more on that below), leading us to wonder if they are simply fishing in the data for associations.
Colon cancer is prevalent and often highly aggressive. According to the American Cancer Society, colorectal cancer is expected to cause more than 50,000 deaths in 2018. It’s tempting to think of dietary changes to prevent cancer as a quick and easy fix–but the truth is much more nuanced. Despite the fact that nutrition studies gain a lot of media coverage (this study was covered by more than two dozen media outlets in the past week), they often have many limitations including reliance on observational data and questionnaires prone to bias. It’s important to take a close look at these studies as they become ever more prevalent to distinguish what is really helpful, and what is not.
Neither the news release nor the actual study discuss the cost of nuts.
The news release did list numbers to back up its claim, but relied on relative, not the preferred absolute risk figures. “Those who regularly consumed at least two, one-ounce servings of nuts each week demonstrated a 42% improvement in disease-free survival and a 57% improvement in overall survival,” wrote the news release author. But the wording is confusing–what is the difference between disease-free survival, and overall survival? Disease-free survival requires a shorter follow-up period to assess and means the absence of any form of cancer recurrence. Overall survival simply means the number of subjects that were alive at the end of the trial period. While disease-free survival is reported to be highly correlated with overall survival in some studies, overall survival provides a more reliable assessment as to whether patients are deriving a meaningful benefit from a therapy.
The news release did address a popular concern that since nuts are high in fat and calories, they might contribute to obesity. The author of the study was quoted as saying that actually, people who regularly consume nuts tend to be leaner.
Here lies the news release’s biggest problem: it failed to take into account the multiple, very relevant limitations to this study. Nutrition studies in general have many flaws, which is one of the reasons that we seem to be on a constant merry-go-round when it comes to which foods are healthy or not. Let’s start with the fact that this was an observational study–we have written extensively about how it is impossible to infer causation in observational studies, meaning that while nut consumption may be associated with decreased colon cancer risk, it is impossible to conclude that it is the sole cause of cancer survival. One of the many pitfalls of observational studies include the presence of confounders–lurking variables that might influence the final study outcome. In this case, one confounder that wasn’t addressed head-on was the fact that people who eat nuts may be more health-conscious overall, and more unlikely to engage in risky behaviors like smoking or lack of exercise that can contribute to disease. The authors of the study indirectly mentioned this in the news release. “The results highlight the importance of emphasizing dietary and lifestyle factors in colon cancer survivorship,” said one of the lead researchers. In fact, the study found that those people most likely to eat two or more servings of nuts per week were also more likely to exercise. So who’s to say that it’s the nuts specifically that are driving down cancer risk?
Another limitation of the trial: recall bias. This has the potential to occur in every trial based on a questionnaire, like this one. When self-reporting, people may not remember exactly what they ate in a given week or month. They may also be embarrassed to admit to certain unhealthy habits, and will leave them out, thus skewing the results.
Finally, the news release mentions that the same authors, analyzing the same cohort of patients, also found a link between coffee consumption and reduced recurrence and mortality in colon cancer. We actually wrote about that study when it came out in 2015, and we had the same concerns about observational studies then. It’s also worth noting that now there have been two different results from the same study–what is called multiple endpoints. This can be a warning sign that instead of looking for a specific outcome, scientists are fishing to see what correlations they can come up with.
No disease mongering.
This is listed as “satisfactory” because the release did disclose conflicts of interest, but it is worth noting that some funding for the trial came from the International Tree Nut Council Nutrition Research & Education Foundation. Was this perhaps one of the reasons they decided to break the study into subgroups of peanuts and tree nuts? The news release wrote that “the private sponsors did not participate in the design, conduct, or analysis of the study, or in review or approval of the paper.” However, it’s not entirely surprising that a trial partially funded by the Tree Nut Council found benefits to tree nuts.
As mentioned above, this study was ripe for potential confounders, yet none of these were addressed in the news release. They could have analyzed other foods that are typically consumed by the health-conscious, such as fruits and vegetables, to see if it was really nuts that were making the difference, or if health-conscious eaters were just healthier in general.
Not applicable–nuts are widely available at grocery stores and convenience stores across the country.
The news release mentioned that many studies have looked at nuts, and found that they “may help to reduce insulin resistance… [which] leads to unhealthy levels of sugar in the blood and is often a predecessor to type 2 diabetes and related illnesses.” However, it was not mentioned whether or not nuts have been studied in the context of cancer before, or exactly how insulin resistance may lead to colon cancer. “These studies support the hypothesis that behaviors that make you less insulin-resistant, including eating nuts, seem to improve outcomes in colon cancer,” one of the lead authors said, “However, we don’t know yet what exactly about nuts is beneficial.”
No unjustifiable language.