It's happened again. Did you see it? Another study (actually, this time there were five) was published that is being reported on with wildly exciting headlines has sparked controversy in the field of nutrition research over what we should and should not be eating. The target this time: red meat.
I'm going to spoil this whole post for you right here, because here's the gist of what we think you need to know about this latest research: Eat a little bit of meat (if you like it), eat a lot of vegetables (even if you don't), and eat as many different kinds of foods as you can that are as minimally processed as they can be. Do this, and you should be good to go.
Now, for the Nitty-Gritty details.
Before we dive into what this research means, let's take a second to talk about nutrition research generally.
The goal of nutrition research is to try to understand the very specific and nuanced ways food makes us more or less healthy. Let's say we wanted to know if ketchup was good for us or bad for us (and how it was good or bad). In an ideal world, you would take two identical people, randomize them to eat EXACTLY the same thing, in the same way and at the same time, except for the amount of ketchup they ate. One would eat ketchup, one wouldn't (in a truly ideal world you would have a lot of people who were exactly the same do this so you could have varying degrees of ketchup intake). In addition, every other aspect of these two people's lives would be kept consistent between them as well (exercise, sleep, sex, travel, hydration, and on and on and on.)
These identical people would do this same thing for years - their whole lifetime - and every ache, illness, joy, change in vertical height or deterioration in eye site would be tracked. Eventually, you would have a complete picture of two people's health status and, if the the only difference between them was their ketchup intake, you could feel pretty certain that those differences were because of ketchup.
This example is extreme, and rather ridiculous. But that is the essence of a randomized controlled trial (an RCT), which is often considered the gold standard in research): keep as many things as possible the same between two identical groups of people except for the one thing you're interested in (ketchup) and see how they differ with respect to health (heart disease, diabetes, or number of cavities). In the field of nutrition research these are extremely difficult and time consuming (especially over long periods of time) and, perhaps most importantly, do no simulate the real world experience of people (no one eats the same amount of ketchup everyday, nor can they carry that on indefinitely).
So instead, nutrition research often relies on cohort studies (or case control studies to a lesser extend, which we won't talk about here). In a cohort study, you take a large group of people, ask them about what they ate (the way you collect nutrition information from them is a subject for another very long blog post), and following them through some period of time tracking their health. These studies are far simpler to conduct (for many reasons, not the least of which is people are willing to consider participating in something like this as opposed to a lifetime of not eating ketchup), but they are prone to potential error in the conclusions made between food and health and so the evidence coming from them is more likely to be taken with a grain of salt (metaphorically speaking). Still, for many of the kinds of questions we have about food and health, this is the best we can do in a real world setting.
So, what's so different about the new studies on red meat intake?
Four systematic reviews addressed the health effects associated with red meat and processed meat consumption, and one systematic review addressed people's health-related values and preferences regarding meat consumption. A systematic review is an in-depth, detailed, summary of the current state of science on a particular topic. rather than conducting just one RCT, they looked at all the RCTs that have been done on this topic and wrote a summary of those findings. These systematic reviews differed from one another in two important ways:
- The kind of study being conducted: they looked just at RCTs in one study and just at Cohort studies in two others and
- Different health outcomes: cancer cases and cancer deaths, cardiometabolic outcomes, and all-cause deaths
[One last note about these new studies. The researchers were grading the quality of the evidence they reviewed on something called the GRADE system, which is a comprehensive system for evaluating the quality of the data and findings and much more heavily weights things like RCTs compared to any other kind of study design. Okay. Your PhD in nutrition research methodology is now complete. Congratulations.]
Enough already ... what did they find?
Study 1: Cohort studies of the relationship between red meat and cancer. This study examined 56 different cohorts (groups of people) from 78 eligible studies (from the 118 identified) and found "Low-certainty evidence suggested that an intake reduction of 3 servings of unprocessed meat per week was associated with a very small reduction in overall cancer mortality over a lifetime."
Study 2: Cohort studies the relationship between red meat and heart disease. Data from 105 studies concluded "low- to very-low-certainty evidence that dietary patterns lower in red and processed meat intake result in very small or possibly small decreases in all-cause mortality, cancer mortality and incidence, cardiovascular mortality, nonfatal coronary heart disease, fatal and nonfatal myocardial infarction, and type 2 diabetes."
Study 3: RCTs examining the relationship between red meat and cancer, in women only. The authors of this study conclude that there is "low- to very-low-certainty evidence suggests that diets restricted in red meat may have little or no effect on major cardiometabolic outcomes and cancer mortality and incidence" in women but that although "our results ... do not support the recommendations in the United Kingdom, United States, or World Cancer Research Fund guidelines on red meat intake ... neither do they seriously challenge those recommendations."
Study 4: Cohort studies examining the relationship between red meat and all-cause mortality (death). Again the authors find that "The magnitude of association between red and processed meat consumption and all-cause mortality and adverse cardiometabolic outcomes is very small, and the evidence is of low certainty."
Although we believe these papers are well researched (they applied the appropriate safe guards and used rigorous methods and clear inclusion/exclusion criteria, etc, etc), there are still so many potential flaws in these studies and the ones they are based on that this should not be as attention grabbing as it has been. If there's one constant in nutrition research it is that it is always changing.
The researchers of these new studies said it best themselves: "there isn't great evidence to support current recommendations for red meat consumption nor can we refute them because we don't have a lot of certainty in our findings" (or in those of the studies that their findings are based on).
To sum up the summary: the headlines are supposed to grab your attention. Eat a little bit of meat, eat more vegetables. Enjoy food. All kinds. In moderation.