Plant Epidemiology is hard to Swallow

In studies of plant chemicals regarding nutrition, we often see a confirmation bias to support the epidemiology (and what our parents have always told us) about the benefits of vegetables and fruits. I’ve read countless studies on this stuff, and it’s almost comical to see that nearly every paper starts with, “We all know that people who eat fruits and vegetables are healthy.” Then the author goes on to describe a study on some isolated plant compound that shows why fruits and vegetables are good for us.

These researchers aren’t testing a hypothesis; they’re merely trying to confirm it. Hence, we have reports that cruciferous vegetables prevent cancer, even though we have data to show that it can either increase or decrease the occurrence of cancer. However, because the existing epidemiology says cruciferous vegetables prevent cancer, we favor the positive data and tend to ignore the negative data.

If I believed that drinking gasoline was a good thing, perhaps because my grandfather told me it was a good thing, and we also conducted a small epidemiologic study that showed that people who’ve ingested gasoline had less incidence of cancer-related death, I’m sure I could design another study to support that conclusion. For example, I could easily take cultured cancer cells and then expose them to various doses of gasoline until I found one that inhibited the growth of cancer cells. Voilà—we now have a mechanistic method by which to show gasoline drinking is healthy and may lower rates of cancer. These situations abound in the literature. Someone looks at an isolated compound in an isolated scenario, which is then extrapolated to the whole of human physiology to support an epidemiologic claim.

Nutrition science continues to make the same mistakes over and over again; we rely heavily on epidemiology and then merely try to use further study to confirm the findings rather than refute them. If you look at an epidemiologic study that shows people who eat more fruits and vegetables appear healthy, you easily could conclude that eating plant-based foods is a healthy thing to do. That’s a very logical conclusion, and no one would fault you for making it. However, if you ask some different questions, things get more interesting. Let’s say that people who eat fruits and vegetables avoid eating snack cakes, donuts, and sodas.

Perhaps they smoke less, drink less alcohol, wear their seat belt, exercise more, have more wealth, and can live in a nicer area. All these things, and likely dozens of other things, contribute to what is known as the “healthy user bias.” In other words, if your overall lifestyle tends to be healthful, how much of the observed improvements in health outcome is attributable to the other factors versus the one particular food being studied. The epidemiologist will attempt to control for these other factors, but really she’s just guessing how much each factor contributes.

I’ve already mentioned some glaring examples of situations where the epidemiology suggests one thing, but real life suggests another. For example, meat is supposedly bad for us and will shorten our lives, yet the population of Hong Kong eats more meat than any other population in the world, and they also live the longest. This observation sets off the immediate cries of, “But they don’t smoke as much; they’re wealthy; they exercise,” and so on, and it’s fine that people make those arguments.

However, when we make the same argument that fruits and vegetables are bad for us, we tend to hear silence from those same people. Nutrition is like politics, and people fight hard for their team. Results that don’t confirm a particular bias are quickly ignored or dismissed. Questioning the current dogma often is met with anger and an almost religious deference to authority and the “consensus”; however, those questions that challenge the status quo should be embraced in a true scientific community.

It’s heretical to suggest that fruits and vegetables are anything but goodness, rainbows, and unicorns. Yeah, we acknowledge that they may have chemicals in them that can cause issues, but, by golly, we still say you need to eat your five (no wait, it’s now ten) servings per day.

Quick, tell me which fruit, vegetable, or other plant is an absolutely essential requirement for human life? If you can think of one, then I’d like to know whether it grows all year round and in all parts of the world. If we have essential requirements for them—and we don’t—we would have had limited access to them for roughly 99 percent of our time on Earth as a species. Given that, why does it make sense to recommend we eat copious amounts of fruits and vegetables every day?

Excerpted from The Carnivore Diet, By Dr. Shawn Baker.
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