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Diet, health, and evidence

November 11, 2020

Good press is being given to a recent meta-analysis showing that those who eat more chili peppers enjoy almost a quarter reduction in all cause mortality, as well as death due to heart disease and cancer. And those of us who love all things capsicum rejoiced.

But, nutrition science is trickier than many realize. The studies analyzed all were retrospective. It might be that people who report they eat more chilis differ in relevant ways that are incidental to their chili consumption. Perhaps they also eat more tomatillos, and that in fact is the magic food, and the studies didn’t ask that. Perhaps they have genes that influence both their appetite and their health. It could be that people who like chilis live longer, whether they eat them or not. Or perhaps they have higher income. Something we know is associated with mortality. (I suspect, but didn’t check, that the study controlled for that.)

It’s impossible to control for all confounding factors. Which is why randomly controlled trials are so important. The problem with nutrition is that is hard to do with human diet. How do you find a thousand volunteers who are willing to be randomly assigned to the chili / no chili arms for a year? That’s a bit tougher than taking a drug or a placebo.

To make up for that, those looking at nutrition try several things. They do studies on mice. You can assign them to any trial you want. They will eat anything. They don’t live long. All of which makes them handy for experiments. Alas, all of which raises questions about how much the results apply to people.

Another approach is to look for causal mechanisms. If, for example, scientists can figure out that capsaicin has wonderful health benefits, that solves the puzzle. Note, though, that the study above doesn’t provide evidence on what capsaicin does. There might be other ingredients in chili peppers that are playing an important role, either alone or along with capsaicin.

And researchers do meta-analyses, like the one referenced. If the primary data comes from retrospective studies, then it is good to look at many of them, from different researchers and different nations.

So I will relish that there is some evidence that the serranos in yesterday’s pico and that the jalapenos I chop tomorrow for chili may be doing us well. We would eat them anyway.

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