We all want to believe that food is medicine. It just sounds so appealing — imagine if you could cure scary diseases by changing your diet just a bit! Instead of dieting to avoid diabetes, what if you could eat a tad more chili, a handful of blueberries, and drink a kale smoothie once a week?
It’s almost certainly not true, of course, but it’s just plausible enough to sound reasonable.
This week was no different. Headlines everywhere have been joyously crying out that chili peppers are not just an important ingredient in kimchi. No, according to news sources across the world, chilies can stop heart attacks and even save your life!
It sounds like a slam-dunk for the food-is-medicine idea. Unfortunately, the reality isn’t nearly as simple as that. We might want to believe that a good curry can save your life, but the science doesn’t really show that at all.
Chili peppers probably don’t stop heart attacks.
The study that has everyone declaring the wondrous healthiness of haboneros was a long-term cohort study looking at about 20,000 Italian people. Basically, the scientists asked a bunch of people questions about what they ate and how they lived, then tracked their health for about a decade. At the end of this period, they found that those who ate more chili peppers had a lower risk of death, in particular death from heart disease, than those who ate very few chilies. In relative terms, chilies appeared to reduce the risk of death by about 25%, which sounds pretty impressive for a seasoning.
Like most epidemiological trials of this kind, the scientists controlled for a range of factors that might be causing both death and chili consumption. It was a large sample, with long-term follow up, and in many ways ticks all the boxes for what is considered a really good study that should be able to tell you something meaningful about health.
So why am I not convinced of the medicinal value of chilis?
I’ve written about the perils of observational trials before — it’s one of the main features of my blog. If you’ve read my blog before, you probably already know that residual confounding could be an issue, that the absolute risk decrease from chilies was probably small (it was), and that there are innumerable reasons that a study like this may not mean very much to your health.
But this time, none of that was really necessary.
You see, if you read the study, something immediately springs out. The scientists measured chili intake by asking people, once, “how often do you eat chili peppers?”, to which people could answer from “never or almost never” to “very often”. This was then used to estimate, vaguely, how much chili people ate each week, from less than 2 times to more than 4. This was then used to categorize people into groups to compare people’s health, often more than a decade later, in terms of hard endpoints like dying from a heart attack.
If that sounds weird to you, it’s because it really is.
For one thing, chili pepper consumption is almost certainly variable over the course of a lifetime — what’s known as a time-varying exposure. Think about how much chili you eat now — is it the same as the amount you ate ten years ago? Twenty? When you were a small child? Probably not. It’s even variable across the year, with chilies going in and out of season regularly. You can’t just infer from a single measurement how much chili someone eats.
Even worse, we know that these food frequency questionnaires can be flawed. There’s a good chance that, not only did people’s intake of chili change over the years, but the measurement wasn’t right to being with. It’s possible that the observed results had nothing at all to do with chilies, because the data gathering didn’t actually measure chili consumption.
This is backed up by the fact that the study didn’t find a biological gradient. Biological gradients are a foundational part of inferring that one thing causes another — what you expect to find is that your risk changes as you are exposed to more of the substance in question. For example, if you smoke more, your risk of lung cancer increases. If chilies are good for your health, then a lot of chili is better for you than a little bit, especially if — as the scientists theorized — the health benefits are due to chemicals present in the chilies.
And, unfortunately, the study didn’t find this. Eating any chilies was as good as eating loads of them, which is what in the scientific world is known as a “massive red flag”.
So, the headlines were wrong. But really, it’s hard to blame the media for this one. There’s not much you can take away from this study except that there are probably fundamental differences between people who choose to eat chilies and people who don’t.
This is, of course, not surprising at all.
The explosion of wonderful headlines is just a by-product of the science being done in the first place. We all want these findings to be true, so when we’re told by fancy scientists that they are, it’s easy to nod along. The reality is that these studies require a lot of expertise to interpret, and probably aren’t worth a huge fanfare of any kind.
Ultimately, it’s extremely unlikely that chilies alone can reduce your risk of heart disease by 25%. Even if you eat a ton of chili, you aren’t getting very much of the compounds that could potentially have health benefits. This study was almost certainly just measuring associations that have nothing to do with whether chilies actually cause a reduced risk of heart disease and death.
If you’re worried about your health, see a doctor, don’t eat a curry. Realistically, it’s unlikely that any single food — from blueberries to jalapeños — is going to make a really big difference to your health, no matter what the headlines say.
Chilies aren’t going to save your life.
But, honestly, I think we all knew that already.