The latest issue of PLOS One contains an article titled Crohn’s Disease and Early Exposure to Domestic Refrigeration. The major result?
This study supports the opinion that CD is associated with exposure to domestic refrigeration, among other household factors, during childhood.
Say whaaaa? Refrigeration is causing disease? Well, okay, that’s not quite what the researchers are saying. (Remember correlation does not equal causation). So what are they saying? Well, to understand, we first need a quick primer on Crohn’s Disease:
We all have trillions of bacteria living in our gut, and for the most part, it’s a peaceful co-existence. They eat a little of the food, and help us digest some of it, and we provide a warm safe environment for growth. When a person gets CD, though, their immune system suddenly begins seeing these little buggers as a threat and starts attacking them. The result is a chronically inflammed intestinal tract, and some nasty GI problems. No one knows exactly why some humans get it, but we do know that it’s a mix of genetic and environmental factors.
This study aimed to find out what environmental factors might be associated with the disease. After comparing people with and without CD, refrigeration turns out to be one of several domestic technologies associated with increased prevalence of the disease. So, it could be that refrigeration itself is the cause, or it could be a standard-of-living thing.
CD is in an interesting class, because it’s a mostly first-world disease – that is, incidence is highest in people with higher incomes and higher standards of living. Most diseases that we think of (say, AIDS or malaria) are associated with the poor. It’s easy to see why that’s so – contaminated water, poor access to doctors, limited education, etc.
So why the first-world association? Well, one hypothesis is that it’s all about hygiene. Specifically, it says that our germophobic lifestyles are to blame. Since we don’t get exposed to as many microbes on a regular basis, when we see an unfamiliar one, our immune system is more likely to go all kung-fu on it.
Based on the results of this and one other study, the authors float a second idea, which they call the “cold chain” hypothesis. The gist is that while most bacteria grow very very slowly at cold temperatures, there are a few species that can grow better than others. When we buy cold foods at the supermarket, they’re likely to be on ice for a few days in the supply chain, then a few more days in our fridges, giving these cold-resistant bacteria time to grow up.
Of course, this all hinges on the idea that these cold-tolerant bacteria are uniquely responsible for CD. I’ve read a fair amount of the literature on CD, and frankly, I’m not convinced. Several individual types of bacteria have been proposed as causative in the past, and so far, none of those ideas have panned out. (MAP, enteroadherent E. coli). It seems more likely that many different types of bacteria can take advantage of a susceptible patient.
I’m also skeptical because, as they state in the paper, ownership of televisions, cars and washing machines were also associated with the disease, nearly to the same extent. Given all these confounding factors, I’m more inclined to believe that the association with refrigerators is just a byproduct of increased standard of living. In that case, this study can be seen as just another data point cementing CD’s status as a first-world disease and perhaps even lending support to the hygiene hypothesis.
Fatemeh Malekzadeh, Corinne Alberti, Mehdi Nouraei, Homayoon Vahedi, Isabelle Zaccaria, Ulrich Meinzer, Siavosh Nasseri-Moghaddam, Rasoul Sotoudehmanesh, Sara Momenzadeh, Reza Khaleghnejad, Shahrooz Rashtak, Golrokh Olfati, Reza Malekzadeh, Jean-Pierre Hugot (2009). Crohn’s Disease and Early Exposure to Domestic Refrigeration PLoS ONE, 4 (1) DOI: 10.1371/journal.pone.0004288