We're used to thinking of nutrition the way we think about computers - input and output. For example, you eat carrots, you get lots of vitamin A. You eat strawberries, you get vitamin C. If you're deficient in a vitamin, take a supplement... or eat more foods containing that vitamin.
So far so good. Only, now science is teaching us it's more complicated.
For one thing, the exact nutrients you need may depend on your genes: see this article, "The nutrigenomics frontier". Which makes the ubiquitous "recommended daily intake" (RDI) nothing more than a ballpark estimate.
But there's another emerging facet of nutrition. There's increasing evidence that if we're going to make use of the nutrients we ingest through asparagus or chick peas or any other food, the body has to act on those nutrients. Sometimes the body's actions make things better for us, and sometimes they produce a bad result.
|Hamburger (source: Wikimedia Commons, PDphoto.org)|
Let's take red meat. Recently in Nature Medicine, a study found that omnivores who took a compound (found in red meat) called L-carnitine had higher markers of arterial plaque buildup than vegans or vegetarians who also took the compound. It also happened that the omnivores had very different gut bacteria, as measured through fecal samples, than the vegans/vegetarians. That would be correlation, not causation.
Enter the mouse part of the study. Here, the researchers found that arterial plaques increased in mice with normal gut bacteria when they were given L-carnitine. The red meat compound and the gut bacteria appeared to be partners in contributing to heart disease.
As a point of comparison, other L-carnitine-eating mice were given antibiotics that cleared their gut bacteria. Arterial plaques did not increase in these mice.
What are we left with? Certainly NOT that we should pop antibiotics alongside our red meat in order to prevent heart disease. (For reasons why antibiotics are not the answer, check out this article by Carl Zimmer.)
We're left with a caution against red meat, I guess. An explanation of why vegans and vegetarians might have a lower risk of heart disease. But the research is preliminary, so personally I'm not going to swear off burgers forever.
Most importantly, I think we're left with a picture of nutrition that's about more than just input. It's a picture of an ecosystem, where each thing interacts with other things to produce an output - in this case, heart disease. Gut bacteria seems to be a big player in these actions. In this article, Stanley Hazen, head of cardiovascular medicine at the Cleveland Clinic, says: "Bacteria make a whole slew of molecules from food... and those molecules can have a huge effect on our metabolic processes."
One more thing: after reading this study, I wouldn't be too quick to take L-carnitine supplements. Certain gut flora might love them... but it's possible that could have negative results.Koeth, R., Wang, Z., Levison, B., Buffa, J., Org, E., Sheehy, B., Britt, E., Fu, X., Wu, Y., Li, L., Smith, J., DiDonato, J., Chen, J., Li, H., Wu, G., Lewis, J., Warrier, M., Brown, J., Krauss, R., Tang, W., Bushman, F., Lusis, A., & Hazen, S. (2013). Intestinal microbiota metabolism of l-carnitine, a nutrient in red meat, promotes atherosclerosis Nature Medicine DOI: 10.1038/nm.3145