In a paper just published in PNAS, scientists use stable isotopes to estimate the contribution of corn to the standard American diet of meat and fries from fast food. They sampled a disgustingly large number of not so happy meals from Burger King, McDonald’s, and Wendy’s and used this form of analysis to determine that a very large percentage of the tissues that make up these meals originate as corn.
How did they do this, and does this finding matter? Well, it’s complicated but I’m happy to explain, and yes, it matters.
This research used carbon (C) and nitrogen (N) stable isotopes to reconstruct the history of meat (beef and chicken). The carbon isotope signature tells us whether or not the animal ate a particular type of grass, which would include but not be limited to corn, during later stages of development. The more grass of this particular type, the more shifted the carbon signal is in a certain direction. The nitrogen isotope signature is not as easily interpreted, and in fact, it’s meaning is empirically derived. It is reasonable to assume that certain nitrogen values mean that the original plant matter from which animal tissue is formed comes from highly fertilized industrialized farms, but N values can also vary because of geographical factors and other things that I will not go into here.
The reason the carbon signal works is this: There are two isotopes of carbon that are reasonably common, stable, and that make up the index number used in this kind of analysis. Meanwhile there are two (simplifying slightly) basic ways in which plants conduct photosynthesis.
In most contexts either of these two carbon isotopes work the same way … which is why they are called isotopes of one element! Different stable isotopes vary only a tiny bit in molecular weight owing to the presence or absence of one little bit in the atom that is seemingly not important as to how each atom behaves. However, it turns out that these two different processes of photosynthesis treat each of these two carbon isotopes differently, differentially using one more often than the other in building complex molecules. So, if you start with a known ratio of these two carbon isotopes in the CO2 being used by the plant, the resulting carbohydrates the plant produces will have a shifted ratio, and this shift will be different for one type of photosynthesis than the other.
Tropical grasses, including corn (which evolved as a tropical grass) have one system of photosynthesis, and temperate grasses have the other. So, if cattle are fed sometimes on the range in North America, and sometimes with corn silage, there will be a mixture of temperate (wild) and tropical (corn) grasses contributing to the C ratios in the food, and thus in the tissues the cattle make out of this food as they grow. If cattle are fed only corn, they will have a very different ratio of carbon isotopes than if they are only range fed in temperate North America.
The long and the short of it is that fast food hamburgers and chickens are astonishingly corn-fed, if this analysis is correct.
I can think of one possible problem with this analysis, which the researchers address but not adequately. Cattle from tropical countries will look just like corn-fed cattle. The nitrogen signatures are consistent with this not being true, but in fact, do not rule it out. The fact that we don’t really know where these cattle come from is a bit disturbing in its own right. While I suspect this effect may be important, I very much doubt the conclusions of this analysis would be affected very much by the presence of tropical beef or chicken in the samples.
Why is this disturbing? Two reasons, depending on your perspective.
If you are someone who eats, you should know that our food supply is based on corn. This means that the food supply is vulnerable to crash if this one element … corn … crashes. And it can crash.
It does not matter that you don’t ever eat fast food. (Who does, after all? Certainly not you or me!) As pointed out in this paper, this apparent corn dependence is not confined to the three big fast food chains and their beef and chicken. It does not matter if you are a vegetarian either. If the corn supply crashes, the meat supply crashes, and the meat eaters, who tend to be well armed and organized, will come after your vegetables.
The other reason this matters is because of the financial structure of the food industry. I do not honestly know how much of an effect subsidies have on corn production, but it is the case that corn production is subsidized. This means that not only are there a few varieties of a single species grown using one method to provide us with a large percentage of our food, but the financial structure on which this industry is built is to some extent similarly undiversified and thus possibly vulnerable.
What is the solution? First, plant a garden! If everyone who could plant a garden did, a measurable percentage of our food supply would be not based on the industrialized corn industry and we would all get more exercise. Then, while you are waiting for your tomatoes to ripen, ask our politicians what they are doing about this. Make this an issue for the next election. Go to your party’s platform conventions and bring this up as an issue for discussion. Learn about it. Blog about it.
Speaking of learning and blogging, here are some more posts on this research.
A. H. Jahren, R. A. Kraft (2008). Carbon and nitrogen stable isotopes in fast food: Signatures of corn and confinement Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.0809870105