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56% of McNugget Carbon is Corn

I recently came across this stat in The Omnivore’s Dilemma:

Amount of corn-derived carbon present in Pollan's McDonald's family meal, as measured by a mass spectrometer: 
Soda, 100%; milk shake, 78%; salad dressing, 65%; chicken nuggets, 56%; cheeseburger, 52%; and French fries, 23%.

My b*llshit alarm went off, so I started doing a sniff test:

Mass Spectrometric Determination of Cane Sugar and Corn Syrup in Maple Syrup by Use of 13C/12C Ratio: Collaborative Study

Use of C3 carbon dioxide (CO2) fixation cycle by the sugar maple ... and C4 carbon fixation cycle by 2 monocotyledenous plants, corn and cane, results in a physiological discrimination between 13C and 12C isotopes. Therefore, determination of 13C/12C ratio of maple syrup by mass spectrometry can be used to detect adulteration with cane and corn sugars.

H*ly shit!

There are only a few distinct methods of fixing carbon from the air. 13C is only 1% of Earth's carbon, so it stands out. Not all plants can grab 13C equally. And once an organism has 13C from the air (or diet), no biochemical process is going to jettison those extra neutrons.

The mass spec can't seem to differentiate between carbon from corn vs. sugar cane because they're both C4 plants, but the general idea totally checks out.

One sec, I need more research…

Apparently fish show an isotopic signature from their diet:

Similarly, marine fish contain more 13C than freshwater fish, with values approximating the C4 and C3 plants respectively.

A less dramatic effect (with poor p-values) can be seen in grass-fed vs. grain-fed beef:

Natural carbon isotopes, 12C, 13C, and 14C, help to authenticate/trace foods and beverages. Levels of total carbon (TC), 13C (δ13C), and 14C in muscle and lipid tissues from grass-fed versus grain-fed steers are reported. The δ13C in muscle versus lipid of steaks were around 5‰ higher in grain over grass-fed (p<0.05).

Corn isotopes everywhere! Has science gone too far?