Dining on DNA

I’ve been reading a lot about genetically modified critters lately–in particular, the U.S.’s first commercially available GM pet: GloFish. The pets are zebrafish–a black-and-white striped tropical fish–that have been modified to express fluorescence genes from jellyfish and other marine organisms. As a result, they glow brightly in shades of red, orange, and green.

Of course, the fish aren’t meant to be eaten, but there’s no predicting what toddlers will put in their mouths, so one of the concerns that opponents to the fish, and genetic modification in general, have raised is: What happens if you snack on something that’s been adulterated with some unusual DNA?

Every time this question comes up, scientists and experts reply with the same, reasonable answer: We’re already gobbling down DNA. I knew this, but only recently did I stumble upon a quantification of the DNA we dine upon: By one estimate, the average human eats 100 trillion more than 100 trillion genes every day. (Of course, genes are tiny, so, by weight, humans are consuming something like 0.1 to 1.0 gram of DNA daily.)

And yet, we hardly turn into orange trees when we eat orange DNA or trout when we eat their fishy genes. So what does happen to the DNA we eat? Most of it gets destroyed in the bubbling cauldron of acids and enzymes in the gut. Of the DNA that remains, some passes the rest of the way through the digestive system, and, a tiny bit, it seems, may be able to sneak into the bloodstream.

Studies done in mice in 1994 and 1997, for instance, documented what happened when mice were fed some foreign DNA. Within two hours, fragments of DNA were detectable in the rodents’ blood. But it was an exceedingly small amount–the 1997 study revealed that between 0.04% and 0.08% of the genetic material that the mice had ingested made its way into the blood.

It’s not clear what exactly happens to the DNA that winds up in the blood, but given how much DNA animals eat and how rarely it ends up expressed in their own cells, scientists suspect that this circulating foreign DNA is somehow destroyed, disposed of, or inactivated. (The fragments that appear in the bloodstream are also sometimes so small that they don’t contain full genes.)

There is also evidence that DNA from the food we eat–whether it’s genetically modified or not–can make its way into the genomes of the many microbes that live in our digestive systems, but scientists still have a lot more to learn about what the consequences of this transfer may be.

There will surely be many more studies of the fate of DNA in our guts as GM food proliferates. But for now, I’d like to leave you with this one thought: A  smidge of DNA–what doesn’t get digested or absorbed by bloodstream–travels all the way through the digestive tract. And so, for this curious correspondent, I have an answer: Yes, Virginia, there is DNA in your poop.

Further Reading

Alexander, Trevor W. (2006-12-15) Conventional and real-time polymerase chain reaction assessment of the fate of transgenic DNA in sheep fed Roundup Ready rapeseed meal. British Journal of Nutrition, 96(06), 997. DOI: 10.1017/BJN20061935

Forsman, A. (2003-12-1) Uptake of amplifiable fragments of retrotransposon DNA from the human alimentary tract. Molecular Genetics and Genomics, 270(4), 362-368. DOI: 10.1007/s00438-003-0930-3

Hohlweg, U. (2001-4-23) On the fate of plant or other foreign genes upon the uptake in food or after intramuscular injection in mice. Molecular Genetics and Genomics, 265(2), 225-233. DOI: 10.1007/s004380100450

Jennings JC, Kolwyck DC, Kays SB, Whetsell AJ, Surber JB, Cromwell GL, Lirette RP, & Glenn KC. (2003) Determining whether transgenic and endogenous plant DNA and transgenic protein are detectable in muscle from swine fed Roundup Ready soybean meal. Journal of animal science, 81(6), 1447-55. PMID: 12817492

Palka-Santini, M. (2003-11-1) The gastrointestinal tract as the portal of entry for foreign macromolecules: fate of DNA and proteins. Molecular Genetics and Genomics, 270(3), 201-215. DOI: 10.1007/s00438-003-0907-2

Schubbert, Rainer. (1994) Ingested foreign (phage M13) DNA survives transiently in the gastrointestinal tract and enters the bloodstream of mice. MGG Molecular , 33(5), 103-504. DOI: 10.1007/BF00285273

Schubbert, R. (1997-2-4) Foreign (M13) DNA ingested by mice reaches peripheral leukocytes, spleen, and liver via the intestinal wall mucosa and can be covalently linked to mouse DNA. Proceedings of the National Academy of Sciences, 94(3), 961-966. DOI: 10.1073/pnas.94.3.961

Sharma, Ranjana. (2006-03) Detection of Transgenic and Endogenous Plant DNA in Digesta and Tissues of Sheep and Pigs Fed Roundup Ready Canola Meal. Journal of Agricultural and Food Chemistry, 54(5), 1699-1709. DOI: 10.1021/jf052459o

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