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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14 Responses to Dining on DNA

  1. Eric Baumholder says:

    I really like this:

    “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 genes every day. ”

    And I’d like to pass it along as a valuable datum. But to be safe, I’d need a cite. Do you know what the source is? Thanks in advance.

  2. Zoonotica says:

    Oooh – so if you ate the glowing zebrafish you could potentially have glowing bacteria in your gut! That’s so cool! (Although I appreciate I may be alone in thinking this…)

  3. Zoonotica says:

    Oh I so fail at html – sorry!

  4. Emily Anthes says:

    Sure. This came from a letter that a researcher wrote to the California Fish & Game Commission. But I have a query into the researcher about where he got the figure from. Will let you know.

  5. Ed says:

    Which reminds me of what was one of my favourite stories from last year – the gut bacteria of Japanese people contain seaweed-digesting genes (which Americans lack). They got these genes from marine bacteria, that stowed away on seaweed eaten by Japanese people in the days when they didn’t cook the stuff first.

  6. Emily Anthes says:

    Wow–what a great post and study, Ed. Fascinating stuff.

  7. Emily Anthes says:

    Ok, spoke with the researcher, who says that he just made a back-of-the-envelope calculation. And, checking his math, he figures that the 100 trillion number is probably low. So the true figure, which–again, he’s just estimating theoretically–is probably even higher than that.

  8. Carl says:

    I’ve never “back of the envelope” calculated it either, but it has to be a lot. Every cell of all the plant or animal material that you eat has DNA in it. Secondly, there are trillions of microbes on our skin and in our gut that die and we also ingest their DNA.

    Gene transfer is very common in prokaryotes. As far as eukaryotes it has only been documented in some fungi. And as you say in your article, the data suggesting it happening in humans is inconclusive. If we could ingest DNA like bacteria, gene therapy would have been a lot more successful by now. Currently the only suspected foreign DNA we have is in our mitochondria.

    Just to clarify Ed’s comment:
    “They got these genes from marine bacteria,… “

    It’s not the DNA in the gut of the Japanese, but the DNA in the marine bacteria that assist in breaking down the seaweed. These bacteria have become part of most Japanese people’s microbiome due to the large amount of seaweed they consume.

  9. Eric Baumholder says:

    Doing the computation as described, on the back of an envelope, is problematic.

    If you’re talking about the total genes consumed, that would actually be the total exons per chromosomal set. Ploidy varies dramatically between different species, meaning total exons can vary 400 percent or more depending on species. Also varying between species is the size of individual cells; larger cells means less-abundant DNA per unit of volume.

    Food preparation is also an issue — extremes of heat, cold, pH, etc. can badly disrupt DNA, vastly reducing the number of exons per mouthful. You’d have to stick with raw, fresh animal or plant matter to ensure you ingested all available exons.

    In short, I don’t see how this can be an easy computation. Unless you make some truly remarkable assumptions.

  10. Bacteria have entire sets of genes dedicated to bringing DNA into their cells. The conventional view is that they take up this DNA to get new genes, but I’ve spent much of the last 20 years trying to convince people that bacteria use the DNA they take up mainly as food.

    From our old web site: “Arguing that bacteria take up DNA to get new genes is like arguing that a hungry person eats steak in the hope of getting cow genes that will let them eat grass.”

  11. @Eric: Let’s just consider the bacteria we swallow in saliva. We swallow about 2 l of saliva/day. The only value I can find for the bacteria content is 5×10^8/ml, but this is offered as an upper limit and seems way too high. Say 5×10^7/ml, with about 5000 genes/bacterial cell. If I haven’t dropped any decimals, I think that’s 5×10^14 genes/day. A trillion is 10^12, right, so that’s 500 trillion genes just from the bacteria in our saliva.

  12. Yogi-one says:

    Where can I get some T-rex DNA to chow down on? Bwa-ha-ha-ha-ha!

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  14. Candide says:

    I’d like to keep eating the DNA that was prepared for me by nature, not the one Monsanto tries to feed me. I know that nature’s eventually going to kill me, but Monsanto wants to cash in before that happens.

    Life’s too short to let them.