Dietary Preference: Hold the Pickles

Written by Kendal Hirschi

DNA barcoding can be used to determine dietary preference, telling us how species can co-evolve when resources are limiting.
My son always orders his hamburger without pickles. He is six and this has been his preference ever since he could communicate. Meanwhile, his twin sister appears to live on a diet of chips and chocolate milk. While the boy avoids pickles, he consumes Korean, Japanese, Thai, and Texas cuisine with zeal. I am often left wondering what causes her to be a picky junk-food junkie while he experiments – except with pickles! Maybe part of the difference is permissive parenting (guilty!)– but is there also a genetic component to dietary preferences?

Pickle600
Pickles are a common food aversion. Credit: National Institute of Health. Wikimedia Commons

Some eating habits are genetic

frank-chips-small
“I’ll just have one I think…” Credit: KJHvM

Our species has set boundaries on eating habits, but we are remarkably varied within these boundaries, as my twins demonstrate. Research has shown that these variations are not entirely cultural or learned. Genes and culture appear to have co-evolved to produce variation in dietary habits (1). In such cases as lactose tolerance in adults, this co-evolution is well understood. It is less well understood in other cases, such as favism, where sufferers cannot make an enzyme that affects the circulation of sugar in their bodies. Very often, this condition is found in regions where there is also malaria. This may mean that the mutant red blood cells found in people with favism protect them from malaria.

Dietary preference and ecology

Differences in dietary preference appear to also allow otherwise similar species to coexist in the same environment. A guild (or ecological guild) is any group of species that utilize the same resources. Let’s imagine we live in an ice-cream store that has 31 flavors but only limited amounts of each flavor. If we all preferred chocolate-chip ice cream, life would be stressful. Life would be easier if we had different preferences. Even if we have different preferences, life gets complicated if we run out of our preferred flavor!
In the real-world, evolutionary biologists look at dietary differences between grazers and browsers. Grazing animals feed primarily on grass. Grass can be difficult to digest and is wearing on the teeth. A browsing animal, on the other hand, feeds on leaves, buds, and the like. Such food is easier to digest, so that the digestive system does not need to be quite so big. A browser and a grazer should be able to live in the same guild without much conflict. But what happens when multiple grazers occupy the same guild? This sounds complicated and confrontational!

DNA barcoding can tell us about dietary preference

A recent paper in the Proceeding of the National Academy of Sciences (PNAS) used DNA barcoding to investigate dietary preferences among large herbivores in Africa (2).

Zebra eating
Grevy’s Zebra eating, by bobosh_t. Wikimedia Commons

DNA barcoding compares short genetic markers in the specimen DNA with reference sequences. Like the barcodes on items in the supermarket, each DNA barcode indicates a particular species. The authors collected feces from large mammalian herbivores in the semi-arid African savanna and used barcoding to see what plant material the animals had consumed. They measured diet breadth, composition, and overlap for seven abundant herbivore species. These species ranged from almost-exclusive grazers to almost-exclusive browsers. For example, zebras get most of their calories from grasses (99%) while small antelopes called dik-diks get less than 1% of their calories from grasses. While this technology is new for dietary studies and ecology, some findings confirmed previous studies. For example, dietary overlap was greatest within species and between species that were similar in body size.
DNA barcoding also allowed more sophisticated data analysis. Diets were strongly divergent across species, irrespective of feeding guild. Grazers ate similar total amounts of grass but different suites of grass species. For example, two different zebra species each ate about 45 plant species, but 15 grasses differed significantly between their diets. DNA barcoding allowed the researches to identify nuances in the diets that could not be seen using ‘older technology’. These results support the theory that large mammalian herbivores partition food resources in order to coexist. This partitioning of resources may be due to dietary preference.
Unlike our fantasy ice cream shop, diets in the savanna can vary seasonally. These data from herbivore feces were gathered in a wet season when food was abundant. Such abundance likely masks some of the differences among species that occur in times of food insecurity.

Closing thoughts about dietary preference

This research shows that DNA barcoding can be used as a universal tool for dietary traceability. The technique has created the clearest picture yet of grazing habits of animals on the semi-arid African savanna.

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Jack Spratt and spouse, Mother Goose’s Nursery Rhymes

To go back to the ice cream shop analogy, this technology is like being able to tell retroactively which kind of ice cream a person prefers, including the specific flavors and quantities. It’s like being able discriminate between chocolate-chip ice cream lovers and chocolate-chip cookie dough lovers. For people in an ice cream shop, we can just ask or observe what types of ice cream they like, which is a good thing, because it’s unlikely there’s enough DNA in ice cream to detect in a person’s poop. But for herbivores on the savanna that would be difficult to observe all the time, this DNA barcoding technology could help us learn which plants the animals eat. [Paragraph edited for clarity on Sept 30 by Anastasia.]
Thankfully, I am no longer dealing with twins in diapers. While this does not diminish my curiosity about their differences in dietary preferences – it does negate my ability to readily research the issue.
Finally, have I overlooked a gender effect? After all, such a difference could greatly affect demand for scarce resources. Remember Jack Sprat? He could eat no fat. His wife, unnamed, could eat no lean. But between them both, they licked the platter clean.

References

  1. Krebs JR. The gourmet ape: evolution and human food preferences. Am J Clin Nutr. 2009 Sep;90:707S-11S.
  2. Kartzinel TR, Chen PA, Coverdale TC, Erickson DL, Kress WJ, Kuzmina ML, Rubenstein DI, Wang W, Pringle RM. DNA metabarcoding illuminates dietary niche partitioning by African large herbivores. Proc Natl Acad Sci U S A. 2015 Jun 30;112:8019-24.

Written by Guest Expert

Kendal Hirschi works at the Children’s Nutrition Research Center at Baylor College of Medicine and is Associate Director of Research at the Vegetable and Fruit Improvement Center at Texas A&M. His research program centers on many biomedical issues and has published papers using bacteria, yeast, crops, zebrafish, mice, and human subjects. His research goal is to increase the nutritional content of crops, in collaboration with clinical faculty at Baylor College of Medicine. His long-term goal is to bridge the chasm between plant biology and nutritional sciences.

25 comments

  1. And I thought “zebra” was one species!
    Could it be that large mammal herbivores can coexist because their resource partitioning allows them to do so? That takes out the purposeful “in order to coexist” from the story.

  2. How much DNA is there in ice cream? I haven’t seen that data. And how exactly would you tell sugar from sugar beets from that of sugar cane?
    Citations needed.

  3. “And how exactly would you tell sugar from sugar beets from that of sugar cane?”
    A sugar psychic could help with that, and they would also tell you about the sugar canes past lives.

  4. I know. I don’t know who you are, hyperzombie , but I know how your brain works. And I’m down with that.
    I am only more baffled by this post the more I think about it. It’s not you, it’s me.
    So I went away to look up some ice cream I eat. I avoid Ben & Jerrys, for reasons probably obvious to most readers here. But let’s say this one–Breyer’s Mint Chocolate Chip.

    MILK, CREAM, SUGAR, CHOCOLATE FLAVORED CHIPS [SUGAR, COCONUT OIL, COCOA (PROCESSED WITH ALKALI), MILKFAT, SOY LECITHIN, VANILLA EXTRACT], WHEY, TARA GUM, PEPPERMINT EXTRACT. Ingredients and Nutrition Facts are current as of August 2015. Please see shelf packaging for any changes. Nutrition Facts may vary in high altitude areas.

    [ok, I am separately baffled by the high altitude ingredients, but let’s forget that for now.]
    Can you DNA bar code some of this for me and show me the DNA?

  5. High altitude cooking often requires some minor ingredient changes, pretty common to see recipes with a high altitude version. Generally the only change is in the ratio of a few ingredients.

  6. Yeah, I have heard of this, but not in ice cream. Please tell show me high-altitude ice cream. Or, perhaps I shall go to Denver, get high, and find out myself….

  7. Hauling ice cream poses unique challenges when changing altitudes. It will push out of the container if not dealt with. (It’s not a true sold, it still flows)
    I’m guessing that the ingredient difference has something to do with that.
    Some of the cream from my cows goes to Ben and Jerry’s. I was an unimportant figure in the corner in a meeting where the possibilities of entirely Non-GMO feed by 2018 was discussed.
    I have to say the answers to the questions and the $$$ proposed brought a small smile to my face.

  8. Excellent–it’s ice cream expansion. I am seriously interested in the chemistry that will fix that. I really hadn’t known of this. I live at sea level.
    But good luck to you and profiting from non-GMO production. If the well-fed need different ice cream ingredients and can afford that, swell.
    Still, though, I’d be interested in seeing the DNA profiles of non-GMO and GMO ice cream compared. Quite frankly, it smells like manure to me.

  9. The smile to my face was from the surprise that the Unilever reps showed at how much more $$$ it’d take to go all non GMO feed
    Production would drop, and costs would go up with all non-GMO feed.
    And I’ll bet anything there could never be any difference whatsoever in the milk.
    I’m looking forward to raising all GMO in the future.

  10. ha ha ha ha ha ha ha
    Ah. I see now. Snicker.
    I’d still be happy to see the data. That’s all I’m askin’.
    And I might buy B&J again if they get off this anti-GMO path. I hope they listen to that data.

  11. I’m sure I’m missing a joke here somewhere but the ice cream is just an analogy. The researchers sequenced herbivore poop and found a variety of grasses and other plants. And they hypothesize that the variation in diets allows more herbivores to live there in different niches than if they all are the same thing. Just like if we were locked in an ice cream store, if all of us only wanted chocolate we’d run out sooner than if each person liked a different type of ice cream.

  12. “These data from herbivore feces were gathered in a wet season when food
    was abundant. Such abundance likely masks some of the differences among
    species that occur in times of food insecurity.”
    Couldn’t that have the opposite effect? In times of plenty, there would likely be more differentiation based on preferences, but under drought conditions, you could get different species competing for the same scarce resources. Previous preferences going by the wayside as survival becomes critical.

  13. Ok, tell me how I’m misreading this:

    To go back to the ice cream shop, we can now use DNA barcoding to tell us not only the kind of ice cream a species consumes, but the variety of flavors and quantities that a particular species consumes. We can now discriminate between chocolate chocolate-chip ice cream lovers and chocolate chocolate-chip cookie dough lovers.

    Look, this is–at best–misleading. But I think it’s more than that. However, let’s give the author a chance to clear it up.

  14. I understand that there is always a risk that someone might take something literally, even an obvious analogy. I’ve changed the paragraph so hopefully it’s clear that no one thinks DNA barcoding could be used to tell what types of ice cream someone ate.

  15. It was not an obvious analogy. It is somewhat better with the fixes. But it’s still an unhelpful choice to try to explain this to people who are already wound up about their Ben & Jerry’s and about what might be in breast milk. And certain folks have been fanning flames about breast milk components.
    Perhaps talking about colleagues at a salad bar, and their choices, might have worked.

  16. It seems the 31 flavors of ice cream at Baskin Robbins may less of a widespread social construct than I thought. Seems to be the go to example for choices, but maybe I’m old. Or is it regional. Or I just eat too much ice cream.
    Anyway, I’m not sure we can avoid all the foods that folks might be concerned about. Anything with milk, fish, tomatoes, corn, soy… I’m sure there’s more.

  17. I think with the changes, it is okay. It sounds much better, and less misleading to say, “It would be as if we could determine the flavor of ice cream someone ate from their poop” than the original that said “we can determine”.

  18. I’m glad my blog post got people thinking about dietary preference and the technical aspects of metabarcoding.
    I apologize for the turbidity of the “ice cream” construct. Anastasia should not be taking the heat for my errors.
    I will try and do better in the future.

  19. Do you understand why this is a problem? Those of us who talk with the general public in lots of forums are continually trying to explain to them that there’s no DNA in a lot of the things that worry them. We are constantly using sugar as an example of how you cannot tell if it’s from sugar cane or sugar beets. An example like ice cream undermines all the education we are trying to do.

  20. Your quote:
    “And I’ll bet anything there could never be any difference whatsoever in the milk.” would very likely be proven to be a conceptual error, if a well designed chemical contaminant and total biochemical assessment were to be done by a dedicated collective of all biochemists selected in a bipartison manner, with independent funding and a dedication to scientific method were goals. If you don’t ever look for differences in the milk, of course, you won’t find any. We have the scientific capability to see a host of differences… but the politics trumps the science…. and then… the ‘science’ is no longer SCIENCE.

  21. I made a mistake and overstated.
    I was thinking of tests ran at the processor when they accept the milk, including a taste test.

  22. While I don’t have any data, I would not be in the least surprised by low, but detectable, levels of DNA in commodity ingredients such as sugar, oil or lecithin. A small, specific, probe for a transgene of interest might still be able to find a match, even with highly degraded DNA.
    I no longer have a lab, but it seems as though this would be relatively trivial to test.

  23. I am fairly sure that using mass spectrometry, sugar from C3 plants (sugar beets) can be distinguished from that of C4 plants (sugar cane) by virtue of the C13 to C12 ratio. C4 plants discriminate less against the C13 isotope of carbon and therefore sugars derived from these plants will have a relatively greater amount. Hopefully someone more conversant with photosynthetic biochemistry will correct me if I’m wrong.

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