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Gutopia: A Microbial Paradise

The development of the microscope in the 17th century magnified our awareness of a microbial universe previously invisible to the naked eye. Anton van Leeuwenhoek, a Dutch textile draper and science hobbyist, was one of the first individuals to glance into the microbial looking glass and identify unicellular organisms (so-called animalcules) such as protozoa and bacteria [1]. His colleague, Robert Hooke, went on to publish the seminal text, Micrographia, which described his observations of microfungi [2]. Two centuries later, Louis Pasteur validated the role of microbes in fermentation. However, Pasteur also gave weight to Ignaz Semmelweis’ controversial germ theory of disease stating that microbes have the capacity to cause pathological effects on our human health [3].

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Early microbial drawings by Anton Van Leeuwenhoek [Photo Credit: Yale University Press]

These early studies in microbiology have provided significant insight on the human-microbe interaction characterized by either mutually beneficial or lethal outcomes. The duality of microbial behavior has dramatically impacted our perception of these microorganisms. Our cultural germophobia often precludes our ability to recognize the naturally transformative and symbiotic properties of microbes from the fermentation of grape juice into wine to their invaluable role in human digestion.

Michael Pollan, author of Cooked, explores a myriad of cooking traditions including those that directly involve the action of microbes such as bacteria and yeast. In his depiction of ancient sourdough recipes, he lists four ingredients: whole grain flour, water, salt, and the repertoire of microbes in the air [4]. The microbes catalyze slow-fermentation reactions (taking up to 24 hours) that leaven the bread while transforming molecules into digestible nutrients for us to absorb.

Whether we can stomach it or not, we all possess unique microbial signatures that are composed of trillions of microorganisms living both inside our bodies and on the surface of our skin [5]. These microbial communities, referred to as the human microbiome, cohabitate in our various mucosal, gastrointestinal, and epidermal surfaces. Symbiotic microbes are tolerated by our immune system and work collaboratively with our own bodies to digest the foods that we eat at the molecular level.

Our evolutionary history with bacteria is fueled by the currency of nutrition. In other words, our diet has a significant impact on the composition of our gut microbiota. Food consumption habits can either encourage the intestinal bloom of beneficial bacteria or opportunistic, disease-causing bacteria. Certain foods contain vital prebiotic molecules that encourage the expansion of beneficial bacterial species in the gut. High fiber foods including whole grains (brown rice, oats), vegetables (broccoli, peas) and legumes (lentils, black beans) contain an invaluable source of metabolic substrates that are converted into short-chain fatty acids by bacteria [6]. These short-chain fatty acids, such as butyrate, help to propagate microflora such as Bifidobacterium and Lactobacillus and maintain gastrointestinal tissue health [7].

If we were to design a perfect microbial habitat–a Gutopia, if you will, it would be a homeostatic organ city of diverse symbiotic microbiota that is rich in fiber economy and free of any harmful pollutants. However, our dietary choices can tip the balance of this intestinal paradise and create a dystopic environment suitable for the expansion of pathogenic microbes.

Contemporary eating habits that are characteristically high in fat and carbohydrates are responsible for the emergence of modern diseases such as diabetes, colorectal cancer, and inflammatory bowel diseases [8-11]. Recent studies suggest that compositional changes in the intestinal microbiome can encourage the bloom of disease-causing microflora. The dramatic alteration of today’s eating behavior introduces gastrointestinal disturbances or challenges that our bodies and microbial counterparts have not evolved to accommodate. For example, a study investigating the consumption of different dietary fats in immunocompromised IL-10/ mice identified the expansion of B. wadsworthia; a gram-negative, sulfur-reducing species of bacteria flourished in mice fed a diet high in saturated fat , but not in low-fat or polyunsaturated fat diets [12-13]. The bloom of B. wadsworthia was attributed to the unregulated production of a bile salt known as taurocholic acid brought about by the overconsumption of saturated fat.

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Imbalanced dietary intake and overconsumption of foods high in saturated fats can impact the ecology of gut microbiota [Illustration by Grace Danico]

Taurocholic acid is an important source of sulfur that can stimulate the growth and maintenance of the pathogenic B. wadsworthia in these mice. The presence of this bacterium activates murine proinflammatory immune defense mechanisms that compromise the permeability of gut mucosal tissue causing intestinal inflammation. This pathogenic etiology of gut inflammation is implicated in the onset of Crohn’s disease and ulcerative colitis [12-13].

Much like any city there are complications that arise, which can tip the balance between utopic and dystopic environments. In the case of our gut health, we should imagine ourselves as landscape architects cultivating the balanced ecology of our microbiome. If we feed our intestinal gardens with the right balance of foods we can foster the growth of symbiotic bacteria while discouraging the bloom of pestilent, pathogenic microbial weeds.

The human microbiome not only affects our gut health but can have some profound effects on our behavior and brain function. Dr. Elaine Hsiao, Assistant Professor in the Department of Integrative Biology and Physiology at UCLA, investigates the interplay between our commensal microbes and their role in neurological development and function. Her work has linked the perturbations in gut microbiota with the onset of neurological disorders such as autism [14].

As we continue to study the ecology and diversity of microbes living within and around us, we are faced with many fundamental challenges in testing the dynamics of these microbial communities. From a clinical perspective, physicians and researchers alike have utilized human fecal samples to identify unique microbial gut profiles in their patients. These samples serve as powerful investigative tools in our pursuit to understand how certain commensal microbes can cause or serve as diagnostic readouts. The power of the sequencing technology used to characterize microbes in stool samples (16s RNA sequencing) comes from its level of coverage-the ability to identify the majority of bacteria in a sample. However, sequencing depth-the resolution at which a species can be identified remains challenging. Additionally, many microbial species have been challenging to culture in vitro, making it difficult for researchers to repeatedly perform experiments in a laboratory setting and gain a deeper mechanistic understanding of microbial behavior and ecology.

Dr. Rachel Dutton, Assistant Professor in the Division of Biological Sciences at UCSD, addresses some of these technological limitations by studying the establishment and maintenance of microbial communities in different types of cheeses. With this model, her lab can investigate the interactions of different types of microbes to better understand them as ecological systems [15-16].

Science & Food is honored to host Elaine Hsiao and Rachel Dutton for the 2016 UCLA Science & Food public lecture series to elaborate on their findings.  They will be accompanied by Sander Katz, author of Wild Fermentation, who will discuss the transformative properties of microbes in the production of foods like sauerkraut.

Join us on Wednesday, May 11th at 7PM in Schoenberg Hall at UCLA for “Microbes: From Your Food to Your Brain” to learn more about the intriguing world of microbes!

References cited

  1. Gest H. “The discovery of microorganisms by Robert Hooke and Antoni Van Leeuwenhoek, fellows of the Royal Society”. Notes Rec R Soc Lond. 5 (2004). 187-201.
  2. Hooke R. “Micrographia” Jo. Martyn & Ja. Allestry (1665).
  3. “The History of the Germ Theory” The British Medical Journal. 1 (1888).
  4. Pollan M. Cooked: A Natural History of Transformation. Penguin Books. (2013).
  5. Abbott A. “Scientist bust myth that our bodies have more bacteria than human cellsNature. (2016).
  6. Leone V, Chang EB, Devkota SD. “Diet, microbes, and host genetics: the perfect storm in inflammatory bowel disease” J. Gastroenterol 48 (2013). 315-321.
  7. Sartor RB,.“Microbial influences in inflammatory bowel disease: role in pathogenesis and clinical implications” Elsevier (2004). 138-162.
  8. Hotamisligil, GS. “Inflammation and metabolic disorders” Nature. 444 (2006). 860-867.
  9. Parkin DM, Bray F, Ferlay J, et al. “Global cancer statistics” CA Cancer J Clin. 55. (2005). 74-108.
  10. Loftus EV Jr. “Clinical epidemiology of inflammatory bowel disease: incidence, prevalence, and environmental influences” Gastroenterology 126. (2004). 1504-1517.
  11. Molodecky NA, Soon IS, Rabi EM, et al. “Increasing incidence and prevalence of the inflammatory bowel diseases with time, based on systemic review” Gastroenterology 142. (2012). 46-54.
  12. Devkota SD, Wang Y, Musch MW, et al. “Dietary-fat-induced taurocholic acid promotes pathobiont expansion and colitis in Il10-/- mice” Nature. 487 (2012). 104-108.
  13. Devkota SD, Chang EB. “Diet-induced expansion of pathobionts in experimental colitis” Gut Microbes. 4:2 (2013). 172-174.
  14. Hsiao E.Y., “Gastrointestinal issues in autism spectrum disorder”, Harv Rev Psychiatry, 22 (2014). 104-111.
  15. Wolfe BE, Dutton RJ. “Fermented Foods as Experimentally Tractable Microbial Ecosystems” Cell. 161(1) (2015). 49-55.
  16. Wolfe BE, Button JE, Sanarelli M, Dutton RJ. “Cheese rind communities provide tractable systems for in situ and in vitro studies of microbial diversity” Cell. 158 (2014). 422-433.

Anthony MartinAbout the author: Anthony Martin received his Ph.D. in Genetic, Cellular and Molecular Biology at USC and is self-publishing a cookbook of his favorite Filipino dishes.

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The International Year of Pulses

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Photo credits: (flickr/Jessica Spengler)

The 68th United Nations General Assembly has declared 2016 the International Year of Pulses. [1] Pulses – that throbbing sensation of your carotid artery after a workout or during a first date, right? Nope. The UN suggests we celebrate the pulses that are leguminous crops harvested solely for their dry seeds. All lentils, and all varieties of dried beans, such as kidney beans, lima beans, butter beans and broad beans are pulses, as are chick peas, cowpeas, black-eyed peas and pigeon peas. Seeds that are harvested green, like green peas or green beans are classified as vegetable crops, not pulses. Legumes used primarily for oil extraction, like soybeans, are also not pulses. [2]

Why are pulses getting a year-long, world-wide campaign?

A global push for pulse production would address many problems of our global food system. The Food and Agriculture Organization of the United Nations’s campaign highlights these key benefits to pulse cultivation [1]:

  • Pulses are highly nutritious – they are excellent plant source of protein, and contain the B vitamins that our bodies require to convert food to energy
  • Pulses are economically accessible and contribute to food security at all levels – from farmers to consumers
  • Pulses foster sustainable agriculture, thus addressing agriculture’s role in climate change
  • Pulses promote biodiversity in agriculture

 

Now that we know the basics of pulses and why they’re important, let’s get scientific.

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Photo credits: (flickr/Kelly Garbato)

Pulses in the nitrogen cycle

Pulses are legumes, or plants in the family Leguminosae. Thanks to their symbiosis with many members of the diazotrophic, or nitrogen-fixing bacterial genus Rhizobium that live in their roots and feed them with nitrogen from the air, pulses have a particularly high protein content compared to non-legumes. [3] Within the bacterium, atmospheric nitrogen (N2), which is typically unusable to plants, is converted to ammonium (NH4+) via the activity of the enzyme nitrogenase. The nitrogen of ammonium is converted to other more complex compounds that are beneficial to humans, like amino acids – the building blocks of protein. In exchange for fixing nitrogen, the bacterium receives food from the plant — carbon in the form of glucose (C6H12O6).

 

This remarkable bacterial symbiosis also enriches the soil in which pulses grow with nitrogen compounds like nitrite (NO2) and nitrate (NO3), which is the preferred nitrogen source for other green plants. For this reason, farmers who crop-rotate with legumes don’t need to apply nearly as much fertilizer as farmers who don’t. [3]

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Pulses in a changing climate

Many pulses are also hardy and drought tolerant crops – lentils, broad beans, peas, and chick peas are all native to the Fertile Crescent of the Near East, and have adapted to sprout quickly and reproduce in the rainy season before the hot, dry summer [3].

Anatomy of the pulse

All food seeds consist of three basic parts: an outer protective coat, the small embryonic portion that develops into the mature plant, and the storage tissue that feeds the plant embryo. [3]The bulk of the seed consists of storage cells are filled with particles of concentrated protein and granules of starch, or organized masses of starch chains.

Cooking and starch retrogradation

When we cook pulses, hot water permeates the starch granules. As the water molecules work themselves between the starch chains, the granules swell and soften. When the pulses later cool down, the starch chains bond to each other again in tighter, more organized associations, resulting in firmer granules. (This process is called retrogradation.) [3] Consider leftover lentils or beans: they’re always harder and drier the next day, and they never get quite as soft as when they were first cooked. This is because during the process of retrogradation, some starch molecules form granules that are even more tightly associated than the bonds in the original starch granule. They form small crystalline regions that resist breaking even at boiling temperatures. [3]

Retrogradation of starch might foil your plans for leftover lentils, but it does do our bodies good: Our digestive enzymes cannot easily digest retrograded starch, so eating it results in a more gradual rise in blood sugar compared to the effects of non-retrograded starch. [3] Our intestines need help breaking down this tough starch, and the beneficial bacteria in our large intestines are happy to be of assistance. Just as the diazotrophic bacteria in soil work in harmony with leguminous plants, our intestinal bacteria digests what we cannot. Thus the retrograded starch functions as a prebiotics, or food for the probiotic bacteria in our guts. Well-fed gut bacteria make for healthy digestive tracks and happy bowels.

Will this pulse promotion save the world and fix the global food economy? Perhaps. We can all do our part by making a hearty spinach dal for dinner tonight, and sweet red bean paste for dessert.

 

Works Cited

  1. “”Save and Grow in Practice” Highlights Importance of Pulses in Crop Rotations and Intercropping.” Pulses – 2016 | 2016 International Year of Pulses. Food and Agriculture Organization of the United Nations, n.d. Web. 05 Feb. 2016.
  2. “What Are Pulses? | FAO.” What Are Pulses? | FAO. Food and Agriculture Organization of the United Nations, 15 Oct. 2015. Web. 05 Feb. 2016.
  3. McGee, Harold. “Seeds.” On Food and Cooking: The Science and Lore of the Kitchen. New York: Scribner, 2004. N. pag. Print.

Elsbeth SitesAbout the author: Elsbeth Sites received her B.S. in Biology at UCLA. Her addiction to the Food Network has developed into a love of learning about the science behind food.

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Blood Orange

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While traditional oranges are available at your local supermarket all year long, the best time to enjoy the juicy, crimson flesh of blood oranges is during these winter months. So while you venture out for some delicious blood oranges, consider these fascinating tidbits. How do they get their characteristic color? How are they different from everyday oranges?

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Eunice LiuAbout the author: Eunice Liu is studying Linguistics at UCLA. She attributes her love of food science to an obsession with watching bread rise in the oven.

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Pumpkin Domestication & Fruit Benefits

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If pumpkins are on your menu this Thanksgiving, be thankful that hundreds of years of human domestication has turned this once super-bitter squash into a sweet dessert. Furthermore, human intervention may have prevented gourds and squashes from extinction. As for reasons to be thankful for cranberries, scientific research shows that a compound within cranberries could prevent urinary tract infections.
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Eve Lahijani

Eve Lahijani graduated from UCLA with a B.A. in Economics and Business and went on to earn her Masters in Nutritional Science at CSU Los Angeles. She is now a registered dietitian for Vitamineve, a nutrition counseling service, and a nutrition health educator at UCLA. Eve’s Fiat Lux seminars on body image and proper nutrition have given many UCLA freshmen the tools necessary to maintain a healthy relationship with food.

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What hooked you on cooking?
I love learning about eating behavior. What, how and why people eat is intriguing to me. Especially when the eating is not related to physical hunger.
The coolest example of science in your food?
The process of denaturing an egg white and turning that into a soufflé is like magic to me.
The food you find most fascinating?
Ice cream is cool. Couldn’t help myself with that pun 🙂 I do appreciate the endless array flavors, textures, colors and combinations that can be created!
What scientific concept–food related or otherwise–do you find most fascinating?
How complicated eating behavior and food has become for some people (especially in harmful ways including over and under-eating and other compulsive eating behaviors) – and each individual’s process of understanding, simplifying and ultimately healing their relationship with food.
Your best example of a food that is better because of science?
I love Boysenberries and they are a blackberry/raspberry hybrid. Thank you science! And of course seedless watermelon.
We love comparing the gluten in bread to a network of springs. Are there any analogies you like to use to explain difficult or counter-intuitive food science concepts?
Yes: Eating in a balanced way is like a pendulum in a grandfather clock. You know, it swings back and forth. If the pendulum swings really far in one direction, due to the laws of physics it will swing far back in the opposite direction. Same holds true with eating. That is, if someone restricts (or goes on a diet) it pushes the pendulum too far in one direction so the better someone gets at depriving themselves the more likely the pendulum would swing far back in the opposite direction which may result in binges, cravings or overeating.
How does your scientific knowledge or training impact the way you cook? Do you conduct science experiments in the kitchen?
I like to plan to have well balanced meals that include components that bring about satisfaction. So I like to make sure my cooking involves carbohydrates, protein and fat – as well as fruits and vegetables. My science experiments include cupcake decorating along with trying new recipes with ingredients I get from the farmers market.
One kitchen tool you could not live without?
Sharp knife
Five things most likely to be found in your fridge?
Oranges, soy milk, Brussels sprouts, peanut butter, eggs and garlic so I guess that’s six!
Your all-time favorite ingredient?
Does chocolate flavored coconut ice-cream count as a food ingredient?
Your standard breakfast?
It’s always evolving. Right now I am into a mushroom, onion and garlic omelet or whole grain waffles with peanut butter. Whatever I choose I usually include some fruit and/or milk.

Hangry Hoarders & Juice Processing

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Hunger may motivate eating, but a team of researchers recently investigated how hunger can influence people to hoard items that can’t be eaten. Thirsty for juice instead? It turns out that cold-pressing juice isn’t as beneficial as people may tout, due to the multitude of factors that go into how micronutrients are absorbed by the human body.
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Lard Legacy: Does Your Diet Doom Your Child’s Health?

Cheeseburger with Fries [photo credit: TheCulinaryGeek]

Cheeseburger with Fries [photo credit: TheCulinaryGeek]

Now you can feel even more guilt about how that greasy cheeseburger might affect your future. A study by the National Institutes of Health suggests eating a high fat diet may also impact your child’s health1.

Growing evidence links increased caloric and fat consumption to the rise in immune-mediated diseases, like arthritis, food allergies, and inflammatory bowel disease2. These diseases result from abnormal swelling and inflammation that occur when the immune system produces exaggerated responses or reacts to false signals. Studies suggest the high fat consumption typical of western diets may be responsible for confusing our immune systems. For example, dietary fats promote inflammation and trigger immune responses specific to bacteria3.

Because much of this evidence is based on short-term or population studies, for this study, Myles and collaborators explored the longer-term effects of increased parental fat consumption on their offspring’s immunity using mice1. Specifically, scientists fed a high fat “western” diet to one group and a low fat control diet to the other. After giving birth, their pups were fed the control diet and exposed to a battery of tests examining their immune response. Compared to the control diet, the western diet had 10% more calories from fat, twice as many carbohydrates, and a higher ratio (as much as 15:1) of omega 6 to omega 3 fats. While both omega fats are essential, healthy diets contain a close balance (2:1) of the animal-derived omega 6 fats relative to the fish- and vegetable-derived omega 3 fats.

While the mice pups showed no differences in weight or blood sugar, the pups whose parents had western diets surprisingly showed significantly lower immune function: these pups were less resilient to bacterial related disease. They had higher mortality rates from internal infections and more severe skin infections. Furthermore, their skin cells displayed a lower level of bacterial defense proteins. Additionally, their colons and spleens did not work as effectively. The colons of these animals, which are critical sites for developing the immune system, showed exaggerated inflammation when exposed bacterial toxins. Both their colons and spleens showed lower levels of immune cells and proteins.

B0008203 E.coli on the surface of intestinal cells

E.coli on the surface of intestinal cells [Photo Credit: Wellcome Images]

Interestingly, researchers suggest that the diet itself did not directly cause the compromised immunity of these mice. Instead, they conclude that the western diet negatively impacted their gut microbiome—the sum of all bacteria present in their gut. In follow-up experiments, pups fed the western diet showed normal immune function if their parents were fed the control diet. Furthermore, DNA characterization of mouse stool revealed that pups from parents on western diets had less diverse bacteria than control pups. Increased fat consumption may have changed available nutrients and limited the bacteria in the parents on western diets. Because mothers shape their offspring’s microbiomes during the birthing and nursing process, pups of parents on the western diet received less diverse bacteria. Therefore in follow-up experiments, the western diet did not affect the immunity of the pups whose mothers ate control diets, because they already had received diverse microbiomes.

Changes in the microbiome may impact the immunity of the pups because of the “hygiene hypothesis”; this essentially suggests we have become too clean. Because we are not exposed to enough bacteria and other immune system triggers growing up, our immune systems don’t develop as extensively as those exposed to a more diverse range of microorganisms. The ‘hygiene hypothesis’ has been fueled by research showing that children in homes with more bacteria have lower asthma and allergy rates. A similar scenario was recapitulated for the immune compromised pups in this study. Final experiments with the mice showed that when the researchers raised pups from both parental groups in the western and control diet together, they both displayed similar bacterial diversity and immune function. By living with the pups with more diverse bacteria, the immune compromised pups exhibited increased diversity in their microbiome and negated the effects of their parents’ western diet. In other words, this result may suggest that even if you lived on a diet of greasy cheeseburgers, your kid’s may still have healthy immune systems if they are rolling around in the dirt with the kids whose parents stuck to their kale and whole grains!

While this study is promising for the hygiene hypothesis, more research is necessary to understand this effect in people. For example, these mice had simpler microbiomes and diets than the average human. It is unclear how this complexity may change the effect for people. Additionally, while this study only looked at fat consumption, other factors such as genetics can impact microbiome diversity and the respective impact on immunity. In any case, assuming this research translates, it suggests eating a lean turkey burger now, may help save your kids from arthritis, food allergies, or inflammatory bowel disease in the future.

References cited:

  1. Myles, I.A. et al. 2013. Parental Dietary Fat Intake Alters Offspring Microbiome and Immunity. Journal of Immunology. 191 (6) 3200-3209.
  2. Kau, A.L. et al. 2011. Human nutrition, the gut microbiome and the immune system. Nature. 474: 327-336.
  3. Calder, P.C. 2011. Fatty Acids and Inflammation: The Cutting Edge Between Food and Pharma. European Journal of Phamacology. 668 (Suppl. 1): S50-S58
  4. Gereda, J.E. et al. 2000. Relation Between House-dust Endotoxin Exposure, Type 1 T-cell Development, and Allergen Sensitisation in Infants at High Risk of Asthma. Lancet 355: 1680-1683

Vince ReyesAbout the author: Vince Reyes earned his Ph.D. in Environmental Engineering at UCLA. Vince loves to explore the deliciousness of all things edible.

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Big Soda & Food Perspectives

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An advocate for anti-obesity and healthful diets worked at PepsiCo to change the way food companies marketed junk food. Also, our understanding of how what we eat affects our biology may change with an alternative perspective on food as a hormone.
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Gluten Sensitivity & Gluten-Free Baking

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This week we’re all about gluten. NPR summarizes recent research on gluten sensitivity, while America’s Test Kitchen gives NPR the lowdown on gluten-free baking. Read more

Why Do We Bother to Eat Bitter?

Photo credit: Melissa McClellan/Flickr

Mustard Greens (photo credit: Melissa McClellan/Flickr)

Through exploration of the ancestral context of taste, scientists can better understand how modern humans use the sense of taste to make decisions and survive. Evolution has shaped our sense of taste to guide us to seek the food we need to survive, while steering clear of foods harmful to us. It is understandable that early humans who avoided spoiled meat and poisonous berries were able to pass down their genes, giving modern humans the ability to avoid them too. But what explains the countless humans who voluntarily consume, and even enjoy, some bitter foods? Why do we eat bitter greens? Brussels sprouts? Hoppy beers? Why do we tolerate some bitter flavors and not others?

Tastes can be positively or negatively palatable depending upon their context among other food flavors. Sour fruit flavors like grapefruit or cranberry can be refreshing and delicious to eat, but sour milk clearly signals that the food has expired. These matches between tastes and flavors are called flavor congruencies.

Most taste-odor flavor pairings are learned associatively through eating. Flavors associated with calories and nutrients become more pleasurable with time, whereas poisoning and illness teach us to associate foods with an unpleasant taste or disgust. For omnivores like us, learning the consequences of eating different foods is an indispensable survival tool. Because our range of food option is so vast, it is essential to sample many foods and connect their post-ingestive consequences with their perceived tastes. Bitter-tasting substances are innately disliked by infants and children presumably because most bitter compounds are toxic. Most children are drawn to all things sickeningly sweet, but as adults enjoy eating eat bitter Brussels sprouts. We learn to enjoy the taste of mildly bitter foods, especially when paired with positive metabolic and pharmacological outcomes. The more your body benefits from an ingested food, the more palatable it becomes [1].

Our bodies require phytonutrients such as flavonoids that cannot be physically separated from their vegetable carriers. Humans learn to tolerate low levels of bitterness in foods as they co-occur with nutrients in plants through a post-digestive reward/punishment system. For example, rhubarb contains 0.5% oxalic acid by weight, a substance that in large doses can cause joint pain and fatal kidney stones. The first time a child eats rhubarb, the initial taste response tells the brain that the food is bitter, toxic, and should be avoided. However, as the body begins to benefit from the essential nutrients in rhubarb without suffering any damage, the rhubarb becomes more and more palatable. Experiments show that rats can very quickly learn associations between tastes and metabolic and physiological consequences, perhaps in a matter of days. These associations occur after only a single trial and are strong enough to resist fading away even after multiple presentations of the food with no physiological consequences [2].

In humans, a large sugar molecule called maltooligosaccharide (MOS) presents a sweeter case of taste association. Human saliva transforms starch into MOS. Although MOS is tasteless, it activates brain reward centers in a manner similar to sugar, while non-nutritive sweeteners do not. Thus, a tasteless molecule that has positive metabolic outcomes can activate brain reward areas more effectively than a sweet-tasting substance that has little nutritional value [3].

The next time you eat mustard greens, stop to appreciate the complex process that allows you to taste and enjoy your leafy meal. Consider how your perception of taste has evolved, which has protected your ancestors from poisoning themselves. Reflect upon the incredible and complex mechanisms humans have developed to keep you well nourished. And if you still haven’t warmed up to greens, consider introducing them gradually into your diet.  By exploiting the body’s associative adaptation to taste, you could learn to love them.

References Cited

  1. Breslin, P. 2013, An Evolutionary Perspective on Food and Human Taste Current Biology, Vol. 23 Issue 9
  2. Sclafani, A., Azzara AV., Lucas, F. 1997, Flavor preferences conditioned by intragastric polycose in rats: more concentrated polycose is not always more reinforcing, Physiology & Behavior
  3. Chambers ES, Bridge MW, Jones DA., 2009, Carbohydrate sensing in the human mouth: effects on exercise performance and brain activity, The Journal of Physiology

Elsbeth SitesAbout the author: Elsbeth Sites is pursuing her B.S. in Biology at UCLA. Her addiction to the Food Network has developed into a love of learning about the science behind food.

Read more by Elsbeth Sites