Wednesday, January 29, 2014

Sweet, Salt, Bitter, Sour - They Ain't The Half Of It

Biology concepts – umami, taste, flavor, gustation, glutamate, chemoreception, CD36, fat taste, water receptor, calcium

Perhaps I was a little hasty when I said umami wasn’t the name
of a new band. Apparently “Umami” is the name of a band from
Minneapolis. They are described as an electro/psych band,
whatever that is. I like it when obscure science words are used
in culture – makes me think I’m in on some secret. Umami isn’t
that obscure a word, but I used to know a band made up of
statisticians called The Outliers.
Ever heard of umami? It’s not the name of a new band, or even a bad Robin Williams movie. It’s a taste; the fifth taste that humans can sense. Umami is the taste of savory; meats and other high protein foods. And what do we have to thank for umami? Seaweed.

Until 1908, science believed that most flavors were just combinations of the four traditional tastes - sweet, salty, sour, and bitter. But don’t get the idea that taste and flavor are the same - oh no. Taste is our gustatory sense, but that isn’t the same as flavor – flavor is something bigger than taste.

You know how having a head cold makes food bland? Well, that’s because smell is a big part of flavor; you don’t smell your food when you have a cold. Food stimulates all your senses - temperature, touch, smell, what it looks like and even how it sounds as you chew it. All these things add up to flavor.

This is why chefs say you eat with your eyes first, and why they try to incorporate different textures into a single dish. They’re trying to appeal to all your senses. Therefore, eat slowly to enjoy your food more. Give all your senses time to participate. And you might just eat less, since your satisfaction will come from the total experience, not just the craving for a particular taste.

But back to the origins of umami. The Japanese had an idea that there was another taste, mostly since their traditional cuisine used so much seaweed, and this flavor couldn’t be accounted for by the other four tastes.  Chemist Kikunae Ikeda wanted to identify the active molecule in seaweed, the one that gave it taste. He called it umami, from the Japanese words for delicious (umai), and taste (mi).

Ikeda’s biochemical studies led to the identification of glutamates as the molecules to which people reacted. And they aren’t just in seaweed, most living organisms contain truckloads of glutamates. When cooked, all glutamates convert to L-glutamate, the amino acid. Ikeda determined that this is what some of our gustatory (taste) receptors sense. Gustation is from Latin gustare = taste, and this is where the word gusto comes from; to taste life.

It turns out that umami taste is not produced by just L-glutamate.
Glutamate comes mostly from meat, as they are high in proteins,
but the nucleotides inosinate and guanylate also perceived as
savory tastes. Inosinates are also found in meats, but also in
seafood. Vegetables are a major source of guanylates, but so are
mushrooms – and we all know that mushrooms are fungi – right?
When you taste something, a chemical signal in the taste cells on
the tongue is converted to an electrical impulse. This is carried by
either the facial nerve or the glossophayrngeal nerve to the brain.
The most palatable form of glutamate that Ikeda could identify was monosodium glutamate (MSG), so he immediately set out to produce and sell it, starting in 1909. Made a pretty penny, he did. Now MSG is a common flavor enhancer in Japanese cooking, including soy sauce. World-class chefs are designing “U-bombs” (umami-filled dishes) to take advantage of the new official taste.

This is why identifying an umami taste receptor for L-glutamate makes sense. This is nature telling you that you need to eat protein, and by giving it a favorable neural response (it tastes good), it increases the chances that you will seek out protein sources for nutrition.

Glutamate has several functions, even beyond its role as one of twenty protein building blocks. Glutamate is the most common neurotransmitter in the central nervous system, and plays a crucial role in long-term potentiation (LTP) and learning. Glutamate is also an intermediate in synthesizing many of the molecules in glycolysis, gluconeogenesis, and the citric acid cycle. I’ve said it before and I hope it jumped into your mind just now – nature hates a unitasker.

So you sense L-glutamate through a gustatory receptor that is specific for that molecule, and the electrical impulse is converted to a specific taste – we call it savory. The cloning of the taste receptors in the late 1990’s (actually umami was the first) started people thinking about other possible tastes. Could there be a sixth taste sense – how about fats? Do we taste fats?

When in the insula of the brain, the input is sorted with other
input and interpreted as a taste. It is the perception that is
important. For the fatty acid receptors, they are receptors, they
are located in the taste cells, and they do carry information via
the same two nerves to the same part of the brain. But are they
then interpreted as a taste?
Much research has been performed in this area in the past few years and a couple of fatty acid receptors on the tongues of rodents and primates have been identified, specifically, CD36 and GPR120. But does this mean we “taste” fat? We said taste doesn't equal flavor – we should now add that sensation may not be the same as taste. Just because there are specific chemoreceptors on taste cells for different fatty acids doesn’t mean that we perceive the sensation as taste.

It has been shown that fatty acids in the oral cavity do have a threshold level for sensation, and that the fatty acid taste receptors do lead to specific changes in physiology. When subjects were given fatty acids on their tongues, they very quickly showed increased serum triglyceride levels, increased pancreatic hormone release, increased release of GI lipases (enzymes to breakdown fat) and a slowly of the GI tract (it takes more time to digest fats).

What is lacking here is a conscious perception of the discernable nature of the fatty acid (like how sugars are sweet or glutamates are savory). The 2009 studies by Mattes and colleagues controlled for the mouth feel, smell, and so forth of fats, so it was definitely the fatty acid receptor that was stimulating the responses, but no where did it say the subjects tasted something. However, his 2011 paper says that fat may very well be a basic taste.

This gives us a new way thinking about taste receptors. Taste is a type of chemoreception, but perhaps it’s only one subset of oral chemoreception. Gustatory chemoreception is a lot more than just tasting something. However, some researchers challenge this division, saying that participants do have a measurable psychophysical response when fatty acids on the tongue reach a threshold level – they do taste something.

Again I say, “nature hates a unitasker.” CD36 is the fatty receptor in
taste cells, but it also works in macrophage recognition of oxidized
fatty acids and the onset of atherosclerosis, and in the activation of
platelets by fatty acids. You can see in the cartoon that CD36 sticks
into the membrane at two places and loops out of the cell. If you
change the order of its amino acids, it’s shape will change. How well
it binds to fatty acids, or activates all those downstream signals will
also be affected. This is why people with different versions of CD36
may eat different amounts of fat. I wonder if people with poor CD36
versions also have more trouble with CD36 functions in other cells?
The CD36 glutamate receptor has been especially well studied in the past couple of years. It comes in several slightly different forms (polymorphisms – slight differences accounted for by single or few amino acid differences in the sequence of the protein), but these differences have a big effect.

When divided into groups based on which variant of CD36 they possessed, a couple of studies from 2012 (here and here) show that responses to fat and how much the subjects craved fats were different. Those who sensed fats most readily (at lowest concentrations) tended to eat less than those who needed more fat in order to trigger the responses.

The hypothesis of a 2013 review of fat taste and obesity says that those who sense less fat are more likely be tipped toward a hunger stimulating hormonal profile, while those who more readily sense the fatty acids in their food tip toward satiety (fullness). There are many hormones involved here and we could get bogged down very fast, so let’s leave it at that for now; undoubtedly someone is trying to make a diet pill based on it.

So, could there be more oral chemoreception events going on – a seventh taste? An eighth?  Let’s talk very briefly about two possibilities. Maybe we can taste calcium. Yes, you could taste your Tums. A 2008 study indicated that mice can perceive calcium as a specific taste. A single 2012 study extends this to humans as well. Calcium is sensed via a certain receptor (Tas1R3), which works with other proteins to sense sweet and umami. But here, it apparently works on its own (we will talk more about the receptors in the posts to come). I need to see more research before I buy calcium taste completely.

Taste number eight - do you think you can taste water? The common argument is that you can taste what is in the water, not the water itself. How or why would you taste water; you’re 65-70% water all the time! What good is it to taste the main ingredient of life? How about this – do you sense the water by taste receptor, not just by temperature, sound, smell, or mouth feel?

There are voluntary swallows an involuntary swallows. But even
in voluntary swallows there are involuntary parts. You don’t think
about closing off your trachea with your epiglottis, it just happens.
This closing is how you keep food and liquid from ending up in
your lungs. Water in your laryngeal pharynx is one stimulus to get
you to swallow and close off the wind pipe until the possible
problem is gone.
Yep, mammals have receptors in the oral cavity that specifically sense the presence of water. In some mammals, like dogs and rabbits, using salt water inhibits the firing of the laryngeal nerve fibers connected to the water receptors; not so in cats and rats. We’ll get to why in a second.

What is the purpose for water receptors in the oral cavity (really, they are in the entrance to the throat, the laryngeal pharynx)? It may be that this is an evolutionary protection from aspirating (breathing in) liquid to the lungs. Liquid in the lung is a bad idea, since it stops gas transfer and promotes bacterial growth. If acids or other liquids that could damage the lungs or throat get in their somehow, it would definitely be better to swallow them than to breathe them in.

When you were a fetus and a very young infant, stimulation of the water receptors in your throat caused you to swallow immediately. As you aged and gained more muscular control, the reflex was replaced by coughing – this is the hypothesis of a 2001 study on the reflex. But the water receptors are still there and still aid you as a stimulus for voluntary swallowing. Whether we taste water or not, I’m glad I have the chemoreceptors.

So, the dampening of the reflex by Cl- in salt might be helpful keeping you from constantly having the urge to swallow. This leads to another point – the power of suggestion. Can you do anything right now other than think about the saliva in your mouth and whether you should be swallowing? Creepy, isn’t it.

Don’t count out the idea of water as a basic tastant (something you can taste). A 2010 study showed by monitoring brain waves that people respond to water using the same pathways as taste, and the responses look the same. And a 2012 study indicates that rats have distinct portions of the gustatory cortex of the brain for identifying both salt and water. If we can taste umami to make sure we eat enough protein, and sweet to make sure we eat enough carbohydrate, why not water to make sure we keep hydrated?

The idea here is that everything seems better if you are in love.
With love, this is a fantastic summer day in a beautiful place.
Without love, it’s just sand in a whole lot of uncomfortable places.
Same with taste, water is water – unless you're in love.
One final point that reflects just how complex taste is – did you know that being in love makes water taste sweeter? Participants in a December 2013 experiment were asked to think or write about love, hate, or jealousy. Then they were asked to describe the taste of a new product (really just distilled water). Those who wrote or thought about love rated the water to be sweeter than those who contemplated hate or jealousy.

It seems that the brain pathways for rewarding feelings in love and in consuming sweet are the same. You can’t discern between the two, and one can stimulate the other. So when you say you love eating sweets, maybe you really do!

 Next week, we can go further into taste. Do people who are supertasters taste good or taste well?

Newman L, Haryono R, & Keast R (2013). Functionality of fatty acid chemoreception: a potential factor in the development of obesity? Nutrients, 5 (4), 1287-300 PMID: 23595136

Pepino MY, Love-Gregory L, Klein S, & Abumrad NA (2012). The fatty acid translocase gene CD36 and lingual lipase influence oral sensitivity to fat in obese subjects. Journal of lipid research, 53 (3), 561-6 PMID: 22210925

Keller KL, Liang LC, Sakimura J, May D, van Belle C, Breen C, Driggin E, Tepper BJ, Lanzano PC, Deng L, & Chung WK (2012). Common variants in the CD36 gene are associated with oral fat perception, fat preferences, and obesity in African Americans. Obesity (Silver Spring, Md.), 20 (5), 1066-73 PMID: 22240721

Chan KQ, Tong EM, Tan DH, & Koh AH (2013). What do love and jealousy taste like? Emotion (Washington, D.C.), 13 (6), 1142-9 PMID: 24040883

MacDonald CJ, Meck WH, & Simon SA (2012). Distinct neural ensembles in the rat gustatory cortex encode salt and water tastes. The Journal of physiology, 590 (Pt 13), 3169-84 PMID: 22570382

For more information or classroom activities, see:

Umami –

Fat taste –

Water receptor –

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