This episode covers a topic that does not – at first pass – appear related to medicine: umami! Here is a link to the Tweetorial Avi posted on this topic:
What is umami? Who discovered it?
Umami is the flavor of savory deliciousness (it means savoriness in Japanese), and is separate from the other four types of taste that we all learned about in school. The term was coined by the chemist Kikunae Ikeda who was the first to describe/discover umami.1
In 1907 Kikeda was sitting down for dinner with his family and he was eating a bowl of dashi broth made from bonito and kelp. He found it to be particularly delicious and perceived its savory flavor. He called this flavor umami and since he was a chemist he went about isolating the source of the flavor, which he eventually found was glutamate.
By 1909 he had extracted crystals that delivered a pure umami flavor. He called them ajinomoto, which means “origins of flavor” in Japanese but we know this compound today by its chemical abbrevation, MSG, or mono-sodium glutamate.
What is umami’s ‘mechanism’?
For years there actually was some controversy about whether umami was a distinct flavor. This was probably because the umami receptor hadn’t yet been identified. In 2003, a group of researchers knocked out the glutamate receptor complex known as T1R1+3 in mice. This eliminated the mice’s response to umami stimulation! This simultaneously both proved umami’s existence and identified its mechanism.
There are other sources of umami, as well, including most prominently inosine monophosphate (IMP). IMP is a purine nucleotide that signals through the same umami receptor, T1R1+3, though it binds at a different site than glutamate.
Can we use this information to become better cooks?
Some umami-rich foods only contain glutamate (e.g., tomatoes and cheese) while others have glutamate and IMP (e.g., bonito and beef). One key idea is that you can get more intense umami flavor by combining different umami sources.
When you combine glutamate and IMP, the signal strength through the T1R1+3 receptor synergistically increases about 30-fold due to conformational receptor changes. The result: an umami bomb!
This synergistic signaling explains how cooking with different umami sources like soy sauce and shitake mushrooms (which have glutamate) with chicken or beef or sardines (which have IMP) can be so delicious.
Is there an evolutionary explanation?
There are a number of physiologic purposes that umami serves:
- Umami signalling increases saliva production and you get a similar synergy when glutamate and IMP are combined.
- Glutamate is the most common amino acid in breast milk.
- Umami appears to be the way we sense the presence of protein in food (just like saltiness signals minerals).
Here is one fascinating theory offered in a paper by Paul Breslin.
Thousands of years ago our early homo sapien ancestors started to cook and ferment food. This provided a major survival advantage compared to other human species as cooking releases micro/macronutrients and kills parasites. Hydrolysis from cooking and fermenting breaks down protein and nucleotides, which releases glutamate and IMP. In effect cooking can accelerate release of umami flavor and may have incentivized early humans to eat healthier cooked or fermented food.
Are there other clinical correlates?
Not surprisingly, given what we have learned about IMP as a purine and a source of umami, a low purine diet for gout control is also low in umami. This may explain why it is notoriously hard to adhere to. But there are umami-rich foods that are not high in purines (which would be those that get their umami flavor exclusively from glutamate), such as tomatoes, red peppers, shitake mushrooms, or parmesan cheese. And of course MSG.
There is also interesting data around umami and, ironically, appetite suppression. One study found that rats who received MSG in their diet, in comparison to rats not exposed to MSG, actually had reduced leptin levels, lower weight gain overall, and smaller amounts of abdominal fat.
There’s also some human data suggesting appetite suppression and reduced postprandial hunger with the use of umami.
Learning Objectives
- Understand that glutamate and inosine monophosphate are the mediators of umami flavor byway of the T1R1+3 taste receptor.
- Appreciate that umami signalling has several physiologic roles including increased salivation, signaling the presence of protein in food, and possibly inducing postprandial satiety.
- Learn that the enhanced umami flavor yielded from cooking or fermenting food may have had an adaptive advantage for early humans.
CME/MOC
Click here to obtain AMA PRA Category 1 Credits™ (1.00 hours), Non-Physician Attendance (1.00 hours), or ABIM MOC Part 2 (1.00 hours).
Listen to the episode
Credits & Citation
◾️Episode written by Avi Cooper
◾️Audio edited by Hannah Abrams
◾️Show notes by Tony Breu and Avi Cooper
Cooper AZ, Breu AC, Abrams HR. Why is umami so delicious? The Curious Clinicians Podcast. September 16, 2020. https://curiousclinicians.com/2020/06/03/episode-9-why-is-umami-so-delicious/
Image credit: http://bcbstwelltuned.com/2018/03/28/the-science-of-taste/tongue-taste-areas-sweet-sour-salty-bitter-umami/
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