the 20-something’s guide to the gut-brain axis & your metabolism (aka:: understanding how what you eat affects how your brain signals the rest of your body!)
the gut-brain axis can be one of the most confusing topics in our overall physiology. it has so many communication pathways between so many different organs communicating about so many various topics that it can be hard to keep up! this article dives a bit more into the gut-brain axis & its role in our metabolism. if you missed our last series, we went over blood glucose regulation, our metabolism, & how our body helps to regulate our blood glucose levels. it would be an awesome read prior to reading this blog post since it sets up the foundation of some of the deeper topics covered here. take a look! & if you’re more of an auditory learner, check out our podcast episodes about the individual components of our metabolism on youtube, spotify, or apple music!
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15:: your metabolism & the role stress plays in your body composition & energy goals!
overview
as a broad overview, there are various glucosensors throughout the digestive tract & the brain that help to sense changes in glucose levels & initiate the right response based on what the glucosensors are finding. they communicate via both neurons (part of the nervous system) & hormones both locally, at the level they are sensing the changes in blood glucose, & globally, through signaling cascades & communication pathways to the brain & beyond. the gut-brain axis utilizes these glucosensors to detect changes in blood glucose levels & send communication to the hypothalamus in the brain. the hypothalamus then utilizes the autonomic nervous system (the parasympathetic & sympathetic nervous systems) to communicate with the rest of the digestive tract on how to best handle & respond to the message…whether through utilizing the glucose or storing it! it is this interplay between the digestive tract & the brain that make up our gut-brain axis! i like to think of the gut/digestive tract as part of a basketball team, with the hypothalamus (the brain component) as the coach. a player senses a change is needed on the court, she goes to the coach, who then directs the entire team on how to proceed!
let’s take a look at the gut-brain axis starting line-up
oral cavity
small intestine
pancreas
portal vein
liver
hypothalamus (coach)
oral cavity
the oral cavity is the first line of communication for the rest of the gut-brain axis. in the oral cavity, we have what are called type II taste receptor cells (type II TRCs) located on the tongue. these type II TRCs sense increases in glucose & initiate a cascade of events to let the rest of the system know what is coming down the pipeline…literally. it allows the digestive tract & brain to be better prepared for whatever the oral cavity is sensing in regards to glucose. (enter fathers everywhere saying “if you fail to prepare then you’re preparing to fail”…anyone else’s POPS? well, they were right with the gut-brain axis.) as a matter of fact, the oral administration of glucose has been found to have a 2-3x higher insulin response compared to IV injection of glucose, meaning our oral cavity & the subsequent communication pathways are very effective at “preparing to succeed!”. a few fun facts about the mouth in regards to metabolism & the gut-brain axis::
in obese individuals, the type II TRCs aren’t as sensitive to changes in local levels of glucose increases, so the “pipeline preparation pathway” is affected
in folks with diabetes, the ability to sense the increase in glucose & prepare appropriately is affected, leading us to possibly seek more sugar
small intestine
next in the line-up is the small intestine, & the small intestine has a few tricks up her sleeve. she’s got back-up. just like the oral cavity with the type II TRCs, we also have glucosensors (glucose-sensing cells) in the small intestine. it is the main place we sense the carbohydrates we have actually eaten, & it is also the place where we break down the carbohydrates into glucose & signal the rest of the body via special cells that line our small intestine called enterocytes. the small intestine also has communication pathways with the brain through a specialized cell called an entero-endocrine cell, or EECs. EECs secret GLP-1 once cued by the glucose levels in order to signal the rest of the gut & brain on what the body is going through. & the final back-up the small intestine brings to the line-up are enteric neurons, which are activated to help with glucosensing directly, or activated indirectly by secretion fo GLP-1 from the EECs.
* a note about GLP-1:: GLP-1 can be thought of as the body’s natural ozempic! it helps us better manage blood glucose levels by signaling various parts of the brain & digestive tract to make sure our gut-brain axis is functioning optimally! GLP-1’s roles include::
cueing insulin to help better manage elevated blood glucose levels
inhibits glucagon secretion, (which acts in opposition to insulin & actually mobilizes glucose when blood glucose levels are too low
inhibits glucose mobilization from the liver
reduces “gastric emptying” which slows the transfer of food from the stomach to the small intestine, helping the body to absorb glucose over a longer period of time
promotes glucose storage
GIP is also an important digestive hormone released to help assist GLP-1, primarily with the long-term storage of glucose!
pancreas
up next? the pancreas…on the team because it helps to release 2 of the most important hormones in glucose regulation & metabolism:: insulin & glucagon. insulin is cued by the parasympathetic nervous system, or the “rest & digest” part of the nervous system, for the utilization or storage of glucose. glucagon is released in response to cueing from the sympathetic nervous system, or the “fight/flight” part of our nervous system that helps us to mobilize glucose to be utilized for energy to “run from the bear”.
the portal vein & liver
& for the 4th & 5th gals, the portal vein & liver! the portal vein is probably the one you haven’t heard too much about when it comes to the human body, but a player that is super important to the gut-brain axis. once glucose is absorbed in the small intestine, it is released into the portal vein, where one of the main players in the parasympathetic nervous system (the vagus nerve) has nerve ending sensors in the portal vein that communicate with the brain when they sense elevated glucose levels. when glucose levels are elevated, the liver is cued to communicate to the coach/brain, the hypothalamus, on the message it received. in addition to communicating with the brain, the portal vein also communicates with skeletal muscle & brown fat to utilize the glucose to help better manage those elevated blood glucose levels. lastly, the portal vein helps to signal the liver (our 5th in the starting line-up!) on glucose levels in comparison to the hepatic artery. if the liver senses higher glucose levels in the portal vein compared to the hepatic artery, then the liver will help to store glucose… remember? the liver is our short term storage solution!
hypothalamus
lastly, let’s introduce the coach, the hypothalamus! this is the primary “brain” component of the gut-brain axis in terms of your metabolism. the hypothalamus is the ultimate coordinator of all the signals & communication it receives from the starting line-up. as various hormones are released & glucose levels are sensed in the digestive tract, there are also chemicals & hormones released to cue the hypothalamus on what to do next. the hypothalamus then utilizes the autonomic nervous system to communicate back to the “gut”. for example, GLP-1, the body’s natural ozempic, is released locally into the portal vein once it senses an increase in blood glucose levels. however, this release is minimized as various enzymes start to break down the GLP-1, so as GLP-1 is released from the intestine, GLP-1 is also cued to be released from the brainstem to signal the hypothalamus on what is actually happening in the digestive tract. the hypothalamus then coordinates a response to help lower blood glucose levels by sending signals via the parasympathetic nervous system (rest & digest) for the release of insulin to help manage the elevated blood glucose levels via utilization of that glucose for energy by skeletal muscles, or storage by the liver and adipose tissue. in states of lower glucose levels, the hypothalamus is also signaled & responds via the sympathetic nervous system (fight or flight) to help cue glucagon to mobilize glucose, decrease storage of glucose, & help us raise those levels back to baseline. so whether in hyperglycemic states or hypoglycemic states, the hypothalamus is cued to help better regulate how our body responds on a larger scale…just like the best coach!
final thoughts
it is incredible the interconnectedness of our body. what we eat affects our brains, & how our brain senses those communication signals affects our entire body. so next time you grab the sugary latte, notice how your body responds throughout the rest of the day. do you notice more cravings? it could be your body trying to recalibrate! & if you do choose that sugary latte & have cravings later in the day, maybe make a mental note how your brain is driving that decision & it might be time to choose a more neutral-glycemic food, fiber, or protein as a mid-afternoon snack rather than the sugary treat your body might be craving.
lastly, i am by no means perfect at any of this, but i truly do believe that awareness of how our bodies are built gives me so much freedom & power to understand & analyze the signals my body gives me. we are al working on feeling like our best selves, & this is just a piece of the puzzle that we all have more knowledge on now!
as always, if you’ve made it this far, thank you. i am so grateful for the time you took to read all of this & become that much more aware of how we can take control of our bodies & physiology!
love always wellness sisters & misters,
kelsy
resources::
Fournel, A., et al. Glucosensing in the gastrointestinal tract: impact on glucose metabolism. Am J Physiol Gastrointest Liver Physiol. Mar 3 2016. Accessed June 29, 2024. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4867329/
