Always happens and it would be awesome to dig a bit into the science of it as well.
Screw-Driven: Much like the ‘Fight or Flight’ response related to real-world dangers and how we react to them, the brain has a hard time telling difference between a physical threat or a psychological one.
Anxiety and Nervousness can be as frightening mentally as a dark alley way or a guy with a knife across the street.
When your body goes into ‘Fight or Flight’ blood is moved away from the stomach and to areas where it might be used to run or fight, such as the legs or arms. Less activity in the abdominal region and an increase in adrenaline are some of the many causes to losing your appetite.
TLDR- Your body is allocating bodily resources effectively for a threat it perceives to be real and life threatening, but is really just mental anguish that it cannot differentiate.
screwstd: To go along with other answer, your gut can also sometimes induce vomiting if you are under very heavy stress In order to free up even more resources for the fight or flight response.
The sympathetic nervous system, I believe, is responsible for increased heart rate, faster breathing, and tightness in the gut in order to prepare you to respond to threats. Even though it may just be as simple as a math test. Our body releases the same stress hormone so we have the same reaction.
Even though the stress we feel today is mostly social anxiety stress or such, we still react as though we are physically in danger and food can wait to digest later. Surviving now is more important than getting a bite to eat to our brain.
Edit: changed reaxy to react. What the hell autocorrect
Phil_Growlers: To tag on to OP’s question, why do some of us do the opposite and stress eat?
CelticAssWhisperer: The exact response is determined by the type of anxiety or stress you’re feeling.
Long term stress such as a final coming up/big event you have to get ready for are handled by the internal medulla of the kidney. This area produces a “compliment” to adrenaline called cortisol. Cortisol causes you to have a large craving for fats and carbs so that your body has energy reserves for when the big event occurs. It expects you to be in a big fight or have to run far, so it can turn the fat stores into energy. Thats why stress eating is such a problem. It’s natural and can’t truly be fixed
The external kidney, the adrenal gland, produces adrenaline. Adrenaline acts to stim blood flow away from non-essential parts of your body to your lungs and heart so you can do things faster (important to note NOT LONGER). It primarily pulls it away from the gastro-intestinal region, the extremities, etc.
This is why great cold medicine uses pseudoephedrine. This copy of adrenaline is made as an extended release to open airways so you can get more oxygen and counter that stuff nose
eiko85: Anxiety is fun:
The liver releases sugars and fats which flow into the blood stream to provide energy.
Breathing becomes faster, providing more oxygen.
Heart beats faster and blood pressure rises to make sure blood reaches the areas needed.
Blood clotting mechanisms are activated in case of injury and bleeding.
Muscles tighten, ready to run or fight.
Saliva dries up and digestion stops so that more blood can be diverted to the muscles.
Sweating increases to cool the body.
Bowel and bladder muscles may become loose.
Pupils in the eyes widen to let in more light and help clearer vision.
All senses are heightened.
Brain thinks of other things that could be a threat.
Blood supply is diverted from the skin to reduce bleeding if injured.
BUUBTOOB: for questions like this I find its always very helpful to adopt a “primitive” mindset. by this I mean lets think about this if we were living in caves in the year 10000BC
what would make us nervous/anxious back then? a predator coming to eat us. under these conditions would it be useful to be distracted by a nagging sense of hunger? Nope.
to effect the necessary changes we have the Sympathetic and Parasympathetic nervous system. Simply put; just equate sympathetic to “fight or flight” and parasympathetic to “rest and digest”.
unscriptable: Also neurotransmitters which ordinarily are sympathetic, (speed up/stimulate) others area of the body such as the heart and noradrenaline, have a parasympathetic effect on the stomach and digestive organs! Slows down digestive tract and gastrin release when released!
kriahfox: Same reason you might have to poop when you’re nervous. It’s part of the fight or flight response. Your body might need to move, FAST – less weight can make that happen.
Schroeder226: To add to what everyone has been saying about fight or flight, I’ll add a little bit of Neuroscience.
Your stomach has a lot of neurons in it, therefore there’s a lot of neurotransmitter action happening there. This is where our “gut feeling” comes from. When it comes to fight or flight, you’re dealing with the transmitters epinephrine (adrenaline) and norepinephrine (noradrenaline). Norepinephrine acts first. It’s function is to get the body ready for action. Specifically, in the digestive tract, it activates this thing called gastrointestinal motility, which, for the digestice tract, is the ability to move food down the tract.
To recap, fight or flight is triggered then a transmitter called norepinephrine inhibits the digestion process and it can do this because the stomach has a lot of neurons.
Okay, now why does this need to happen? Yes, part of it is that you have better shit to worry about in the moment but that just doesnt have to do with the presence of danger. Part of it is to counter the negative effects of adrenaline. Adrenaline is the thing that gives you an increased heart rate and a higher blood pressure when you are in a fight or flight situation (also helped by noradrenaline which releases glucose in your body) and when it’s released, it increases stress and gives anxiety and in the stomach, it makes it rumbly. Noradrenaline comes in and controls this effect, which ends up as you not feeling hungry! Tada!
Sources: a few psych/neuroscience classes and I have adrenal issues.
Nadenoh: The stomach and intestines don’t need much blood when you’re ready to run. If they ain’t got blood they don’t need food.
brianwantsblood: Your brain is so focused on whatever is going on in your head that it forgets about your stomach. Basically it’s saying “I’m hungry but I’ll worry about that later because this shit’s more important.”
cakolin: Follow up question: Why do I eat my weight in Doritos when I’m nervous?
resolvetotonic: here is a physiological explanation for this. When your are frightened, anxious or nervous, your body increases the activity of the sympathetic nervous system. This makes your body ready to run if there were some sort of danger. Sympathetic activity includes
– Increased heart rate and increased heart contractility to increase the amount of blood reaching your muscles
– Increased blood shunting (taking blood away) from non important parts of the body in times of danger (such as the stomach, intestines, prefrontal cortex, kidneys etc)
-Increased blood flow to the muscles and heart to react
Anxiety is the result of chronic activation of this system which explains the seemingly permanent loss of appetite.
Something_Syck: When you’re scared your body thinks you are in imminent, physical danger
If you’re in danger of death, you really don’t need to digest any food inside of you, you have more important things to worry about
Part of your body’s response is to take blood away from your stomach and to the muscles in your arms and legs, since you will need to either run or fight
nosferobots: Probably merits an addendum on why others, like me, tend to eat more when we are nervous or anxious.
Johnny3_sb: I’m literally studying this for anatomy right now. Anxiety/nervousness is caused by the release of certain hormones because of your sympathetic nervous system. These hormones are the same hormones released when you are scared/frightened and are meant to shut down certain processes/senses in your body in order to heighten the functionality of the processes/senses needed for the stressful situation you are in. One of these processes shutting down just happens to be your appetite!
goodguy_asshole: Fight or flight vs rest and digest.
Anxiety causes a flight or fight type response. Your body kicks into gear to respond to the threat, real or not. Well, turns out fighting and running with a stomach full of food isnt the best of options. So the fight or flight redponse shifts your body away from wanting food and digesting food and storing energy mode to a no time for food lets use your stored energy mode.
DonutPsychology101: I would like to add to the comment of u/Screw-Driven/: When being in a fight-or-flight mode, your body is 100% ready to flee. In order to run as fast as possible, your body also need to be as light as possible. Hence, many people experience the need to go to the loo quite often and some may even vomit.
Tl;dr the body needs to be light in order to run fast. Doo does not go well with that.
SoNuclear: You have two branches of the autonomous nervous system –
There is the parasympathetic nervous system or the feed and breed one, it governs calming actions generally and as the name would suggest actually stimuates the GI system as well as your reproductive system(erections).
The adrenal fight or flight one – everyone has heard of it and it dominates, meaning activation of this counteracts the other one. Adrenalin raises blood pressure, contracts your blood vessels, increases hearth rate, opens up airways. This system is called the sympathetic nervous system and a good analogy i was told is that it does everything that happens when you see someone you fancy(sympathize)(except erection) – not that this system turns on in that case unless your anxious about it, but a good way to remember what it actually does. And also it turns off unnecessary functions for fight or flight, such as your GI tract, because those are activated by the parasympathetic nervous system. That means anxiety and stress can cause you to lose appetite, as well as make you constipated