this is usually the fault of glucagon.
most people know insulin, which is a hormone that, among many other things, brings glucose (sugar) in the bloodstream into cells, where it can be used. in doing so, it lowers blood sugar. insulin is supposed to work when you've just eaten and need to get sugar out of your blood and into the rest of your body. (famously, it does not work very well for everyone in various ways, which we might call diabetes or insulin resistance.)
glucagon can be thought of as the opposite of insulin in this regard. glucagon works to raise blood sugar.
you do actually need some amount of glucose in your blood at all times, because that's how it's transported around your body to get to different locations. if there's not enough glucose available in your blood, you will start feeling tired and weak, because your cells are not functioning very well.
so, if your blood sugar is too low - like, say, you're hungry and need to eat - your pancreas will (ideally) make glucagon, which tells your liver to dump some of the glucose it has stored, and also makes glucose from scratch from various other things floating around your body (like lipids). these are emergency protocols designed to make sure that you do not simply die.
unfortunately, glucagon also fucks with your gastrointestinal system and your nervous system in several different ways that can amount to nausea.
this effect is often most pronounced in people with diabetes, but anyone with any derangement of their glucose management system (insulin resistance and conditions like pcos that can cause insulin resistance, persistent inflammation, chronic stress, certain medications like antidepressants, mast cell activation) can have problems with glucagon-induced nausea, because the way that glucagon is released and used in their body may not be happening optimally.
there is not a specific number at which you experience hypoglycemia or hyperglycemia. the numbers that doctors use are estimates based on population averages, and they're totally meaningless for an individual. some people's bodies just do not use glucose well. some people's brains consume way more glucose than they should (perhaps because they have chronic migraine or epilepsy/epileptiform discharges). antidepressants and other medications that "may cause weight gain" can permanently change the way that your body responds to and processes glucose.
so, if you are feeling nausea from mild hunger frequently, consider that you could be having blood sugar problems, and potentially ones that are not picked up by the things that doctors - or even endocrinologists - typically test for, because medical tests are not developed to test individuals, and because the many ways that glucose processing can be disturbed are not common knowledge.
the solution to this problem is generally eating smaller but more frequent meals, and making adjustments based on how you feel on a day to day basis. keeping a diary also helps some people (especially with finding problems that make this worse, like "oh, i get migraines when the pressure changes, and that makes me need to eat more often," which is completely normal). some people might even discover medical problems that they did not know about from tracking the circumstances surrounding when they start feeling hunger-related nausea.
this is an extremely common medical problem that is criminally underdiscussed even by medical professionals, and it can often turn into more serious problems regulating glucose, despite the fact that the solution is fairly simple. it can also turn into problems with disordered eating, unfortunately, because your body may start to interpret "we need to eat" as "we feel bad."
to be absolutely clear, this is not a design flaw. that is because the human body - the animal body - was not designed. it evolved from tiny unicellular creatures that were just fucking around. those creatures, at some point, managed to start using glucose to do powerful and cool things with their bodies, and then they needed to regulate glucose in their new blood-tubes so that the glucose could go everywhere it needed to be in the big complicated bodies they were developing and also not eat holes through their blood-tubes or damage the nerves that were piloting their complicated bodies.
life is complicated in this way, and our bodies have always been doing their best to keep us alive. that is just how evolution works. it is often inconvenient, and often silly, but it's something that we as humans with cognition, language, and communication skills can learn about, understand, and respond to.
all this is to say, it does not have to be this way! i am sorry from the bottom of my heart that we were never taught about basic things in our bodies that help us stay alive, and ways to attend to them to prevent our own suffering. we all deserve better than this. i hope that this information helps someone.
