You can learn to feel/tell if your stomach has emptied yet, if your blood has run out of fat or carbs, and if you’re dipping into either fat or carb (glycogen) storage, or if your body has decided to stop liberating something and you’re starting to lightly chew on muscle or lean tissue and/or produce ketones.
Being low or topped off on Vitamins A and B, and Choline are all distinct for me, too. I haven’t noticed anything yet from other vitamins or micronutrients.
I don’t have a good way to describe the sensations (yet?). It is like describing subtle anger or subtle sadness or happiness if those weren’t in the language.
I will say that, for the stomach emptying thing, each time your stomach empties a little bit into your small intestine, you sometimes get a spreading, fizzy warmth. The fizziness is autonomic activity. The warmth is probably a combination of things. This is very faint. When your stomach is completely empty, there’s a sense of “doneness” or “completeness,” which might be from reduced autonomic activity in the stomach as well as faint interoception of reduced stomach stretch receptor activity.
For when your blood runs out of stuff, take a slow, meandering walk for at least forty minutes. This should be after your stomach is completely empty. (If you go fast, your body will be supplementing from storage tanks at the very beginning.) Around that time, depending, you’ll run out of blood fuel and/or muscle-stored glycogen, and there will be a cutover from blood supplied fuel to fuel from storage tanks (liver, fat stores). If your body doesn’t cut over cleanly, your arms and legs will feel temporarily heavy and your walking speed will slow down. You might faint, transient dizziness. You might feel a faint uptick in anxiety, cortisol release, and fizzing autonomic activity, as your hormones and nervous system trigger fuel release. The better everything is working, the smoother the cutover is, and the less likely you are to feel anything.
For the other stuff–I’m NOT saying this is important or useful for people, necessarily!! This is just a thing!!
Noticing being low on fat:
Eat, for example, only rice and lean meats for a 12-72 hours. (Probably take a multivitamin or something during this.) Notice how you’ll eventually be wanting to sit down all the time (depending on a bunch of stuff). Notice it partially, temporarily goes away if you exercise hard (if you have the energy; this depends on your current metabolic regime). Notice how your “stamina profile” changes, what kinds of activity is harder or easier. Now, eat butter, lard, tallow, heavy cream, etc. Forty minutes to three hours later, what has changed? You’ll likely be almost unable to sit. You’ll want to stand up and/or walk around. Also, how has thinking changed, etc., etc.
Noticing being low on carbs:
Do “cardio” for a couple hours or simply eat “low carb” for 6-30 hours. Make sure you’re getting enough potassium. Notice mood, energy, motivation, “stamina profile.”
Noticing gluconeogenesis and ketone production:
Do the carb thing and keep going for 30-120 minutes. Or, try to sleep after eating relatively less carbs that day and then when you wake up… Your liver and muscles will be relatively empty of glycogen. Notice your bad breath, possible faint dizziness and faint nausea. Notice any uptick in sensory experience and clarity (cortisol release and partially from the ketones) and possibly anxiety. The negative stuff happens less and less the more your body is used to it. I also start to feel pain in my forearms.
If you ingest a lot of sugar or orange juice while in this state, you may eventually experience a rush of increased autonomic activity, involuntary heavy breathing, and feeling like you’re going to faint for twenty seconds to a couple minutes. Your glycogen depleted muscles and liver are sucking sugar out your bloodstream and transiently your brain is getting less glucose than it’s used to.