is there a way to know when my body is using stored fat as energy after glycogen storage is used up? Is that what ketosis is?
You don't really know when you've used up your glycogen stores, unless you have access to a lab.
Endurance runner's tend to know the moment they start burning fat vs. cho because their pace drops significantly (this is because fat is less efficient as a fuel source compared to carbs).
Susan Macfarlane, MScA, RD
Registered Dietitian Nutritionist
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Is it that urgent, ravenous hunger feeling that all of a sudden comes over one, and it feels like one is on empty; as distinct from a gentle reminder that one maybe ought to eat a little something?
Or is that lethargic feeling of being constantly, somewhat hungry and worn, and maybe being able to use a day or two of slacking? I used to get this when younger, with super low body fat content, and high activity level.
Pillage, then burn.
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You can know when you are in ketosis by purchasing a ketone meter and using it.
When consuming a mixed diet, it's unlikely to deplete your glycogen stores fully through activities of daily living. However, what you are describing sounds like different stages of physiological hunger that are often accompanied by drops in blood sugar levels.
It sounds like you had a very fast metabolism and as such, your cues for hunger were very strong.
Actually being in ketosis gives you energy. I can usually tell because I can think more clearly, I have more energy and I'm NOT hungry.
Ravenous hunger usually is a sign of low blood sugar, not glycogen depletion. You "know" when your glycogen is extremely low (not depleted completely as @Susan_RD_101 pointed out) when you "hit the wall" in runner-speak, or "bonk" in cycling-speak. If lifting weights, glycogen depletion of the particular muscle you are using results in the "not one more" feeling.
After readily available glycogen muscle stores run out, the liver will continue to try to catch up and produce blood glucose to keep runners and cyclists going and indeed the fat metabolism will kick in and aerobic exercisers will experience that blissful "second wind." Keep in mind that the old adage "fat burns in the flame of carbohydrate" means that once the "bonk" happens, some sort of glucose is necessary to kick the fat metabolism in gear (glucose or ketones too! @Wackiejac ).
Also keep in mind that we don't "switch" cleanly between one tank or another for energy, we are always burning a mix of carbs, protein and fat. In a lab, we measure the CO2 and O2 ratio expelled by our poor graduate student volunteers while exercising. Since each of the fuel sources burns differently, with a breath/gas analyzer we can tell what mix an athlete is using at any given moment and what fuel is most predominantly used at any given pace or effort. I miss lab days!
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Well-said @VegasTortoise !