You can function just fine for prolonged periods of time without ingesting any glucose sources.
It does take the body a while to adjust and ramp up ketone production, which can lead to feeling drained/low energy and induces sugary-rich food cravings.
But that mostly subsides after a few days.
Nice side effect: it is quite common to get a certain sense of eurphoria and well-being after not getting sugar for a few days. The current hypothesis is that one of the ketone bodies (BHB) might have similar effects on the brain as GHB, which is a psychoactive drug. (they have a similar structure, namely they are isomers)
Personally, I feel much more energetic and healthy when skipping carbs and fruits entirely. Even mentally! At least after the initial adjustment. Sadly it's hard to fall back into eating sugar all the time.
Glucose is needed for some red blood cells though, and the body has ways of producing it in absence of food. Those are insignificant amounts.
No.
In deep ketosis there's gluconeogenisis where the small amount you need will be made for you.
In fact, people on keto or fasting actually report vastly better mental clarity.
- converts carbohydrates to glucose on the way in
- stores glucose as glycogen (fast-release energy mostly in the liver and muscles) or storage fat (slow-release energy mostly in dedicated tissue)
- converts glycogen[0] back to glucose when you need energy to burn
You don't need dietary carbohydrates very frequently. Your body will be fine converting glycogen[0] to glucose at an appropriate rate, and you store about a day of glycogen. The problem with dietary carbohydrates is that they always start as glucose and then get stored. If you dump a bunch of simple sugars into your stomach, they all end up in your bloodstream at once (hyperglycemia), your body releases a lot of insulin to cope with the surplus, and then there's a dip (hypoglycemia) afterwards. Once you're through the dip, though, everything should go back to normal.
I think this is the same as what you're saying, just with more detail on why it works that way.
[0] or other things, like fat and protein, in a pinch
Sugar is a shortcut to get glucose, however it burns fast so if you want to keep the same buzz you'll have to take more sugar. So a vicious cycle of ups and downs will arise.
In contrast, one can take slower-burning glucose sources. The good news is that this stuff is objectively measured by the Glycemic Index; here's a table for a good variety of foods. https://www.health.harvard.edu/diseases-and-conditions/glyce...
On a good week I'll stick to low GI foods and take no chocolate / red bulls. Will feel productive, and notoriously energetic.
Still one can mix and match - sometimes out of a self-assessment will intentionally seek a short-term boost which (I hope) will not decline sharply because I also ate low-GI foods.