I experienced this. I never needed to lose weight, always been lean. But I wanted to see if I could reduce my blood sugar since it was borderline high. So I went very low carb for a while. Did not intend to but lost 5 kilos right away and felt fine, although it seemed like I was a bit underweight. A few weeks later got a miserable case of flu and couldn't eat for a few days. Lost even more weight, felt terrible, looked like death. It seems like the little reserves I previously had were gone from my change in diet.
My research indicates those 5 kilos were probably mostly water because when you go low carb your body uses up all of its glycogen, which is stored with 3-parts water, and that takes a lot of salt with it. So you have to drink more often and consume more salt. Then it can take 2-4 weeks for your body to ramp up the right chemicals to switch to fat burning.
Does it really need stored energy? Parenteral nutrition in hospital works fine with plain glucose. One would assume you could just eat fruit (or drink soda, I guess) when sick, to get the energy you need right then.
One problem for athletes is that they take an energy drink or bar and then feel dopey. This is because as soon as a surge of glucose hits your blood your pancreas pumps out insulin and sweeps it all up leading to a crash. Instead drink milk or eat complex un processed carbs like oat cakes (but not ones that have a ton of honey).
Yes, your body can't carry that much energy in your blood - it gets shunted off to glycogen or fat quickly because if it didn't the chemistry would start to go horribly wrong.
You could maybe do what you suggest by just slowly sipping a glucose solution over the course of a few hours but it would be very inefficient.
