You die of starvation when you can no longer produce glucose.
Or do you think all sugar is the white stuff you get at the store?
Now, I don't advocate for the keto diet, and if you indent to do that you should probably talk to a doctor, like for all restrictive diets.
EDIT: I misread your statement. Of course you would die of starvation if you couldn't make glucose. But why couldn't you make glucose, assuming you don't have a specific disease preventing it.
1. https://en.wikipedia.org/wiki/Ketosis
2. Sugar is 50/50 fructose and glucose, the fructose half is arguably poison to humans. It's important to distinguish glucose from sugar when it comes to human health/nutrition.
Fructose is usually considered healthier than sucrose or glucose - the whole "natural fruit sugars are OK" premise - though that seems likely to be nonsense given it's immediately and easily converted to glucose after you eat it. I don't know of any argument that says it's poisonous, or in any sense worse than glucose. That also seems unlikely to be true for the same reason - it's immediately converted to glucose.
(replies suggest my recollection of fructose being metabolised by trivial conversion to glucose followed by the usual glucose pathway is incorrect, interesting)
Fructose is metabolized by the liver[0] ~identically to ethanol. This is why Robert Lustig (the Sugar: The Bitter Truth[1] dude) argues fructose is a poison, and taxing it like alcohol.
AIUI it's arguably the cause of non-alcoholic fatty liver disease:
https://en.wikipedia.org/wiki/Non-alcoholic_fatty_liver_dise...
Your comment strikes me as confusing several factors... fructose does not turn into glucose upon eating it. But something like that is applicable to Sucrose vs. HFCS, rendering them ~identical in consumption (a split of glucose and fructose). That's orthogonal to what the body does with the fructose though.
Furthermore it's intact fruit that is seen as healthier because of the dietary fiber, not because of the fructose. The fructose is always a problem for the liver to deal with when absorbed. Fiber helps by slowing/preventing its absorption, in the best case letting it reach the large intestine to turn into flatulence via guttural flora before the body can absorb it.
[0] https://en.wikipedia.org/wiki/Fructose#Fructose_metabolism
The “just-so” evolutionary story behind this goes: when fructose calories are available in excess, you want to make sure you’re storing all you can, because a fruit bonanza is often associated with an upcoming lean period. So, creatures whose bodies treated large amounts of fructose as a metabolic trigger to fatten up and reduce energy expenditure were more likely to make it through the winter.