[Babcock_Hall]

Here are some very general principles.  Some amino acids are glycogenic, only two are ketogenic (leucine and lysine), and some are both glycogenic and ketogenic.  The carbon skeletons of glycogenic amino acids can be made into pyruvate or oxaloacetate, either of which can be converted by liver into glucose via gluconeogenesis.  Ketogenic amino acids can be turned into ketone bodies but not into glucose

Under starvation conditions, the brain uses a mixture of glucose and ketone bodies for fuel, in contrast to its otherwise strong preference for glucose.

[Enthalpy]

Feeding humans from dead, dying or living biomass looks trivial.

Cellulose makes much vegetable material and is a polymer of D-glucose
  Cellulose
we know already the enzyme that humans lack to break cellulose to D-glucose, one fundamental source of energy
  Glucose
so we can just use the enzyme, or some chemical processing instead, to produce D-glucose that we eat. Or?

So would we compete for the unpolluted biomass after a nuclear war? Perhaps not, because the radionuclides with significant life are inorganic: I, Cs, Sr and the others. So we might remove the radionuclides by chemical processing apparatus, perhaps some sort of resin or electrolysis or just precipitation.

Again, providing the calories amount to humans is easy. The vitamins, essential elements and others are more difficult.

[Enthalpy]

[...] radionuclides with significant life [...]

I meant: half-life.

==========

It doesn't need a nuclear catastrophe. Local famines abound at any time. Humans could then feed on cellulose by trees and grass, at least for the diet part that D-glucose can replace.

Must the transformation happen in reactors, or could humans just ingest the proper enzyme and swallow tree dust? I suppose our stomach is too small and simple in comparison with cows.

Hey, I'm perfectly zero on biochemistry, so someone else could usefully enlighten us!

[Babcock_Hall]

I don't see how it would be possible to ingest the proper enzyme and swallow tree dust.  The acid in our stomach might denature this enzyme, and it would itself be acted upon by proteases.

[Enthalpy]

Thanks Babcock_Hall!

Then cellulose must be broken outside our body. It's already done for lactose-free milk and diaries, and for agave syrup.