The Viability of Food Pills as a Source of Sustenance

In Is there any single food that can sustain a person? the OP (post #2) proscribes science fiction references to food pills in any contributions to that thread.

My question exclusively concerns the viability of food pills as a source of sustenance because, as the links below would indicate, the concept is not entirely exclusive to the SF genre.

From here:

and from there :

This link is 3 years old and I’m unacquainted with any progress which may have been made during the interim.

So the the military is, or was, looking at a pill which would provide sustenance for 5 days or so. I’m curious to know where the boundaries might lie in a project such as this. Would 5 days without conventional food be a safe upper limit or is it possible to go far longer using only food pills? What would happen to the human body the longer it goes without food as we know it?

Many thanks.

define pill.

Theres no way you can get 666.666(2000/3) calories into something the size of a advil. It’s a physical impossibility.

Take 100% fat which is pretty energy dense. 9 calories/gram. A ‘fat’ pill which would provide you 2000 calories would be just under half a pound. Divide that by 3 and you get a pill thats 1/6th of a pound. Thats 75X the heft of a 500mg advil.

What the military seems to be researching is not a sustenance pill, but a pill which controls metabolism. The pill it self provides no calories. Instead it shifts the metabolism to preferentially burn fat. Those fat stores only last so long, and i’m guessing thats where the 5 day limit comes in.

Thanks.

To clarify, sustenance is used in the sense of a ‘means of sustaining health’ or a ‘means of (bodily) maintenance’.