[Pseudoscience-is-malarkey (OP)]

How possible is it to invent a pill that contains all the nutritional elements needed to sustain ourselves?

[Bored chemist]

Yes, but it would need to contain something like half a pound of lard to provide the energy and also a lot of water.

[alancalverd]

I've always preferred the Whopper but many swear by the Big Mac.

As the Buddhist said to the burger flipper: "Make me One With Everything."

My great grandfather used to say, "If you eat that for a hundred years, you will live a long time". Though I think he was referring to borscht.

[evan_au]

It would be a very bulky pill to contain all the fibre that is needed by our guts and microbiome.

And you would need to wash it down with 2 litres of water...

[alancalverd]

So it's a Big  Mac,  coleslaw, and a large Coke for lunch, with tea and coffee ad lib per diem. Sounds pretty normal to me.

[Bored chemist]

ad lib per diem

I don't think per diem makes sense after ad lib.

[alancalverd]

Sheldon Copper had a strict hydration regimen and some folk wear "fitbits" to control their lives, but most normal people drink when they are thirsty rather than by the clock, and you have suggested a daily norm of 2 liters. If you score zero over more than about 3 diems you will get very sick.