What are ADVENT burgers made of?

According to this image found within the SDK, ADVENT burgers (according to ADVENT) are made up of (translation from the image):

Buckaroo Burger

• Soft juicy and full of flavor - cheese pickle tomato lettuce and fresh angus beef all wrapped up on a freshly baked sesame bun

The Buffalo Chicken Classic

• Soft melted cheddar cheese melted onto a panko breaded chicken breast rolled in juicy buffalo sauce made fresh in house. Sesame bun perfection

Cowboy Burger Diggyup

• Thick cut applewood smoked bacon fresh off the farm with melted American cheese, pickles, and iceberg lettuce with two smokin kobe beef patties

So according to ADVENT, their burgers are made out of cows, with all the fillings you'd expect, like lettuce, cheese, and sesame bun. But to be honest? I think they're made from different kind of cows.

Mootons.

The only references in the game I'm aware of are the off-hand comments made by the crew:

• Tygen says he misses Advent Burgers
• Bradford hates them, and says if he finds one more wrapper there's gonna be hell to pay
• Shen wonders where they come from, since all the cows are gone
• In one of the guerrilla ops missions, you find conveyors of advent burgers with "no clear source of meat". During that same mission, Bradford says something like "no one questions where the meat comes from even though there's no cattle coming in"

Additionally, the game has an official prequel novel. I have not read it, but according to one post,

In the book, while burgers are never mentioned no one gets much in the way of meat anymore because Advent have a strict policy on domesticated animals which turns out to be because some sort of rabies-like alien contagion that makes wild animals stand around all creepy and if you shoot them they [explode] in a shower of rainbow dust that spreads. [..]

What they eat in the cities is called "CORE", featuring yummy " reclaimed protein"...

The lack of cows comment + "reclaimed protein" makes it pretty clear they're hinting that Advent Burgers are made from humans. It's probably a reference to the classic 1970's Sci-Fi film Soylent Green, which is famous for that same twist.