I'm doing research on WW II and came across this from a former Golden Lion (106th Infantry Division) PoW, Sgt. Russell Lang.

German Black Bread Recipe
This recipe comes from the official record of the Food Providing Ministry which was published in Berlin, on 24 November 1941 (Top Secret) by the Directors in Ministry Herr Mansfeld and Herr Moritz.

50% bruised rye grains,
20% sliced sugar beets,
20% tree flour (saw dust*) and
10% minced leaves and straw

Note: “From our own experience with the black bread, we also saw bits of grass and sand.
Someone was cheating on the recipe.” Joseph P. O’Donnell, Robbinsville, New Jersey.

The German PoWs in the US were fed very well too.

link

https://www.askaprepper.com/shocking-foods-that-people-ate-during-the-leningrad-siege/
Non food items as additives, and I guess that really could be anything.

I just found this after looking up what you posted. Lots of interesting stuff on WW1 rations too. Good post.