Iirc the lore gets a bit weird about this, but essentially their nutrient requirements are so high that unless they’re eating an entire butchered hog for every meal, they’re basically starving to death. Slowly, yes, but it takes a lot to maintain the astartes’ physiology. Normal humans just die if they eat astartes food (of… food overdose? I guess? Too much nutrition? its unclear.), but inconsistency is rife on this subject. Basically tho, bigger picture, if things are so bad even the astartes supply line is cut off you’ve got bigger problems than food to worry about so the point is a bit moot.