They are not absorbed and may be seen as greasy feces. Proteins that escape digestion in the small intestine may be putrefied by microbes in the large bowel. The odor of feces will be offensive. The lower bowel milieu of hind-gut fermenters also yields B vitamins, synthesized by microbes. The nutritional value of these vitamins formed at a postabsorptive site has been long debated.
Spinach kills herbivorous reptiles.
Oxalates would likely clinically impact mineral metabolism only if mineral intakes were marginal. These non-nutritive substances include saponins, phenols, and alkaloids and serve as deterrents to bacteria, fungi, and herbivores. Spinach, as with kale, is a nutritious and tasty green that can be included as part of a mixed salad for herbivores. Just as I discourage feeding a diet of only kale, so I suggest avoiding diets of only spinach. However, spinach is safe to include as part of a balanced diet.