Why is spinach shit black?

Eat spinach stool black is generally caused by the reaction of certain substances in spinach in the intestinal tract.

Spinach is a nutritious vegetable, which contains a lot of oxalic acid and tannic acid. Oxalic acid can be combined with calcium, iron and other minerals in the intestinal tract, affecting the body's absorption of minerals; tannic acid can be combined with proteins, fats and other substances in the intestinal tract. If spinach is ingested too much, oxalic acid and tannic acid in spinach may chemically react with hydrogen sulfide in the intestinal tract to produce some black substances, which are then excreted with the stool, making the stool appear black.