One potential cause of green-colored stool is that stool has passed through the intestinal system too rapidly, according to MedicineNet. When food passes too quickly through the intestines, the normal process of breaking it down is hastened, and the lack of time spent in the intestines is insufficient for bile to break the food down. The process of breaking food down generally results in stool that is brown in color but with reduced transit time allows the greenish color of bile to be seen in the stool.
Not all green stool is indicative of a short transit through the intestines, advises MedicineNet. Green stool can also be caused by a diet that is abundant in green vegetables, particularly spinach. Additionally, iron supplements may cause green stools, although it is more likely that iron turns stool a black color.
A less innocuous cause of green stool is food additives and artificial coloring, according to Mayo Clinic. Ice pops and flavored drink mixes often have this coloring added to them.
Food generally takes around three days to be digested, and stool that is green is indicative of food making it through just the initial stage of digestion, notes WebMD. Generally, green stool is not cause for alarm. Black stool, however, is an alarming color for stool, since it can indicate internal bleeding.