According to Health.com, humans cannot fully digest corn because it contains the fiber cellulose. The human digestive system cannot completely break down cellulose because it lacks the enzyme to do so, leading to the trouble people often have fully digesting corn.
Health.com notes that humans may have been able to digest corn fully in prehistoric times since they had sharper teeth. Today, corn can still be digested if it is chewed on long enough to break down the cellulose fibers. If not, eating corn can cause abdominal discomfort and gas, and undigested kernels may show up in stool. Chewing corn fully also allows the nutrients in it to be absorbed better.