Developing diarrhea after eating corn is a common gastrointestinal complaint. For most people, this reaction is not due to an allergy or illness, but rather a normal physiological response to the corn kernel’s unique composition. The core issue lies in how the human gut processes certain components of the corn, including its tough outer shell and the type of starches found within the kernel. Understanding these specific digestive challenges explains why corn often appears undigested and triggers a loose bowel movement.
The Indigestible Shell: Cellulose and Fiber Content
The primary reason corn kernels often pass through the digestive tract appearing intact is the structure of their outer layer, known as the pericarp or hull. This outer shell is composed of cellulose, which is a type of insoluble fiber that provides structural rigidity to the plant. Humans do not possess the necessary digestive enzyme, called cellulase, to break down the specific chemical bonds in cellulose.
Because the body cannot chemically dissolve this material, the cellulose hull remains largely unchanged as it moves through the digestive tract. This undigested, bulky fiber acts as a physical irritant and accelerator within the gut. The presence of this roughage speeds up intestinal transit time, meaning the entire digestive process happens faster than normal. This rapid movement reduces the time the colon has to absorb water from the stool, resulting in the looser, watery consistency associated with diarrhea.
Starch Malabsorption and Osmotic Effects
Beyond the indigestible shell, the contents of the corn kernel, which are mostly starch, also contribute to diarrhea through malabsorption. While most of corn’s starch is digestible, a portion known as resistant starch is not broken down in the small intestine. This undigested carbohydrate travels to the large intestine, acting as a food source for gut bacteria. The bacterial fermentation of these starches produces gas, causing bloating and cramping, and creates short-chain fatty acids. The presence of these high concentrations of unabsorbed particles creates an osmotic effect.
Osmosis draws excess water from the body’s tissues into the intestinal lumen to dilute the unabsorbed material. This increased volume of water in the large intestine results in the characteristic watery stool of osmotic diarrhea. The same mechanism is at play when people experience diarrhea after consuming poorly absorbed sugars like fructose or sugar alcohols.
When the Reaction Signals a Deeper Issue
For most people, corn-related diarrhea is a simple intolerance caused by the mechanisms of fiber bulk and osmotic water retention. However, a reaction to corn can sometimes signal or exacerbate an underlying digestive condition. Individuals with disorders like Irritable Bowel Syndrome (IBS) or Small Intestinal Bacterial Overgrowth (SIBO) may find that corn’s high fiber and resistant starch content triggers a more severe response. Corn intolerance may also be related to a rare congenital enzyme deficiency, such as congenital sucrase-isomaltase deficiency, which impairs the digestion of certain corn sugars.
A true corn allergy is a distinct, immune-system-mediated reaction that involves symptoms like hives, swelling, or breathing difficulty. If your reaction to corn is consistently accompanied by intense abdominal pain, unexplained weight loss, or blood in your stool, consult a healthcare provider. To manage typical corn intolerance, try thoroughly chewing corn to break the pericarp, controlling portion size, or consuming processed forms like cornmeal. Cornmeal removes most of the problematic hull.