Monosodium glutamate (MSG) is a flavor enhancer used globally to impart the savory taste known as umami. It is the sodium salt of glutamic acid, an amino acid found naturally in foods like aged cheese, tomatoes, and meat. Despite its common use, a persistent question remains about whether consuming MSG can trigger increased bowel movements or cause diarrhea. This concern often stems from anecdotal reports of digestive upset after eating certain foods, prompting inquiry into the additive’s physiological effect on the gut.
How the Body Processes MSG
When consumed, MSG separates immediately into sodium and free glutamate. The body does not distinguish this free glutamate from the glutamate naturally occurring in other protein-rich foods. Glutamate is one of the most abundant amino acids and is synthesized by the body as part of normal metabolic processes. The vast majority of ingested glutamate is metabolized directly within the gastrointestinal tract.
Over 95% of the free glutamate is used as a primary energy source by the enterocytes, the cells lining the small intestine. This rapid utilization means only a small fraction of the consumed glutamate enters the bloodstream. This processing is fundamentally different from the action of a typical osmotic laxative.
A substance that causes diarrhea by drawing water into the bowel is not metabolized and absorbed as readily as glutamate. Since intestinal cells quickly consume the glutamate for fuel, MSG does not linger in the colon. Therefore, it cannot exert a significant osmotic effect that would lead to increased bowel movements.
Scientific Consensus on MSG and Bowel Function
Controlled scientific studies have not established a reproducible causal link between MSG consumption and diarrhea or a change in bowel frequency. Regulatory bodies worldwide, including the U.S. Food and Drug Administration (FDA), classify MSG as generally recognized as safe (GRAS). This safety designation is based on decades of research showing no adverse effects on the general population when consumed at typical dietary levels. The average adult consumes approximately 13 grams of glutamate daily from natural food sources, with only about 0.55 grams coming from added MSG.
Double-blind, placebo-controlled trials are the gold standard for testing such sensitivities. These trials have often struggled to confirm a connection between MSG and gastrointestinal distress. Researchers administer MSG in capsules to eliminate the flavor factor, and the results show no consistent reaction.
In the few instances where minor, transient symptoms have been observed in self-identified sensitive individuals, they typically occurred only after consuming very large doses. These doses were often three grams or more of MSG, administered without food and on an empty stomach. This quantity is substantially higher than the amount typically found in a single serving of a meal containing the additive. Therefore, the scientific evidence suggests that MSG does not act as a direct irritant or laxative at the levels commonly used in food preparation.
Why People Report Gastrointestinal Sensitivity
Despite the scientific consensus, some individuals report experiencing mild, short-term gastrointestinal distress, such as loose stools or stomach upset, after consuming foods with added MSG. This experience falls under the “MSG symptom complex.” These symptoms are generally mild and transient, usually resolving without medical intervention.
The reported symptoms often occur when MSG is part of a large, complex meal that is also high in fat, sodium, or other ingredients known to affect digestion. For example, a heavy, high-fat meal can naturally slow gastric emptying and cause digestive discomfort. This discomfort may be mistakenly attributed to the MSG itself.
Research indicates that the symptoms are often not reproducible when tested under blinded conditions, suggesting a potential nocebo effect for some individuals. The threshold for experiencing any mild symptoms is high, requiring an intake significantly greater than what is typically found in a flavored dish. The overall scientific evidence does not support the idea that MSG is a substance that causes increased bowel movements.