Green onions, also known as scallions, are a common ingredient used to add a mild, fresh onion flavor to many dishes. While generally safe for consumption, they can cause diarrhea in some individuals. This reaction typically depends on the amount consumed, the person’s underlying digestive health, or external factors like food contamination.
The Role of Fructans and Dietary Fiber
The most common reason green onions may lead to diarrhea is their high content of fructans and dietary fiber. Fructans are a type of fermentable carbohydrate known as a FODMAP (Fermentable Oligosaccharide, Disaccharide, Monosaccharide, and Polyol). The human body lacks the necessary enzymes to fully break down these complex carbohydrates in the small intestine, meaning only a small percentage is absorbed.
The unabsorbed fructans travel to the large intestine, where gut bacteria rapidly ferment them. This fermentation produces gas, leading to bloating and cramping. Fructans also create an osmotic effect, drawing extra water into the large intestine, which accelerates bowel movements and results in diarrhea.
When Underlying Health Issues are the Cause
For individuals with sensitive digestive systems, green onions can act as a trigger. The high fructan content in the white bulb and pale green sections makes them a common culprit for those with Irritable Bowel Syndrome (IBS). People managing IBS often have a lower tolerance threshold for FODMAPs, meaning the normal digestive process causes more severe symptoms.
The fermentation and water-drawing effects of fructans intensify IBS symptoms, leading to pain, bloating, and diarrhea. Interestingly, the dark green, leafy tops of the scallion contain significantly fewer fructans and are considered low-FODMAP. This distinction allows many people with sensitivities to still enjoy the onion flavor.
Addressing Food Safety and Contamination Risks
Diarrhea caused by green onions can also stem from food safety issues and contamination. Green onions, like other fresh produce, are susceptible to contamination from water, soil, or handling. They can harbor pathogenic bacteria, such as Salmonella or E. coli, which can cause sudden, severe food poisoning.
This type of illness is distinct from digestive intolerance and usually presents with more intense symptoms like fever, vomiting, and bloody diarrhea. Since green onions are frequently consumed raw as a garnish, the bacteria are not destroyed by heat. Contamination can occur at any point, from the farm’s irrigation water to cross-contamination in the kitchen.
Prevention and Safe Consumption Practices
To reduce the likelihood of digestive upset, paying attention to preparation and portion size is helpful. Thoroughly washing green onions under running water helps remove potential surface contaminants, mitigating the risk of bacterial food poisoning. Since the white and pale green parts are highest in fructans, sensitive individuals can minimize symptoms by consuming only the darker green tops.
Cooking green onions can also help, as the heat may reduce the bacterial load and the fructans are water-soluble. For sensitive individuals, limiting the portion size is the most effective strategy, as symptoms are often dose-dependent. If diarrhea is severe, persistent, or accompanied by blood, fever, or vomiting, consult a healthcare professional, as this may indicate a serious infection or underlying condition.