Why Does Junk Food Cause Diarrhea?

Many people would likely guess that “junk” food can cause diarrhea.

But the exact reason for this digestive problem is less obvious, especially because junk food — aka highly processed food — often contains a number of unknown additives.

Here’s what the experts say about highly processed foods and why they might be causing you tummy trouble.

Defining ‘Junk’ Food

“Junk” food, or fast food, is often used to describe highly processed foods. These are foods that have had other ingredients added to them during their preparation and packaging, according to the Harvard T.H. Chan School of Public Health.

These extra ingredients are called additives. They include preservatives, artificial flavors and colorings, sugars, fats, salts, and even nutrients like vitamins.

Technically, the majority of all packaged food items have been processed in some form. However, this level of processing can vary from minimally processed to highly processed.

Minimally processed foods have been only slightly altered to preserve them, such as canning, freezing, drying, or baking. For highly processed foods, additives are used to increase taste, texture, color, and shelf stability, according to the Harvard T.H. Chan School of Public Health.

Why Processed Food Could Cause Diarrhea

Diarrhea, or watery, loose bowel movements, can have many causes, like viral and bacterial infections, parasites, and diseases such as Crohn’s or ulcerative colitis, according to Mayo Clinic.

However, diarrhea can also be a (literal) by-product of the foods we eat.

“Fast food is generally high in a combination of ingredients that, even on their own, can contribute to digestive distress in high amounts,” says Will Cole, a functional medicine practitioner and senior clinical director of the Cole Natural Health Centers in Monroeville, Pennsylvania. Cole says these ingredients include unhealthy oils, preservatives, and sugar.

Certain sugars like fructose have a particularly bad reputation, because not everyone can properly absorb them. Many people who eat more than 40 to 80 grams of fructose a day will experience diarrhea, according to Harvard Health Publishing.

Amber Romaniuk, a certified holistic nutritional consultant based in Calgary, Canada, says a highly processed or fast food intake can have a negative impact on the gut flora in the large intestine, which could then cause diarrhea and other health issues.

“Healthy gut flora support proper digestion, production of some B vitamins, which are important for energy, and proper bowel movements,” Romaniuk says.

Research continues to support the relationship between a nutritious diet and a healthier gut.

A review published in Cureus in 2023 found that people who ate mostly processed foods tended to have a more imbalanced, less diverse gut microbiome, with a higher presence of harmful bacteria, compared with people who ate mainly whole foods.

You Could Be Sensitive to Certain Foods

Romaniuk notes that a condition like celiac disease or another food sensitivity could cause your stomach upset.

“If you find you are sensitive to any particular foods, like wheat or dairy, this can trigger diarrhea, as the body sees the food as a stressor and wants to eliminate it from the body as soon as it can,” she says.

Talk to your doctor if you are experiencing frequent diarrhea. They can rule out any underlying conditions or allergies and recommend the right treatment.