Synthetic food dyes give many foods their bright colors, from cereals and candies to sports drinks. One of the most common is Allura Red AC, better known as Red 40 or E129. A study from McMaster University in Canada suggests that long-term exposure to this dye may make the gut more prone to inflammation, at least in mice.

Why researchers looked at food dyes

Inflammatory bowel diseases (IBD), including Crohn’s disease and ulcerative colitis, are long-term conditions that cause inflammation in the digestive tract. Their causes are complex, involving genes, the immune system and environment. Growing evidence suggests diet plays a role, and some food additives have been shown to affect the gut. Meanwhile, the use of synthetic dyes has risen sharply over the past 50 years, yet little was known about how they affect gut inflammation.

How the study worked

The research, published in Nature Communications, used mouse models. Mice were exposed to Red 40 at levels meant to reflect typical human intake, either continuously over 12 weeks or on and off.

What they found

• Long-term, continuous exposure made the mice more susceptible to colitis, a form of gut inflammation, and some developed mild colitis.
• On-and-off exposure did not have the same effect.
• Early-life exposure also appeared to raise later susceptibility, which matters because many dyed products are marketed to children.
• Serotonin played a role. The dye raised levels of serotonin in the gut, a chemical messenger that affects gut function, and the researchers identified this as a key part of how the inflammation developed.

The effect appeared mainly in mice already prone to developing colitis, suggesting the dye may act as a trigger in people who are susceptible because of genetics or other factors.

What we don’t know yet

This research was done in mice, and the authors stress that studies in people are needed to know whether the same thing happens in humans. Red 40 is approved for use in food in the U.S. and many other countries, though synthetic dyes have faced growing scrutiny. For now, the findings are a reason for more research, not a reason to panic.

Where Red 40 shows up

Red 40 is found in many brightly colored foods and drinks, including some breakfast cereals, candies, gummies, sports drinks, sodas, flavored yogurts, frosting and snack foods. It can also appear in some medicines and vitamins. On U.S. labels, look for “Red 40,” “FD&C Red No. 40” or “Allura Red.” In Europe it’s listed as E129.

What this means for you

• Read labels if you’d like to cut back on synthetic dyes, especially in foods for children.
• Choose more whole foods. Fruits, vegetables, whole grains and minimally processed foods are naturally free of synthetic dyes and support gut health.
• Look for naturally colored options. Many products now use colors from beets, paprika or other plant sources.
• If you have IBD, talk with your doctor or a dietitian about which foods seem to affect your symptoms. Food triggers vary from person to person.
• Don’t ignore gut symptoms. Ongoing diarrhea, abdominal pain, blood in the stool or unexplained weight loss should be checked by a doctor.

The bottom line

In mice, long-term exposure to the common food dye Red 40 increased the risk of gut inflammation. Human studies are still needed, but limiting highly processed, artificially colored foods is a reasonable step for overall health.

This article is for general information and isn’t medical advice. Talk with your healthcare provider about your individual needs.

Sources

• Kwon YH, Khan WI, et al. Chronic exposure to synthetic food colorant Allura Red AC promotes susceptibility to experimental colitis via intestinal serotonin in mice. Nature Communications, 2022.
• New Atlas: coverage of the McMaster University study, 2022.
• Mayo Clinic: Inflammatory bowel disease (IBD) symptoms and causes.