Red 40 Tricks Your Gut Into Making Too Much Serotonin

Most of your serotonin is made in your gut, not your brain

Cells in your gut lining pump out about ninety percent of the serotonin in your body. They dose it in careful bursts… just enough to keep food moving through the intestines.

But a petroleum-based food dye can push those cells to pour out far more serotonin than the gut needs. A team at McMaster fed the dye to mice for twelve weeks… and the animals developed a broken gut barrier, shifted bacteria, and a path toward colitis. The dye is called Red 40, and it sits on more American labels than any other synthetic color.

22,216 The number of US food product labels that list Red 40, more than any other synthetic dye on the market.

PlainFoodSafe indexed label analysis, 2026

The dye colors candy, cereal, sports drinks, fruit snacks, and gelatin desserts. It shows up in M&M's, Lucky Charms, and even Tylenol PM. So what does all that Red 40 do once it reaches your colon?

What twelve weeks of Red 40 did to mice at McMaster

Waliul Khan and first author Yun Han Kwon, at McMaster's Farncombe Digestive Health Research Institute, fed mice Red 40 at doses built to match human intake. Twelve weeks later the animals had a disrupted gut barrier, shifted bacteria, and higher colitis susceptibility. Their findings appeared in Nature Communications in 2022.

A 2023 study in Toxicology Reports found more. Red 40 decreased Lactobacillaceae, a whole family of protective gut bacteria. It caused DNA damage in colon cells, and a high-fat diet made every measure worse.

The chain ran through one specific molecule. But how does a color cause that kind of damage inside your gut?

The dye floods your colon with serotonin

Red 40 passes through the stomach and small intestine without breaking down. It reaches the colon whole. Once there, it pushes the cells that make serotonin to pump out far more than normal. Normally those cells release just enough to keep food moving. Red 40 throws off that balance.

The extra serotonin weakens the seals between gut-wall cells. Gaps open. Bacteria and food particles slip through, triggering swelling. At the same time, protective Lactobacillaceae decline and the bacterial mix shifts toward a colitis profile. Look at a label and you might not catch it — Red 40 goes by three different names.

Red 40 goes by three names on the label

Red 40 is the most widely used synthetic food dye in the country. On ingredient lists it appears as Red 40, Allura Red AC, or FD&C Red No. 40. Check candy bags, cereal boxes, sports drinks, and strawberry-flavored yogurt.

In April 2025, the FDA asked companies to voluntarily phase out petroleum-based synthetic dyes. The agency set a target of the end of 2027. Dyes remain legal and sit on shelves right now.

But cutting one dye out of your cart is easier than it sounds.

Six foods that skip the dye and feed your gut lining

The bacteria Red 40 decreases — Lactobacillaceae — can be rebuilt with the right foods. And none of them need a single drop of synthetic color to look good on the plate.

🥬 Sauerkraut Live Lactobacillus may help replace the bacteria Red 40 depletes	🌶️ Paprika Capsanthin gives food natural red color
🍒 Tart cherries Deep red plus fiber for butyrate-producing bacteria	🍓 Whole strawberries Fiber feeds bacteria that hold the gut wall tight
🟡 Turmeric Curcumin supports gut-barrier repair in animal studies	🟣 Beet powder Betaine supports cell repair, betalains give natural red

Once you know where to look on the label, the swap takes five seconds.

Flip the bag and scan the ingredient list

Every bag, box, and bottle has the ingredient list on the back. The dye appears under one of three names.

🚫 Grab a bright-red snack without checking the back

✅ Flip the bag and scan for Red 40 or Allura Red before it goes in the cart

If you find Red 40, Allura Red, or FD&C Red No. 40, put the box back. Pick the one that lists fruit juice, beet juice, or turmeric for color instead. One label flip. That is the whole move.