New Study Links Arthritis to Tryptophan Consumption

When broken down by gut bacteria, the essential amino acid produces a compound that could potentially trigger arthritis.

Step away from the turkey.

Tryptophan, an essential amino acid typically found in poultry, meat, fish, dairy products, and some nuts, is widely known — and blamed — to be the cause of your post-Thanksgiving somnolence. And now, it looks like it could be doing more damage to your body than previously thought. A new study out of the University of Colorado found that tryptophan, which plays a vital role in the production and preservation of muscles, protein, enzymes, and neurotransmitters, could be broken down by common gut bacteria into a chemical that induces an inflammatory response — thus triggering rheumatoid arthritis or spondyloarthritis (a.k.a. arthritis of the spine).

The aforementioned byproduct, called indole, is a malodorous compound more commonly associated with human waste. But it’s important to note that tryptophan doesn’t always turn into indole: “If tryptophan hits our body’s cells, it tends to go get broken down into anti-inflammatory products versus when it hits the bacterial cells and goes more inflammatory,” said Dr. Kristine Kuhn, the study’s co-author and head of Colorado University’s Division of Rheumatology. “The ways we think about how this could lead to therapies are: How do you keep that balance tipped so that tryptophan goes towards that anti-inflammatory pathway? How can you manipulate intestinal bacteria to tip that balance? That's where we’re interested in going in the future.”

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To get to the bottom of things, Kuhn and her colleagues made use of the collagen-induced arthritis (CIA) model in mice — an oft-used autoimmune representation of rheumatoid arthritis. And the result “identified alterations in tryptophan metabolism, and specifically indole, that correlated with disease.”

“We found that when indole is present, the mice start to develop autoreactive T-cells that are more inflammatory,” Kuhn said. “They have less of those regulatory T-cells that help maintain balance in the immune system, and they start to develop antibodies that are more pathogenic. We found that the antibodies had flags for being more inflammatory when indole was present.”

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So where does that leave us? Well, it’s something we’ve all heard before. Because tryptophan is ever-present in protein-rich foods, the recommendation is to aim for “a diet that’s rich in plant-based fibers and lean meats — this whole Mediterranean diet — seems to push the microbiome into a healthier state, so that you are getting the anti-inflammatory properties of tryptophan, whereas the typical western diet seems to go more toward the inflammatory pathway.”

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