Study Finds Link Between Arthritis and an Amino Acid Found in Many Common Foods

The same amino acid blamed for putting people to sleep after Thanksgiving dinner has now been linked to arthritis, in a first of its kind study.

Researchers at the University of Colorado say that tryptophan, a common amino acid found in most protein-based foods, can break down in the body and cause inflammation. Tryptophan is found in high levels in Western dietary staples such as red meat, eggs, fish, poultry, nuts, legumes, and dairy including milk, yogurt, and cottage cheese. Although there is a misconception that turkey contains an abnormal amount of tryptophan, it's actually the average amount found in most poultry.

The body uses tryptophan to build proteins, muscles, enzymes and neurotransmitters, but what's remaining gets broken down. Once this happens, it can produce an inflammatory chemical that can trigger rheumatoid arthritis, an autoimmune and inflammatory disease that affect the joints.

In the study, which was published in the Journal of Clinical Investigation, researchers conducted lab tests on mice and found that tryptophan becomes broken down into an organic compound called indole, which occurs naturally in human feces. When gut bacteria breaks down tryptophan into these byproducts, depending how they're used in the body it may activate inflammatory T-cells.

"It’s been known that the microbiome—the bacteria in our gut—can break down tryptophan into byproducts," said Dr. Kristine Kuhn, Scoville Endowed Chair and head of the CU Division of Rheumatology, who co-authored the study. "Some of those byproducts are anti-inflammatory, but we’ve also associated some inflammatory causes of those products. We’re the first to highlight which products are contributing to inflammation, and how they are doing that."

“We put mice on antibiotics to wipe out their microbiome, and they didn’t get arthritis, and they didn’t have indole,” Kuhn continued. “So we said, OK, what if they do have a microbiome and we put them on a diet with little tryptophan? The microbiome can’t break down tryptophan into indole, and the mice didn't get arthritis. So two different ways, we showed that it’s tryptophan that’s broken down by the microbiome into indole.”

As a result of the study, Kuhn says the team has begun to understand the at-risk stage and can now actually identify people who are likely to develop rheumatoid arthritis in the next few years based on blood markers. She said that the data suggests there might be a way to intervene during that period and prevent disease, but haven't found the right way to go about it yet.

In the meantime, those concerned about their tryptophan can simply adjust their diet to avoid the types of foods that might end up causing inflammation.

“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," Kuhn added.