If your diet is heavy on packaged snacks, fast food and sugary drinks, you may have another reason to rethink what's on your plate — your diet could be depressing your mood. Research from the University of California, San Francisco (UCSF) suggests that sharply reducing ultraprocessed foods can improve symptoms of depression.

The small pilot study was led by D. Nyasha Chagwedera, an assistant professor in UCSF's Department of Psychiatry and Behavioral Sciences and founder of the UCSF Metabolic and Mental Health Program. It involved 20 adults with depression who also consumed high amounts of ultraprocessed food.

Nineteen people with an average age of 43.7 completed the study; seventy percent were women. Everyone in the study had a high body weight or another metabolic problem, such as high blood pressure, abnormal cholesterol or insulin insensitivity. Fourteen of the participants also had anxiety.

The goal was not to eliminate every packaged food, but to substantially reduce this category for meals.

People in the study experienced both diets. For four weeks, one group continued eating its usual diet, while the other followed a low-ultraprocessed food diet with guidance from researchers on shopping and purchasing food themselves.

After four weeks, the group switched diets. Participants were asked not to change their medications, therapy, exercise or sleep during the study.

Ultraprocessed foods generally include products made with industrial ingredients or techniques not found in home kitchens, such as packaged snacks, soft drinks, ready-to-eat meals, processed meats and some fast foods. They may contain preservatives, emulsifiers, artificial colors, flavor enhancers or other additives. In this study, the goal was not to eliminate every packaged food, but to substantially reduce this category for meals.

But the dietary change ended up being substantial. Participants' ultraprocessed food consumption fell by more than 85 percent during the low-ultraprocessed food phase. Depression scores were lower during that period on three different measures. Anxiety scores also improved modestly. The researchers reported no adverse events, and participants generally found the dietary intervention acceptable.

This very small preliminary study is not evidence that changing your diet can treat depression, but its results suggest it could help. It was a pilot designed primarily to determine whether a larger clinical trial is feasible. The study also relied on participants' reports of their food intake rather than an objective measure of ultraprocessed foods. Larger and more rigorous studies will be needed to determine whether the dietary change itself caused the improvement in mood.

The findings do not mean people with depression should abandon medication or psychotherapy in favor of dietary changes, but they add to research asking whether diet might be one piece of a broader approach to mental health.

Why might food affect depression? The answer remains uncertain. Chagwedera has suggested that ultraprocessed foods may damage the gut's protective lining, contributing to chronic, low-grade inflammation that could affect the brain. Other possibilities include the amounts of sugar and fat in some ultraprocessed foods, their calorie density, additives used during processing and the possibility that these foods displace more nutritious choices.

“Reducing the amount of ultraprocessed food in our diet can be challenging, because it's highly palatable, cheap, and convenient," Chagwedera said in a press release. She also noted that ultraprocessed foods can promote cravings, although the mechanisms connecting these foods with depression have not been established in humans.

The findings do not mean people with depression should abandon medication or psychotherapy in favor of dietary changes. Rather, they add to a developing area of research asking whether diet might be one piece of a broader approach to mental health. The researchers expect to begin enrollment in a phase two study in 2028.

For now, the most reasonable takeaway is modest. Eating fewer ultraprocessed foods may be worth discussing with a health professional, particularly if those foods make up a large share of your diet. But this intriguing early study is a starting point, not a prescription for treating depression.

The research letter is published in JAMA Psychiatry.