Food choices may affect more than physical health. A small preliminary study from the University of California, San Francisco (UCSF) found that people who sharply reduced their intake of ultra-processed foods also reported fewer symptoms of depression.

The findings, published in JAMA Psychiatry and funded by the National Institutes of Health, add to growing research on the possible connection between diet and mental health.

The study is notable because researchers assessed depression symptoms in the same participants before and after they changed their diets. Still, the trial involved only 20 people, so the results need confirmation through larger studies.

Depression Symptoms Improved

Pexels | Cutting ultra-processed foods by over 85% significantly reduced depression and modestly lowered anxiety.

Participants reduced their intake of ultra-processed foods by more than 85%. After the dietary change, researchers recorded moderate to large improvements across three different measures of depression. Anxiety scores also improved, although the change was more modest.

Ultra-processed foods can include packaged snacks, fast food, sugary drinks, and other heavily manufactured products. They may contain preservatives, artificial colors, flavor enhancers, emulsifiers, and other additives.

Reducing these foods can be difficult because they are inexpensive, convenient, and designed to taste appealing. Nyasha Chagwedera, MD, PhD, assistant professor in the UCSF Department of Psychiatry and Behavioral Sciences, noted that these foods can stimulate dopamine, which is involved in pleasure and reward.

She also said food companies use carefully selected amounts of sugar, salt, and fat that can encourage people to keep eating.

Possible Gut-Brain Link

The 20 participants had an average age of 44, and 70% were women. Fourteen participants also had anxiety. Everyone had a high body mass index and at least one metabolic disorder, such as hypertension, diabetes, or elevated LDL cholesterol.

Researchers placed participants into two groups. One group kept its usual diet for four weeks, while the other followed a diet low in ultra-processed foods. Participants bought their own food with guidance from the research team. After four weeks, the groups switched diets.

Pexels | Reducing processed foods may improve mental health, but larger studies are needed to confirm the results.

Chagwedera, who founded the UCSF Metabolic and Mental Health Program, suggested that changes in the gut could help explain the results. One theory focuses on damage to the gut’s protective lining and the inflammation that may follow.

According to this theory, a damaged gut barrier could contribute to low-grade, chronic inflammation throughout the body. That inflammation may affect depression directly or increase vulnerability to depressive symptoms. However, this remains a possible explanation rather than a proven cause.

Larger Study Planned

Andrew Krystal, MD, Ray and Dagmar Dolby Distinguished Professor in UCSF’s departments of Psychiatry and Behavioral Sciences and Neurology, said the pilot study “successfully established the viability of carrying out this research in a larger-scale trial.”

The researchers plan to continue studying the connection with a larger group. Enrollment for phase 2 is expected to begin in 2028. A larger trial could help determine whether reducing ultra-processed foods directly improves depression symptoms or whether other dietary and lifestyle changes also contribute.

The UCSF study offers an early look at how major changes in food choices may relate to mental health. The sharp reduction in ultra-processed foods was followed by improvements in depression symptoms, but the small sample size limits what researchers can conclude.

Larger research will be needed to clarify the gut-brain connection and determine whether the results hold across broader groups of people.

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