The study, published September 23 in JAMA Psychiatry, followed 20 adults diagnosed with depression who regularly consumed high amounts of ultra-processed food. Researchers found that participants reported substantially lower depression scores during a period when they sharply reduced their intake of these foods. Anxiety scores also declined, although the improvement was more modest.
Researchers stress that the findings are preliminary. Depression and anxiety were exploratory outcomes rather than the study's primary endpoints, and the trial was small, open-label, and relied on participants' self-reported food intake. The results therefore cannot establish that ultra-processed foods directly cause depression or that eliminating them will treat depression in everyone.
What the Study Found
The trial enrolled 20 participants between the ages of 18 and 80. All had a primary diagnosis of depression, high consumption of ultra-processed foods, and at least one reported metabolic abnormality. Nineteen participants completed the study.
Participants followed their usual diet during one period and a diet with very little ultra-processed food during another. The order was randomized, allowing researchers to compare the same participants under the two dietary conditions. Participants were asked not to change their psychiatric medications, therapy, exercise, or sleep during the study.
Ultra-processed food consumption fell dramatically during the intervention. The study's food-processing score declined from an average of 10.10 at baseline to 1.42 after the intervention. Researchers reported no adverse events, and participants generally considered the dietary intervention feasible, although the ease of making the change received a more moderate rating.
Several measures of depression improved during the low-ultra-processed-food period:
- PHQ-9: The average score was 5.53 during the low-UPF period compared with 8.68 during the usual-diet period.
- MADRS: the average score was 8.05 during the low-UPF period compared with 18.9 during the usual-diet period.
- QIDS: The average score was 5.21 during the low-UPF period compared with 8.89 during the usual-diet period.
- GAD-7: The average anxiety score was 4.63 during the low-UPF period compared with 7.37 during the usual diet.
The researchers reported statistically significant differences between the dietary periods for all four measures. However, because this was a small pilot study, these results should be viewed as signals for further research rather than definitive evidence of a treatment effect.
What Counts as Ultra-Processed Food?
Ultra-processed foods are generally industrially formulated products that often contain ingredients, additives, or processing techniques uncommon in ordinary home cooking.
Examples can include:
- sugary soft drinks and other sweetened beverages
- packaged chips and snack foods
- cookies, cakes, and confectionery
- some packaged breakfast cereals
- instant or ready-to-eat meals
- processed meats such as hot dogs and some types of luncheon meat
- packaged desserts
- some fast-food products
Importantly, not every packaged or processed food is necessarily ultra-processed. Processing exists on a spectrum, and foods such as frozen vegetables, canned beans, or plain yogurt can be processed without fitting the same category as highly industrially formulated products.
Why Might Diet Affect Mood?
Scientists do not yet know exactly why reducing ultra-processed food might influence depression or anxiety.
The UCSF researchers point to several possibilities, including differences in sugar and fat content, total calorie intake, and exposure to additives or other substances associated with industrial food processing. They also note that the study was not designed to determine which component was responsible for the changes in mood.
Other research has proposed several possible pathways involving blood-sugar regulation, inflammation, nutrient intake, and the gut microbiome. However, these mechanisms remain an active area of investigation, and a direct causal pathway between ultra-processed foods and depression has not been established in humans.
One possibility is that diets dominated by ultra-processed foods may displace foods containing nutrients and dietary components important for overall health. Another is that the combination of high palatability, energy density, and rapid consumption may influence eating behavior.
Researchers caution against treating any single proposed mechanism as proven.
The Findings Fit a Larger Research Picture
The new trial is notable because it moves beyond simply asking whether people who eat more ultra-processed food also report poorer mental health.
Previous research has largely been observational. A systematic review and meta-analysis of 17 observational studies involving more than 385,000 people found that greater ultra-processed food consumption was associated with higher odds of depressive and anxiety symptoms. The prospective studies in that review also found an association between greater consumption and subsequent depression. However, the authors emphasized that observational evidence cannot by itself establish causation.
A later meta-analysis of six prospective studies likewise found that higher adherence to an ultra-processed dietary pattern was associated with a greater risk of depressive outcomes.
More recent research continues to examine the relationship, but much of the evidence remains observational. A 2025 systematic review of 13 studies involving more than 264,000 adults reported that all included studies found a positive association between greater ultra-processed food consumption and depression severity, while also highlighting variation among the studies.
The 2026 UCSF trial is therefore important mainly because it begins testing the reverse question: what happens when people with depression substantially reduce their consumption of ultra-processed food?
Why the Study Does Not Prove That Ultra-Processed Food Causes Depression
The researchers themselves identify several limitations.
First, only 20 people participated, and 19 completed the study. Second, the participants knew which diet they were following, meaning the study was not blinded. Third, depression and anxiety were exploratory outcomes rather than the primary outcomes. Finally, ultra-processed food intake was assessed using participant self-report rather than an objective measurement.
The participants also had specific characteristics—including diagnosed depression, high ultra-processed food consumption, and reported metabolic abnormalities—so the results should not automatically be generalized to the entire population.
The study was conducted from June 2024 through June 2025 and published online in JAMA Psychiatry on September 23, 2026.
What Should People Do?
The findings do not mean that people with depression should stop taking medication or discontinue psychotherapy in favor of dietary changes.
Instead, diet may eventually become one component of a broader approach to mental health. For people who want to reduce ultra-processed food, practical changes can include replacing sugary drinks with water or unsweetened beverages, choosing minimally processed foods more often, adding fruits and vegetables to meals, and preparing simple meals at home when possible.
There is also no requirement for an all-or-nothing approach. For many people, gradually reducing the proportion of ultra-processed foods in the diet may be more realistic than attempting to eliminate every packaged food.
People experiencing persistent depression, anxiety, suicidal thoughts, or significant changes in mood should seek appropriate medical or mental health care rather than relying on dietary changes alone.
The Bottom Line
The new UCSF study provides early evidence that substantially reducing ultra-processed food intake may be accompanied by improvements in depressive and anxiety symptoms among people with depression.
But the evidence is not yet strong enough to conclude that ultra-processed foods directly cause depression or that eliminating them is a stand-alone treatment. The researchers say larger and more rigorous studies are needed, including trials with larger samples, blinding where possible, objective measurements of food intake, and investigation of the biological mechanisms involved.
For now, the study adds another reason for researchers to investigate the connection between diet and mental health—and for consumers to consider making gradual, sustainable shifts toward a diet containing more minimally processed foods.
0 Comments