Ultra-processed foods, such as packaged snacks, fast food and sugary drinks, have already been tied to obesity and heart disease. A new pilot study from the University of California, San Francisco, published in JAMA Psychiatry, asks a different question: what happens to depression symptoms when people with depression stop eating them?
A first of its kind, but very small
The researchers believe this is the first study to compare depression symptoms in the same people while they ate their usual ultra-processed diet and after they switched to a healthier one. It is important to be clear about its size: only 20 adults took part. The team describes it as a feasibility study, meant to show that a larger trial can be done, rather than proof that diet treats depression.
How the study worked
Participants had an average age of 44, and about 70% were women. All had depression, 14 also had anxiety, and every participant had a high body mass index plus at least one metabolic problem such as high blood pressure, diabetes or raised LDL cholesterol.
The design was a crossover. One group kept eating as usual for four weeks while the other moved to a diet low in ultra-processed food, buying and preparing their own meals with guidance from the researchers. After four weeks, the groups swapped.
What changed
- Participants cut their intake of ultra-processed food by more than 85%.
- Scores on three different depression measures showed moderate to large improvements.
- Anxiety scores also improved, though more modestly.
Why might food affect mood?
The authors offer a hypothesis involving the gut. In their view, ultra-processed food may damage the gut’s protective lining, letting low-grade inflammation spread through the body. That inflammation could either contribute directly to depression or make a person more vulnerable to it. The team also points to the way these foods are engineered: a carefully tuned mix of sugar, salt and fat triggers dopamine release and promotes craving, which helps explain why cutting back is so hard when such foods are cheap, tasty and convenient.
This gut-inflammation route is a proposed explanation, not something this small study demonstrated.
What to keep in mind
- Tiny sample. Twenty people cannot tell us how the results would look in the wider population.
- A specific group. Everyone had a high BMI and a metabolic condition, so the findings may not apply to people without these.
- Diet studies are hard to blind. Participants know what they are eating, and the switch involved many changes at once, so it is difficult to pin the effect on one factor.
- Not a replacement for treatment. Nothing here suggests that diet should replace therapy or medication for depression.
What comes next
The researchers plan to begin enrolling participants for a larger, phase 2 study in 2028. Until then, the takeaway is modest but interesting: the food we eat may be one piece of the puzzle of mental health, and it is now being tested more rigorously. The work was funded by the US National Institutes of Health and the UCSF Weill Institute for Neurosciences.
News release: EurekAlert! / University of California, San Francisco
Original paper: Chagwedera N, Krystal A, et al. Study on a low ultra-processed diet and depression symptoms. JAMA Psychiatry, 2026. https://doi.org/10.1001/jamapsychiatry.2026.2320
Leave a Reply