Older adults who ate two cups of blueberries a day felt less low, but the result missed significance

Eighteen older adults finished a three-month trial in which half received 48 grams of blueberry powder a day, roughly two cups of fresh berries, and the other half received a look-alike placebo. Those on blueberries showed a bigger drop in depressive symptoms on paper. The gap between the two groups was not statistically significant, and that null result is the finding the paper actually supports.

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A randomized pilot with 18 completers

The work comes from the Hinda and Arthur Marcus Institute for Aging Research at Hebrew SeniorLife, and it was published in the Journal of Nutrition, Health and Aging in 2026, volume 30, issue 10, article 100958. Courtney L. Millar, PhD, of Hebrew SeniorLife, Beth Israel Deaconess Medical Center and Harvard Medical School, is the lead author, working with colleagues including Mohit P. Chopra, Alex Wolfe, Alyssa B. Dufour, Douglas P. Kiel and Lewis A. Lipsitz of Hebrew SeniorLife, along with collaborators at the Harvard T.H. Chan School of Public Health, the Broad Institute and the University of Connecticut. The design was randomized, double-blind, parallel-arm and placebo-controlled, per the ScienceDaily summary of the paper.

Participants were adults aged 65 and older who were physically inactive and had mild depressive symptoms.

The mood comparison, and the gut findings that went with it

Ten completers were in the blueberry group, with a mean age of 71, and eight were in the placebo group, with a mean age of 75. Most were women and the group was predominantly white. According to the institution’s own release, the difference in depressive-symptom change between the blueberry and placebo groups was not statistically significant. The authors wrote that the reduction in symptom severity appeared greater and less variable in the blueberry group, a descriptive observation that stops well short of a demonstrated benefit. In a trial this small, a gap of that kind can appear by chance, and a nonsignificant result cannot distinguish a real small effect from none at all.

Neither the release nor the secondary coverage reports p-values, confidence intervals or effect sizes for the mood comparison, and those numbers were not available for this article. The depression questionnaires are described only as validated, without being named.

The primary endpoint and the microbiome results

A formally designated primary endpoint is not named in the summaries that were reachable. The stated aim was to test preliminary effects of blueberries on the gut microbiome, gut metabolites and depressive symptoms, and the authors hypothesized that blueberries would change the microbiome, raise short-chain fatty acids and lower symptoms. The mood hypothesis was not borne out at the level of statistical significance. The overall makeup of the gut bacteria, their metabolic pathways and the fecal short-chain fatty acids did not change either, according to the trade-press account of the paper, which quotes the authors directly.

What did change was narrower. Within the blueberry group only, the abundance of genes for several microbial enzymes shifted significantly, including EC 3.6.3.31, a polyamine-transporting ATPase involved in GABA production. A within-group change is not a comparison against placebo, so it cannot by itself show that blueberries caused the shift, and it does not connect to mood without larger studies. The trial registration number and funding source are not stated in the summaries, and the full paper behind its DOI could not be opened for this article.

Blueberries as a hypothesis, not a treatment

Millar framed the rationale in terms of biology, saying that “Fiber and anthocyanins in blueberries have potential to modify the gut microbiome and metabolites that are relevant to depression in older adults.” The word potential is doing the work in that sentence. ScienceDaily summarized the outcome by saying the findings do not establish that blueberries can relieve depression, and the authors describe the data as preliminary and call for larger studies to link the enzyme changes to mood.

There are other limits visible in the numbers. The group that received blueberries was about four years younger on average than the placebo group, completers were mostly white women, and the 18 people who finished are a subset of whoever enrolled; how many dropped out is not stated in the summaries. The institute’s site carries the same account on its Marcus Institute news page, under a title that calls it a pilot study, which is exactly what it is: a feasibility-scale experiment meant to decide whether a bigger trial is worth running, not to tell anyone what to eat for their mood.

It helps to be concrete about what a nonsignificant result means. With eight people in one arm and ten in the other, a single participant who improved or worsened a lot could move an average noticeably, which is why the authors’ note about smaller variability in the blueberry group is a description of the data and not a test result. A larger, adequately powered trial would be needed to say whether the apparent gap is real, and the pilot’s job is to supply the numbers that make such a trial plannable.

The practical reading is therefore modest. Blueberries are a food with fiber and anthocyanins, and this small trial found no statistically reliable mood advantage over placebo. Whether a larger trial would find one remains an open question.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

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