The Impact of Dietary Fiber Intake on Gut Microbiota and Mental Health in Adults with Mood Disorders

Introduction

The connection between diet and the brain has been growing in clinical discussion, as recent studies show that the gut may be just as important as the brain in regulating mood through a complex communication system known as the gut-brain axis. This axis is a bidirectional communication system of signals between the central nervous system and gastrointestinal tract, directly linking digestion, mood, and health. The gut harbors trillions of beneficial bacteria that play vital roles in processes such as immune function and behavior. An imbalance of bacteria in the gut microbiome, known as gut dysbiosis, has been associated with inflammatory responses in the brain, as well as increased symptoms of mood disorders, such as anxiety and depression (Mayer, et al., 2015). Previous studies have confirmed that dietary fiber plays a key role in shaping a healthy, diverse gut microbiota by promoting the production of short-chain fatty acids (SCFAs) through fermentation. These SCFAs possess anti-inflammatory properties that help modulate the gut environment and may also protect neurons from cell degradation, providing a biological pathway linking gut health to brain function (Mantri et al., 2024). 

The established relationship between fiber and gut microbiome modulation suggests a need to investigate the potential of dietary fiber to improve mental health outcomes. Several studies have begun exploring this connection, but the findings have remained inconsistent. This is due to differences in fiber sources, measurement tools, and the way the psychological and microbial outcomes were analyzed. Some randomized controlled trials (RCTs) report improvements in mood disorders following fiber-based interventions. In contrast, other RCTs show little to no effect, particularly when fiber is administered as a supplement rather than through whole-food sources. This review aims to further analyze evidence from clinical trials and large-scale observational studies to clarify how increased dietary fiber intake may influence mental health through gut microbiome pathways.

Methods 

A systematic approach was used to identify and evaluate current literature exploring the effects of increased dietary fiber intake on gut microbiota composition and mood disorder symptoms in adults. Searches were conducted using databases such as PubMed, PsycINFO, and CINAHL. Search terms included combinations of “dietary fiber,” “prebiotics,” “gut microbiome,” “mood disorders,” “anxiety,” “depression,” and “gut-brain axis.” Boolean operators and filters were used to limit results to peer-reviewed English-language articles with adult human subjects, all published within the past decade. 

Inclusion criteria required quantitative primary studies with adult participants experiencing symptoms of mood disorders. Additionally, an intervention involving dietary fiber intake through a whole-diet approach and/or supplementation was required, along with outcomes measuring gut microbiota and/or mental health. Studies were excluded if participants were under the age of 18, included non-human populations, were not available in English, focused on unrelated nutrients, lacked outcome measures relevant to mood or the gut microbiota, or were published more than 10 years ago. 

Seven central sources were selected for full review, including RCTs, a longitudinal observational study, and a systematic/meta-analysis. Additionally, each study was critically appraised to ensure its quality, validity, and relevance. Specifically, each appraisal was based on population, fiber dosage, fiber source, intervention duration, microbial analysis method, and psychological measurement tools. Studies were then grouped thematically based on the differentiation of microbial and psychological outcomes separately or concurrently, as well as the effects observed in participants. 

Results 

Long-Term Observational Associations Between Fiber Intake and Mood

 In two large-scale studies, fiber intake was found to be related to mood symptoms over time. Aslam et al. (2024) conducted a systematic review and meta-analysis that assessed the relationship between dietary fiber intake and mental health outcomes, focusing on depressive and anxiety disorders. Across observational studies, higher fiber intake was consistently associated with lower levels of depressive and anxiety symptoms. However, RCTs primarily used fiber supplementation rather than whole-diet interventions and included participants who were not clinically diagnosed with a mood disorder. These results did not demonstrate significant improvements in depressive and anxiety symptoms in comparison to a placebo, suggesting that supplementation alone may be insufficient to produce psychological benefit.

These findings are supported by Ramin et al. (2020), who conducted a 17-year cohort analysis of data from the Iowa Women’s Health Study, involving 14,129 postmenopausal women who completed baseline dietary assessments in 1986. Increased fiber intake was associated with better mental health-related quality of life, with women in the highest-fiber intake quartile having mental health scores approximately 0.6 points higher than those in the lowest quartile. Although this association was attenuated after full statistical adjustment, fiber from whole grains remained positively correlated with improved mental health outcomes, highlighting the potential importance of fiber-rich foods over supplementation.

Together, these studies show a consistent link between higher fiber intake and improved mental health outcomes. While neither study assessed mental health outcomes, both cited microbial fermentation via short-chain fatty acid (SCFA) production as a potential pathway to improved mood, linking improved mental health symptoms to a more diverse and rich microbial composition. The findings are also strengthened by the large-scale nature of both studies; however, causality cannot be ensured due to the lack of microbiota data and a whole-diet intervention. 

Whole-Diet Fiber Interventions and Mental Health Outcomes 

The SMILES trial, conducted by Opie et al. (2018) was the first RCT to investigate the impact of a whole-diet intervention on adults with major depressive disorder. In this parallel-group, single-blinded study, fiber serves as an important component of a broader lifestyle change in eating patterns as an intervention. 67 adults who ate highly processed foods and were clinically diagnosed with major depressive disorder were prescribed a modified Mediterranean diet (ModiMedDiet) over a 12-week period. This dietary intervention provided approximately 50 grams of fiber per day and emphasized whole grains, fruit, vegetables, legumes, fish, and extra-virgin olive oil, alongside reduced intake of ultra-processed foods (Opie et al., 2018). The SMILES trial successfully implemented fiber-rich intervention into participants’ daily lives and contrasts with the supplement-heavy RCTs in the review by Aslam et al. (2024). Notably, the intervention group experienced significant reductions in depressive symptoms and reported that the diet was feasible, enjoyable, and emotionally beneficial by the end of the 12 weeks. This study suggests that fiber is more effective when delivered in whole foods and as part of a continuing dietary pattern. 

Strengths include transparency and replicability through a clearly prescribed dietary model, as well as a high retention rate of 93.9% in the diet group (Opie et al., 2018). Limitations include a modest sample size and self-reported food diaries, which introduce potential reporting bias. In addition, no gut microbiota data were collected, which restricts the ability to draw mechanistic conclusions. Taken together, the findings suggest that fiber may have greater mental health benefits when part of a nutrient-dense diet than as an isolated supplement. 

Further supporting whole-diet approaches, the HELFIMED RCT, conducted by Parletta et al. (2019), evaluated the effectiveness of a Mediterranean-style dietary intervention supplemented with fish oil in adults diagnosed with depression. This diet consisted of many fiber-rich foods, such as vegetables, legumes, whole grains, nuts, and fruits, along with dietary education. The intervention group demonstrated significant improvements in depressive symptoms and overall mental health in comparison to controls. The authors acknowledged that improvement may not be due to fiber alone, as factors such as nutritional education, increased routine, and overall lifestyle structure could have contributed to symptom improvement. However, the control group, which also received support, strengthens the validity of the findings by reducing the likelihood of external factors driving mood and depressive symptoms. Although gut microbiota outcomes were not directly addressed, the authors proposed modulation of the gut-brain axis and inflammatory regulation as potential mechanisms. This trial strengthens evidence that whole-diet, fiber-rich interventions are associated with improved mood outcomes in individuals with mood disorders such as depression.

Dual Microbiota and Mood Outcome Studies 

Jackson et al. (2023) examined the effects of prebiotic fibers, oligofructose (OF), 2’fucosyllactose (2’FL), and their combination on gut microbiota composition and mood in 92 healthy adults with mild-to-moderate depression and anxiety. This RCT was conducted over a 5-week period and was double-blind and placebo-controlled, including 92 healthy adults aged 18-50 years in the UK. Participants were split into the following groups: OF, 2’FL, OF + 2’FL, or a placebo, with a total intervention dose of 10 grams per day. 16S rRNA sequencing and FISH-FLOW cytometry were used to measure gut microbiota, while mood state indicators were the Beck Depression Inventory (BDI), State-Trait Anxiety Inventory (STAI), and Positive and Negative Affect Schedule (PANAS-SF). 

The OF+2’FL groups showed significant increases in beneficial bacteria and improved mood scores from baseline. However, the 2’FL groups alone showed no significant benefit compared with placebo. As a result of this study, fiber's role in modulating the gut-brain axis was directly linked with mood outcomes, providing statistically significant dual-outcome evidence supporting fiber's therapeutic role in mental health. Accordingly, this study addresses the gaps noted by Aslam et al. regarding the effects of fiber on mental health outcomes and expands the microbiota-focused findings of Mantri et al. by integrating both gut microbial outcomes and psychological measures into a single study. Unlike Opie et al. (2018), who measured mood outcomes without gut microbial data, Jackson et al. provide experimental causality rather than correlation. However, the short time frame and non-clinical population limit generalizability, as these factors don't capture long-term effects or sustainability. 

The results of this study highlight the importance of measuring both gut microbial and psychological outcomes to understand how fiber influences mood. Through integrating both components, Jackson et al. address gaps left by Ramin et al. and Aslam et al. and provide stronger evidence that changes in the gut microbiome are a plausible path to addressing fiber intake and mental health. These findings support the need for future studies that simultaneously examine both microbial and mood-related outcomes.

Fiber and Prebiotic Supplementation Effects on Gut Microbiota 

To investigate whether prebiotics and probiotics improved the gut microbiome in adults with low-fiber intake, Mantri et al. conducted a secondary analysis of an RCT involving 117 healthy men aged 20 to 60 in Germany. This placebo-controlled, double-blind trial consisted of a high-fiber group and a low-fiber group. The high-fiber group had participants who ate more than 25 grams of fiber per day, and the low-fiber group consumed less than 25 grams per day. The intervention lasted seven weeks and, similar to Jackson et al., used 16S rRNA sequencing before and after intervention to measure gut microbiome diversity. The low fiber group showed statistically significant increases in beneficial SCFA-producing bacteria, which is known to improve gut health. Although no mental health outcomes were measured, this study provided valuable insights into the mechanisms underlying synbiotic supplementation and microbiota modulation in fiber-deficient adults. 

These findings align with Aslam et al. (2024), who reported that while fiber supplementation can influence gut microbiota, RCTs using supplements alone did not consistently produce improvements in depression or anxiety. These findings suggest that while fiber may modulate the gut, these changes alone do not necessarily translate into psychological benefits. In contrast, Jackson et al. (2023) advanced this understanding by measuring both microbial and mood outcome measures within the same intervention. Their results demonstrated that combining two dietary supplements improved both gut and mood symptoms, providing stronger evidence of a connection to the gut-brain axis.

Discussion 

This review examined how dietary fiber intake influences gut microbiota composition and mood-related symptoms in adults. All included sources indicated a link among dietary fiber, mood, and modulation of the gut microbiome. Although this evidence is still evolving, the effect that fiber has on the gut-brain axis is present, as supported by studies that used whole diet interventions or those that measured microbial and psychological outcomes simultaneously, such as the SMILES trial (Opie et al., 2018), HELFIMED (Parletta et al., 2019), and Jackson et al. (2023). All of these sources provided direct experimental evidence of gut-brain axis modulation, showing that fiber is more effective when part of a sustained dietary pattern or delivered as a prebiotic. Trials using isolated fiber supplements without altering the overall diet pattern showed minimal effects on mood, as reviewed by Aslam et al. (2024). This result shows the importance of both fiber diversity and source, since whole-food interventions typically consisted of complex mixes of fiber that supported microbial diversity. The study by Mantri et al. (2024) illustrates this point by supplementing synbiotics and significantly altering gut microbiota composition in people with low fiber intake, without directly considering psychological outcomes.

Strengths and Limitations 

The strength in this review lies within its systematic approach and PICO-based research question that used a very diverse set of study designs, ranging from randomized control trials to long-term observational cohorts, to see the relationship between dietary fiber intake, gut microbiota, and mood symptoms in adults. Through thematic organization and cross-referencing, the review effectively highlighted patterns and gaps across interventions, including how whole-diet approaches are unique and used in dual-outcome studies.

Limitations were also present in this review, as most RCTs lasted only 5-12 weeks, which may not be sufficient time to observe lasting microbial and psychological outcomes. In addition, only one study directly measured both microbial and mental health outcomes, which limits the ability to establish causality. Inconsistencies in fiber type, dosage, and intervention design also appeared to be a limitation, as they make it difficult to determine which combination of interventions yields better outcomes. 

Future Research 

Future research should prioritize long-term, large-scale clinical trials that simultaneously evaluate gut and mood outcomes using standardized tools. This would improve comparability, reliability, and clinical relevance across studies. Standardized psychological assessment tools allow researchers to measure mood changes consistently across different populations and intervention types. Additionally, studies that compare fiber types in relation to mood would clarify best practices for intervention. 

Conclusion 

This literature review explored the impact of increased dietary fiber intake on mood disorder symptoms and gut modulation. Analysis of seven studies shows that higher dietary fiber intake may improve both microbial diversity and mood symptoms. These results were more prominent in studies that utilized whole-diet interventions and reinforced the gut-brain axis as a promising therapeutic target for improving symptoms of mood disorders in adults. Although this correlation is present, limitations such as short study durations, diverse fiber types, and small sample sizes underscore the need for further research. These future studies should prioritize whole-diet interventions that measure both psychological outcomes and microbial diversity. These studies should also be standardized and occur over longer durations to establish causality and inform evidence-based dietary recommendations.

About the Author: Hailee Nielsen

I am a UC Davis graduate with a degree in Human Biology and a minor in Human Development. I am currently pursuing a Master of Science in Biomedical Science at California Health Sciences University while applying to medical school. I have a passion for public health, health equity, and improving access to healthcare for underserved communities. Through my clinical experience, public health research, and global health experience in South Africa, I hope to bridge clinical medicine and community-based approaches to improve patient outcomes and nutritional deficits.

Author’s Note

I wrote this piece to explore the intersection between diet, gut health, and mental wellness. This area of science relates to many college students, including myself. As a student attending a university in the United States, where fiber is often lacking in the typical diet and stress is a constant part of student life, I recognize the importance of understanding how our eating habits influence both our mental and physical well-being. This paper is intended for students, researchers, and anyone curious about the body’s interconnected systems.

References

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