PDI reflects overall plant food intake levels and helps to identify the health effects associated with plant-based diets. Furthermore, it highlights the importance of a diverse range of plant foods. In this index, plant-based foods receive positive scores, whereas animal-based foods receive negative scores. The total PDI is calculated as the sum of scores across all food groups.

Results on the relationship between PDI and depression are inconsistent. This may be due to ignoring differences in plant-based diet quality. Some studies demonstrate a protective effect of PDI against depression, including research on Chinese elderly populations (15). However, other studies show no significant association between PDI and depression. For example, a cross-sectional study from a multicenter community trial found no significant association between PDI and depressive or anxiety symptoms (16). Additionally, a Spanish population-based survey study that defined plant-based diet adherence as complete avoidance of meat, fish, and animal products based on a food frequency questionnaire found this adherence associated with increased risks of depressive symptoms and stroke (17). However, such binary definitions may obscure differences in food quality and processing within plant-based dietary patterns. Therefore, current research shows contradictory results. One possible reason is that participants with high PDI may consume both healthy plant foods and substantial amounts of less healthy plant foods, such as refined grains and high-fat or high-oil items. Taken together, these mixed results suggest that plant-based classification alone may be insufficient, and that diet quality likely drives much of the variability in observed associations.

The hPDI emphasizes healthy plant foods including whole grains, vegetables, and fruits, which are nutrient-dense. It highlights not only the quantity of plant foods but also their quality. The hPDI is calculated by assigning positive scores to healthy plant foods and negative scores to unhealthy plant foods and animal foods.

Most studies have found that higher hPDI is linked to lower depressive symptoms and reduced anxiety risk. This protective effect of a healthy plant-based diet on mental state is consistent across different populations. A systematic review found that adherence to a healthy plant-based diet was associated with lower anxiety, depressive symptoms, and psychological distress among adults (18). This suggests that healthy plant-based diets may influence psychological well-being by improving nutritional status. Additionally, a study of Iranian adolescent females found that a higher hPDI was associated with lower insomnia rates (19). However, a multicenter cross-sectional study found no significant association between hPDI and depression or anxiety (16). This may be related to differences in study design and scale selection. Despite some inconsistent findings, numerous studies support that dietary patterns rich in healthy plant foods have a protective effect against depression.

The uPDI assesses intake of unhealthy components in plant-based diets. These include processed plant foods, high-sugar and high-fat plant foods. The uPDI assigns positive scores to unhealthy plant foods and negative scores to healthy plant foods and animal foods.

Although plant-based diets are generally considered healthy, not all plant-based foods are equally nutritious or beneficial. For example, some processed plant-based foods may contain high levels of sugar, salt, and saturated fat. Excessive intake of these components is closely associated with a range of health issues, such as obesity, diabetes, and cardiovascular diseases. Most studies show that a higher uPDI significantly increases depression risk. For example, a 10-year study found that people with the highest uPDI were more likely to experience moderate or highly progressive depression trajectories (20). A study of middle-aged and elderly people also found that uPDI was significantly associated with depression risk (15). Therefore, while plant-based diets may support psychological well-being, consumption of unhealthy plant foods may have adverse effects. Together with the hPDI findings, these results collectively demonstrate that food processing level and nutrient composition, rather than plant-based classification per se, determine mental health.

Plant-based diets are characterized by reduced or no animal food intake. Vegan diets exclude all animal-derived foods, whereas vegetarian diets encompass several subtypes defined by the selective inclusion or exclusion of specific animal foods (21), including pescatarian and lacto-ovo vegetarian diets. Well-planned vegetarian diets benefit health by controlling weight, blood sugar, and blood lipids, bringing metabolic and cardiovascular benefits (22). However, findings on vegetarian diets and depression are mixed (Table 2). Some studies link vegetarian diets to increased depressive symptoms (23, 24), and several meta-analyses also report a higher risk of depression among vegetarians and vegans (25–27). Several factors may underlie these associations. Self-selection is plausible, as individuals with depressive symptoms may adopt vegetarian diets in an attempt to improve health. In addition, the cultural background also matters because when vegetarianism is uncommon, limited food availability, reduced social support, or minority stress may contribute to higher depressive symptoms. Motivations for adopting vegetarian diets and the degree of dietary planning also vary across populations, contributing to heterogeneity in diet quality and nutrient adequacy (28). Collectively, these factors may help explain the observed associations, which are unlikely to reflect a direct causal effect of vegetarian diets alone.

Nevertheless, some studies find no significant association between vegetarian diets and depression or anxiety (29), and cross-cultural differences have also been reported. For example, in some Western regions such as the United States, Russia, and Germany, vegetarian diets have not been clearly linked to mental problems, whereas in China they have been associated with higher levels of anxiety and depressive symptoms (28). Conversely, some studies also show vegetarian diets reduce depression risk. A prospective cohort study of 12,062 participants in Taiwan found that vegetarians had lower depression risk than non-vegetarians (30). Another cohort study on depressive symptoms and vegetarianism found that depression was associated with excluding any food group (21). The risk of depression was higher when excluding meat, fish, or vegetables, and increased further as more food groups were excluded, regardless of the specific types. Taken together, no solid evidence shows vegetarian diets per se directly lead to depression. Rather, vegetarians may lack certain nutrients. To improve dietary balance, diets should include key components such as iron, calcium, magnesium, potassium, vitamins (especially B12, B2, and D), quality protein, and unsaturated fatty acids (31).

The Mediterranean, DASH, and MIND diets all emphasize high intake of plant foods such as vegetables, fruits, and whole grains. However, as these patterns also include specific animal-based foods such as fish, dairy, poultry, we consider them plant-forward rather than strictly plant-based. Studies have shown that they are all closely linked to reduced depressive symptoms (32–34). In a longitudinal study on dietary patterns and depressive symptoms, a diet characterized by an abundance of vegetables, olive oil, whole grains, fruits, and fish, alongside moderate consumption of wine, red meat, and processed meats, was consistently linked to a reduction in depressive symptoms (35). These dietary patterns commonly emphasize dietary diversity and high-quality plant foods. In contrast, Western diets high in processed foods, red meat, sugar and low minimally processed plant foods are linked to increased depressive symptoms, and this association becomes strengthens over time (36). Therefore, these findings likely reflect the health effects of plant-forward diet quality.

In summary, associations between plant-based dietary patterns and depression vary and appear to depend on context, motivations for dietary choice, and the degree of dietary planning. Across dietary patterns, maintaining dietary diversity and nutrient adequacy is a key consideration when interpreting depression-related outcomes.

Depression in older adults is a major public health challenge in aging societies worldwide. It significantly increases disease burden, functional impairment, and difficulty managing chronic diseases, while raising suicide risk. Early screening and intervention are urgently needed. In middle-aged and elderly populations, many studies have found significant associations between plant-based diets and depression incidence. A cross-sectional study of 3,153 Chinese adults aged ≥45 found that hPDI was associated with lower depressive symptoms, while uPDI correlated with higher risk (37). Here, the hPDI emphasizes higher intakes of healthy plant foods such as whole grains, fruits, vegetables, nuts, legumes, and vegetable oils, whereas the uPDI reflects higher consumption of unhealthy plant foods, such as fruit juices, sugar-sweetened beverages, refined grains, potatoes, and sweets or desserts. Prospective cohort evidence from the UK Biobank study further strengthens this finding, demonstrating that high hPDI was associated with reduced risks of dementia and depression, whereas high uPDI increased risk for both diseases (38). Additionally, a study found that greater adherence to hPDI was associated with better health-related quality of life in older adults, while greater adherence to uPDI was associated with poorer quality of life (39). Similarly, a cohort study from the Health and Retirement Study of 4,262 participants with a median follow-up of 7.8 years demonstrated that higher adherence to hPDI was inversely associated with multimorbidity risk (40). In another cohort study of 1,880 participants with a mean follow-up of 3.3 years, higher uPDI was associated with increased frailty risk (41). Therefore, these findings highlight the importance of plant-based diet quality.

The protective effect of healthy plant-based diets on mental state may partly come from improving overall health status. However, a study in Sardinia, Italy (42) found that diets overly focused on plant foods may increase depression risk in the elderly. This is especially true when animal foods are lacking. Depression incidence increased significantly without animal foods. Conversely, moderate animal food consumption was associated with better emotional status. This suggests that dietary patterns combining high-quality plant foods with nutrient-dense animal products may optimize psychological well-being in elderly populations.

Previous studies suggest older adults consume more fresh fruits, vegetables, plant oils, and legumes daily. Adding eggs to a healthy plant-based diet may also reduce depression risk (43). Future research should identify optimal food combinations for middle-aged and elderly populations. Intervention designs should include nutritional monitoring and individualized adjustments to help diet play its role in psychological well-being.

Plant-based diets’ effects on depression may vary in different populations. These include pregnant women, chronic disease patients, and youth population. Physiological, hormonal, and life stress differences may amplify or weaken dietary effects. Systematic evaluation of high-risk populations helps identify crucial nutritional factors, develop targeted interventions, and reduce depression risk.

Inflammation is a protective immunological reaction that encompasses immune cells, blood vessels, and various molecular mediators. Normally, the immune system produces both pro-inflammatory and anti-inflammatory mediators. However, if the anti-inflammatory mediators cannot control pro-inflammatory responses, chronic inflammation may develop. Psychological stress can trigger inflammation responses, raising cytokine levels and affecting mood. Many studies show inflammation plays a crucial role in depression pathophysiology. Depressed patients typically have elevated pro-inflammatory cytokines including CRP, IL-6, TNF-α, and IL-1ra (51). Anti-inflammatory treatment can reduce depressive symptoms (52). Many plant foods contain anti-inflammatory components. Fruits, vegetables, nuts, herbs, spices, and legumes are rich sources of anti-inflammatory components, including plant-derived omega-3 fatty acids, polyphenols, vitamins, essential minerals, and probiotics (9). These may have protective effects against depression. A meta-analysis of 16 articles on high fruit/vegetable intake and plant foods’ effect on CRP showed significant CRP reduction (53). Additionally, Anthocyanin and plant oil intake also significantly reduced CRP. Plant-based diets may reduce inflammation through antioxidants and anti-inflammatory components, helping to alleviate depressive symptoms. Nevertheless, these research are based on human observational studies, and plant-based foods have indirect biomarker associations with depression. Diet-induced reductions in inflammatory markers translate to improvements in depressive symptoms are currently sparse.

Dysbiosis of the gut microbiota is considered a potential trigger for depression. Compared with healthy individuals, depressed patients show altered microbial diversity and shifts in the relative abundance of key bacterial groups. Firmicutes, Actinobacteria, and Bacteroidetes exhibit the most significant alterations. The increase in the Bacteroidetes/Firmicutes ratio is reflected by the enrichment of Bacteroides and the reduction of Blautia, Faecalibacterium, and Coprococcus (54). In addition, alterations in gut microbiome composition also affect microbial metabolomics. These include short-chain fatty acids (SCFAs), vitamins, and neuromodulators, which can signal through neural, endocrine, and immune pathways within the bidirectional brain-gut-microbiota axis. Notably, growing evidence shows the brain-gut-microbiota axis plays an important role in depression pathophysiology. It affects satiety, digestion, behavior, and mood. Research indicates that depressive phenotypes accompany gut microbiome changes. These changes further affect depression-like behavior and increase depression risk (55).

Diet is an important factor determining gut microbiota composition and function. In particular, plant-based diets are rich in macronutrients (dietary fiber, protein, fatty acids) and micronutrients (minerals, vitamins) that shape microbial diversity and community structure. A systematic review (56) on plant-based diets and gut microbiota found that individuals on such diets have higher Lachnospiraceae and lower Bacteroidaceae. In vegans and vegetarians, analyses of plasma microbial metabolites, including SCFAs and trimethylamine N-oxide (TMAO), further suggest that gut microbiota serves as a critical mediator in diet-host interactions. SCFAs are vital for neuronal function, nerve cell maturation, and blood–brain barrier stability (57), whereas elevated TMAO levels are linked to inflammation and metabolic diseases. Plant-based diets, rich in fiber and healthy fats, promote SCFA production and reduce TMAO levels (58). Another cross-sectional study (59) on protein intake in healthy women found that high plant protein intake was negatively associated with mental disorders, kynurenine levels, and the Firmicutes/Bacteroidetes ratio. Another study (60) found that restoring gut microbiota and gut-brain axis function with probiotics, prebiotics, and healthy diets may relieve depressive symptoms. As an adjunct to antidepressants, this approach can improve psychiatric status.

In general, plant-based dietary patterns may help regulate gut microbiota composition and function. These changes may include increased abundance or diversity of certain taxa and altered production of microbial metabolites such as SCFAs. However, direct links to depression outcomes in humans have not yet been fully characterized.

The “monoamine hypothesis” of depression suggests that imbalances in monoamine neurotransmitters such as serotonin (5-HT), dopamine (DA), and norepinephrine (NE) are crucial mechanisms. Most antidepressants restore emotional balance by modulating these monoamine systems. Some specific nutrients act as cofactors or precursors for key enzymes involved in neurotransmitter biosynthesis and related metabolic pathways. For example, folate and B vitamins are considered vital nutrients for promoting neurotransmitter synthesis. Folate participates in one-carbon metabolism—a process that donates methyl groups essential for neurotransmitter synthesis. B vitamins act as coenzymes or coenzyme precursors in amino acid metabolism, thereby affecting monoamine neurotransmitter production (61). In addition, research shows 12 antidepressant nutrients are linked to depression prevention and treatment. These include folate, iron, long-chain omega-3 fatty acids (EPA and DHA), magnesium, potassium, selenium, thiamine, vitamin A, vitamin B6, vitamin B12, vitamin C, and zinc (62). Plant foods rich in these nutrients include leafy greens, lettuce, peppers, and cruciferous vegetables, as well as animal foods include bivalves like oysters and mussels, various seafood, and organ meats (62). This supports the possibility that improving overall diet quality and micronutrient adequacy may complement established treatments.

Additionally, it should be noted that strict vegetarian diets can also cause deficiencies in nutrients essential for brain function. This may increase the risk of depression. Vitamin B12 is found almost exclusively in animal products and is crucial for DNA and neurotransmitter synthesis (63). One study found that community-dwelling elderly with Vitamin B12 deficiency or low levels at baseline had a higher likelihood of developing new-onset depression 4 years later (64). Moreover, iron serves as a cofactor for key enzymes in dopamine and serotonin synthesis. Plant-based foods contain mainly non-heme iron, which has lower absorption efficiency than heme iron. Iron deficiency may cause low mood and fatigue. In a cross-sectional study of 1,875 participants aged 65 and older, researchers found that depressive symptoms were associated with anemia and low serum ferritin levels (65). Equally important, Omega-3 fatty acids, particularly long-chain EPA and DHA, are primarily derived from fish oil (66). They are important components of neuronal cell membranes and have anti-inflammatory effects while regulating immune system and neural signal transmission (67, 68). Plant-derived Omega-3, mainly alpha-linolenic acid (ALA), converts to EPA and DHA in the body with low efficiency. This may lead to insufficient intake in vegetarians (69). Some evidence suggests omega-3 supplementation may be beneficial as an adjunctive therapy for depression in some people (70). Therefore, the impact of plant-based diets on depression largely depends on whether they can ensure adequate supply of these key nutrients while avoiding animal products.

Besides neurotransmitters, brain-derived neurotrophic factor (BDNF) is vital for neuronal survival, growth, and synaptic plasticity. BDNF levels are usually low in patients with depression (71). A meta-analysis found that polyphenol supplements significantly raised BDNF levels in healthy young and middle-aged adults (72), and this effect was linked to better cognitive function. Plant foods contain many phytochemicals, including polyphenols, flavonoids, and carotenoids, that show potential for preventing and treating depression. Studies show polyphenols can relieve depressive symptoms, but more research is needed (73).

In summary, while micronutrient adequacy and phytochemical intake from plant foods may theoretically support neurotransmitter metabolism and neurotrophic signaling, direct evidence that these pathways mediate the relationship between plant-based dietary patterns and depression outcomes in humans is largely absent. Prospective intervention studies are needed to validate these mechanisms.