Connections Between Diet and Mental Health: Comparing Participants Randomized to Vegan and Omnivorous Diets in the Nutritious Eating With Soul (NEW Soul) Study

Abstract

Research on vegan diets and mental health shows mixed results but most of the studies have been cross-sectional and among white populations. This study examined changes in perceived stress and mental wellbeing among African American adults aged 18-65 years (N = 159) with overweight/obesity, who were randomized to vegan or omnivorous diets in a 2-year nutrition intervention. Changes in outcomes were assessed using intent-to-treat analysis with mixed model repeated measures. Both groups experienced decreased stress from baseline, seen at 6, 12, and 24 months for the omnivorous group and at 12 and 24 months for the vegan group. The omnivorous group showed improved mental wellbeing at 24 months. There were no differences in changes between the groups at any timepoint (P > 0.05) for stress and mental wellbeing. There were no differences between adherent vegan vs adherent omnivorous participants in changes in either outcome at any timepoint (P > 0.05). Prior research suggests increased stress and reduced mental wellbeing or reduced stress and increased mental wellbeing among individuals following a vegan diet, but this study found no difference in changes in stress and mental wellbeing between participants randomized to vegan or omnivorous diets.

“Transitioning to a vegan diet might introduce temporary stress as individuals adjust to new eating patterns.”

Introduction

Mental health issues affect a significant portion of the global population and have profound implications for individuals’ wellbeing and overall quality of life. Data suggest that the number of people living with mental health disorders has increased post COVID-19 pandemic. ¹ Among the most prevalent mental health issues are stress-related conditions and challenges to overall mental wellbeing, including anxiety, depression, and psychological distress. ²

Perceived stress is one’s subjective assessment of stress and ability to cope with challenges in daily life which can have significant implications for one’s physical and mental health. ³ Chronic and excessive perceived stress has been linked to the emergence of mental health issues, ⁴ and increased chronic pain risk. ⁵ The profound impact of mental issues on health, quality of life, and lifespan is well documented, ⁶ highlighting the need for comprehensive investigations into their etiology and potential interventions. African American adults often report higher perceived stress and poorer mental wellbeing compared to other racial groups.^7-9 Mental health issues are also associated with overweight and obesity, ¹⁰ and research suggest that this relationship is bidirectional. ¹¹ African Americans with overweight or obesity are more likely to experience mental health issues due to factors such as internalized weight stigma and body image dissatisfaction.^12,13 On the other hand, African American adults with mental health issues such as high stress are more likely to be obese due to factors such as emotional eating and reduced physical activity. ¹⁴ However, African American adults remain underrepresented in literature examining psychological distress. ¹⁵

Traditionally, the focus of mental health interventions has been on pharmacology and psychotherapy. ¹⁶ Emerging research suggests that diet plays a role in modulating stress ¹⁷ and mental wellbeing. ¹⁸ Plant-based diets, rich in fruits and vegetables, have been associated with better mental health outcomes such as reduced risk of depressive symptoms, potentially due to reduced oxidative stress from high antioxidant levels. ¹⁹ Additionally, eating foods high in prebiotic fibers promotes a healthy gut microbiota, and a healthy gut microbiota produces chemicals like short-chain fatty acids and neurotransmitters like serotonin, potentially reducing depressive symptoms. ²⁰ Chronic inflammation, often linked to depression, has also been lowered through the intake of plant-based diets high in polyphenols, which inhibit the production of pro-inflammatory cytokines.^21,22 Furthermore, plant-based diets high in phytoestrogens may regulate hormones, particularly estrogen, which plays a key role in mood regulation.^23(p5)

Healthy plant-based diets consisting of higher amounts of dietary fiber, whole grains, nuts and vegetables and lower intakes of refined grains, and sugar-sweetened beverages have been linked to better mental health. ²⁴ On the contrary, vegan, and vegetarian diets have also been linked to poorer ratings of mental health status than omnivorous diets. ²⁵ One possible explanation for such conflicting findings may be due to variations in vegetarian diets, sample sizes, and assessment tools.

A systematic review of cross-sectional studies, prospective cohort studies, and randomized controlled trials (RCTs) has also shown conflicting results between vegetarian diets and likelihood of depression. ²⁶ One pilot RCT showed no differences between omnivorous and vegetarian diets over 2 weeks ²⁷ whereas another RCT showed that providing weekly instructions to follow a vegan diet improved depression indicators after 18 weeks, compared to a control group receiving no instruction. ²⁸ Cross-sectional studies on vegetarian diets have shown mixed results, with some studies indicating lower rates of depression among vegetarians than non-vegetarians,^29,30 others reporting an association between following a vegetarian diet and a higher risk of depression, ³¹ and some finding no association at all. ³² Following vegan and vegetarian diets was also associated with a higher risk of depression and lower anxiety scores in a systematic review and meta-analysis. ³³

Recent research from observational studies further highlights the importance of adherence, with one study indicating that individuals who adhered to a vegetarian diet were 53% more likely to experience depression compared to omnivores. ²⁵ Conversely, another cross-sectional study found that participants following a healthy plant-based diet (comprising of whole grains, fruits, legumes, vegetables, nuts, vegetable oils) had lower levels of psychological disorders whereas those following an unhealthy plant-based diet (comprising of refined grains, fruit juices, sugar-sweetened beverages, potatoes, and desserts) had higher levels of anxiety and distress. ²⁴ These findings suggest that adherence to diet may be a critical factor influencing mental health, warranting further study.

While most studies on diet and mental health have examined specific dietary components, such as foods consumed by an individual on a regular basis,^24,25,30,34 there is a need to investigate the dietary pattern, which entails the types, quantities, and variety of foods, ³⁵ and beverages consumed habitually. Diet patterns reflect the complex interactions between various foods and nutrients and may have synergistic effects on perceived stress ¹⁷ and mental wellbeing. ³⁶ By assessing mental health outcomes among individuals following different diet patterns, we can better understand the cumulative effects of these diets on mental health. The current study therefore aims to conduct a comparative analysis of participants following vegan and omnivorous diets to investigate the relationship between the 2 diet patterns and mental health.

Research on the link between diet and mental health has been conducted in multiple countries including Australia,^37-39 Iran, ³⁰ United Kingdom, ⁴⁰ North America, ⁴¹ Brazil, ⁴² Peru, ⁴³ and France. ⁴⁴ Most studies (Iran, Australia, North America, Brazil, Peru, France) were cross-sectional and all except the study in France, showed that vegetarian diets were associated with increased risk of depression and anxiety. The connection between diet and mental health among African Americans is insufficiently researched, despite some evidence from the United States (US) indicating increased mental health implications compared to White counterparts. ¹⁵ Given that different cultural and sociodemographic factors can influence dietary choices, stress experiences, and overall wellbeing, ⁴⁵ it is essential to address this research gap. This study aims to fill this gap by examining how following vegan and omnivorous dietary patterns may be linked to mental health within an African American population.

Previous research on diet and mental health has mostly been cross-sectional and focused on individuals who self-select plant-based diets, limiting the ability to infer causal relationships. The present study allows for the examination of changes by randomized group (regardless of adherence to the assigned diet) and among those who were adherent to the diets. In this study, mental health was operationalized as perceived stress and mental wellbeing. The research questions were as follows: 1) What are the differences in changes in stress between African American participants randomized to follow vegan or omnivorous diets? 2) What are the differences in changes in mental wellbeing between African American participants randomized to follow vegan or omnivorous diets? 3) Do changes in mental health differ based on adherence to one’s assigned diet? We hypothesized there would be no differences in changes in stress and mental wellbeing between randomized groups and between adherent vegan and omnivore participants.

Methods

Study Design

Data for this analysis were from the Nutritious Eating with Soul (NEW Soul) study, a 2-year randomized trial conducted to examine whether a vegan diet affects cardiovascular disease risk factors and weight as compared to an omnivorous diet, both of which focused on soul food cuisine. ⁴⁶ Soul food refers to cuisine rooted in the African American culture in the Southern, US and is characterized by foods such as collard greens, corn bread, black eyed peas, okra, fried chicken, and sweet potatoes. ⁴⁷ While its origins date back to the 1400s when many West African tribes primarily had plant-based diets, the history of soul food took shape during slavery in the US whereby enslaved Africans adapted cooking methods to work with limited ingredients provided to them including, cornmeal, chitlins (intestines) and pork. ⁴⁸

The study recruited African American adults aged 18-65 years with a body mass index (BMI) between 25.0 and 49.9 kg/m² living in South Carolina (US) via radio ads, TV interviews, social media posts, referrals from friends and family, and community events. ⁴⁹ Participants were excluded from the study if they were pregnant, were currently following a vegan diet, taking weight loss medication or had recently lost weight. Participants were randomized to follow one of 2 culturally tailored diets, vegan or omnivorous. Participants in both diet groups received similar behavioral interventions which entailed weekly diet group-based classes for the first 6 months, biweekly for the second 6 months, and monthly for the final 12 months (24 months in total). All participants received diet specific food group recommendations, example meal ideas and starter recipes followed by additional handouts and recipes related to the topics covered in class.

The intervention was conducted in 2 cohorts to accommodate at least 130 participants whereby cohort 1 begun in May 2018 until August 2020, while cohort 2 began in June 2019 until May 2021 (Cohort 1, n = 67; Cohort 2, n = 92). A total of 159 participants were included in the study, with 77 in the vegan group and 82 in the omnivorous group. The main outcomes for the study were change in body weight and lipid measures at 12 months obtained for 121 participants (vegan, n = 57; omnivorous, n = 64). Data on dietary intake, perceived stress and mental wellbeing were collected at baseline, 6, 12, and 24 months from participants throughout the study. The study adhered to the Consolidated Standard of Reporting Trials (CONSORT) guidelines for reporting randomized clinical trials. ⁵⁰ A completed checklist has also been included (see Supplementary Material). The intervention description, procedures, measures, inclusion and exclusion criteria and recruitment procedures have been described elsewhere in detail.^46,49

Measures

Stress – Perceived Stress Scale (PSS)

The Perceived Stress Scale-10 (PSS-10) ⁵¹ contains 10 items that ask participants to reflect on their stress levels in the last month. Participants responded using a 5-point Likert scale ranging from 0 – never, 1 – almost never, 2 – sometimes, 3 – fairly often, and 4 – very often. Scores ranging from 0-13 were considered low stress; scores ranging from 14-26 were considered moderate stress; and scores ranging from 27-40 were considered high stress. ⁵¹ The PSS-10 demonstrated good internal consistency at baseline (α = .90), 6 months (α = .90), 12 months (α = .92), and 24 months (α = .90).

Mental Wellbeing—Mental Health Component of SF-12

The Short Form Health Survey (SF-12) ⁵² is a 12-item questionnaire designed to capture the physical health and mental health components and the response format of the overall scale varies by item. ⁵² To capture mental wellbeing, participants were asked to respond to 6 items on a 5-point Likert scale ranging from 1 – all of the time, 2 – most of the time, 3 – a good bit of the time, 4 – a little of the time and 5 – none of the time. The mental health component scores were summed at each timepoint to reflect mental wellbeing. Lower scores (below 50) indicate poorer mental functioning, while higher scores (above 50) indicate better functioning. The mental health component of the SF-12 for this sample demonstrated good internal consistency at baseline (α = .88), 6 months (α = .88), 12 months (α = .89), and 24 months (α = .89).

Adherence

Data on adherence were obtained through Automated Self-administered 24-hour (ASA24) ⁵³ dietary recalls. A scoring system with 0 to 5 points was used to represent the presence or absence of 5 different animal-based food groups (eggs, seafood, poultry, red meat, and dairy) and the serving sizes per day that each diet group was expected to consume. Vegan group participants received points for avoiding these food groups, and 0 points for consuming more than 0 servings per day. For the omnivorous diet, participants received a point if they met the recommended threshold of serving sizes per day or 0 points if they did not meet the recommended threshold. The overall continuous scores therefore ranged from 0 to 5 and were categorized into adherent (≥2.5) and non-adherent (<2.5) groups to facilitate analysis. The cut off score of 2.5 was not arbitrary but was informed by the scoring criteria established for this study and further detailed information on the scoring of adherence has been described elsewhere. ⁵⁴ Adherence was assessed in a way that reduced the likelihood of socially desirable responses, to the extent possible. The purpose of the study was reiterated throughout the intervention, highlighting that the goal was to understand the effects of the diet rather than enforce strict compliance, which ensured participants felt comfortable being truthful about their intake. Participants were assured of confidentiality, and emphasis put on the importance of honest reporting for the study’s integrity.

Data Analysis

This study entailed a secondary data analysis based on data from the parent study, ⁴⁶ and analytical decisions were made after collection of the data. A descriptive analysis was previously performed for baseline characteristics, including continuous covariates such as age, and BMI (25.0-49.9 kg/m²), and categorical covariates such as sex (male or female), level of education (high school or some college, college graduate, advanced degree), employment status (employed for wages, self-employed, retired, other) and marital status (single, married, divorced or separated, widowed, other). ⁵⁰ Covariates were identified based on prior literature identifying age, sex, education, employment status as potential confounders in dietary interventions. ⁵⁵

Using SAS 9.4 (SAS Institute), intent-to-treat analysis with mixed model repeated measures was used to examine changes in PSS-10 and SF-12 scores over time (from baseline to 6 months, 12 months, and 24 months) and between omnivorous and vegan diet groups. The distributional assumptions for outcomes were checked. The models were specified with group, time, (within group) and the group by time (between group) interaction as fixed effects. This interaction allowed for testing of whether the diets influence changes in stress and mental wellbeing across timepoints. A subgroup analysis was done to assess conditional effects of adherence to diet on perceived stress and mental wellbeing within groups and between groups, at 6, 12, and 24 months. In the analysis of adherence, the same variables were included in the model with limited samples. The models were adjusted for 6 baseline covariates: age, gender, education, employment, BMI, and cohort membership, and there was no missing data on these covariates. For the outcome data, participants were included in the analyses with sample sizes of n = 122, 107, and 93 at 6, 12, and 24 months respectively for stress; and n = 121, 109, and 93 at 6, 12, and 24 months respectively for mental wellbeing. Dietary adherence data were obtained for all the participants with perceived stress and mental wellbeing data. In the event of participants being lost to follow-up, the mixed models provided unbiased estimates using restricted maximum likelihood estimation assuming that losses to follow-up were missing at random. Missing data across all time points did not show particular patterns of participants’ missingness. Therefore, there was no evidence that the data were not missing at random. Available data were used as much as possible. In cases when participants were lost at a particular time point, any data collected at subsequent timepoints was used.

Ethical Approval and Participant Compensation

This study was approved by the University of South Carolina’s institutional review board (approval number Pro00064855). The trial was registered on https://clinicaltrials.gov as NCT03354377. The study was supervised by a data safety and monitoring board, composed of 3 NIH-funded researchers from the University of South Carolina who were not affiliated with the study. All participants gave informed consent and were given financial incentives after completing assessments (US$10 at 3 months, US$50 at 6 months, US$50 at 12 months, and US$100 at 24 months).

Results

The study sample at baseline included 159 African American participants (mean [SD] age, 48.3 [10.6] years; 126 females [79.2%]) who were randomized to either omnivorous (n = 82) or vegan (n = 77) diet groups. Figure 1 presents a CONSORT diagram showing participant enrollment, randomization, group allocation, and inclusion in the final analyses of the outcomes reported in this manuscript. At 6, 12, and 24 months, the percentage of participants who completed assessments was 77%, 69%, and 58%, respectively, and was similar in both diet groups. More than half of the participants had obtained an advanced level of education [40.3%] and were employed [74.2%]. Participants in both diet groups displayed comparable average BMI values with an approximate mean BMI of 36.9 [SD = 6.9]. Baseline demographic findings in the NEW Soul study have been previously published. ⁵⁰

Figure 1.

CONSORT Diagram

Changes in Stress and Mental Wellbeing by Diet Group

Stress

Participants in the omnivorous group had a mean decrease in stress between baseline and 6 months (−2.68, 95% CI [-4.61, −0.74]), baseline and 12 months (−2.26, 95% CI [-4.33 to −0.19]) and baseline and 24 months (−2.56, 95% CI [-4.70 to −0.42]) (Table 1). Participants in the vegan group had a mean decrease in stress at 12 months (−2.52, 95% CI [-4.63 to −0.42]) and 24 months (−2.51, 95% CI [-4.76 to −0.27]), with no change between baseline and 6 months (−0.75, 95% CI [-2.80 to 1.30]). There were no differences in changes in stress between groups at any time point.

Table 1.

Changes in Stress (Perceived Stress Scale) and Mental Wellbeing (SF-12) From Baseline to 6, 12, and 24 months Among Participants Randomized to Vegan or Omnivorous Diets in the NEW Soul Study.

Lsm (SE) [95% CI]a
Outcome	N	Omnivorous	N	Vegan	Difference Between Omnivorous and Veganb	P Value for Difference
Changes in stress (as assessed by perceived stress scale) from baseline
6mo	65	−2.68 (0.99) [-4.61 to −0.74]	57	−0.75 (1.04) [-2.80 to 1.30]	1.93 (1.43) [-0.89 to 4.75]	0.18
12mo	54	−2.26 (1.05) [-4.33 to −0.19]	53	−2.52 (1.07) [-4.63 to −0.42]	−0.27 (1.50) [-3.21 to 2.68]	0.86
24mo	49	−2.56 (1.09) [-4.70 to −0.42]	44	−2.51 (1.14) [-4.76 to −0.27]	0.04 (1.57) [-3.05 to 3.14]	0.98
Changes in mental wellbeing (as assessed by SF-12 c ) from baseline
6mo	62	1.12 (1.21) [-1.25 to 3.49]	59	0.43 (1.23) [-2.00 to 2.86]	−0.69 (1.72) [-4.08 to 2.69]	0.69
12mo	54	0.51 (1.27) [-1.98 to 3.00]	55	1.22 (1.27) [-1.27 to 3.72]	0.71 (1.79) [-2.80 to 4.23]	0.69
24mo	49	2.69 (1.31) [0.11 to 5.26]	44	0.89 (1.37) [-1.81 to 3.59]	−1.80 (1.89) [-5.52 to 1.92]	0.34

^aAll models were adjusted for baseline cohort, age, gender, education, employment, and BMI.

^bUsing intent-to-treat analysis, mixed models were specified with main effects (group and time) and interaction effects (group by time) to estimate change of means and mean differences over time.

^cSF-12, The Short Form Health Survey. ⁵²

Mental Wellbeing

In the omnivorous group, there were no changes in mental wellbeing scores from baseline to 6 months (1.12, (95% CI [-1.25 to 3.49]), or baseline to 12 months (0.51, 95% CI [-1.98 to 3.00]). There were changes from baseline to 24 months (2.69, 95% CI [0.11 to 5.26]). In the vegan group, there were no changes in mental wellbeing scores at any timepoint; 6 months, (0.43, 95% CI [-2.00 to 2.86]), 12 months, (1.22, 95% CI [-1.27 to 3.72]) and 24 months (0.89, 95% CI [-1.81 to 3.59]). There were no between group differences in changes in mental wellbeing scores at 6 months (−0.69, 95% CI [-4.08 to 2.69]), 12 months (−0.27, 95% CI [-3.21 to 2.68]) and 24 months (0.04, 95% CI [-3.05 to 3.14]).

Changes in Stress and Mental Wellbeing among Adherent vs Non-Adherent Participants

Stress

Adherent omnivorous participants reported a decrease in stress from baseline to 6 months (−3.77, 95% CI [-7.23 to −0.30], baseline to 12 months (−3.94, 95% CI [-7.52 to −0.36]) and baseline to 24 months (−3.40, 95% CI [-6.79 to −0.01]) (Table 2). Adherent vegan participants reported a decrease in stress from baseline to 12 months (−3.99, 95% CI [-7.02 to −0.96]) and baseline to 24 months (−3.43, 95% CI [-6.60 to −0.27]). Non-adherent omnivorous participants had a decrease in stress from baseline to 6 months only (−2.54, 95% CI [-4.88 to −0.20]), while non-adherent vegan participants did not have changes in stress at any timepoint.

Table 2.

Changes in Stress and Mental Wellbeing by Adherence to Diet From Baseline to 6, 12, and 24 months Among Participants Randomized to Vegan or Omnivorous Diets in the NEW Soul Study.

Lsm (SE) [95% CI]a
Outcome	Adherentb						Non-Adherent
	N	Omnivorous	N	Vegan	Differenced	P c	Omnivorous	N	Vegan	N	Differenced	P c
Changes in stress (as assessed by perceived stress scale) from baseline
6mo	17	−3.77 (1.75) [-7.23 to −0.30]	20	−2.06 (1.58) [-5.20 to 1.07]	1.70 (2.36) [-2.97 to 6.38]	0.47	−2.54 (1.19) [-4.88 to −0.20]	48	0.13 (1.34) [-2.50 to 2.77]	37	2.68 (1.79) [-0.85 to 6.20]	0.14
12mo	17	−3.94 (1.81) [-7.52 to −0.36]	25	−3.99 (1.53) [-7.02 to −0.96]	−0.05 (2.37) [-4.74 to 4.64]	0.98	−1.56 (1.28) [-4.07 to 0.95]	37	−1.46 (1.46) [-4.33 to 1.41]	28	0.10 (1.93) [-3.70 to 3.90]	0.96
24mo	17	−3.40 (1.71) [-6.79 to −0.01]	20	−3.43 (1.60) [-6.60 to −0.27]	−0.03 (2.34) [-4.67 to 4.60]	0.99	−1.99 (1.38) [-4.70 to 0.72]	32	−1.72 (1.56) [-4.79 to 1.35]	24	0.27 (2.07) [-3.81 to 4.36]	0.89
Changes in mental wellbeing (as assessed by SF-12 e ) from baseline
6mo	16	−0.65 (1.99) [-4.60 to 3.30]	24	1.45 (1.64) [-1.80 to 4.70]	2.10 (2.58) [-3.01 to 7.22]	0.42	1.87 (1.47) [-1.03 to 4.78]	46	−0.42 (1.67) [-3.71 to 2.88]	35	−2.29 (2.22) [-6.67 to 2.09]	0.30
12mo	17	−0.32 (2.15) [-4.57 to 3.93]	26	1.77 (1.79) [-1.77 to 5.32]	2.10 (2.80) [-3.44 to 7.63]	0.45	1.02 (1.54) [-2.02 to 4.06]	37	0.57 (1.73) [-2.85 to 3.98]	29	−0.45 (2.31) [-5.02 to 4.11]	0.84
24mo	18	0.10 (1.98) [-3.83 to 4.03]	20	1.33 (1.88) [-2.40 to 5.06]	1.23 (2.74) [-4.18 to 6.65]	0.65	3.84 (1.70) [0.49 to 7.20]	31	0.43 (1.90) [-3.33 to 4.18]	24	−3.42 (2.55) [-8.44 to 1.61]	0.18

^aAll models were adjusted for baseline cohort, age, gender, education, employment, and BMI.

^bMissing data in adherent were treated as non-adherent.

^cP represents P-value for difference between omnivorous and vegan groups.

^dUsing intent-to-treat analysis, mixed models were specified with main effects (group and time) and interaction effects (group by time) to estimate change of means and mean differences over time.

^eSF-12, The Short Form Health Survey. ⁵²

There were no differences between adherent omnivorous and adherent vegan participants from baseline to 6 months (1.70, 95% CI [-2.97 to 6.38]), baseline to 12 months (−0.05, 95% CI [-4.74 to 4.64]), and baseline to 24 months (−0.03, 95% CI [-4.67 to 4.60]). Similarly, there were no differences between non-adherent omnivorous and non-adherent vegan participants from baseline to 6 months (2.68, 95% CI [-0.85 to 6.20]), baseline to 12 months (0.10, 95% CI [-3.70 to 3.90]), and baseline to 24 months (0.27, 95% CI [-3.81 to 4.36]).

Mental Wellbeing

Only non-adherent omnivorous participants reported an increase in mental wellbeing from baseline to 24 months (3.84, 95% CI [0.49 to 7.20]) (Table 2). The magnitude of each intervention effect was computed by a contrast of follow-up to baseline change in the group minus follow-up to baseline change in another group, adjusted for covariates. This type of comparison is often referred to as difference-in-differences analysis. There were no differences between adherent omnivorous and adherent vegan participants from baseline to 6 months (2.10, 95% CI [-3.01 to 7.22]), baseline to 12 months (2.10, 95% CI [-3.44 to 7.63]), or baseline to 24 months (1.23, 95% CI [-4.18 to 6.65]). Similarly, there were no differences between non-adherent omnivorous and non-adherent vegan participants from baseline to 6 months (−2.29, 95% CI [-6.67 to 2.09]), baseline to 12 months (−0.45, 95% CI [-5.02 to 4.11]), or baseline to 24 months (−3.42, 95% CI [-8.44 to 1.61]).

Discussion

The present study aimed to investigate the relationship between diet and mental health in an African American population, contributing to the growing body of literature on the topic. We hypothesized that there would be no differences in changes in stress and mental wellbeing between participants assigned to follow either a vegan or omnivorous diet, or in changes in stress and mental wellbeing based on adherence to diet. Previous studies have shown varying results, with some indicating benefits of plant-based diets on depression while others have found no effects or negative associations. ²⁶ Given this inconclusive evidence, we chose a neutral null hypothesis to objectively assess the potential differences. Participants in both diet groups reported reduced stress and improved mental wellbeing over time but there were no differences in changes in stress and mental wellbeing between participants randomized to follow vegan or omnivorous diets. Changes in perceived stress and mental wellbeing did not differ based on adherence to either diet. The observed decrease in stress among participants in both diet groups at varying timepoints aligns with existing research indicating that dietary modifications can influence mental health. ⁵⁶

Previous intervention studies have indicated reduced stress and depression among individuals following a vegan diet vs non-vegan diet.^27,28 Findings from the present study are in line with other previous intervention studies that found vegan diets do not influence mental health. ²⁷ This findings can be explained by the comprehensive nutritional profiles of both the vegan diets and omnivorous diets in the parent study, ⁵⁰ which emphasized intake of fruits, and vegetables; antioxidant rich food sources that have been associated with reduced risk of depressive symptoms. ¹⁹ The study placed emphasis on limiting intake of saturated fats and simple sugars which are detrimental to mental health. ⁵⁷ Additionally, lifestyle factors such as engagement in health promotion behaviors such as regular physical activity (participants in both intervention groups were encouraged to meet the Physical Activity Guidelines for Americans, ⁵⁰ ) may enhance mental health in both groups. ⁵⁸ While this study excluded participants with eating disorders, previous studies suggest that vegan diets are associated with poorer mental health because individuals with disordered eating may adopt vegan or vegetarian diets as a socially acceptable way to limit their food intake. ⁵⁹ Excluding participants with eating disorders allowed us to reduce the potential confounding influence of restrictive eating behaviors. ⁵⁰

Adherent omnivorous participants reported a decrease in stress from baseline to 6, 12, and 24 months, suggesting a consistent trend towards decreased stress over time. Adherent vegan participants, however, reported a decrease in stress from baseline to 12 months and baseline to 24 months only. Since this impact was not observed at 6 months, a possible dynamic influence that extends beyond the first 12 months of adherence to the vegan diet may exist. An alternative explanation for the observed trend among vegan participants could be related to the stress associated with the initial dietary change. Transitioning to a vegan diet might introduce temporary stress as individuals adjust to new eating patterns. Over time, as participants become more accustomed to new eating patterns, the stress may diminish leading to observed reduction in stress at 12 and 24 months. This finding suggests the benefits of sustained adherence to diets, ⁶⁰ particularly for vegan diets which may require a longer adjustment period before they become evident.

There were no differences between adherent omnivorous and adherent vegan participants at any timepoint over the 24-month period, suggesting that both dietary patterns may have similar effects on stress reduction when adhered to consistently. This finding is contrary to other studies that have found a direct association between adherence to diet and mental health, ⁶¹ including those that found a higher risk of depression and anxiety among participants who adhered to unhealthy plant-based diets^24,34 and greater risk of depression among adherent vegetarians. ²⁵

Among all adherent participants, there was a decrease in stress from baseline to 12 and 24 months without a corresponding increase in mental wellbeing. A ceiling effect in baseline mental wellbeing potentially limited the observable improvement in mental wellbeing, as participants had relatively high scores of mental wellbeing at the beginning of the study.

Strengths and Limitations

Strengths of the study included its longitudinal and randomized design, which allowed for control of common causal factors that could influence mental health while following the assigned diets. The study’s length of 24 months allows for a thorough exploration of short- and long-term effects of dietary patterns on mental health. The inclusion of 2 diet groups also enhances the generalizability of findings and facilitates a comprehensive comparison. The study’s focus on analyzing adherence to diet adds depth to the investigation, shedding light on the behavioral aspects of adherence and effect on mental health. The study also fills a gap by looking at these relationships among African American adults.

Some limitations should be acknowledged in interpreting the findings of the study. First, the reliance on self-reported mental health and dietary adherence measures introduces the possibility of social desirability bias. Social desirability bias might lead participants to report improved mental health or adherence to diet thus affecting the validity of the findings. Second, this study is limited in its generalizability because it was conducted among African American adults in the Southeast of the US, the majority of whom were women and highly educated. Third, all participants had overweight or obesity prior to participating in the dietary intervention, and the psychological challenges associated with obesity could overshadow the potential effects of dietary differences on mental health outcomes. Fourth, the study was not powered to examine specific diet-sex interactions as the sample size was largely female, and menopausal status was not assessed, which may limit interpretation of sex-specific effects on stress and mental wellbeing. The study’s findings may also be limited by high non-adherence rates, as the imbalance in adherence rates between groups and low adherence in some participants may hinder their generalizability. Lastly, while randomization reduces potential confounding influences, it may not fully capture the complexity of individual psychological, social, and lifestyle factors that can affect mental health and might still vary across participants.

Conclusion

In conclusion, there were no differences in changes in stress and mental wellbeing among participants following vegan and omnivorous diets. Additionally, the relationship between both diets and mental health did not differ based on adherence. Future research should seek to obtain a more comprehensive understanding of the intricate interplay between diet, adherence, and mental health.

Ethical Approval

This study was approved by the University of South Carolina’s institutional review board (approval number Pro00064855).

Consent to Participate

All participants provided written informed consent prior to participating.

ORCID iDs

Enid A. Keseko https://orcid.org/0000-0001-6961-4500

John A. Bernhart https://orcid.org/0000-0001-6710-4195

Data Availability Statement

Upon completion and publication of all study-related manuscripts, data and analytic code described in the manuscript will be accessible upon request.*