Adherence to a Vegetarian Diet is not Associated With Depression: Results From the National Health and Nutrition Examination Surveys

Maximilian Andreas Storz; Alvaro Luis Ronco

doi:10.30773/pi.2022.0251

Psychiatry Investig > Volume 20(4); 2023 > Article

Storz and Ronco: Adherence to a Vegetarian Diet is not Associated With Depression: Results From the National Health and Nutrition Examination Surveys

Original Article

Psychiatry Investigation 2023;20(4):315-324.

Published online: April 20, 2023

DOI: https://doi.org/10.30773/pi.2022.0251

Adherence to a Vegetarian Diet is not Associated With Depression: Results From the National Health and Nutrition Examination Surveys

Maximilian Andreas Storz¹

, Alvaro Luis Ronco²

¹Department of Internal Medicine II, Center for Complementary Medicine, Freiburg University Hospital, Faculty of Medicine, University of Freiburg, Freiburg, Germany

²Unit of Oncology and Radiotherapy, Pereira Rossell Women’s Hospital, Montevideo, Uruguay

Correspondence: Maximilian Andreas Storz, MD Department of Internal Medicine II, Center for Complementary Medicine, Freiburg University Hospital, 55, Hugstetter Str., Freiburg 79106, Germany Tel: +49 157 545 43852, Fax: +49 761 270 83230 E-mail: maximilian.storz@uniklinik-freiburg.de

Received September 2, 2022 Revised October 17, 2022 Accepted January 11, 2023

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/4.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

Objective

Adherence to vegetarian diets has been associated with physical health benefits, whereas vegetarian mental health is less well documented. We sought to investigate whether adherence to a vegetarian diet was associated with depression in a nationally-representative sample of US adults.

Methods

We used population-based data from the US National Health and Nutrition Examination Surveys to examine said associations. Depression was assessed using the Patient Health Questionnaire (PHQ-9), vegetarian status was self-reported. Multivariate regression was used to measure the magnitude of associations, controlling for a range of covariables known to be associated with depressive symptoms.

Results

Our analysis included 9,584 individuals, of which 910 individuals had PHQ-9 scores suggestive of depression. Vegetarian diet was associated with reduced odds of PHQ-9 defined depression (odds ratio [OR]: 0.49 [95% confidence interval {CI}: 0.24-0.98], p=0.047) in a model adjusting for sex, age, ethnicity, income, and marital status. When adjusting for additional factors (educational level, smoking status, serum C-reactive protein, and body mass index) in a second model, the previously observed association was no longer significant (OR: 0.66 [CI: 0.34-1.26], p=0.203).

Conclusion

Vegetarian diet was not associated with PHQ-9 defined depression in this nationally-representative sample of adults. Additional longitudinal examinations are warranted to evolve the understanding of the role of vegetarian diets in mental health.

Keywords: Mental health, Depression, Vegetarian, Diet, Epidemiology, NHANES

INTRODUCTION

Over the past decades, the interest in vegetarian and vegan dietary patterns has increased in the general population and within the scientific community [1,2]. Ethical, environmental, and health motivations are the main drivers of this development [3-6]. Adherence to vegetarian and vegan diets has also been associated with numerous physical health benefits, including a lower risk for cardiovascular disease [7], improved glycemic control in type-2-diabetes, and a more favorable body weight [8,9]. Notably, vegetarian mental health is less well documented [10], and the number of studies investigating potential associations between vegetarian diets and depression is limited [11,12].

Some studies suggested that a vegetarian diet is associated with a lower risk of depression and does not adversely affect mood states, whereas other studies reported opposite findings, highlighting differences in the prevalence of depressive symptoms between vegetarians and omnivores [10,13-15].

In nutritional psychiatry, studies suggested positive associations between a higher intake of fruits, vegetables, and whole grains and a lower depression risk [13,16,17]. Diets high in sugar and processed fatty foods, on the other hand, have been associated with an increased risk for depression [18-20]. A 2020 analysis showed a significant association between red and processed meat intake and depression risk, reporting an overall effect size of 1.08 (95% confidence interval [CI]=1.04-1.12, p<0.001) [21]. In contrast, a 2021 review-supported by the National Cattlemen’s Beef Association-suggested that vegetarians are more likely to suffer from major depression [22]. Regrettably, the authors did not report an overall effect size of depression for red and processed meat. Of note, a recently published meta-analysis by Askari et al. [23] could not confirm the aforementioned association. The authors found no significant associations between the consumption of a vegetarian diet and depression.

Whether vegetarian diets are associated with depression is thus subject to an ongoing and controversial debate. Additional studies investigating potential association between vegetarian diets and depression are therefore warranted [24]. Using data from the National Health and Nutrition Examination Surveys (NHANES), the present analysis investigated whether adherence to a self-reported vegetarian diet was associated with depression in a nationally representative sample of US adults.

METHODS

Study design and population

The present study is based on cross-sectional, populationbased data from the NHANES [25]. The NHANES is a nationally representative survey of the US’ civilian non-institutional population designed to assess the health and nutritional status of adults and children in the US [26,27]. Data from the NHANES has been frequently used to examine mental health-related questions [28,29]. Special NHANES aspects and features have been described elsewhere in great detail [30].

The sample for the present study was drawn from a multistage probability sample of US civilians [30]. Representative of the non-institutionalized resident population, it excluded all persons in supervised care or custody, in institutional settings and all active-duty military personnel and their family members. The survey consisted of questionnaires administered in the home, and a standardized physical examination in a specially equipped mobile examination center. Approximately 10,000 individuals participated in every 2-year-cycle [29].

For this particular analysis, we appended 2 NHANES survey cycles (2007-2008 and 2009-2010) to increase the sample size for analyses stratified by population subgroups. The overall response rates exceeded 75% for both cycles [31]. NHANES was approved by the National Center for Health Statistics research ethics review board (Protocol #2005-06) [32]. Informed written consent was obtained from all participants [32]. The present analysis was conducted in May 2022.

Outcome variable and covariables

For the present analysis, we merged demographic data, examination data and questionnaire data. The respective NHANES modules and their usage are explained hereafter.

We assessed depression (outcome variable) using the Patient Health Questionnaire (PHQ-9). The PHQ-9 is a nineitem screening instrument that covers questions about the frequency of symptoms of depression over the past 2 weeks [33]. Response categories to each question included “not at all,” “several days,” “more than half the days,” and “nearly every day” and were given a score ranging from 0 to 3. Thus, potential total scores range from 0 to 27. The PHQ-9 is a frequently used tool in the primary care setting and considered reliable in the diagnosis of depression in the general population [29,33-35]. We treated the data dichotomously and defined depression as a PHQ-9 score of 10 or higher [29]. This cut-off point has been validated and deemed suitable in previous studies [36,37]. A-cut off score of 10 or above maximizes combined sensitivity and specificity [38]. We defined individuals with a score ≤9 as having no depression.

Our covariates included sociodemographic factors known to be associated with depression and/or impaired mental health (e.g. female gender, smoking history, elevated C-reactive protein [CRP] levels). For this, we carefully screened the literature and checked previously published publications that used comparable datasets [29].

Demographic data included: sex, age, race/ethnicity, marital status, annual household income, and education level. Race and ethnicity included the following categories: Mexican American, Other Hispanic, Non-Hispanic White, Non-Hispanic Black, and Other Race [39]. Marital status included the following categories: married, widowed, divorced, separated, never married, and living with a partner. Annual household income comprised two categories: under $20,000 and over $20,000. NHANES education level categories were not modified and can be obtained from Table 1. Body mass index (BMI) was obtained from the body measures dataset and categorized into four categories: obesity (BMI ≥30), overweight (BMI 25.00-29.99), normal weight (BMI 18.50-24.99), and underweight (BMI ≤18.49).

CRP levels, a classical acute-phase reactant and one of the best measures of the acute-phase response to an infectious disease or other causes of tissue inflammation, were obtained from the laboratory dataset. CRP was quantified by latex-enhanced nephelometry and categorized in two groups (based on an approach by O’Neil et al.29): below 0.5 mg/dL or ≥0.5 mg/dL.

Vegetarian status was obtained from the NHANES diet behavior and nutrition section, which provides personal interview data on various dietary behavior and nutrition-related topics. This component included the question “Do you consider yourself to be a vegetarian?” that we used for the group assignment. Assessment of vegetarian status was thus self-reported and based on a yes/no question [40].

Smoking status was assessed using data from the NHANES smoking (cigarette use) module and categorized based on a method by O’Neil et al. [29] We differentiated “current smokers” from “former smokers” and “never smokers,” based on the questions “Do you now smoke cigarettes?” and “Have you smoked at least 100 cigarettes in your entire life?”

Statistical analysis

We used Stata 14 statistical software (StataCorp. 2015. Stata Statistical Software: Release 14; StataCorp LP, College Station, TX, USA) for the entire statistical analysis. We used Stata’s “. svyset” and “. svy” commands to account for the complex NHANES survey design characteristics and the population weights. Prior to the analysis, we generated an appropriate 4-year-sample-weight for the Mobile Examination Center data. Unconditional subclass analyses (preserving the main survey design and providing larger standard errors) were performed to compare demographic, anthropometric, and examination data between depressed and non-depressed individuals (as assessed by the PHQ-9 questionnaire) [41].

To build an adequate logistic regression model, we followed the model-building steps proposed by Heeringa et al. [41] and Hosmer et al. [42] In a first step, we conducted exploratory bivariate analyses to check the eligibility of potential candidate predictors (other than vegetarian status) [41]. We investigated the association between PHQ-9-defined depression and vegetarian status (and other demographic and anthropometric predictors, respectively) using Rao-Scott F-tests of associations (Table 1). Candidate predictors of scientific interest and a bivariate relationship of significance p<0.25 with the response variable were included in the multivariate logistic model (Table 2). All variables were entered as categorical variables into the regression models.

In a first model, we investigated whether PHQ-9-defined depression was associated with vegetarian status after adjusting for the effects of various demographic factors (sex, age, race/ethnicity, annual household income, and marital status). Smoking status, educational level, BMI, and CRP were then added in a second model to examine whether the relationship was independent of these.

The aforementioned covariables were identified based on the literature and adjustment was performed because individuals on a vegetarian diet oftentimes exhibit a healthier lifestyle [43], characterized by lower smoking rates [44] and lower biomarkers of systemic inflammation [45]. Odds ratios (OR) and their CIs are presented in Table 2. We considered a p-value of less than 0.05 statistically significant. All tests were twotailed. Ultimately, we used Archer and Lemeshow’s [46] designadjusted test to assess the Goodness-of-fit for our logistic regression models.

RESULTS

Our analysis included 9,584 individuals with a complete dataset, of which 212 were self-reported vegetarians (unweighted). A total of 910 individuals (9.49%) had PHQ-9 scores ≥10, suggesting depressive symptoms. A total of 8,674 individuals (91.51%) reported PHQ-9 scores <10, suggesting no depressive symptoms.

More than 65% of individuals with PHQ-9 scores ≥10 were female and more than 27% were between 45 and 54 years of age. Almost 64% were non-Hispanic White and 40.25% were married. Approximately 70% had an annual household income over $20,000. Only 1.24% of individuals suffering from PHQ-9-defined depression were self-reported vegetarians. A large proportion of individuals with PHQ-9 scores ≥10 had an elevated body weight: more than one-forth were overweight and almost 44.60% were obese. Table 1 shows the other demographic, anthropometric and clinical data characterizing our sample. The weighted proportions describing our sample characteristics may also be obtained from Table 1.

Self-reported vegetarians differed from non-vegetarians with regard to the male to female ratio (32.78% vs. 67.22%, vs 49.35% vs. 50.65%) and with regard to the proportion of individuals with a pathologically elevated body weight (BMI >25, 52.10% vs. 70.09%). The proportion of current smokers was significantly smaller among self-reported vegetarians (11.03% vs. 21.59%), whereas the proportion of participants with a college degree or above was significantly higher (43.31% vs. 27.12%). No significant differences between self-reported vegetarians and non-vegetarians were found with regard to mean age, annual household income, and marital status.

After adjustment for sex, age, race/ethnicity, annual household income, and marital status, vegetarian status was not associated with increased odds for PHQ-9-defined depression (Table 2). To the contrary, we found lower odds (OR: 0.49 [CI: 0.24-0.98], p=0.047) for PHQ-9-defined depression in individuals reporting a vegetarian diet.

After additionally adjusting for educational level, BMI, CRPlevels, and smoking status (Table 2), vegetarian status was no longer associated with PHQ-9-defined depression. Other factors that increased the odds for PHQ-9-defined depression include female sex, Hispanic race as well as divorce and separation status. From the covariates in the second model, obesity, smoking and CRP increased the odds for PHQ-9-defined depression in the employed model.

Archer and Lemeshow’s [46] design-adjusted test to assess the goodness of fit for both of our models revealed p-values of 0.26 (FA-L[9,24]=1.35) and 0.99 (FA-L[9,24]=0.99), respectively-suggesting that the null hypothesis stating that the models fit the data well is not rejected.

DISCUSSION

The present study sought to investigate whether self-reported vegetarianism was associated with PHQ-9-defined depression in a nationally representative sample of the US civilian non-institutionalized population. After adjustment for various sociodemographic factors in multivariate logistic regression, we did not find an association between both entities. A discussion in the context of previous studies and potential pathomechanisms as well as explanations for our findings is thus warranted.

Several studies suggested a potential relationship between dietary patterns and mental health [13]. Dietary approaches limiting the intake of processed foods and emphasizing whole grains, vegetables, and fruits were associated with a reduced likelihood of depression [47,48].

A recently published prospective cohort study by Shen et al. [13] demonstrated that a Taiwanese vegetarian diet may reduce the odds for depression. The authors followed more than 12,000 participants from the Taiwanese Buddhist Tzu Chi Foundation since 2005. The vegetarian group in this particular cohort had a lower incidence of depressive disorders (2.37 vs. 3.21 per 10,000 person-years) than non-vegetarians. A landmark study in the field of nutritional psychiatry, this was the first prospective study investigating the long-term effects of vegetarian diet on the development of depressive disorders. Main strengths included the large sample size, the robust study design, and the assessment of diet through a reliable and validated 57-item food frequency questionnaire [13,49].

A previously published Austrian matched sample study revealed opposite findings [15]. Burkert et al. [15] found significantly higher rates of mental illness (including anxiety disorders or depression) in vegetarians (9.4% vs. 5.8% in individuals who consumed carnivorous diets rich in meats). The results, however, were also based on cross-sectional data, and the authors acknowledged that no statements were possible as to whether poorer mental health in vegetarians was the result of the chosen diet or whether the diet was consumed due to poorer health.

Li et al. [50] also reported that a vegetarian diet may be associated with depressive symptoms in older Chinese men. Compared to men consuming a meat-based diet, those who consumed a vegetable-based diet had a higher rate of depression (OR [CI]: 1.62 [1.07-2.46]), and severer depression symptoms. Yet again, the cross-sectional design of the study did not allow for determining causality. Likewise, Hibbeln et al. [12] reported a higher frequency of depressive symptoms in vegetarian men after adjustment for sociodemographic factors. Vegetarians in this particular cohort from South West England had higher average depression scores than non-vegetarians (mean difference: 0.96 points [CI: +0.53 to +1.40]). The authors discussed nutritional deficiencies as a potential cause but also acknowledged that reverse causation could not be ruled out.

More recently, Jin et al. [51] investigated the association between a vegetarian diet and depression among South Asians in the US. The authors reported a 43% lower odds of depression among vegetarians (p=0.023) and called for longitudinal examinations confirming their findings.

Unfortunately, the number of longitudinal intervention studies in this particular field is limited. One example for a sound intervention study in this field is the trial by Kahleova et al. [52] The authors reported significant improvements in depressive symptoms (using Beck Depression Inventory) after a 24-week vegetarian intervention in 74 individuals suffering from type-2-diabetes. A 2012 study by Beezhold and Johnston [53] investigated mood changes following a 2-week dietary intervention. The authors randomized omnivorous individuals to either a lacto-ovo-vegetarian group (avoiding meat, fish, and poultry) or to a group consuming fish 3-4 times weekly (avoiding poultry and meat) or to a control group consuming meat, fish, and poultry daily. Those who adopted the lacto-ovo-vegetarian diet significantly improved their mood, whereas the other two groups did not.

There are many potential explanations how a vegetarian diet could affect mood and depression risk. It is now widely accepted that depression is associated with chronic and systemic low-grade inflammation [54,55]. Individuals suffering from depression exhibit significantly higher concentrations of proinflammatory cytokines, including tumor necrosis factor-α and interleukin (IL)-6 [56]. A recently published meta-analysis confirmed depression as a pro-inflammatory state and emphasized the role of elevated inflammatory markers in depression (particularly CRP and IL-12) [57]. Vegetarian and vegan diets are abundant in antioxidants, polyphenols and low in pro-inflammatory precursors, such as arachidonic acid [17]. They have been associated with favorable reductions in inflammatory biomarkers in various studies [58,59], and appear to protect against inflammation and hence, potentially depression [13].

According to Shen et al. [13], vegetarian diets convey mood protection via a reduction of oxidative stress. Oxidative stress is the consequence of an imbalance between the production of reactive oxygen species and their removal. A moderate production and activity of oxygen species is necessary for body health [60]. Pathologically elevated levels, however, are detrimental and play an important role in depressive disorders [61]. A regular consumption of antioxidant-rich plant-foods has been recommended to increase the body’s antioxidative [17,62]. A vegetarian diet contains more antioxidant vitamins (betacarotene, vitamin C, and vitamin E) than an omnivorous diet and has been associated with lower oxidative stress [63,64]. This could be one explanation why some studies reported lower odds of depression in vegetarians [13,51].

The reservation must be made, however, that not all vegetarian diets are created equal. The Taiwanese vegetarian diet, as reported in the Tzu Chi Vegetarian Study and Tzu Chi Health Study, excluded meat and fish and included only few dairy products [65]. Moreover, the daily amount of animal protein intake in those cohorts was quite low and Taiwanese vegetarians differ from Western vegetarians in many ways [66]. The vegetarian diet consumed by vegetarians in our sample was different. Several participants occasionally reported consumption of some type of flesh products (meat, poultry, and/or seafood), and the amount of eggs and dairy was higher in this cohort as compared to the Taiwanese studies [67]. This must be kept in mind when comparing our investigation with the one by Shen et al. [13] and could partly explain the lack of an association between vegetarian diet status and PHQ-9-defined depression in our sample.

It is often difficult to disentangle whether the reported poorer mental health in vegetarians in some studies was the result of the chosen diet or whether the diet was consumed due to poorer mental health. However, one must keep in mind that an unbalanced and restrictive vegetarian diet may result in nutritional deficiencies. [15]. Said potential deficiencies, particularly in omega-3-fatty acids, vitamin B 12, iron, and folate, could be a potential explanation for the higher odds of depression in vegetarians observed in several studies [12,15,50].

Finally, it is of paramount importance to consider lifestyle factors and behaviors when discussing about vegetarian diets and their association with depression. Vegetarians frequently yield a healthier lifestyle with higher levels of physical activity as compared to non-vegetarians [68]. Some studies also suggest that vegetarians are also less likely to smoke [69], and are more likely to have a higher educational level as compared to meateaters [70]. Adjustment for these co-variables is warranted in logistic regression models and frequently done [51]; yet not all studies considered a complete set of factors potentially impacting depression [15]. This could partially explain the aforementioned contradictions between studies and should be carefully considered in future investigations.

In our case, the initially significantly reduced OR in model 1 was not retained when adjusting for lifestyle factors in model 2. Said phenomenon could be interpreted in multiple ways. One potential explanation is that BMI and inflammation (as assessed by CRP levels) could be potentially mediators instead of confounders. As discussed earlier, vegetarian diets may decrease systemic inflammation and associate with a healthier body weight. It is not inconceivable that when we adjusted for said (hypothetical) mediators, we were potentially blocking “the causal” path from diet to depression, resulting in the change in significant OR.

Strengths and limitations

Our study has several strengths and limitations that warrant further consideration. The current status of being vegetarian diet was self-reported. Previous studies, however, showed that self-identified vegetarians do not necessarily completely abstain from meat [11,67]. The same applies to our vegetarian sample. Juan et al. [67] examined the consumption of selected food groups and food components in self-identified vegetarians from the same cycles of the NHANES (2007-2010). Although self-identified vegetarians consumed significantly more plant-based foods and whole grains (with less solid fats and added sugars), several vegetarians also reported consumption of some type of animal products, such as meat, poultry, and/or seafood. Thus, caution is warranted in interpreting the term “vegetarian” from self-reports. After all, our results are not applicable to strict vegetarians. We clearly acknowledge this important limitation (which has also been discussed elsewhere in detail71), and highlight that the self-reported nature of vegetarian status could be an important issue when discussing the results of our present analysis. On the other hand, this approach resembles real life field-research and considers participants’ self-perception.

Moreover, our results are based on a cross-sectional study design, and we acknowledged that no causal interference is possible with this kind of data. A potential reverse causal relationship between vegetarianism and PHQ-9-defined depression is always possible in a way that individuals who have depression may be more inclined to adopt a vegetarian diet in order to improve their mental health. This must be kept in mind when discussing our results in either way. Individuals suffering from depression and other chronic health conditions associated with an increased risk of depression may potentially be more likely to adopt a vegetarian diet to improve their health, and could bias beneficial effects towards the null. In addition, depression was not diagnosed by a medical professional but is supposed by the PHQ-9 results of participants. While this approach has limitations, it is frequently used in comparable studies [29].

Another potential limitation of this manuscript is the examined sample size and the underestimation of the actual vegetarian prevalence in the US. As we only included individuals with a complete dataset, the number of (self-perceived) vegetarians is limited to 212 individuals (unweighted). In this context, it is important to note that adding another NHANES examination cycle would not have solved this problem. The vegetarian status variable was only included in two NHANES cycles (2007-2008 and 2009-2010) and newer NHANES cycles no longer contain the vegetarian status variable [39]. The estimated prevalence of vegetarianism in the US (approximately 2%) might nowadays be actually higher, ranging from 5% to 10% [72,73]. Finally, we acknowledge that some covariables known to be associated with depression (including physical activity) were not included in this analysis.

Strengths of this paper include the nationally representative nature of NHANES and the adjustment for a variety of covariables that have been shown to influence depression risk. Another strength is the PHQ-9, which is an established and validated questionnaire. Finally, the estimated depression prevalence in our analysis is essentially in line with other studies which suggested a past 12‒month depression prevalence among nearly 1 in 10 Americans in 2020 [74].

Conclusions

Results from our study suggest that a vegetarian diet is not associated with depression when adjusting for demographic variables, educational level, BMI, CRP-levels, and smoking status. These findings are of importance, given the ongoing debate about mental health risks and benefits of vegetarian nutrition. To the best of our knowledge, this is the first study to examine depression risk in a special cohort of NHANES vegetarians with unique features. Additional longitudinal examinations or intervention studies in different populations are warranted to evolve the understanding of the role of vegetarian diets in mental health.

Notes

Availability of Data and Material

Data is publicly available online (https://wwwn.cdc.gov/nchs/nhanes/Default.aspx). The datasets used and analyzed during the current study are available from the corresponding author on reasonable request.

Conflicts of Interest

The authors have no potential conflicts of interest to disclose.

Author Contributions

Conceptualization: Maximilian Andreas Storz. Data curation: Maximilian Andreas Storz. Formal analysis: Maximilian Andreas Storz, Alvaro Luis Ronco. Funding acquisition: Maximilian Andreas Storz. Investigation: Maximilian Andreas Storz. Methodology: Maximilian Andreas Storz, Alvaro Luis Ronco. Project administration: Maximilian Andreas Storz, Alvaro Luis Ronco. Resources: Maximilian Andreas Storz. Software: Maximilian Andreas Storz. Supervision: Maximilian Andreas Storz, Alvaro Luis Ronco. Validation: Maximilian Andreas Storz, Alvaro Luis Ronco. Visualization: Maximilian Andreas Storz. Writing—original draft: Maximilian Andreas Storz. Writing—review & editing: Maximilian Andreas Storz, Alvaro Luis Ronco.

Funding Statement

The authors received no financial support for the research, authorship, and/or publication of this article. The OA APC was paid by the Open Access Publication Fund of the University of Freiburg.

ACKNOWLEDGEMENTS

We thank the National Center of Health Statistics for the availability of NHANES survey data.

Table 1.

Demographic, anthropometric, behavioral, and clinical characteristics by depression status in individuals aged 20 years or older (weighted)

Characteristics		Not depressed (PHQ-9 score 0-9): (N=8,674)	Depressed (PHQ-9 score ≥10): (N=910)	p^*
Sex				<0.001
	Female	4,240 (49.82)	594 (65.05)^‡
	Male	4,434 (50.18)	316 (34.95)^‡
Age				<0.001
	18-24 yr	720 (9.27)	77 (8.53)
	25-34 yr	1,364 (17.89)	131 (15.79)
	35-44 yr	1,464 (19.23)	194 (22.80)
	45-54 yr	1,476 (20.57)	217 (27.71)^‡
	55-64 yr	1,432 (15.43)	169 (15.11)
	≥65 yr	2,218 (17.62)	122 (10.05)^‡
Ethnicity				<0.001
	Mexican American	1,522 (8.21)	169 (9.52)
	Other Hispanic	863 (4.50)	120 (7.31)^‡
	Non-Hispanic White	4,373 (71.65)	414 (63.94)^‡
	Non-Hispanic Black	1,556 (9.94)	175 (14.87)^‡
	Other race-including multi-racial	360 (5.70)	32 (4.37)
Education level				<0.001
	Less than 9th grade	956 (5.41)	148 (9.66)^‡
	9-11th grade^†	1,372 (12.20)	230 (21.67)^‡
	High school graduate/GED or equivalent	2,071 (23.59)	211 (26.01)
	Some college or AA degree	2,395 (30.02)	241 (30.70)
	College graduate or above	1,880 (28.77)	80 (11.96)^‡
Marital Status				<0.001
	Married	4,756 (58.71)	338 (40.25)^‡
	Widowed	699 (5.48)	84 (7.37)
	Divorced	930 (9.69)	155 (17.72)^‡
	Separated	260 (2.04)	61 (6.33)^‡
	Never married	1,388 (16.81)	180 (18.88)
	Living with partner	641 (7.25)	92 (9.46)
Annual household income				<0.001
	Under $20,000	1,738 (13.10)	360 (30.68)^‡
	Over $20,000	6,936 (86.90)	550 (69.32)^‡
Vegetarian status				0.096
	Yes	194 (2.22)	18 (1.24)
	No	8,480 (97.78)	892 (98.76)
Smoking status				<0.001
	Never smoker	4,672 (55.06)	361 (38.84)^‡
	Former smoker	2,251 (25.33)	181 (18.95)^‡
	Current smoker	1,751 (19.61)	368 (42.21)^‡
Body mass index (BMI)				0.001
	Underweight (BMI ≤18.49)	117 (1.43)	17 (1.85)
	Normal weight (BMI 18.50-24.99)	2,300 (29.23)	204 (24.03)^‡
	Overweight (BMI 25-29)	3,004 (34.36)	259 (29.51)^‡
	Obese (BMI ≥30)	3,253 (35.00)	430 (44.60)^‡
C-reactive protein				<0.001
	≤0.5 mg/dL	6,808 (81.04)	626 (69.01)^‡
	>0.5 mg/dL	1,866 (18.96)	284 (30.99)^‡

Values are presented as unweighted observations (%). Percentages may not total 100% due to rounding.

^* p-value is based on a designbased Rao-Scott F-Test and tests for a potential association between depression status and the respective demographic, anthropometric or clinical characteristics;

^† includes 12th grade with no diploma;

^‡ indicates significant differences in the weighted proportions.

PHQ-9, Patient Health Questionnaire; GED, general educational development

Table 2.

Multivariate logistic regression models examining potential associations between selected predictor variables and the odds for depression

Independent variables		Model 1			Model 2
Independent variables		OR	CI	p	OR	CI	p
Gender
	Female	1.75	(1.46-2.10)	<0.001	1.86	(1.55-2.24)	<0.001
Age
	18-24 yr	1.55	(0.90-2.68)	0.107	1.51	(0.88-2.57)	0.123
	25-34 yr	1.86	(1.30-2.51)	<0.001	1.70	(1.20-2.40)	0.003
	35-44 yr	2.06	(1.91-3.71)	<0.001	2.42	(1.73-3.37)	<0.001
	45-54 yr	3.05	(2.23-4.17)	<0.001	2.69	(2.00-3.63)	<0.001
	55-64 yr	2.14	(1.62-2.78)	<0.001	2.01	(1.52-2.64)	<0.001
Ethnicity
	Mexican American	1.07	(0.81-1.42)	0.580	0.95	(0.73-2.23)	0.704
	Other Hispanic	1.47	(1.14-1.89)	0.044	1.38	(1.13-1.70)	0.002
	Non-Hispanic Black	1.21	(0.98-1.49)	0.065	1.08	(0.89-1.31)	0.409
	Other race^*	0.83	(0.52-1.32)	0.423	0.96	(0.60-1.55)	0.893
Annual household income
	Under $20,000	2.48	(2.00-3.09)	<0.001	1.83	(1.50-2.23)	<0.001
Marital Status
	Widowed	2.03	(1.26-3.27)	0.005	1.85	(1.13-3.03)	0.016
	Divorced	2.01	(1.61-2.51)	<0.001	1.79	(1.42-2.27)	<0.001
	Separated	3.04	(2.20-4.20)	<0.005	2.61	(1.90-3.57)	<0.001
	Never married	1.53	(1.14-2.07)	0.006	1.46	(1.11-1.92)	0.008
	Living with partner	1.67	(1.20-2.31)	0.003	1.26	(0.90-1.76)	0.158
Vegetarian status
	Yes	0.49	(0.24-0.98)	0.047	0.66	(0.34-1.26)	0.203
Education
	Less than 9th grade				2.92	(1.87-4.56)	<0.001
	9-11th grade^†				2.40	(1.54-3.74)	<0.001
	High school graduate/GED or equivalent				1.88	(1.26-2.80)	0.003
	Some college or AA degree				1.79	(1.23-2.60)	0.003
Body mass index
	Underweight (BMI ≤18.49)				0.93	(0.39-2.19)	0.870
	Overweight (BMI 25-29)				1.12	(0.89-1.40)	0.307
	Obese (BMI ≤30)				1.40	(1.07-1.83)	0.014
Smoking status
	Former smoker				1.14	(0.87-1.51)	0.312
	Current smoker				2.37	(1.85-3.03)	<0.001
C-reactive protein
	≥0.5 mg/dL				1.36	(1.10-1.68)	0.005

Reference categories: gender, male; age group, ≥65 yr; annual household income, ≥$20,000; ethnicity, non-Hispanic White; education, college graduate or above; marital status, married; vegetarian status, no; body mass index, normal weight; smoking status, never smoker; C-reactive protein, ≤0.5 mg/dL. Model 1 includes the following parameters: gender, age, race/ethnicity, annual household income, marital status, and, vegetarian status; Model 2 includes all the aforementioned parameters in Model 1 plus education level, BMI, smoking status, and CRP level.

^* includes multi-racial;

^† includes 12th grade with no diploma.

OR, odds ratios; CI, 95% confidence interval; GED, general educational development; BMI, body mass index

REFERENCES

1. Ruby MB. Vegetarianism. A blossoming field of study. Appetite 2012;58:141-150.

2. Marrone G, Guerriero C, Palazzetti D, Lido P, Marolla A, Di Daniele F, et al. Vegan diet health benefits in metabolic syndrome. Nutrients 2021;13:817

3. Williams KA, Patel H. Healthy plant-based diet: what does it really mean? J Am Coll Cardiol 2017;70:423-425.

4. Rosenfeld DL, Burrow AL. Vegetarian on purpose: understanding the motivations of plant-based dieters. Appetite 2017;116:456-463.

5. Hopwood CJ, Bleidorn W, Schwaba T, Chen S. Health, environmental, and animal rights motives for vegetarian eating. PLoS One 2020;15:e0230609

6. Storz MA. Will the plant-based movement redefine physicians’ understanding of chronic disease? New Bioeth 2020;26:141-157.

7. Kahleova H, Levin S, Barnard N. Cardio-metabolic benefits of plantbased diets. Nutrients 2017;9:848

8. Jardine MA, Kahleova H, Levin SM, Ali Z, Trapp CB, Barnard ND. Perspective: plant-based eating pattern for type 2 diabetes prevention and treatment: efficacy, mechanisms, and practical considerations. Adv Nutr 2021;12:2045-2055.

9. Barnard ND, Alwarith J, Rembert E, Brandon L, Nguyen M, Goergen A, et al. A Mediterranean diet and low-fat vegan diet to improve body weight and cardiometabolic risk factors: a randomized, cross-over trial. J Am Nutr Assoc 2022;41:127-139.

10. Beezhold BL, Johnston CS, Daigle DR. Vegetarian diets are associated with healthy mood states: a cross-sectional study in seventh day adventist adults. Nutr J 2010;9:26

11. Michalak J, Zhang XC, Jacobi F. Vegetarian diet and mental disorders: results from a representative community survey. Int J Behav Nutr Phys Act 2012;9:67

12. Hibbeln JR, Northstone K, Evans J, Golding J. Vegetarian diets and depressive symptoms among men. J Affect Disord 2018;225:13-17.

13. Shen YC, Chang CE, Lin MN, Lin CL. Vegetarian diet is associated with lower risk of depression in Taiwan. Nutrients 2021;13:1059

14. Forestell CA, Nezlek JB. Vegetarianism, depression, and the five factor model of personality. Ecol Food Nutr 2018;57:246-259.

15. Burkert NT, Muckenhuber J, Großschädl F, Rásky É, Freidl W. Nutrition and health - the association between eating behavior and various health parameters: a matched sample study. PLoS One 2014;9:e88278

16. Lai JS, Hiles S, Bisquera A, Hure AJ, McEvoy M, Attia J. A systematic review and meta-analysis of dietary patterns and depression in community-dwelling adults. Am J Clin Nutr 2014;99:181-197.

17. Storz MA. Lifestyle adjustments in long-COVID management: potential benefits of plant-based diets. Curr Nutr Rep 2021;10:352-363.

18. Akbaraly TN, Brunner EJ, Ferrie JE, Marmot MG, Kivimaki M, Singh-Manoux A. dietary pattern and depressive symptoms in middle age. Br J Psychiatry 2009;195:408-413.

19. Wattick RA, Hagedorn RL, Olfert MD. Relationship between diet and mental health in a young adult appalachian college population. Nutrients 2018;10:957

20. Zhang Y, Yang Y, Xie MS, Ding X, Li H, Liu ZC, et al. Is meat consumption associated with depression? A meta-analysis of observational studies. BMC Psychiatry 2017;17:409

21. Nucci D, Fatigoni C, Amerio A, Odone A, Gianfredi V. Red and processed meat consumption and risk of depression: a systematic review and meta-analysis. Int J Environ Res Public Health 2020;17:6686

22. Dobersek U, Wy G, Adkins J, Altmeyer S, Krout K, Lavie CJ, et al. Meat and mental health: a systematic review of meat abstention and depression, anxiety, and related phenomena. Crit Rev Food Sci Nutr 2021;61:622-635.

23. Askari M, Daneshzad E, Darooghegi Mofrad M, Bellissimo N, Suitor K, Azadbakht L. Vegetarian diet and the risk of depression, anxiety, and stress symptoms: a systematic review and meta-analysis of observational studies. Crit Rev Food Sci Nutr 2022;62:261-271.

24. Fazelian S, Sadeghi E, Firouzi S, Haghighatdoost F. Adherence to the vegetarian diet may increase the risk of depression: a systematic review and meta-analysis of observational studies. Nutr Rev 2022;80:242-254.

25. National Health and Nutrition Examination Survey [Internet]. Available at: https://www.cdc.gov/nchs/nhanes/index.htm. Accessed July 16, 2022.

26. Lee WC, Fu E, Li CH, Huang RY, Chiu HC, Cheng WC, et al. Association between periodontitis and pulmonary function based on the Third National Health and Nutrition Examination Survey (NHANES III). J Clin Periodontol 2020;47:788-795.

27. About the National Health and Nutrition Examination Survey [Internet]. Available at: https://www.cdc.gov/nchs/nhanes/about_nhanes.htm. Accessed July 16, 2022.

28. Yu B, Zhang X, Wang C, Sun M, Jin L, Liu X. Trends in depression among adults in the United States, NHANES 2005-2016. J Affect Disord 2020;263:609-620.

29. O’Neil A, Berk M, Venugopal K, Kim SW, Williams LJ, Jacka FN. The association between poor dental health and depression: findings from a large-scale, population-based study (the NHANES study). Gen Hosp Psychiatry 2014;36:266-270.

30. NHANES Survey Methods and Analytic Guidelines [Internet]. Available at: https://wwwn.cdc.gov/nchs/nhanes/analyticguidelines.aspx. Accessed July 16, 2022.

31. NHANES Response Rates and Population Totals [Internet]. Available at: https://wwwn.cdc.gov/nchs/nhanes/responserates.aspx. Accessed July 16, 2022.

32. NCHS Ethics Review Board (ERB) Approval [Internet]. Available at: https://www.cdc.gov/nchs/nhanes/irba98.htm. Accessed October 16, 2022.

33. Beard C, Hsu KJ, Rifkin LS, Busch AB, Björgvinsson T. Validation of the PHQ-9 in a psychiatric sample. J Affect Disord 2016;193:267-273.

34. Kroenke K, Spitzer RL, Williams JB. The PHQ-9: validity of a brief depression severity measure. J Gen Intern Med 2001;16:606-613.

35. Martin A, Rief W, Klaiberg A, Braehler E. Validity of the brief Patient Health Questionnaire mood scale (PHQ-9) in the general population. Gen Hosp Psychiatry 2006;28:71-77.

36. Manea L, Gilbody S, McMillan D. Optimal cut-off score for diagnosing depression with the Patient Health Questionnaire (PHQ-9): a meta-analysis. CMAJ 2012;184:E191-E196.

37. Ghazisaeedi M, Mahmoodi H, Arpaci I, Mehrdar S, Barzegari S. Validity, reliability, and optimal cut-off scores of the WHO-5, PHQ-9, and PHQ-2 to screen depression among university students in Iran. Int J Ment Health Addict 2022;20:1824-1833.

38. Levis B, Benedetti A, Thombs BD, DEPRESsion Screening Data (DEPRESSD) Collaboration. Accuracy of Patient Health Questionnaire-9 (PHQ-9) for screening to detect major depression: individual participant data meta-analysis. BMJ 2019;365:l1476

39. 2007-2008 Demographics Data - Continuous NHANES [Internet]. Available at: https://wwwn.cdc.gov/nchs/nhanes/search/datapage.aspx?Component=Demographics&CycleBeginYear=2007. Accessed September 16, 2021.

40. Storz MA, Müller A, Lombardo M. Diet and consumer behavior in U.S. vegetarians: a National Health and Nutrition Examination Survey (NHANES) data report. Int J Environ Res Public Health 2022;19:67

41. Heeringa SG, West BT, Berglund PA. Applied survey data analysis. 2nd ed. London: Chapman and hall/CRC, 2017, p.121.

42. Hosmer DWJ, Lemeshow S, Sturdivant RX. Model-building strategies and methods for logistic regression. In: Hosmer DWJ, Lemeshow S, Sturdivant RX, editor. Applied logistic regression. 3rd ed. Hoboken: John Wiley & Sons, Inc, 2013.

43. Fresán U, Errendal S, Craig WJ. Influence of the socio-cultural environment and external factors in following plant-based diets. Sustainability 2020;12:9093

44. Davey GK, Spencer EA, Appleby PN, Allen NE, Knox KH, Key TJ. EPIC-Oxford: lifestyle characteristics and nutrient intakes in a cohort of 33 883 meat-eaters and 31 546 non meat-eaters in the UK. Public Health Nutr 2003;6:259-269.

45. Haghighatdoost F, Bellissimo N, Totosy de Zepetnek JO, Rouhani MH. Association of vegetarian diet with inflammatory biomarkers: a systematic review and meta-analysis of observational studies. Public Health Nutr 2017;20:2713-2721.

46. Archer KJ, Lemeshow S. Goodness-of-fit test for a logistic regression model fitted using survey sample data. Stata J 2006;6:97-105.

47. Psaltopoulou T, Sergentanis TN, Panagiotakos DB, Sergentanis IN, Kosti R, Scarmeas N. Mediterranean diet, stroke, cognitive impairment, and depression: a meta-analysis. Ann Neurol 2013;74:580-591.

48. O’Neil A, Quirk SE, Housden S, Brennan SL, Williams LJ, Pasco JA, et al. Relationship between diet and mental health in children and adolescents: a systematic review. Am J Public Health 2014;104:e31-e42.

49. Chiu TH, Huang HY, Chen KJ, Wu YR, Chiu JP, Li YH, et al. Relative validity and reproducibility of a quantitative FFQ for assessing nutrient intakes of vegetarians in Taiwan. Public Health Nutr 2014;17:1459-1466.

50. Li X, Cao HJ, Xie SY, Li KC, Tao FB, Yang LS, et al. Adhering to a vegetarian diet may create a greater risk of depressive symptoms in the elderly male Chinese population. J Affect Disord 2019;243:182-187.

51. Jin Y, Kandula NR, Kanaya AM, Talegawkar SA. Vegetarian diet is inversely associated with prevalence of depression in middle-older aged South Asians in the United States. Ethn Health 2021;26:504-511.

52. Kahleova H, Hrachovinova T, Hill M, Pelikanova T. Vegetarian diet in type 2 diabetes - improvement in quality of life, mood and eating behaviour. Diabet Med 2013;30:127-129.

53. Beezhold BL, Johnston CS. Restriction of meat, fish, and poultry in omnivores improves mood: a pilot randomized controlled trial. Nutr J 2012;11:9

54. Berk M, Williams LJ, Jacka FN, O’Neil A, Pasco JA, Moylan S, et al. So depression is an inflammatory disease, but where does the inflammation come from? BMC Med 2013;11:200

55. Dinan TG. Inflammatory markers in depression. Curr Opin Psychiatry 2009;22:32-36.

56. Dowlati Y, Herrmann N, Swardfager W, Liu H, Sham L, Reim EK, et al. A meta-analysis of cytokines in major depression. Biol Psychiatry 2010;67:446-457.

57. Osimo EF, Pillinger T, Rodriguez IM, Khandaker GM, Pariante CM, Howes OD. Inflammatory markers in depression: a meta-analysis of mean differences and variability in 5,166 patients and 5,083 controls. Brain Behav Immun 2020;87:901-909.

58. Turner-McGrievy GM, Wirth MD, Shivappa N, Wingard EE, Fayad R, Wilcox S, et al. Randomization to plant-based dietary approaches leads to larger short-term improvements in Dietary Inflammatory Index scores and macronutrient intake compared with diets that contain meat. Nutr Res 2015;35:97-106.

59. Eichelmann F, Schwingshackl L, Fedirko V, Aleksandrova K. Effect of plant-based diets on obesity-related inflammatory profiles: a systematic review and meta-analysis of intervention trials. Obes Rev 2016;17:1067-1079.

60. Shao A, Lin D, Wang L, Tu S, Lenahan C, Zhang J. Oxidative stress at the crossroads of aging, stroke and depression. Aging Dis 2020;11:1537-1566.

61. Barbosa ML, de Meneses A-APM, de Aguiar RPS, de Castro e Sousa JM, de Carvalho Melo Cavalcante AA, Maluf SW. Oxidative stress, antioxidant defense and depressive disorders: a systematic review of biochemical and molecular markers. Neurol Psychiatry Brain Res 2020;36:65-72.

62. Pellegrini N, Serafini M, Colombi B, Del Rio D, Salvatore S, Bianchi M, et al. Total antioxidant capacity of plant foods, beverages and oils consumed in Italy assessed by three different in vitro assays. J Nutr 2003;133:2812-2819.

63. Rauma AL, Mykkänen H. Antioxidant status in vegetarians versus omnivores. Nutrition 2000;16:111-119.

64. Galor SW, Siu PM, Benzie IFF. Antioxidants, vegetarian diets and aging. In: Preedy VR, editor. Aging San Diego: Academic Press, 2014, p.81-91.

65. Chiu THT, Chang HR, Wang LY, Chang CC, Lin MN, Lin CL. Vegetarian diet and incidence of total, ischemic, and hemorrhagic stroke in 2 cohorts in Taiwan. Neurology 2020;94:e1112-e1121.

66. Lin CL. Reply from author for letter to the editor concerning: stroke and diets: a review. Tzu Chi Med J 2022;34:253

67. Juan W, Yamini S, Britten P. Food intake patterns of self-identified vegetarians among the U.S. population, 2007-2010. Procedia Food Sci 2015;4:86-93.

68. Baines S, Powers J, Brown WJ. How does the health and well-being of young Australian vegetarian and semi-vegetarian women compare with non-vegetarians? Public Health Nutr 2007;10:436-442.

69. Lehto E, Kaartinen NE, Sääksjärvi K, Männistö S, Jallinoja P. Vegetarians and different types of meat eaters among the finnish adult population from 2007 to 2017. Br J Nutr 2022;127:1060-1072.

70. Allès B, Baudry J, Méjean C, Touvier M, Péneau S, Hercberg S, et al. Comparison of sociodemographic and nutritional characteristics between self-reported vegetarians, vegans, and meat-eaters from the NutriNet-Santé study. Nutrients 2017;9:1023

71. Storz MA, Rizzo G, Müller A, Lombardo M. Bowel health in U.S. vegetarians: a 4-year data report from the National Health and Nutrition Examination Survey (NHANES). Nutrients 2022;14:681

72. Le LT, Sabaté J. Beyond meatless, the health effects of vegan diets: findings from the Adventist cohorts. Nutrients 2014;6:2131-2147.

73. Norwood FB, Bir C. 1 in 10 Americans say they don’t eat meat - a growing share of the population. Alliance for Science [Internet]. 2022 Mar 10 [cited 2022 Sep 1]. Available at: https://allianceforscience.org/blog/2022/03/1-in-10-americans-say-they-dont-eat-meat-agrowing-share-of-the-population/.

74. Goodwin RD, Dierker LC, Wu M, Galea S, Hoven CW, Weinberger AH. Trends in U.S. depression prevalence from 2015 to 2020: the widening treatment gap. Am J Prev Med 2022;63:726-733.