Fresh vs. processed: How diet quality holds the secret to managing depression, shedding light on the true impact of what’s on your plate.

Study: Omnivore, vegan and vegetarian diet quality associations with depressive symptoms: A comparative cross-sectional analysis of the Australian Longitudinal Study on Women’s Health. Image Credit: 910732akubi / Shutterstock

In a recent study published in the Journal of Affective Disorders, researchers in Queensland, Australia, investigated the relationship between diet and depression in Australian women across two time points (2003 and 2009). Previous studies have provided confounding evidence for the mental health benefits of vegetarian versus omnivorous diets, suggesting that the choice of vegetarian, vegan, or omnivorous diets may not sufficiently explain depression risk.

Study findings reveal that while diet type was not significantly associated with depressive symptoms, diet quality (fresh foods versus processed ones) shows a small yet significant correlation with depressive symptoms, with higher-quality diets generally decreasing these symptoms.

Background

Depression is a global pandemic, estimated to affect one in every 20 humans (5%; ~350 million people). Data from the World Health Organization (WHO 2023) suggests that women are more susceptible to depressive symptoms than their male counterparts. Australian women are particularly susceptible to these symptoms – the Australian Bureau of Statistics (2019) has documented that 12% of Australian women suffer from depression compared to 10% of men.

Depression is a multifaceted condition, with genetics, environment, and health behaviors (sleep, diet, physical exercise) implicated in its manifestation and management. Dietary choices, particularly the type (plant-only/vegetarian versus vegan versus omnivorous) and quality (fresh foods versus ultra-processed and sugary foods), have sparked extensive debate and, in turn, research on the healthiest nutritional choices to prevent and manage depressive symptoms.

Unfortunately, thus far, results from this research have proved confounding. Some studies report that exclusively plant-based diets are beneficial in positively modifying depressive symptoms (due to their high concentrations of prebiotics, fibers, and antioxidants). In contrast, others have reported that nutritional deficiencies due to the lack of meat in vegetarians may increase depressive symptoms. However, emerging evidence suggests that diet quality, rather than diet type, may play a more significant role in determining depressive symptoms.

About the Study

The present study aims to address this debate in the context of Australian women by using nationwide longitudinal secondary data to elucidate the role of dietary choices in depressive symptoms. Study data was acquired from the Australian Longitudinal Study on Women’s Health (ALSWH), a long-term (1996–2018) record of the impacts of social, economic, and behavioral factors on health.

The present study focuses on data obtained from ALSWH participants born between 1973 and 1978. Since ALSWH time points one (1996) and two (2000) did not collect data relevant to the present study (diet quality and depressive symptoms), they were excluded from downstream analyses in favor of ALSWH time points three (2003; n = 9,081) and five (2009; n = 8,199).

Data acquisition was questionnaires-based and included the Centre for Epidemiological Studies Depression (CESD-10) questionnaire (a test of the prevalence and severity of depressive symptoms), the Dietary Questionnaire for Epidemiological Studies version 2 (DQES v2) questionnaire (an evaluation of dietary intake and nutrient value), and the Australian Recommended Food Score (ARFS) datasheet (a measure of diet quality).

Hierarchical and moderated regression models were used to evaluate the associations between diet and depression. Alcohol consumption, smoking status, body mass index (BMI), marital status, and clinical anxiety were considered covariates and confounders of depressive symptoms and were adjusted for in regression models. Age and other variables not associated with depressive symptoms due to ‘cohort effects’ (education, geographical location, and income) were considered control variables in regression models.

Study Findings

The study cohort comprised 9,081 women in 2003 (ALSWH time point 3) and 8,199 women in 2009 (ALSWH time point 5). These participants had a mean age of 33.70 (in 2009); 77% were partnered, 41% smoked, and 88% consumed alcohol. The proportion of omnivores at both time points substantially outweighed those consuming an exclusively plant-based diet.

“In 2003, 500 women were categorized as plant-based and 6,110 as omnivores. In 2009, 333 women were categorized as plant-based and 6,276 as omnivores.”

Notably, despite 1,079 participants categorizing themselves as ‘vegetarian/vegan,’ analysis of the sample cohort’s dietary data revealed that more than 86% consumed some form of animal-derived nutrition, leaving only 147 truly exclusive plant consumers. This finding highlights challenges in accurately categorizing dietary patterns and suggests that many self-reported plant-based eaters follow a low-meat rather than strictly plant-based diet.

The plant-based group was shown to have lower diet quality scores and mean BMIs than their meat-eating counterparts across both time points. Depressive symptoms in the plant-based cohort were revealed to be higher than those in meat consumers.

Regression models evaluating diet quality (fresh versus processed foods) found a small yet significant association between these variables and depression—higher consumption of fresh and minimally processed foods was associated with decreased depressive symptoms across both time points three and five.

Unfortunately, due to the low sample size of exclusive vegetarian/vegan participants, additional research is required to confirm the clinical validity of these results. It is important to note that while statistical significance was achieved, the small effect sizes raise questions about the clinical relevance of these findings.

Conclusions

The present study used ALSWH data to identify the impacts of different dietary choices (type and quality) on depressive symptoms in Australian women. Women who included meat in their diets generally experienced fewer depressive symptoms than those who restricted their diet to plant-based sources. However, diet quality, rather than diet type, emerged as the more significant determinant of depressive symptoms.

Notably, dietary quality (fresh versus processed foods) was found to be a significant determinant of depressive health, with higher quality (lower processing) associated with decreased depressive symptoms. Unfortunately, a large (~86%) proportion of women identifying as vegetarian or vegan were found to inadvertently consume small proportions of meat-derived foods, thereby placing them in the omnivorous category in regression models. This resulted in the final vegetarian dataset comprising only 147 individuals, which was insufficient to statistically establish these findings’ clinical validity.

Further research is needed to explore the nuanced relationship between diet quality, dietary motivations, and mental health outcomes.