Better Food, Better Mood: Diet Quality & Mental Health

In a recent study published in the Journal of Affective Disorders, researchers from Queensland, Australia, looked at the association between food and depression in Australian women during two time periods (2003 and 2009). Previous research has found conflicting evidence for the mental health benefits of vegetarian versus omnivorous diets, implying that the option between vegetarian, vegan, and omnivorous diets may not adequately explain depression risk.

While diet type was not found to be significantly associated with depressive symptoms, diet quality (fresh versus processed foods) did indicate a tiny but significant correlation, with higher-quality diets generally reducing these symptoms.

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, so far, the findings of this study have been perplexing. Some research suggest that plant-based diets can help improve depressed symptoms. Others, however, have noted that nutritional inadequacies caused by vegetarianism may exacerbate depression symptoms. However, accumulating data suggests that food quality, rather than diet type, may be more important in predicting depressed symptoms.

About the Study
The current study intends to address this argument in the context of Australian women by analyzing statewide longitudinal secondary data to determine the role of dietary choices in depressed symptoms. The study’s data came from the Australian Longitudinal Study on Women’s Health (ALSWH), a long-term (1996-2018) study of the effects of social, economic, and behavioral factors on health.

The current study focuses on data from ALSWH individuals born between 1973 and 1978. Because ALSWH time points one (1996) and two (2000) did not gather data pertinent to the current investigation (diet quality and depression symptoms), they were omitted from subsequent analyses in favor of ALSWH time points three (2003; n = 9,081) and five (2009; n = 8,199).

Data were collected using questionnaires, including the Centre for Epidemiological Studies Depression (CESD-10) questionnaire (which assesses the prevalence and severity of depressive symptoms), the Dietary Questionnaire for Epidemiological Studies version 2 (DQES v2) questionnaire (which assesses dietary intake and nutrient value), and the Australian Recommended Food Score (ARFS) datasheet.

The correlations between diet and depression were evaluated using hierarchical and moderated regression models. Alcohol use, smoking status, body mass index (BMI), marital status, and clinical anxiety were all considered factors and confounders of depressive symptoms and were factored into regression models. In regression models, age and other variables that were not linked with depressive symptoms due to ‘cohort effects’ (education, geographic location, and income) were used as controls.

Study findings
Diet quality trumped diet type: When diet quality was taken into account, the study discovered that the type of diet (omnivorous vs. plant-based) had no significant impact on depressive symptoms, underlining the importance of fresh, minimally processed foods.

The study cohort consisted of 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 partners, 41% smoked, and 88% drank alcohol. At both time points, the proportion of omnivores was significantly higher than that of individuals eating only plants.

“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 self-identifying as’vegetarian/vegan,’ a review of the sample cohort’s dietary data revealed that more than 86% ingested some sort of animal-derived nourishment, leaving only 147 really exclusive plant consumers. This finding underscores the difficulties in precisely defining dietary habits and implies that many self-identified plant-based eaters consume less meat rather than a purely plant-based diet.

The plant-based group had lower diet quality scores and mean BMIs than their meat-eating peers at both time points. Depressive symptoms in the plant-based group were found to be greater than in meat consumers.

Regression models comparing diet quality (fresh versus processed foods) revealed a small but significant relationship between these variables and depression: higher consumption of fresh and minimally processed foods was associated with fewer depressive symptoms at time points three and five.

Unfortunately, because to the small sample size of only vegetarian/vegan participants, more research is needed to prove the clinical validity of these findings. It is important to emphasize that, while statistical significance was obtained, the tiny effect sizes cast doubt on the therapeutic usefulness of these findings.

Conclusions

The current study analyzed ALSWH data to determine the effects of various food choices (kind and quality) on depressive symptoms in Australian women. Women who ate meat reported fewer depression symptoms than those who ate only plant-based foods. However, food quality, rather than diet type, emerged as the most important predictor of depression symptoms.

Notably, dietary quality (fresh versus processed foods) was identified as a major predictor of depressive health, with better quality (lower processing) related with less depressive symptoms. Unfortunately, over 86% of women who identify as vegetarian or vegan consume minor amounts of meat-derived items, putting them in the omnivorous category in regression analyses. The final vegetarian dataset consisted of just 147 participants, which was insufficient to statistically verify the clinical validity of these findings.

More study is needed to elucidate the complex link between diet quality, dietary motivations, and psychological effects.