This longitudinal study of 3483 twins (aged 45-90) from four HICs found a modest association between higher fruit and vegetable intake and lower depressive symptoms over 5-11 years. Using linear mixed-effects models and controlling for confounders, high fruit intake was associated with a 0.29-point reduction, and high vegetable intake with a 0.26-point reduction in depressive symptoms (based on a median score of 18) compared to low intakes. The ICE FALCON method suggested no familial confounding for vegetables, but results for fruit were inconclusive. Sensitivity analyses revealed that including potatoes attenuated the association for vegetables, and excluding one cohort altered the significance. These findings suggest a potential, though small, benefit of fruit and vegetable consumption for depressive symptoms, but causal claims are limited by the observational design and self-reported dietary data.
This longitudinal twin study suggests a modest association between higher fruit and vegetable intake and lower depressive symptoms over time in middle-aged and older adults from high-income countries (HICs). While the twin design and statistical adjustments for confounders strengthen the study, the reliance on self-reported dietary data and the modest effect sizes limit causal interpretations and practical utility. The observed associations may be influenced by residual confounding or other unmeasured factors. While the findings contribute to the growing body of evidence suggesting a link between diet and mental health, they do not definitively establish a causal relationship. Future research with more robust dietary assessments, clinical depression measures, and diverse populations, including those from LMICs, is needed to strengthen causal inference and explore the clinical significance of these findings. The practical implications for practitioners remain limited, though the study supports the general recommendation of promoting fruit and vegetable consumption as part of a healthy lifestyle.
The abstract effectively summarizes the key elements of the study, including the research question, methodology, main findings, and implications. It provides a concise overview of the study's purpose, design, and results, allowing readers to quickly grasp the essential information.
The abstract clearly states the main finding that higher fruit and vegetable intakes are associated with lower depressive symptoms in middle-aged and older adults. This direct presentation of the primary outcome helps readers understand the study's key contribution.
This is a medium-impact suggestion that would enhance the clarity and precision of the abstract. The abstract should clearly define the categorization of fruit and vegetable intake. While the abstract mentions "high" and "low" intakes, it doesn't quantify these categories. This lack of specific information can lead to misinterpretations of the findings. Providing clear definitions in the abstract, even if detailed explanations are in the main text, ensures that readers immediately understand the intake levels being compared. This clarity is crucial for accurately conveying the study's findings and their implications. Adding these definitions would strengthen the abstract by providing essential context for interpreting the results. It would also enhance the reproducibility of the study by allowing other researchers to clearly understand the intake categories used. Clearly defining the intake categories in the abstract improves the transparency and accuracy of the reported findings.
Implementation: Include quantifications for "high" and "low" fruit and vegetable intakes in the abstract. For example, specify the number of servings or grams per day that constitute each category. Refer to the methods section for detailed explanation of the categorization.
This medium-impact improvement would enhance the study's methodological transparency and reproducibility. The abstract should briefly mention the specific statistical methods used to control for familial confounding, such as the co-twin method. While the main text provides details, briefly mentioning this in the abstract reinforces the study's rigor and addresses a key methodological consideration. Adding this methodological information would strengthen the paper by providing essential context for interpreting the results and enabling other researchers to build upon this work effectively. This enhancement would also facilitate meta-analyses and systematic reviews in the field. Ultimately, clarifying these methodological details would significantly improve the study's scientific contribution by ensuring its findings can be properly contextualized and replicated.
Implementation: Briefly mention the co-twin control method in the abstract. For example, add a phrase like, "Familial confounding was addressed using the co-twin control method." This concise addition signals the study's methodological rigor to readers without requiring extensive detail in the abstract.
The introduction effectively establishes the context and importance of investigating the relationship between fruit and vegetable intake and depressive symptoms in middle-aged and older adults. It highlights the prevalence and burden of depressive disorders in this age group, emphasizing the need for effective prevention and intervention strategies.
The introduction clearly articulates the research gap and the study's objective to address it. It points out the limited research on older adults and the potential for residual confounding in previous studies, setting the stage for the current investigation.
This is a high-impact suggestion that would strengthen the introduction by providing a more focused and impactful rationale. The introduction should explicitly state the study's hypothesis. While the introduction mentions the research aim, it doesn't explicitly state the expected direction of the association between fruit/vegetable intake and depressive symptoms. Clearly stating the hypothesis enhances the scientific rigor and transparency of the study. A clear hypothesis provides a testable prediction, allowing readers to better evaluate the study's findings in light of the initial expectations. This strengthens the paper by demonstrating a clear scientific approach and facilitating a more focused interpretation of the results. Explicitly stating the hypothesis improves the study's scientific rigor and transparency.
Implementation: Add a clear statement of the hypothesis, such as "We hypothesize that higher intakes of fruits and vegetables are associated with a lower risk of depressive symptoms in middle-aged and older adults."
This medium-impact suggestion would enhance the introduction's clarity and provide a more comprehensive overview of the study's design. The introduction should briefly describe the twin study design and its advantages for addressing confounding. While the abstract mentions the use of twins, the introduction doesn't explain why this design is beneficial. Explaining the twin study design in the introduction reinforces the study's methodological strengths and helps readers understand how it addresses potential confounding factors. This strengthens the paper by providing context for the chosen methodology and highlighting its advantages for causal inference. Briefly describing the twin study design in the introduction clarifies the study's methodological approach and its rationale.
Implementation: Add a brief explanation of the twin study design and its advantages, such as "The use of a twin study design allows us to control for genetic and shared environmental factors, strengthening the ability to isolate the effects of fruit and vegetable intake on depressive symptoms."
The methods section provides a detailed account of the data collection process across the four cohorts, including specific questionnaires and assessment methods used for dietary intake and depressive symptoms. This level of detail strengthens the study's transparency and allows for replication.
This is a high-impact suggestion as it addresses a crucial aspect of data harmonization and comparability across different studies. The Methods section should provide a more detailed explanation of how the different depression scales (CES-D, GDS-15, and CAMDEX) were harmonized. While the section mentions the use of crosswalk tables and item response theory, it lacks specific details about the development and validation of these crosswalks. Providing a more comprehensive explanation of the harmonization process would strengthen the study's methodological rigor and increase confidence in the comparability of depression scores across cohorts. This would also enhance the transparency and reproducibility of the study, allowing other researchers to understand and potentially replicate the harmonization procedure. A detailed explanation of the depression scale harmonization process is crucial for ensuring the validity and comparability of the study's findings.
Implementation: Expand the description of the depression scale harmonization process. Include details about the sample used to develop the crosswalk tables, the specific item response theory model employed, and any validation procedures performed to ensure the accuracy of the converted scores. Consider providing a supplementary table with the crosswalk conversions.
This is a medium-impact suggestion that would enhance the transparency and reproducibility of the study. The Methods section should provide more detail about the FFQs used in each cohort, including the number of food items, specific examples of foods included, and any validation studies performed. While the section mentions the use of FFQs, it lacks specific information about the questionnaires themselves. Providing more details about the FFQs would strengthen the study's methodological rigor and allow for better assessment of the dietary intake data quality. This would also enhance the reproducibility of the study, enabling other researchers to understand the specific dietary assessment tools used and their potential limitations. Detailed information about the FFQs is essential for evaluating the reliability and validity of the dietary intake data.
Implementation: Include more details about the FFQs used in each cohort. Specify the number of food items included in each questionnaire, provide examples of specific foods assessed, and mention any validation studies conducted to assess the accuracy and reproducibility of the FFQs. Consider including the questionnaires as supplementary material.
Table 1. Details of contributing cohorts. *n=number of individuals with dietary and depressive symptom data at baseline and at least one depressive symptom follow-up assessment; **n=number of individuals with dietary and depressive symptom data at baseline and at least one depressive symptom follow-up assessment and having a co-twin who also provided this data.
The results section clearly presents the main findings of the study, including the associations between fruit and vegetable intake and depressive symptoms. The use of tables and specific statistical values (β coefficients, 95% CIs, p-values) enhances the transparency and allows for a clear understanding of the magnitude and significance of the observed associations.
The results section provides a detailed account of the statistical analyses performed, including the use of linear mixed-effects models and the handling of missing covariate data through multiple imputation. This detailed description strengthens the study's methodological rigor and transparency.
This is a high-impact suggestion as it directly addresses the interpretability and clinical relevance of the findings. The Results section should include an interpretation of the effect sizes found, translating the statistical findings into meaningful units of change in depressive symptoms. While the section reports the beta coefficients, it doesn't explain what these values represent in terms of practical changes in depressive symptom scores. Providing an interpretation of the effect sizes, such as the expected change in depressive symptom scores associated with different levels of fruit and vegetable intake, would strengthen the study's clinical relevance and facilitate a better understanding of the practical implications of the findings. This would also enhance the communication of the results to a broader audience, including clinicians and policymakers. Interpreting the effect sizes in terms of meaningful units of change in depressive symptoms enhances the clinical relevance and practical implications of the study's findings.
Implementation: Interpret the beta coefficients in terms of changes in depressive symptom scores. For example, state how many points on the CAMDEX scale a person's score is expected to decrease for each additional serving of fruits or vegetables consumed per day. Provide this interpretation for both the partially and fully adjusted models.
This is a medium-impact suggestion that would enhance the transparency and reproducibility of the study. The Results section should provide more details about the sensitivity analyses conducted, particularly regarding the exclusion of the MADT cohort. While the section mentions the exclusion, it doesn't fully explain the rationale or the specific results obtained. Providing a more detailed explanation of the sensitivity analyses, including the specific results for each cohort and the overall combined results, would strengthen the study's methodological rigor and allow for a better assessment of the robustness of the findings. This would also enhance the transparency and reproducibility of the study, enabling other researchers to understand the potential influence of individual cohorts on the overall results. A more detailed description of the sensitivity analyses enhances the transparency and robustness of the study's findings.
Implementation: Provide a more detailed explanation of the sensitivity analyses, including the specific results obtained when excluding the MADT cohort. Report the beta coefficients, 95% CIs, and p-values for each cohort and the combined results in a supplementary table. Explain the rationale for excluding the MADT cohort and discuss the potential implications of its influence on the overall results.
Table 2. Baseline characteristics of participants by study. SD standard deviation; BMI body mass index, IQR inter quartile range; MADT Middle Age Danish Twins Study; MTSADA Minnesota Twin Study of Adult Development and Aging; OATS Older Australian Twins Study; SATSA Swedish Adoption/Twin Study of Aging.
Table 3. Baseline characteristics of participants by category of fruit and vegetable intake. SD standard deviation; BMI body mass index, IQR Inter quartile range.
Table 4. Associations between fruit and vegetable intake and depressive symptoms over time (5-11 years), n=3483. Mean intakes serves/day combined analysis: Fruit-Low 0.3 0.2, Moderate 1.0 0.2, High 2.1 0.8; Vegetables-Low 0.5 0.3, Moderate 1.1 0.4, High 2.0 0.7. Analysis based on log10 transformed depressive symptoms, transforming back for the fully adjusted model results in the following ratios-Fruit: Moderate 0.986, High 0.984, Vegetables: Moderate 0.989, High 0.985. Based on a median depressive symptom score of 18, a change from low intake is associated with the following reduction in depressive symptoms: Fruit- Moderate 0.25, High 0.29; Vegetables-Moderate 0.20, High 0.26. Partially adjusted model adjusted for age, sex, education and living alone status; fully adjusted model additionally adjusted for BMI, physical health and cognitive ability. MADT Middle Age Danish Twins Study; MTSADA Minnesota Twin Study of Adult Development and Aging; OATS Older Australian Twins Study; SATSA Swedish Adoption/Twin Study of Aging. Significant results indicated in bold.
Supplementary Table S5. Sensitivity analysis including potatoes in vegetable intake.
The discussion effectively summarizes the main findings, highlighting the observed associations between high fruit and vegetable intake and lower depressive symptoms over time. This clear restatement of the key results reinforces the study's primary message.
The discussion acknowledges the limitations of the study, such as the reliance on self-reported dietary intake and the potential for residual confounding. This transparency strengthens the study's credibility and provides context for interpreting the results.
This is a high-impact suggestion as it directly addresses the interpretability and clinical relevance of the study's findings. The discussion section should discuss the clinical significance of the observed effect sizes. While the discussion mentions the modest effect sizes, it doesn't fully explore their practical implications for mental health interventions. Elaborating on the clinical significance would strengthen the study's impact by providing context for how these findings might translate into real-world benefits for individuals experiencing depressive symptoms. This would also enhance the discussion's contribution to the field by informing future research and intervention development. Discussing the clinical significance of the effect sizes provides valuable context for interpreting the study's practical implications.
Implementation: Expand the discussion on the clinical significance of the observed effect sizes. Discuss the practical implications of the modest reductions in depressive symptoms associated with higher fruit and vegetable intake. Consider relating the observed changes in depressive symptom scores to established clinical thresholds for meaningful change.
This is a medium-impact suggestion that would enhance the discussion's contribution to the field and provide directions for future research. The discussion should elaborate on the potential mechanisms underlying the observed associations. While the discussion briefly mentions dietary fiber, vitamins, micronutrients, and the gut microbiome, it doesn't delve into specific pathways or mechanisms. Expanding on potential mechanisms would strengthen the study's scientific contribution by providing a more in-depth understanding of how fruit and vegetable intake might influence depressive symptoms. This would also stimulate future research by suggesting specific pathways to investigate. Elaborating on potential mechanisms provides a deeper scientific understanding and guides future research.
Implementation: Expand the discussion on potential mechanisms linking fruit and vegetable intake to depressive symptoms. Discuss specific pathways, such as the role of antioxidants, anti-inflammatory effects, and the influence of specific nutrients on brain function. Consider incorporating relevant literature on the gut-brain axis and the role of the microbiome in mental health.
The conclusion effectively summarizes the main findings of the study, especially the association between higher fruit and vegetable intake and reduced depressive symptoms. This concise restatement reinforces the study's core message and provides a clear takeaway for the reader.
The conclusion acknowledges the limitations of the study, such as reliance on self-reported data, potential residual confounding, and the modest effect sizes. This transparency strengthens the study's credibility and provides context for interpreting the findings.
This is a high-impact suggestion because it addresses the generalizability and relevance of the findings to diverse populations. The conclusion should explicitly discuss the limitations of focusing solely on HIC populations and suggest the inclusion of LMIC cohorts in future research. This is crucial for ensuring the broader applicability of the findings and addressing potential disparities in diet-mental health relationships across different socioeconomic contexts. Expanding the scope to include LMICs would strengthen the study's contribution to global mental health research by providing a more comprehensive understanding of the relationship between diet and depression. This would also inform culturally sensitive interventions and public health policies targeting diverse populations. Ultimately, addressing the HIC-centric limitation broadens the study's impact and promotes more equitable mental health research.
Implementation: Add a statement in the conclusion explicitly acknowledging the limitation of including only HIC cohorts and emphasizing the need for future research to include LMIC populations. For example, state: "Our study focused solely on HICs, limiting the generalizability of our findings. Future research should include LMIC cohorts to explore potential variations in diet-depression relationships across diverse socioeconomic and cultural contexts."
This is a medium-impact suggestion that would enhance the conclusion's practical implications and provide actionable recommendations for healthcare professionals. The conclusion should provide specific recommendations for incorporating dietary assessments and interventions into routine primary care. While the conclusion mentions monitoring and counseling, it lacks specific guidance for implementation. Providing concrete recommendations would strengthen the study's translational value by offering practical steps for integrating dietary considerations into mental health care. This would empower healthcare providers to address dietary factors in their patients' mental health management and potentially improve patient outcomes. Ultimately, offering specific recommendations for primary care integration enhances the study's practical applicability and potential impact on patient care.
Implementation: Include specific recommendations for incorporating dietary assessments and interventions into primary care. For example, suggest the use of validated dietary screening tools during routine check-ups, provide guidance on how to counsel patients on increasing fruit and vegetable intake, and recommend referral to registered dietitians for personalized dietary advice. Consider providing examples of brief dietary interventions that can be easily integrated into primary care settings.