Flavonoids, a bioactive plant compound found in fruits, vegetables and beverages, have been well cited for their beneficial effects on physical health (1–3) and cognition (4, 5). In addition, systematic reviews have highlighted the overall effects of flavonoid-rich foods on mood (6–8), whereby supplementing participants with flavonoid-rich foods, or extracts, can result in improvements to mood.

Mood benefits have been associated with a range of flavonoid-rich foods across the lifespan. Specifically, effects of flavonoid intervention have been observed acutely over a two-hour period following cocoa consumption (499 mg flavanols) (9) and chronically after 30-day cocoa supplementation in healthy middle-aged adults (500 mg polyphenols/day) (10). Furthermore, berry flavonoids have also been found to improve mood (specifically positive affect) following an acute study in 7–10 year old children (253 mg anthocyanins) (11). In addition to positive affect, flavonoid-rich foods have shown to hold benefits for longer, chronic psychological outcomes such as anxiety and depression. Fisk et al. (12) demonstrated a reduction of depression symptoms after a 4-week wild blueberry intervention (253 mg anthocyanins/day) in adolescents (aged 12–17). Similarly, this finding has been reflected in older adults. Esmaeilpour-Bandboni et al. (13) found a reduction of depressive symptoms in an older population after a five-week green tea intervention (dose not specified). In addition, Calapai et al. (14) showed a significant reduction in anxiety scores following a 12-week grape extract intervention in healthy older adults (250 mg capsule containing V. vinifera extract consisting of proanthocyanidins (>9% w/w) and anthocyanins (4%–5% w/w)/day). Collectively, these findings suggest that a variety of flavonoid-rich foods and extracts could benefit mood and mental health over the lifespan.

Evidence from studies exploring habitual diet also demonstrate protective effects of flavonoid-rich foods on mental health. Higher consumption of fruits and vegetables has shown to promote psychological well-being and mental health in longitudinal studies (15, 16). Furthermore, in a large systematic review, Głąbska et al. (17) found that a higher intake of fruits and vegetables, specifically flavonoid-rich berries, citrus fruits, and leafy green vegetables, reduced levels of psychological distress and had a protective effect against depression, supporting the idea that a greater consumption of flavonoid-rich foods in the habitual diet can promote mental health. Chang et al. (18), Godos et al. (19) and Park et al. (20) also found that high dietary intake of foods rich in proanthocyanidins, flavones, flavanones, and anthocyanins found in berries, citrus fruits and leafy green vegetables, were associated with a reduction of depressive symptoms in a dose response manner. These results indicate that both a higher intake and combination of foods rich in specific flavonoid subclasses could elicit optimal mood effects.

The majority of flavonoid and mood research has been conducted in healthy adults, though flavonoid interventions could be of particular benefit to populations who are susceptible to low mood such as new parents, who are at risk of Postpartum Depression (PPD). PPD is classified as an episode of Major Depressive Disorder [MDD; (21)], lasting longer than two weeks, with onset either during pregnancy (peri-partum onset) or within the first six months to one year following birth [post-partum onset; (21)]. However, the period during which PPD may occur is subject of discussion (22). While the DSM-5 confines the diagnosis of PPD to the initial four weeks after giving birth (21), others propose symptoms can manifest up to one year postpartum (23). While there is discrepancy regarding when PPD arises, there is consensus that PPD is often higher in the initial 6 months (24, 25) which then decreases over 18 months postpartum (26). Despite discrepancies of when PPD arises, research suggests up to 10%–15% of new mothers are affected by PPD, however this number is likely to be far greater as those experiencing symptoms don't often seek a diagnosis (27). Considering this, the period between birth and six-months represents a potentially sensitive period which may benefit from an intervention targeted at preventing mood disturbances.

As PPD is a type of MDD, symptoms are like those of other depressive disorders, including but not limited to feelings of irritability, lack of energy and feeling low or sad. As such, treatment for PPD is very similar to the treatment of depression, where antidepressants and psychological therapies are the main treatment routes. However, these options pose barriers for the postpartum population where there is varying accessibility, success rate and cost-effectiveness. Several risk factors are additionally associated with postpartum mood disorders; maternal age, where both older and younger mothers are higher risk (28, 29); prior history of depressive and anxiety disorders (30); having multiple children (31) and complications with birth, breastfeeding and pregnancy (32, 33), indicating higher risk individuals may benefit the most from treatment or prevention of mood disturbance within the postpartum.

PPD not only affects the mother, but also the family unit. Letourneau et al. (34) highlight that mothers with PPD may have reduced quality of maternal-infant interactions which have been shown to negatively impact infant development, in addition to affecting the relationship with their partner, highlighting the additional collective importance of treating and reducing the prevalence of postpartum mood disorders. Moreover, much of the PPD literature focuses on mothers, however nearly 1 in 10 new fathers (35) experience symptoms of PPD across the postpartum period. This indicates that paternal and maternal mental health should be equitable in prevention or treatment of PPD. In 50% of cases, maternal depression is accompanied by paternal depression (36), suggesting PPD can affect both caregivers of the family simultaneously. Poor paternal mental health is rapidly becoming a public health issue alongside maternal mental health due to its prevalence, impact on child development and on the healthy functioning of the family unit (37).

Prevention or treatment of paternal PPD is rarely considered; subsequently, research exploring flavonoid intervention in new parents should target both mothers and fathers to address the equivalent risks of low mood in both populations. Further, research suggests that lifestyle interventions focusing on families as a whole may be more feasible compared to existing approaches that primarily target individuals (38) likely due to shared motivation and social support (39). It is also significant to explore postnatal mood among fathers, or those in father roles in non-traditional family structures such as those with step-fathers and blended families, where instances of paternal depression have been observed to be more prevalent compared to traditional family structures (40). For the purposes of this study, the focus was postpartum mood in biological mothers and fathers, which remains as the most prevalent family structure of the UK population.

Consideration of mood beyond those diagnosed with postpartum mood disorders is also necessary as low mood, high anxiety and poorer mental health in non-clinical populations have been found to predict onset of postpartum mood disorders (41). Therefore, it may be beneficial to target interventions that focus on non-clinical populations to potentially prevent severity of symptoms that may contribute to the development of mood disorders during this sensitive period.

In view of the evidence for flavonoids and mood, flavonoid-rich foods may offer protection against onset or symptom severity of PPD. Recent evidence has shown that a dietary flavonoid intervention can promote mood regulation in a postpartum population. Barfoot et al. (42) conducted a randomised control trial where 0–12-month postpartum mothers were randomised to a flavonoid intervention group who added one additional flavonoid-rich food item to their normal diet every day for two weeks alongside a control group, who continued with their normal dietary routine. Mothers in the flavonoid group showed significantly reduced state anxiety and increased perceived quality of physical health highlighting that dietary flavonoid intervention has potential to improve mental health and wellbeing in the postpartum period.

The current study follows on from Barfoot et al. (42) by utilising the same two-week parallel groups design, and the same flavonoid-rich intervention foods to investigate postpartum mood. The current study focuses specifically on the 0–6-month postpartum period due to this being a particularly sensitive window for detecting PPD where mood is more labile. This will identify whether a dietary flavonoid intervention closer to birth, where onset risk is higher, offers greater benefits. Additionally, Barfoot et al. (42) required participants to consume one flavonoid item per day, whereas the present study increases this to two items per day. Previous research suggests a dose dependency of flavonoids, whereby a higher flavonoid dose translates to improved mood (18, 19, 43). Therefore, a higher dose may increase sensitivity to detect mood benefits, or benefits observed may be greater.

Furthermore, PPD-specific measures of depression and anxiety (Edinburgh Postnatal Depression Scale and the Postnatal-Specific Anxiety measure) are used in the current study which may increase sensitivity to change in postpartum mood compared to the Patient Health Questionnaire [PHQ-8; (44)] previously used by Barfoot et al. (42). Finally, the current study also recruits a small sample of fathers to pilot whether the dietary intervention would be feasible for fathers during this critical time. Therefore, the aim of this study was to investigate whether a two-week dietary flavonoid intervention would improve parents’ mental health in the postpartum period. Based on Barfoot et al. (42), we hypothesised that mothers and fathers in the flavonoid intervention arm would have a reduction in symptoms of state anxiety and improved quality of life. Due to the increased sensitivity of the other measures, we also hypothesised improved mood and reduced postnatal depression and anxiety as a result of the flavonoid intervention.

There were no significant differences in demographics (Table 1) and outcome data (Table 4) between flavonoid or control groups at baseline.

A significant Condition × Time interaction (F[1,37]= 10.38, p = .003), revealed that mothers in the flavonoid condition reported significantly lower postpartum depression at the end of the two week intervention compared to baseline (p = .002), which was not evident in the control group (p = .276). There were no significant main effects of Condition or Time or any covariate in the model (p > 0.05) (Figure 2).

A significant Condition × Time interaction (F[1,37]= 5.13, p = .029) was also found for positive affect where mothers in the flavonoid group had significantly higher positive affect scores at Week-2 compared to baseline (p = .006) which was not evident in the control group (p = .971) (Figure 3). Additionally, results revealed a significant main effect of Time, where positive affect was greater at follow up regardless of condition (F[1,37]= 4.92, p = .033). Interestingly, Mothers age was a significant covariate (F[1,37]= 4.62, p = .038). Age and positive affect scores were not significantly associated at baseline (r = −.098, p = .557), or change from baseline (r = .285, p = .083), though were marginally significant post-intervention (r = −.308, p = .060), suggesting older ages were associated with lower positive affect at the end of the study, regardless of intervention type.

State anxiety (F[1,37]= 4.10, p = .05), negative affect (F[1,37]= 4.78, p = .035), postpartum anxiety (F[1,37]= 12.46, p = .001) and physical health quality of life (F([1,37]= 5.07, p = .029) were significantly lower at follow up compared to baseline (a significant main effect of Time), however, interactions and main effect of Condition were not significant (p > 0.05). For psychological, social and environmental quality of life scores no significant interaction or main effects were found (p > .05).

Regarding habitual diet, analysis of data from EPIC-Norfolk showed no significant differences between flavonoid and control conditions at baseline (Table 3). Both control (p = <.001) and flavonoid (p = <.001) conditions had a higher flavonoid intake post intervention compared to baseline (F[1,37]= 63.09, p = <.001, see Table 3). Additionally, one sample t-tests were conducted to explore differences between mean nutrient intake at baseline and the UK recommended dietary allowance (RDA) for mothers in the postpartum period. Significant differences were observed for all nutrients (Table 3), whereby mothers were under-achieving RDA's for most macronutrient groups, except for flavonoid intake in both conditions, and vegetable intake in the control condition, where they were higher than the respective RDA (Table 3).

For the qualitative data, a total of 25 mothers provided responses regarding their health experiences during pregnancy. Among these, 18 (72%) mothers acknowledged facing pregnancy challenges. In relation to birth complications, 20 mothers shared their experiences, with all of them highlighting difficulties during childbirth (100%). Elaboration of the birth experience was given by all 38 participants, where 21 (55%) participants described their experiences as more positive, and 17 (44%) more negative. For breastfeeding, 27 mothers responded, all of which indicated encountering difficulties (100%).

The current study recruited both mothers and fathers with infants under six months old with the aim to investigate whether a two-week dietary flavonoid intervention would improve parents’ mental health in the 6-month postpartum period. Mothers who consumed two additional flavonoid-rich foods per day over two weeks had a significant reduction in postpartum depression scores and a significant increase in positive affect post intervention relative to baseline. No such differences occurred in the control group. These results suggest that regular consumption of flavonoid-rich foods in the habitual diet during the first 6-months postpartum may alleviate symptoms of postpartum depression and improve transient mood in the postpartum population. Due to lack of compliance with the intervention and outcome measures, fathers’ data were not analysed, suggesting potential barriers to engagement in this sample. Overall, findings provide evidence to further support the idea that flavonoid-rich foods are an accessible and cost-effective option to promote mental health in mothers in the immediate postpartum period, where risk of mood disturbance is higher than that of the general population.

The overall improvement in positive affect for the flavonoid group aligns with findings from Khalid et al. (11) who found higher positive affect following acute blueberry intervention across separate samples of children and young adults. Considering that low positive affect is linked to depression and high negative affect is linked with anxiety (46, 47) it could be argued that acute and chronic affective states associated with depressive disorders are more sensitive to flavonoid interventions compared with those associated with anxiety, as both negative affect and anxiety measures did not significantly change as a result of the flavonoid intervention. Furthermore, sensitivity of a flavonoid effect on chronic depressive symptomology may also depend on the specificity of the measure used. Barfoot et al. (42) found no flavonoid-related outcomes on the PHQ-8, a measure of depression designed for the general population. However, the current study found significantly reduced depression following 2 weeks of dietary flavonoids using the EPDS, a tool specifically designed to capture PPD in a postpartum population. This highlights that though depressive states may be more sensitive to flavonoid interventions, the outcome measure used is also important, though additional research is needed in this population to further investigate effects of flavonoid supplementation on a range of mood states and underlying mechanisms which may drive this effect.

Maternal age was highlighted as a predictor of positive affect scores, whereby being an older mother was associated with lower positive affect scores post-intervention. In the study, 5 mothers were approaching 40 (aged 38–39) and 2 were over 40, suggesting an older subsample in the study that may have been associated with poorer mood, as has been observed in previous research (28). Upon further exploration, age was not significantly related to positive affect at baseline or change from baseline analysis. Considering these findings, it may be of interest to further explore whether the benefits of nutritional interventions for mood during the postpartum period are influenced by age.

At baseline, postnatal depression and postpartum anxiety (PPA) scores (as assessed by EPDS and PSAS respectively) were noticeably higher than the number of recorded mental health diagnoses in the sample (postnatal depression n = 1, postnatal anxiety n = 1, postnatal anxiety and depression n = 1 anxiety n = 1) whereby 42% of mothers scored over the clinical cut-off for PPD and 60% were above cut-off for PPA. This exceeds respective global population estimates [PPD: 10%–15% (27); postpartum anxiety: 13%–40% (68)], indicating symptoms indicative of PPD and postpartum anxiety are noticeably higher than the number of recorded mental health diagnoses in the sample. This highlights the importance of investigating postpartum mental health within the wider context where risk of missed diagnoses may be high. Furthermore, the qualitative data indicated that the percentage of women reporting a negative birth experience (44%) was also higher than previously recorded prevalence, ranging from 7 to 21% [(69); Henriksen et al., (70)]. It is unclear why this might be and future research should investigate the factors that might affect mothers’ interpretation of birth experience and relation to mood. These data may highlight that the sample were more clinically vulnerable than the general postpartum population. However, the significant reduction in mean postpartum depression scores in the flavonoid group from “possible depression” at baseline to “little or no depression” at 2-weeks (−2.6 scoring points) highlights the potential for a flavonoid-rich diet to alleviate some symptoms of clinical depression in this cohort.

In addition to benefits for postpartum depression and positive affect, results showed significantly lower state anxiety scores for both groups over the two weeks. In comparison, Barfoot et al. (42) found significant state anxiety effects for the flavonoid group only in postpartum mothers. The reason for differences in the STAI scores from Barfoot et al. (42) and the current study remains unclear, although highlights the need for further investigation into the effects of flavonoids in a postpartum sample. Despite this, both the STAI and PSAS-RSF-C seem to be somewhat sensitive to changes across time in this period which supports use of both measures in future postpartum populations.

The EPIC FFQ was utilised to assess baseline habitual diet and to monitor stability of participants’ diets over the 2-week intervention period. The only outcome from the EPIC FFQ to significantly change over the two-week intervention was flavonoid content, where flavonoid intake significantly increased from baseline to post-intervention in both groups. Flavonoids were not explicitly mentioned in study advertisements, rather diet was highlighted as the variable of interest, so this effect cannot be explained by a flavonoid knowledge bias. This finding may have been an effect of food diary completion leading to increased awareness of diet over the intervention period whereby participants consumed food items that happen to be rich in flavonoids. Reassuringly, the mean change between pre and post was highest for the flavonoid group compared to the control (Table 3). As no significant differences were found in other EPIC items such as fruit and vegetable intake it's likely participants consumed other flavonoid-rich foods and beverages such as cocoa and tea which contributed to the significant change in flavonoid consumption in both groups. However, these specific items were not captured in the EPIC analysis, making it difficult to ascertain the main sources of flavonoids in the participant's diet. It is also important to emphasise here that the food diaries did not provide enough data to accurately estimate flavonoid or other nutrient intake, and the main purpose, as previously described, was to identify the intervention food items, to assess and encourage compliance, and to match procedures across the two groups. Future research should consider use or development of other dietary measures which may provide better capture of high flavonoid foods, for example 24-hr food recalls (71) or a more comprehensive collection of flavonoid-rich items in current dietary measures.

Food diaries in the current study did accurately capture the two daily flavonoid-rich items participants added to their diet over the 2-week intervention period. Berry fruits, orange juice and dark chocolate were the most consumed intervention food by mothers (22%, 18% and 17% consumed over the two-week period respectively). Our results are in keeping with previous experimental and epidemiological studies demonstrating antidepressant effects of a high flavonoid, berry-rich diet (11, 12, 18). Regarding potential mechanisms, berry fruits are rich in anthocyanins, which have been hypothesised to have regulatory abilities for mood by crossing the blood brain barrier (BBB) acting as a monoamine oxidase (MAO) inhibitor (72, 73). Elevated MAO has been observed in the immediate postpartum period (4–6 days after birth) (74) which was later associated with postpartum depressive symptoms (75), suggesting the MAO hypothesis for depression may extend to the postpartum. Promising findings from Dowlati et al. (76) additionally showed increased resilience in depressed mood following a blueberry flavonoid dietary supplement in the 0–5-day postpartum period. Collectively, this evidence could explain why depressive mood and positive affect were improved in the present study following an increased consumption of berry fruits, though more research is warranted.

There is also evidence that flavonoids may regulate mood via maintaining neuroplasticity. It has been established that those with mood disorders, such as anxiety and depression have a reduction in circulating brain derived neurotropic factor (BDNF) [see (77, 78) for reviews]. A recent review from German-Ponciano et al. (79) concluded that flavonoid supplementation (minimum duration 14-days) in rodents has the ability to reverse depressive behaviours via increasing BDNF levels, highlighting flavonoid supplementation as a potential for the treatment in depression. Collectively, these results indicate that flavonoids may act as therapeutic candidates for mood disorders by maintaining neuroplasticity, potentially acting as the underlying mechanism in the current study. However, more conclusive research needs to be conducted with flavonoid supplementation, assessing BDNF levels and mood outcomes in humans with interventions lasting minimum 14-days to explore this.

As discussed, fathers’ data was not analysed due to lack of engagement with the study materials and intervention, meaning we were not able to conclude whether a flavonoid intervention benefited mood in fathers. This continues to be an area of research interest due to the significant risk of poor mental health and PPD in the immediate postpartum for fathers and male caregivers. Additionally, there was a large attrition rate (Figure 2), where fathers seemed less motivated than mothers to participate in a nutrition and mood study. Further research should explore engagement of fathers in nutritional interventions and paternal mental health, to evaluate whether certain study methodologies (e.g., online testing) or other factors around motivation for involvement or completion of the research affect engagement. Mitchell et al. (80) stress the difficulty in recruiting and retaining fathers in research, highlighting that high attrition rates in paternal research is common. Under engagement in research has also been associated with lower socio-economic status (81). Our father's sample had a slightly lower household income than UK average at £32,300 (82) and compared to our mothers sample, which may partly explain the lack of engagement in the trial. Additionally, majority of fathers (n = 18) did not have a partner also taking part in the trial which may have resulted in a lack of shared motivation and social support which perhaps could be a barrier for fathers engaging in nutritional interventions. Another major barrier to paternal research is recruitment, which is primarily conducted via their partners (83). Recent research suggests that studies seeking to recruit fathers for parenting research should prioritize using advertisements specifically tailored to fathers, claiming that 79% of participants recruited through father-targeted advertisements were male, while only 14% of participants recruited through advertisements targeting parents in general were male (84). Collectively, this highlights a growing need to explore paternal involvement with research generally and applying these methods to nutritional research. In addition, the structure of families within the UK is as varied as it has ever been, with only 44% of families in a traditional nuclear structure of a biological mother, father and child(ren) cohabiting (85), therefore is important to acknowledge that prevalences in parental mental health may vary depending on changing family structures. Furthermore, same sex couples have been found to have higher rates of postnatal depression than heterosexual couples (86) with research suggesting perinatal mental health may be poorer in Lesbian, Gay, Bisexual, Transgender, Queer or Questioning, Two spirit and additional sexual orientations and gender identity (LGBS+) couples (87). Exploring the mental health of parents and caregivers inside and outside the heteronormative realm is critical to supporting and treating PPD at population level.

In conclusion, this study provides evidence for the benefits of a dietary intervention for mood and mental health to mothers in the immediate postpartum. Specifically, this study demonstrated improvements in postpartum depression symptoms and positive affect following a 2-week flavonoid-rich diet in the 0–6-month postpartum. The research also observed that fathers in this critical period lacked adequate engagement, highlighting a need for further research into diet and paternal mental health.