The Mediterranean diet (MeDiet) is a dietary pattern inspired by the traditional eating habits of countries bordering the Mediterranean Sea, such as Greece, Italy, and Spain (1). This dietary pattern was first coined by Ancel Keys back in 1960 (2). It is characterized by an abundance of fruits, vegetables, whole grains, legumes, and nuts, with olive oil as a primary source of fat. Fish and poultry are consumed in moderate amounts, while red meat is limited (1, 3, 4). The diet is known for its potential health benefits, including reducing the risk of cardiovascular disease (5, 6), diabetes (7, 8), cancers (9, 10), and overall mortality (11), promoting weight management and lowering the risk of metabolic syndrome (12, 13), and providing essential nutrients.

The Mediterranean diet is also said to offer a promising and relatively straightforward approach to mitigating the severity of COVID-19 infection (14). R. Perez-Araluce et al. revealed that individuals demonstrating moderate to high adherence to the Mediterranean diet experienced a significantly reduced likelihood of contracting COVID-19 (15). Notably, observational studies have emphasized a correlation between adherence to the Mediterranean diet and improved outcomes in individuals with COVID-19 (such as mortality and recovery rate), as well as a reduced risk of COVID-19 infection across various populations (16, 17). It is also recommended as a useful nutritional approach for patients with post-COVID-19 syndrome (18).

Moreover, MeDiet is associated with the improvement of female infertility, decreasing the risk of developing pregnancy-associated complications (19). Published evidence also revealed MeDiet’s role in assisted reproduction. A previous cohort study investigated the Mediterranean diet’s effect on in vitro fertilization (IVF) outcomes and it turned out that the higher MeDiet adherence group showed more embryos available (8.40 ± 5.26 vs. 7.40 ± 4.71, p = 0.028) while the pregnancy rate and implantation rate was similar (20). However, another study showed that women with higher Mediet scores had significantly higher clinical pregnancy rates (50.0% vs. 29.1%, p = 0.01) and live birth rates (48.8% vs. 26.6%, p = 0.01) (21). Conversely, an Italian study finds that the Mediterranean diet score was not significantly associated with IVF outcomes (22). A recent meta-analysis including 11 studies also concludes that insufficient current evidence exists to support the clinical application of high adherence to the Mediterranean diet and fertility markers (23). More evidence of well-designed clinical studies is needed to prove the comprehensive role of Mediet in IVF outcomes.

Current evidence demonstrates that COVID-19 infection impairs reproductive function and leads to infertility as well as unsuccess in IVF treatment (24). In a small-sample observational study, a reduction in the proportion of top-quality embryos was observed in women post-COVID-19 infection (25). Additionally, a slight decrease in the blastocyst (day 5 or later embryo) formation rate was recorded in the case group (26). Based on these findings MeDiet is helpful in many pathological situations of COVID-19 infection and is still controversial in IVF outcomes. Thus, our study tries to investigate the effect of MeDiet on IVF outcomes in women after COVID-19 infection.

A total of 635 participants were recruited for the study, while 10 of them refused to fulfill the questionnaire and 10 of them did not meet the inclusive criteria, and the remaining 615 were enrolled in our study. However, 5 of them accepted oocyte cryopreserve due to personal reasons. 3 participants conceived spontaneously and 2 participants did not start the IVF treatment at the end of the study, leaving 605 participants to finish the ART procedure and their data was analyzed (Figure 1). Table 1 shows the 14 questions of the questionnaire and the answer distribution. The MeDiet score distribution is shown in Figure 2A. We divided our participants into three groups according to the MeDiet score: low adherence [MeDiet score 0–4 (n = 191)], medium adherence [MeDiet score 5–7 (n = 326)] and high adherence [MeDiet score 8–14 (n = 88)]. A majority of participants followed low to moderate MeDiet, and only a small group of them followed high MeDiet. There is no difference in demographic information such as age, infertility type and reason, body mass index, waist-to-hip ratio, blood pressure, etc. among these groups. Besides, we also evaluated some serological markers to evaluate the metabolic status of our participants. No difference was found in fasting glucose, insulin level, HOMA-IR (homeostatic model assessment of insulin resistance) as well as thyroid function among groups. Similarly, no differences were found in blood cells and coagulation indicators in our participants (Table 2).

Ovarian stimulation protocol differs among the groups with less gonadotropin-releasing hormone agonist (GnRH-a) protocol while more uncommon protocols (such as mild stimulation, letrozole protocol, etc.) are administrated in the higher MeDiet score group (p = 0.019). Nevertheless, there is no difference in sex hormones such as estradiol (E2), progesterone (P), and luteinizing hormone (LH) on human chorionic gonadotropin (hCG) day. The follicle number on hCG day, yield oocytes (including metaphase II (MII), metaphase I (MI), germinal vesicle (GV), degenerated oocytes), 2 pronucleus (2PN) zygotes, fertilization rate, day three embryos (cleavage embryos) and embryo quality are comparable among the three groups. Interestingly, there is an obvious difference in that the blastocyst formation rate is significantly higher in MeDiet scored 8–14 points women (46.08%), compared to the other two groups (which is 41.75 and 40.07% respectively) (p = 0.044). Further multivariate regression analysis also shows the positive relationship between MeDiet and blastocyst formation [adjusted β:0.077, 95% confidential interval: (0.028, 0.313), p = 0.019]. Moreover, age is negatively related to blastocyst formation [adjusted β: −0.099, 95% confidential interval: (−0.098, −0.018), p = 0.005] (Supplementary Table S1). However, there is no difference in blastocyst quality (Table 3).

There were only 157 women out of 605 participants received fresh embryo transfer. The reasons for other participants’ embryo transfer cancellation are as follows: 1. Ovarian hyperstimulation syndrome (n = 15), 2. No oocytes retrieved (n = 5), 3. No transferrable embryo (n = 35), 4. Desynchronization between the endometrium and the embryo (n = 165), 5. Peri-implantation genetic test (n = 169), 6. Personal reasons (n = 59). The follow-up in this study ends 3 months after the embryo transfer. For those who received embryo transfer, we noticed an obvious trend that with the higher MeDiet score, the higher clinical pregnancy rate (62.37% vs. 76.09% vs. 81.25%, p = 0.197), implantation rate (55.84% vs. 66.44% vs. 69.23%, p = 0.240) and ongoing pregnancy rate (61.22% vs. 75.00% vs. 81.25%, p = 0.152) even though the p values are not significant (Table 4; Figure 2B). Only two miscarriage cases and no ectopic pregnancy cases were observed in the study population.

To further validate the age risk for blastocyst formation, we divided our participants into two age groups: women with age less than 35 years old and those beyond 35 years old. Surprisingly, we observed a similar result in women under 35 that the blastocyst formation rate is higher in the highest MeDiet appliance women(47.03%), compared to low (40.23%) and moderate (39.74%) appliance women (p = 0.033). No difference is observed in pregnancy outcomes (Supplementary Table S2). However, there is no difference in oocyte and embryo quality as well as pregnancy outcomes in women with age above 35 years old (Supplementary Table S3).

In the present study, we noticed that women after COVID-19 infection with high adherence to MeDiet obtained a higher blastocyst formation rate. Meanwhile, MeDiet might play a favorable role in pregnancy outcomes such as clinical pregnancy, implantation as well as ongoing pregnancy. However, there is no relationship between MeDiet and the quantity and grade of oocyte and embryo.

MeDiet is characterized by the incorporation of predominantly plant-based nutritional elements, including fruits, vegetables, legumes, nuts, and olive oil. These items serve as notable reservoirs of bioactive polyphenols. Polyphenols, specifically flavonoids and their derivatives, exhibit diverse health-promoting effects, particularly in cardiovascular and metabolic conditions, owing to their antioxidant, anti-inflammatory, anti-thrombotic attributes, and immunomodulatory effects (31, 32). Accumulating evidence from prospective cohort observational studies suggests that the nutritional status of both the father and mother during the periconceptional period influences early fetal development and the perinatal and long-term health of the offspring (33). Recent studies gradually focus on dietary habits and early embryo development and pregnancy. In an observational study, it was shown that embryos from women reporting higher consumption of fruit and fish had an elevated likelihood of forming a blastocyst. Conversely, there was a decreased probability of blastocyst formation in those who consumed more red meat or were on a weight loss diet (34). Similarly, we found that women with high adherence to Mediet obtained more blastocyst formation in the present study, which is in accordance with the previous results. It is reported that a short MeDiet dietary supplementation alters the rate of embryo cleavage, indicating improved embryo quality (35). However, this study was limited to the cleavage embryos with no data on blastocyst formation.

Mediterranean diet showed a positive correlation with elevated levels of red blood cell folate and vitamin B6 in both blood and follicular fluid. Additionally, following this diet was associated with a reported 40% increase in the likelihood of achieving pregnancy (36). Vitamin B6 serves as a versatile coenzyme engaged in numerous biochemical pathways. Studies indicate that administering vitamin B6 to women experiencing subfertility enhances reproductive performance (37). Additionally, earlier research findings have indicated that the consumption of fruits and vegetables is linked to reduced oxidative stress and enhanced antioxidant status (38), while oxidative stress has been shown to cause defective embryo development in vitro (39). Optimal concentrations of antioxidants in oocytes are essential for regular fertilization and subsequent embryonic development during the preimplantation stage (40). It was demonstrated that women with a high intake of alpha-linolenic acid exhibited elevated baseline oestradiol levels, suggesting that increased intake of alpha-linolenic acid and docosahexaenoic acid may enhance embryo morphology (41). This results in accordance with another study which shows the relationship between fish consumption and the likelihood of blastocyst formation (34).

COVID-19 infection could cause a series of body defensive responses, which damages the reproductive process. One of the important mechanisms is associated with exaggerated immune responses like “cytokine storm” and intense inflammation. Excessive production of proinflammatory cytokines can modulate the cellular microenvironment in a way that impairs reproductive physiology (24). Another mechanism is oxidative stress and disturbed reproductive health. Massive reactive oxygen species (ROS) are produced at the subcellular level during the COVID-19 infection period, which is involved in the etiology of errant embryo implantation and development, ovulatory failure, and hyperandrogenism (42, 43). In patients with past COVID-19 infection, decreased pregnancy rates were observed after embryo transfer (44). In another study, a reduced proportion of top-quality embryos was observed (25). Based on those findings, we designed this study to evaluate whether MeDiet improves IVF outcomes in women with past COVID-19 infection. Unfortunately, we did not observe a significant difference in improvement in pregnancy outcomes, which is attributed to the small sample size of women who received embryo transfer. However, there is a trend that the clinical pregnancy rate, implantation rate, and ongoing pregnancy rate are higher along with higher adherence to MeDiet.

Participants in our study adhered to MeDiet at least 1 month before they started IVF treatment. However, it’s a short period. Whether a longer dietary history could prevent adverse effects of infections and/or promote a healthy pregnancy in general should be verified in further research. Moreover, it is a Western dietary pattern, indicating that it is possible that other healthy dietary patterns that suit Asia people could also be beneficial factors in this regard and needs to be determined.

To the best of our knowledge, the present study is the first to propose the influence of the Mediterranean diet on the IVF outcomes of post-COVID-19 females. However, limitations also exist in our study. On one hand, the sample size for women who received embryo transfer is relatively small, especially in women with high MeDiet adherence, which limited the efficacy of the results. On the other hand, a majority of participants in our study with low to moderate adherence to the MeDiet, leaving a minority of them with high adherence. The sample size difference might restrict the effectiveness of the test to some extent. However, the main reason underlying is that the dietary habits differ substantially from Asian to European, thus, only a small group of people could follow the MeDiet. More importantly, we cannot avoid the potential biases associated with self-reported dietary habits. Lastly, we did not follow up on the long-term outcomes such as live birth, or gestational complications. Further studies should give full consideration to sample size and long-term outcomes such as live birth rates and gestational complications.

High adherence to MeDiet is associated with improved blastocyst formation in women after COVID-19 infection. There is also a trend that high adherence to MeDiet might be beneficial to clinical pregnancy, embryo implantation as well as ongoing pregnancy in these women.

The original contributions presented in the study are included in the article/Supplementary material, further inquiries can be directed to the corresponding author.

The studies involving humans were approved by Ethics Committee of the Reproductive and Genetic Hospital of CITIC-Xiangya. The studies were conducted in accordance with the local legislation and institutional requirements. The participants provided their written informed consent to participate in this study.

HC: Conceptualization, Data curation, Formal analysis, Funding acquisition, Investigation, Methodology, Software, Validation, Visualization, Writing – original draft, Writing – review & editing. JW: Methodology, Formal analysis, Validation, Software, Writing – review & editing. HG: Data curation, Project administration, Writing – review & editing. QZ: Data curation, Project administration, Writing – review & editing. GL: Supervision, Writing – review & editing. BH: Writing – review & editing. PK: Writing – review & editing. ZW: Validation, Writing – review & editing. FG: Methodology, Supervision, Validation, Writing – review & editing.