IVF-ET is supposed to help infertile couples to achieve pregnancy (1). With the breakthrough of novel technologies in ovulation induction protocols and laboratory, the successful rate in IVF has progressively risen (2). However, there is still a considerable number of infertile couples who cannot conceive through IVF. Improving the quantity and quality of embryos is a crucial factor contributing to the success of IVF-assisted reproduction (3–5). Great efforts have been made to discover adjuvant therapy or supplementation attempting to get ideal IVF outcomes (6, 7).

As a supplementation in IVF, inositol has attracted increasing attention. It is a compound that occurs naturally in many foods and is an essential component of the vitamin B group (8). Studies have shown a correlation between the level of inositol in follicular fluid and the quality of oocytes (9). It was also confirmed that women who achieved pregnancy through in vitro fertilization and embryo transfer had higher levels of inositol in their follicular fluid than non-pregnant women (10). MI, one of the nine different forms of inositol, can be converted into inositolphosphoglycan within the human body, which acts as a secondary messenger involved in insulin signal transduction, primarily regulating the activation of glucose transporters and glucose utilization (11, 12). Therefore, its function as an insulin sensitizer is used to treat PCOS of which insulin resistance is one of the typical symptoms (13–16). Although researches conducted to investigate the effectiveness of inositol on PCOS women attending intracytoplasmic sperm injection (ICSI) programs indicated no significant improvement on the quality of oocyte or embryo or pregnancy rates, the combination use of d-chiro-inositol (DCI), another form of inositol, left the conclusion undetermined, because DCI was reported to act differently with MI may even have a negative effect on oocyte quality (17, 18). Moreover, MI is also utilized in non-PCOS women in IVF, including normal responders and poor responders (19, 20), since it plays a crucial role in cell growth and follicle-stimulating hormone (FSH)signal transduction (21, 22), which is associated with oocyte maturation and embryo development.

At present, the data regarding the therapeutic efficacy of MI in infertile women undergoing IVF is still rather unclear. Thus, this systematic review and meta-analysis aims to systematically review and summarize the evidence examining the impact of MI on IVF outcomes to further optimize clinical treatment strategy.

Increasing evidence has emerged to support the use of inositol in human reproduction, particularly for women with PCOS, but its effectiveness on IVF outcomes remains uncertain. This meta-analysis found that pre-treatment with MI may improve the MII oocyte rate in women undergoing IVF. This improvement remained in the PCOS subgroup as well as the non-obese PCOS women, but not in the POR subgroup. In addition, the positive impact on fertilization rate was consistent among the PCOS, the non-obese PCOS and POR subgroups, although the cleavage rate, high-quality embryo rate, or clinical pregnancy did not improve significantly (34). The results of sensitivity analysis by excluding studies with the most extreme dose variation (MI 2g daily) indicated that the variable dose of MI did not unduly influence the results. The effect of myoinositol supplementation on IVF outcomes in PCOS women with insulin resistance was a predetermined outcome. However, it failed to be analyzed since the studies on PCOS either excluded patients with diabetes or hyperinsulinemia, or did not provide data on insulin resistance.

Studies have indicated a significant positive correlation between MI concentration in follicles and various reproductive factors such as estradiol level in follicular fluid, cleavage rate of fertilized oocytes, embryo stage (± 4 cells), and embryo quality (grade) (9). A higher concentration of inositol in human follicular cells serves as a biological indicator for the improvement of oocyte quality (9, 35). Inositol can exist within cells in a free form or as a binding component of phospholipids or inositol phosphate derivatives (such as inositol triphosphate, etc.) in the plasma membrane, where they are crucial for cell growth, insulin, and FSH activity (36). Inositol serves as a precursor of inositol phosphate, which is a key component of the phosphatidylinositol signal transduction system. This signal transduction system involves the hydrolysis of phosphatidyl-inositol bisphosphate-dependent receptors to generate two important second messengers: trisphosphate (InsP3) and diacylglycerol (DAG) (37). InsP3 diffuses into the cytoplasm and binds to inositol InsP3 receptors on the surface of the endoplasmic reticulum, leading to the release of intracellular calcium oscillations. DAG activates protein kinase C, which modulates various cellular processes such as gamete formation, fertilization, cell proliferation, and development by phosphorylating proteins in diverse cell types (38, 39). Inositol performs various functions at the ovarian level, particularly the role of InsP3 in regulating intracellular calcium concentrations in response to the effects of the luteinizing hormone and FSH (40, 41). The function of this molecule seems to be crucial in the maturation of oocytes (42).

As in our research, MI was reported to contribute to the quality and maturity of oocytes, the cleavage rate, blastocyst expansion, and embryo quality (43). Nevertheless, it has been also demonstrated that in women with PCOS, MI was insufficient to improve the MII, embryo quality or pregnancy rate in women with PCOS (17). In this study, any dose and duration of inositol pretreatment, either MI or DCI, were included. Currently, the data on the effects of DCI on the ovary is inconsistent. The usual ratio of MI/DCI in the follicular fluid is 100:1 but is altered to 0.2:1 in PCOS patients (16). A decrease in MI concentration may lead to excessive DCI due to epimerase overactivation, resulting in depleted MI levels and subsequent decline in embryo quality (11). The dosage, duration of supplementation, and the ratio of two types of inositol are likely influencing factors on its effects which warranted further evaluation by large-sample randomized controlled trial (RCT) studies. Meanwhile, there was evidence showing that increasing the dose of DCI gradually deteriorated oocyte quality and the total amount of r-FSH (18). Therefore, this meta-analysis excluded studies with interventions of DCI supplementation or combination with metformin to ensure the accuracy of the data. Only studies with MI supplementation were included.

MI may enhance oocyte responsiveness to intracellular calcium oscillations during the early stages of fertilization, thereby potentially improving fertilization rate and embryo quality (25). It plays a role in the release of cortical granules, inhibition of polyspermy, meiosis, and subsequent activation of the cell cycle (19). In a mouse model, elevated levels of MI lead to an increase in intracellular calcium oscillations and the end of meiosis (44). The exposure of fully grown mouse germinal vesicle oocytes to MI during in-vitro maturation can enhance meiotic maturation, and the subsequent developmental potential of these oocytes following fertilization. Colazingari G et al. found that MI supplemented embryos displayed a faster cleavage rate and by the end of preimplantation development, the majority of MI supplemented blastocysts was expanded and formed by a higher number of blastomeres (45). Mohammadi F et al. showed that MI could decrease Intracellular reactive oxygen species and increase glutathione and mitochondrial membrane potential levels and consequently prevent oocyte quality reduction and improve fertilization potential in mouse (46). In humans, it has been demonstrated that the incidence of gestational diabetes mellitus was significantly reduced in women supplemented with MI. Moreover the incidence of fetal macrosomia (birth weight > 4000 g), gestational hypertension, preterm delivery, caesarean section, shoulder dystocia, neonatal hypoglycemia, and neonatal transfer to an intensive care unit) did not reveal appreciable differences between the MI and the placebo‐exposed groups (47).

In the present study, the MII oocyte rate did not increase in women with POR, which may be attributed to the multifactorial nature of oocyte quality, encompassing nuclear and mitochondrial genomes, as well as the ovarian and follicle microenvironments influencing cytoplasmic maturation, with inositol being just one contributing factor (48).

Achieving clinical pregnancy not only relies on high-quality embryos but also requires an endometrium with good receptivity, normal immune and coagulation functions as well as consideration for the psychological state of the infertile women. Improving clinical pregnancy rates demands multiple intervention measures rather than relying solely on one method since any abnormality at any stage may result in failed embryo implantation.

Overall, data from this systematic review should be interpreted with caution because of the limitations. Firstly, the robustness of the results depends largely on the quality of the primary studies included in this review. Inclusion of both RCTs and observational studies might introduce methodological heterogeneity in some instances. Meanwhile, adverse events were not reported by the majority of studies. Secondly, substantial disparities in patient selection and the variability in sample sizes decreased the certainty of the evidence overall as women with PCOS and POR accounted for a large proportion of participants in this meta-analysis. Significant statistical heterogeneity (I² >50%) in outcomes, might invalidate fixed-effect models, necessitating cautious interpretation of random-effects results. Consequently, the data derived from this systematic analysis should be interpreted with caution until further high-quality evidence becomes available. Future studies should standardize outcome reporting and prioritize individual participant data meta-analyses to address these gaps.

In conclusion, this meta-analysis demonstrates that MI supplementation improves the MII oocyte rate and the fertilization rate for women undergoing IVF. The uncertainty of the results should be considered with individual preferences when making clinical decisions. Further, clinical research and extensive multicenter randomized controlled trials needed to be conducted to assess its supplementation mode and better understand the working mechanism, establishing a more robust evidence base for clinical practice.