Recent progress in metabolomics for analyzing common infertility conditions that affect ovarian function

This review conducted a comprehensive narrative analysis of the targeted literature focusing on metabolomics in endometriosis. Electronic databases, PubMed ( https://pubmed.ncbi.nlm.nih.gov/ ) and Google Scholar ( https://scholar.google.jp/ ), were searched for literature published up to May 1, 2024, using a combination of the following keywords: “metabolomics,” “infertility,” “follicular fluid,” “glycolysis,” “oxidative phosphorylation,” “lipid metabolism,” and “β‐oxidation.” The search terms were combined using Boolean operators, as outlined in Table  1 . Additionally, a manual search of reference lists from published articles was performed. The studies included in this review were original research publications in English, along with reference lists from relevant review articles. Studies that were duplicated, irrelevant to the research topic, or published in non‐English languages were excluded. The study selection process is detailed in the flowchart depicted in Figure  1 , which outlines the inclusion and exclusion criteria. The initial phase involved identifying records through electronic and manual searches. Titles and abstracts were subjected to preliminary screening, and after removing duplicates, non‐relevant studies were discarded. In the final phase of eligibility, full‐text articles were analyzed, and any from which detailed data could not be obtained were excluded. The authors independently evaluated the articles to determine their suitability for inclusion before reviewing the full texts. The keyword and search term combinations. Flowchart depicting the study selection process.

Metabolomics, the study of functions and interactions of various metabolites such as amino acids, nucleotides, carbohydrates, lipids, and others, offers profound insights into the dynamic phenomena of reproductive processes. 5 , 10 , 32 , 33 , 34 Comprehensive untargeted analysis of the human metabolome enables the examination of approximately 3000 metabolites. 5 The first metabolomics investigation of FF for biomarkers of oocyte quality was reported in 2007. 35 FF contains hormones, proteins, amino acids, enzymes, and fatty acids that influence the metabolism, development, and physiology of the oocyte. 10 , 16 , 36 , 37 , 38 , 39 Recent metabolomic methodologies involving the analysis of FF metabolites may enable the identification of infertility causes and the discovery of novel non‐invasive biomarkers. 3 , 17 , 39 , 40 , 41 , 42 , 43 , 44 , 45 Furthermore, extracellular vesicles (EVs), such as exosomes, which carry RNAs, proteins, lipids, and amino acids, are crucial mediators of intercellular communication between various cell types (e.g., cumulus cells and oocytes), thereby influencing age‐related pathological conditions. 36 , 46 , 47 We summarize current knowledge regarding metabolite alterations and evaluate whether these changes vary depending on the underlying condition. Table  2 provides a summary of the references utilized in the construction of Figures  2 , 3 , 4 , 5 . This table includes the citations corresponding to each subsection from Sections  3.2.1 , 3.2.5 . Summary of the references cited for creating Figure  2 . Among original articles, clinical research is highlighted with a yellow background, in vitro studies with green, and review articles with blue. Dynamics of energy pathways in older women as determined by metabolomic analysis. The yellow, orange, green, blue, and gray boxes symbolize glycolysis, oxidative phosphorylation, lipid metabolism, ketogenesis, and nucleic acid metabolism, respectively. The metabolic‐level changes are represented by orange arrows. In Figures  2 , 3 , 4 , 5 , the yellow box indicates glycolysis, the orange box signifies oxidative phosphorylation, the green box denotes lipid metabolism, the gray box corresponds to purine metabolism, and the blue box represents ketone body synthesis. Oocyte quality is known to decline with advancing age. Researchers have explored the relationship between age and reproductive function by examining differences in metabolites within the FF of patients of varying ages undergoing IVF and ET. 48 In Figure  2 , the orange upward and downward arrows indicate age‐related changes in metabolite levels. Patients were divided into younger and older groups, with advanced age typically defined as 35 or 40 years or older. Compared to younger women under 35 with normal ovarian reserves, older women and those with DOR exhibit lower glucose and higher lactate concentrations in their FF. 49 Similar results were observed in women over 40 years of age. 50 This may reflect an age‐related increase in the expression of glycolytic enzymes, such as phosphofructokinase 51 and LDH. 52 Phosphofructokinase was also abundant in bovine cumulus cells. 53 Excessively low glucose levels in FF negatively impact oocyte maturation, necessitating enhanced glycogenolysis and the compensatory conversion of α‐lactose and d‐maltose into glucose. 36 Indeed, analysis of exosomal metabolites in FF revealed higher levels of lactose and maltose in the older group compared to the younger group. 36 Cumulus cells in women of advanced reproductive age must supply energy to the oocyte through further heightened glycolytic activity and anaerobic metabolism. 6 , 54 However, the availability of nutrients like pyruvate in the TCA cycle diminishes in the oocytes of older women due to mitochondrial dysfunction and impaired DNA repair mechanisms, leading to lactate accumulation. The increase in lactate levels results in a lowered pH of the FF, further impairing the quality of cumulus cells and oocytes. 50 Thus, glycolysis and mitochondrial oxidative phosphorylation might be compromised in the cumulus cells and oocytes of older women. To counteract follicular stressors such as reduced pH, hypoxia, and elevated reactive oxygen species (ROS), and to meet the oocytes' demands in aging individuals, a compensatory upregulation of oxidative phosphorylation is necessary to boost ATP production. 6 Additionally, aging is known to affect nucleic acid synthesis and metabolism. Higher levels of cytidine and inosine have been noted in older women compared to younger women, potentially contributing to enhanced nucleic acid synthesis via the upregulation of glycolysis and the pentose phosphate pathway (PPP) (Figure  2 , gray square). 36 The elevated levels of cytidine and inosine may indicate an upregulation of nucleic acid synthesis, alongside enhanced phosphoinositide signaling and lipid synthesis, likely as a compensatory mechanism. 36 Furthermore, it has been reported that the dimethylarginine/arginine ratio is significantly decreased in the FF of older women with DOR (Figure  2 , gray square). 26 , 36 Dimethylarginine functions as an inhibitor of nitric oxide synthase (NOS), thereby suppressing the production of nitric oxide, which is crucial for vasodilation and blood flow regulation. Thus, a decrease in dimethylarginine levels may suggest a loss of compensatory reserve, including anti‐inflammatory and vasodilatory effects. Next, age‐related changes influence not only metabolites related to glycolysis, the PPP, and mitochondrial oxidative phosphorylation but also alter the lipid profile of FF. 48 Women under 35 exhibit higher levels of lipids in FF or extracellular vesicles (EVs) compared to older women. 15 Research indicates a decline in polyunsaturated choline plasmalogen, a complex lipid with antioxidant properties, with advancing age. 48 This reduction is accompanied by a compensatory increase in lipid metabolism in women over 35, including glycosphingolipid, phosphatidylinositol phosphate, and glycerophospholipid metabolism, 6 , 55 as well as arachidonic and oleic acids (Figure  2 , green square). 48 Polyunsaturated choline plasmalogen has antioxidant properties and serves as a reservoir for polyunsaturated fatty acids, such as arachidonic acid and docosahexaenoic acid (DHA). 56 Consequently, the diminished antioxidant capacity of plasmalogen leads to the accumulation of oxidized substances due to oxidative stress. Additionally, excess arachidonic acid heightens vascular permeability through the production of prostaglandins and leukotrienes, triggering inflammatory responses. Collectively, lower lipogenesis and higher lipolysis in older women may alter the availability of lipid metabolites, leading to decreased plasmalogen levels and increased unsaturated fatty acid concentrations, potentially impairing oocyte maturation. 14 , 57 Meanwhile, oleic acid has been reported to positively impact bovine oocyte maturation and subsequent embryo development, mitigating the harmful effects of saturated fatty acids such as palmitic and stearic acids. 36 , 58 Increased concentrations of oleic acid in the FF of older women may protect oocytes from damage due to reduced lipid storage. 36 , 59 Furthermore, an increase in trimethylamine N‐oxide (TMAO) levels and a concomitant decrease in acetoacetate levels have been observed in patients of advanced age (Figure  2 , green square). 50 TMAO is produced by the gut microbiota. 60 Phosphatidylcholine (PC) undergoes metabolism to produce trimethylamine, which is subsequently converted to TMAO. Elevated TMAO levels suggest a reduced conversion of fatty acids to acyl‐CoA and a concomitant inhibition of β‐oxidation. Moreover, a decline in β‐oxidation results in lower acetoacetate levels due to the suppression of ketone synthesis (Figure  2 , blue square). Diminished β‐oxidation leads to a reduction in ATP availability for oocytes, resulting in compromised oocyte quality and meiotic arrest. 61 , 62 Thus, increased levels of TMAO and reduced levels of acetoacetate could potentially serve as indicators of oocyte quality in older women. Collectively, the lack of balance and coordination among glycolysis, oxidative phosphorylation, lipid metabolism, β‐oxidation, and nucleic acid synthesis with aging may contribute to the low success rates of IVF treatment. Fat burning could be accelerated in older women. However, the extent of glycolysis and lipid metabolism varies significantly depending on whether compensatory mechanisms remain intact or have been compromised. It is believed that factors influencing compensatory mechanisms include age‐related alterations in the expression of genes involved in various energy metabolism pathways, although the specifics remain unclear. The yellow arrows in Figure  3 highlight the metabolites that are altered in patients with endometriosis. Consistent findings have been reported across multiple studies regarding glycolysis (e.g., glucose, pyruvate, and lactate) and fatty acid metabolism (e.g., glycerophospholipids and fatty acids) in endometriosis. 10 , 63 Patients with endometriosis exhibit decreased glucose levels and increased lactate and pyruvate concentrations in FF, indicative of enhanced anaerobic glycolysis. 64 , 65 The increased anaerobic glycolysis is associated with the Warburg effect, which mitigates oxidative stress by reducing oxidative phosphorylation and lipid peroxidation. 10 , 66 Moreover, phospholipid concentrations in the FF of endometriosis patients were elevated compared to controls, 65 a finding that contrasts with observations in older women. Specifically, the metabolic profile in FF from women with endometriosis includes increased levels of glycerophospholipids, phosphatidyl serine (PS), 10 PC, 20 sphingolipids, 20 , 67 lysophosphatidylcholine (LPC), 10 , 68 and phosphatidylinositol‐4,5‐bisphosphate. 20 For instance, the increase in LPC levels was particularly notable in large follicles compared to small follicles. 69 The accumulation of phospholipids in endometriosis may be linked to the induction of oxidative stress and inflammatory processes. 2 , 70 Two significant classes of complex phospholipids, PC and sphingomyelin (SM), are also involved in cell proliferation, anti‐apoptosis, and processes of denervation and re‐innervation. 20 , 71 , 72 Phosphatidylinositol diphosphate (PI diphosphate) generates the production of two second messenger molecules, inositol 1,4,5‐triphosphate (IP3) and diacylglycerol (DAG). 20 Additionally, increased PI and decreased lysophosphatidylinositol (LPI) have been identified in follicles of patients with endometriosis‐associated infertility. 63 Indeed, the proper balance of PI and LPI correlates with the number of retrieved and mature oocytes. 63 Since LPI inhibits apoptosis and promotes cell proliferation, a decrease in LPI may lead to cell death. 63 Given that PI is hydrolyzed by phospholipase A2 to generate LPI, phospholipase A2 expression may be reduced in endometriosis. 63 However, phospholipase A2 activity is known to be elevated in peritoneal fluid cells of women with endometriosis. 73 The decreased LPI levels may reflect a breakdown in compensatory mechanisms. Dynamics of energy pathways in patients with endometriosis as determined by metabolomic analysis. The yellow, orange, green, blue, and gray boxes symbolize glycolysis, oxidative phosphorylation, lipid metabolism, ketogenesis, and nucleic acid metabolism, respectively. The metabolic‐level changes are represented by yellow arrows. Furthermore, lipid, glycerol, and ketone body concentrations were elevated in women with deep infiltrating endometriosis (DIE) compared to control participants. 74 These concentrations were further elevated in DIE patients with ovarian endometrioma. 74 Increased glycerol levels lead to the activation of glycolysis via recruitment of the intermediate dihydroxyacetone phosphate (DHAP). Additionally, ketone synthesis is promoted by fatty acid degradation and β‐oxidation, resulting in elevated levels of β‐hydroxybutyrate. β‐Hydroxybutyrate also enhances antioxidant capacity by inducing the expression of manganese superoxide dismutase and catalase. 75 However, sustained upregulation of energy metabolism through fatty acid β‐oxidation may lead to a decrease in certain phospholipid species over time. 10 Since mitochondria are involved in pyruvate oxidation, the TCA cycle, oxidative phosphorylation, fatty acid oxidation, and ketogenesis, compensatory mitochondrial function may result in increased oxidative phosphorylation, fatty acid synthesis, and β‐oxidation, but decreased ketogenesis. Indeed, some women with endometriosis experience reduced levels of acetate and β‐hydroxybutyrate. 10 , 76 Conversely, progressive mitochondrial dysfunction can lead to a diminished role of fatty acid β‐oxidation and an increased reliance on glycolysis in overall energy metabolism. 74 The shift between anaerobic glycolysis and lipid metabolism in different endometriosis phenotypes may depend on the degree of mitochondrial dysfunction. 74 PCOS is the most common endocrine and metabolic disorder affecting women of reproductive age and is associated with insulin resistance, even in the absence of overweight. 22 , 77 Metabolite levels were analyzed and contrasted between normal‐weight women with PCOS and those without the condition. The green arrows in Figure  4 identify the metabolites altered in PCOS patients. An elevated concentration of glucose and reduced concentrations of lactate and pyruvate have been observed in the FF of PCOS patients (yellow square). 41 , 64 , 78 This characteristic is attributed to altered metabolic pathways with decreased aerobic glycolysis in PCOS patients, 64 which contrasts with findings in older women and those with endometriosis. High glucose and low lactate levels in FF have been linked to impaired oocyte maturation and pregnancy failure. 17 , 41 Moreover, alterations in glycerolipid and glycerophospholipid metabolic pathways have been identified in PCOS patients. 24 , 79 The levels of plasmalogens containing oleic acid, arachidonic acid, choline, or ethanolamine, as well as other lipids such as PA, LysoPA, LysoPE, and LysoPC, were significantly decreased in FF from women with PCOS (green square). 79 , 80 Plasmalogens and their metabolites (e.g., prostaglandins, leukotrienes, eicosatrienoic acid, dihydroxyeicosatetraenoic acid, eicosatetraenoic acid, and lipotoxins) are known for their antioxidant potential and immunomodulatory effects. 80 , 81 Consequently, plasmalogens may play a role in the pathophysiological processes of hypoxia, inflammation, oxidative stress, immunomodulation, and ferroptosis in the granulosa cells of PCOS patients. 80 Indeed, decreased plasmalogen levels are associated with reduced oocyte quality. 80 DHAP serves as a precursor for plasmalogen biosynthesis. 80 Therefore, in PCOS patients, compromised glycolysis in granulosa cells may reduce the production of DHAP, subsequently impairing plasmalogen biosynthesis and resulting in lipid metabolism dysfunction. Additionally, PCOS patients are prone to glycolipid metabolism disorders, often linked to insulin resistance and dyslipidemia. 79 Furthermore, carnitine levels have been reported to be significantly lower in women with PCOS. 79 , 82 Carnitine facilitates the transport of long‐chain fatty acids into mitochondria, where they are converted into energy. 79 , 83 In granulosa cells of women with PCOS, reduced carnitine levels contribute to impaired β‐oxidation and mitochondrial oxidative phosphorylation via acetyl‐CoA, leading to an energy deficit. Nevertheless, significant discrepancies exist in the literature concerning the levels of lipid metabolites. 64 Dynamics of energy pathways in patients with PCOS as determined by metabolomic analysis. The yellow, orange, green, blue, and gray boxes symbolize glycolysis, oxidative phosphorylation, lipid metabolism, ketogenesis, and nucleic acid metabolism, respectively. The metabolic‐level changes are represented by green arrows. The black arrows in Figure  5 highlight the metabolites altered in obese women. Obesity leads to significant alterations in metabolites within the FF of women undergoing IVF, largely due to inflammation, oxidative stress, and disrupted lipid metabolism. 84 Previous research has demonstrated that elevated glucose levels in the FF of obese women have been observed, potentially impairing oocyte function and influencing fertilization and embryo development. 41 , 85 Furthermore, the expression of genes in oocytes that regulate inflammation and oxidative stress (e.g., upregulation of aminomalonate, 2‐ketoglucose dimethylacetal, indole‐3‐propionic acid, and isothreonic acid) and lipid metabolism (e.g., upregulation of uric acid and downregulation of carnitine) undergoes considerable changes. 84 , 86 , 87 The black and green arrows indicate similar metabolic trends (e.g., increased glucose, decreased lactate, decreased lipids, decreased carnitine, and increased purine degradation), suggesting that the metabolic patterns of obesity and PCOS are closely aligned. Dynamics of energy pathways in obese women as determined by metabolomic analysis. The yellow, orange, green, blue, and gray boxes symbolize glycolysis, oxidative phosphorylation, lipid metabolism, ketogenesis, and nucleic acid metabolism, respectively. The metabolic‐level changes are represented by black arrows. Several studies have compared the metabolic profiles of FF in patients with various clinical conditions associated with infertility, including endometriosis, PCOS, age‐related reduced ovarian reserve (AR‐ROR), reduced ovarian reserve (ROR), unexplained infertility (UI), and genetic infertility (GI). 88 Patients with endometriosis, AR‐ROR, ROR, and UI exhibited lower glucose and higher lactate levels, 88 indicating that in conditions other than PCOS, there is an elevation in glycolytic activity, accompanied by increased glucose consumption and lactate overproduction during oocyte maturation, leading to reduced mitochondrial oxidation. In comparisons between women who became pregnant following IVF and those who did not, the latter group showed lower lipid levels and altered lipid composition (e.g., PA, PE, PC, PI, glucosylceramides, and 1‐hydroxyvitamin D3 3‐D‐glucopyranoside). 2 These metabolites play roles in cell proliferation, inflammation, signal transduction, and apoptosis, and may contribute to reduced oocyte quality. 2 Additionally, the concentrations of hypoxanthine and xanthine were significantly elevated in the FF of the infertile group. 88 Hypoxanthine, a degradation product of ATP, is generated during energy metabolism and is further converted into uric acid (gray square). Studies have shown that increases in hypoxanthine and xanthine indicate ATP depletion. 89 Energy deficiency signifies a failure of compensatory mechanisms, ultimately leading to infertility and ART failure.

Infertility is a multifaceted condition influenced by various factors such as age, obesity, specific treatment modalities, and conditions like endometriosis and PCOS. Therefore, an ideal biomarker must be distinct and reliable, independent of confounding factors. The predicted metabolic pathways in women with several pathological conditions are visually represented in Figure  6 . Several metabolites related to energy metabolism in infertile patients are significantly affected across five pathways: glycolysis, oxidative phosphorylation, lipid metabolism and β‐oxidation, nucleic acid synthesis, and ketogenesis. Additionally, metabolomic profiling may potentially classify these patients into two categories: infertility associated with aging and endometriosis, and infertility related to PCOS and obesity. Anticipated metabolic pathways in women with infertility. The flame's size visually represents the energy level. First, glycolysis and lipid metabolic profiles differ between the aging and endometriosis group and the PCOS and obesity group. In the aging and endometriosis group, glycolysis and lipid metabolism are upregulated to compensate for mitochondrial dysfunction, whereas the opposite trend is observed in the PCOS and obesity group. 64 In the aging and endometriosis group, glycolysis and lipid metabolism synergize to enhance the FF environment and counteract adverse conditions such as inflammation, oxidative stress, and aging. Metabolomic analysis also revealed activation of nucleic acid synthesis and antioxidant signaling via the PPP. Conversely, in the PCOS and obesity group, lipid metabolism and β‐oxidation are often reduced due to diminished levels of fatty acids and carnitine. Although the glycolytic pathway supports the production of plasmalogens and fatty acids, further impairment in glycolysis and lipid metabolism could lead to irreversible damage to energy metabolism. Therefore, in the PCOS and obesity group, abnormal glycolysis and mitochondrial dysfunction may lead to a decreased number of high‐quality embryos. 90 Ultimately, in both groups, mitochondrial dysfunction results in compromised oocyte quality due to inadequate energy production. Collectively, in the aging and endometriosis group, there may be an activation of glycolysis and lipid metabolism to compensate for the dysregulation of mitochondrial oxidative phosphorylation. Conversely, in the PCOS and obesity group, irreversible changes in glycolysis and lipid metabolism could contribute to mitochondrial dysfunction. It is plausible that the compensatory mechanisms of cumulus cells and oocytes are integrated through the interconnection of five energy metabolic pathways centered around acetyl‐CoA. Second, fatty acids and glycolysis are intricately connected in cellular energy metabolism, adapting to the energy demands of the cell. In energy‐rich FF environments, glucose is metabolized through glycolysis, and fatty acids are stored, whereas during energy shortages, fatty acids are degraded to serve as fuel. In addition to their role as energy suppliers, fatty acids also participate in cellular signaling. 91 , 92 PA undergoes dephosphorylation to form DAG, an intermediate in glycerolipid metabolism that facilitates the activation of protein kinase C (PKC). DAG also plays a role in synthesizing prostaglandins, which are implicated in inflammatory responses, platelet aggregation, and several pathophysiological processes, including cell proliferation, oncogenesis, phagocytosis, and apoptosis. 91 , 93 PKC isotypes are crucial in various biological functions, including the resumption of meiosis in oocytes, spindle organization during meiosis, and chromosome movement. 91 , 93 Hence, the development and maturation of follicles, along with the quality of oocytes, may be reliant on energy metabolism and the quantity and composition of fatty acid metabolites within the follicular fluid. Third, the findings of this metabolomic study, in conjunction with genomic, transcriptomic, and proteomic analyses, offer a deeper understanding of the interplay among energy metabolism, follicular development and maturation, and oocyte quality. For instance, endometriotic cells have developed a range of adaptive strategies to endure the chronic conditions of hypoxia and nutrient deprivation. Mitochondria serve as pivotal regulators, integrating various physiological processes such as energy production, cellular redox homeostasis, mitochondrial dynamics, and apoptosis regulation. 94 To limit ROS generation, endometriotic cells favor glycolysis over oxidative phosphorylation and maintain mitochondrial quality control mechanisms through mitophagy and autophagy. 94 This could lead to compromised oocyte development, maturation, and quality due to the downregulation of oxidative phosphorylation. Conversely, in patients with PCOS, hyperandrogenism triggers altered gene expression associated with glycolysis, mitochondrial biogenesis, mitochondrial fission and fusion dynamics, and mitophagy, culminating in abnormal mitochondrial morphology and disruption of the electron transport chain. 95 Dysfunctional mitochondrial dynamics may undermine quality control mechanisms and impair the compensatory capacity of mitochondrial biogenesis, thereby further compromising mitochondrial function and diminishing oocyte quality. 95 Thus, the phenotype and severity of infertility dictate the shift in energy metabolism from adaptive to compensatory and eventually to pathological conditions. This shift finely adjusts the equilibrium among glycolysis, oxidative phosphorylation, fatty acid synthesis, and β‐oxidation. Glycolysis and lipid metabolism vary greatly depending on whether compensatory mechanisms are still maintained or have already been compromised. In other words, IVF failure may occur along with the deterioration of effective compensatory mechanisms of energy metabolism. Finally, we address current challenges and future research directions. It is often suggested that comprehensive metabolomics data analysis, combined with genomic, transcriptomic, and proteomic data, can help elucidate the mechanisms of IVF success or failure. 61 In recent years, advancements in AI analysis technology have significantly enhanced the interpretation of complex omics data. However, in clinical practice, there is a need for molecular targets that can be easily and inexpensively tested at any time and place. It is worth measuring glucose, pyruvate, lactate, and plasmalogen levels in FF. The dynamics of these parameters may reflect mitochondrial activity required for oocyte growth and maturation. However, it is still premature to determine whether these metabolic changes can serve as reliable indicators of oocyte quality and ART success in clinical practice. Currently, there is no evidence to suggest that altering metabolomic profiles can enhance reproductive outcomes. 17 Thus, while metabolomic analysis of FF may hold potential for predicting female reproductive outcomes, ideal biomarkers remain to be identified. Future research should urgently explore whether combinations of these biomarkers correlate with the number of oocytes retrieved, the quantity of mature oocytes (MII), and the generation of high‐quality embryos.

Infertility affects approximately one in seven couples of reproductive age worldwide. 1 , 2 Over the past two decades, there has been a notable increase in the use of assisted reproductive technology (ART). 3 An analysis of a registry database from 2007 to 2015 in Japan revealed that the live birth rate per embryo transfer (ET) was 15.7%. 4 Further analysis of the ART registry for 2015 demonstrated that 1 in 19.7 neonates was born through ART in Japan. 4 In the United Kingdom, the overall pregnancy and live birth rates per ET in 2014 were 36.3% and 26.5%, respectively. 5 In developed nations, a substantial number of older women are opting for in vitro fertilization (IVF), a trend that contributes to declining birth rates. 3 , 4 It is well established that infertility rates are higher among older women, with advanced age being a primary risk factor for diminished female reproductive capacity due to a decline in oocyte quantity and quality, which is associated with increased infertility. 6 A fundamental aspect of the aging process is the increase in chromosomal segregation errors during meiotic divisions, coupled with metabolic imbalances caused by mitochondrial damage, as well as alterations in mitochondrial DNA (mtDNA) copy number and mutations. 6 , 7 Besides aging, various complications and environmental factors, such as endometriosis, polycystic ovary syndrome (PCOS), obesity, and smoking, negatively impact fertility. 8 Extensive research has been conducted to improve the success rates of fertility treatments. 5 Previous studies have suggested that several factors, including embryo score, treatment history, total dose of FSH, infertility cause, female age, height, and endometrial thickness, are associated with live birth rates following assisted reproduction. 5 A significant body of basic research, focusing on genomic, epigenomic, transcriptomic, and proteomic studies, has aimed to identify biomarkers for oocyte and embryo quality. 9 , 10 These studies have identified several key genes and proteins. 11 Communication between the oocyte and its surrounding cumulus cells is crucial for supplying the energy required for oocyte meiotic maturation. 11 Comparative proteomic analyses have revealed that an imbalance between energy requirements and supply can lead to increased oxidative stress, mitochondrial damage, and reduced energy production, which can have adverse effects on oocytes. 12 Disruptions in processes related to mitochondrial oxidative phosphorylation and energy metabolism have also been observed in the context of maternal aging and infertility due to endometriosis or PCOS, which may further impair oocyte quality. 13 Understanding changes in gene and protein expression is crucial for monitoring molecular pathways and cellular activities, which may elucidate the fundamental mechanisms underlying infertility. 6 The rise of novel “omics” fields, including metabolomics and lipidomics, is aiding in the discovery of new biomarkers for endometriosis or infertility. 10 For example, follicular fluid (FF) and extracellular vesicles (EVs) from patients undergoing ART are abundant in low‐molecular‐weight metabolites that play a crucial role in oocyte maturation. Omics approaches that comprehensively investigate the abundance and composition of these metabolites have become an effective tool for investigating potential markers of oocyte quality and developmental competence, and successful fertilization. 14 , 15 Metabolomics offers the advantage of detecting the ever‐changing biological phenomena due to the relatively small number of metabolites and has therefore become prevalent in the field of reproductive medicine. 5 , 16 Metabolomic analysis is increasingly recognized as one of the valuable integrated multi‐omics strategies for assessing the precision of transcriptomics and proteomics studies. 17 , 18 Researchers have initiated studies to identify alterations in the expression of particular metabolites, including lipids and amino acids, within the FF of patients struggling with infertility. 19 To date, it has been found that the amount and composition of metabolites in FFs collected from patients with various infertility disorders, such as endometriosis, 20 , 21 PCOS, 22 , 23 , 24 , 25 and diminished ovarian reserve (DOR), 26 differ from those in healthy controls. 19 Additionally, various metabolites linked to oocyte quality are proposed as potential biomarkers for ART success, including successful pregnancy, implantation, and delivery. 19 , 27 , 28 , 29 , 30 , 31 However, only a limited number of studies have comprehensively compared patients with different infertility conditions and metabolic pathways. This review explores the latest data on metabolomic analysis of FF in various clinical conditions leading to infertility, including aging, endometriosis, PCOS, and obesity. In particular, we concentrate on the comprehensive landscape of energy metabolism, follicular development and maturation, as well as oocyte quality, and suggest potential future avenues of research.

The authors declare no conflict of interest.