One-carbon metabolism (OCM) supplies one-carbon (1C) units necessary for DNA, proteins, and lipids biosynthesis, as well as for the epigenetic modification of the genome. It plays a critical role in cell proliferation and gene expression, particularly in cell growth process such as embryonic development and malignant tumors (1–3). The prenatal period is characterized by rapid cellular proliferation and differentiation, and epigenetic marks established early in life can be maintained and exert lasting impacts on gene expression and functions later in life. Extensive research has explored the impact of maternal OCM during pregnancy on early development and fetal programming. It has been demonstrated that imbalances in prenatal 1C nutrients (deficiency/excess) can impede fetal growth, neurodevelopment, and cardiometabolic parameters in childhood (4). Furthermore, they have been associated with childhood allergies (5–7).

OCM encompasses the folate cycle and the methionine cycle, involving various nutrients, such as B vitamins, amino acids, minerals and carbohydrates. Among these, folate, choline, betaine and methionine serve as methyl donors; while vitamin B₂, B₆, B₁₂ act as coenzymes, and S-adenosylmethionine (SAM) and homocysteine (Hcy) serve as functional biomarkers of OCM (2). These nutrients and their metabolites interact in a complex manner. In fact, despite the supplementation of folic acid during periconception has been recommended worldwide since the 1990s (8), studies have shown that folate deficiency still existed (9–11). This emphasizes the necessity not only for folate but also for a comprehensive approach to OCM.

The interplay among OCM nutrients holds significance. Animal studies reported that methyl donors regulated metabolism, immune response, and epigenetic events through interactions within the OCM (12). A review suggested that reduced folate levels were associated with enhanced choline oxidation pathway activity during pregnancy; the roles of nutrients such as folate and cobalamin were closely linked and their imbalance has been associated with an increased risk of adverse pregnancy outcomes (13). Plasma total homocysteine (tHcy), an intermediate product of methionine metabolism, might serve as an independent risk factor for cognitive decline, irrespective of B-vitamin deficiency (14). The Reus-Tarragona Birth Cohort revealed that during 24–27 gestational weeks, both plasma folate and betaine exhibited an inverse association with plasma tHcy. Furthermore, the association between betaine and tHcy was dependent on folate status. Another study indicated that blood betaine showed an inverse association with tHcy at labor, irrespective of folate status (15). The precise interactions among OCM indexes remained unclear.

Moreover, while numerous studies have concentrated on dietary intake of OCM nutrients and their effects, data on circulating OCM nutrient/metabolite status and their interactions remain limited. In fact, the levels of blood OCM indices result from an interaction between dietary intake and genetic factors, offering a precise reflection of the OCM status. Perturbations in OCM might establish a significant link between early environmental exposure and later-life offspring development (2, 16). Studies have revealed imbalances in blood OCM during pregnancy, yet uncertainties persist regarding their cutoff values and associated factors (17–19). A case–control study found that pregnant women who supplemented folic acid throughout pregnancy had higher serum folate concentrations compared to those who took supplementation during short periods (20). Maternal serum tHcy was positively associated with maternal age (21), and negatively with pregnancy-associated plasma protein A (22).

Therefore, our study aimed to examine the levels of blood OCM indices among Chinese pregnant women to delineate their distribution, interactions, and associated factors.

OCM plays a critical role in fetal growth and neurodevelopment by transferring methyl group to biomolecules. This process involves folate, methionine, choline and betaine acting as methyl donor nutrients, vitamin B₂, B₆, B₁₂ acting as enzyme cofactors; and homocysteine and SAM, serving as functional intermediate metabolites. In the study, OCM nutrients and metabolites levels were measured in 397 Chinese women in mid-to-late pregnancy. The results revealed serum deficiencies of folate, vitamin B₆, and B₁₂ at 2.5, 1.3, and 8.3% respectively, with a concurrent incidence of 1.8% hyperhomocysteinemia. Within the folate cycle, RBC folate demonstrated a positive correlation with the cofactor vitamin B₁₂. Regarding functional markers of OCM, the study identified serum and RBC folate and serum vitamin B₆ as significant negative predictors of serum tHcy. Additionally, plasma SAM exhibited a positive association with serum betaine, while displaying a negative association with serum vitamin B₆.Previous studies predominantly focused solely on folate status, particularly dietary folate intake and its relationship with health outcomes. Actually, dietary exposure could not reflect OCM metabolism precisely, in which the folate cycle interacts with the methionine cycle. Besides, OCM was related to not only the neural tube defects occurring in early pregnancy, but also the fetal physical and mental development, even childhood health. Therefore, it is imperative to explore the relationships of OCM during different trimesters with both maternal and offspring outcomes.

Since the 1990s, worldwide recommendations for folic acid supplementation during periconception have resulted in very high rates of periconceptional folic acid supplementation, with 71.0% of pregnant women in this study reported to have taken folic acid supplementation. Similarly, another Chinese rural cohort study showed a rate of 83.2% among pregnant women (21). However, folate deficiency remains prevalent. Studies showed 24% of North American women had folate deficiency in their first trimester (9), and 24% of Indian women had folate deficiency (24). In this study, there was 2.5% of serum folate deficiency during mid-to-late pregnancy. One plausible explanation for this finding is that folate metabolism is influenced not only by dietary folate intake (including folic acid supplementation) but also by metabolic cofactors such as vitamin B₂, B₆, and B₁₂. In fact, this study reported a vitamin B₁₂ deficiency rate of up to 8.3% and observed a positive correlation between RBC folate and vitamin B₁₂ levels. A review indicated that vitamin B₁₂ insufficiency affected 21, 19, and 29% of women in the first, second, and third trimesters, respectively, (29). Vitamin B₁₂ deficiency disrupts the one-carbon cycle and impedes the conversion of 5-methyl tetrahydrofolate to tetrahydrofolate (30). Hence, it is imperative to improve not only folate but also metabolic cofactors (such as vitamin B₁₂) throughout pregnancy.

Furthermore, aside from its involvement in neural tube differentiation in early pregnancy, OCM plays a role in mid-to-late fetal development and contributes to the occurrence of maternal anemia and hyperhomocysteinemia (HHcy). The high prevalence of OCM disturbance among mid-to-late pregnant women in this study underscores the importance of continuous folic acid supplementation throughout pregnancy, which has been recommended in Chinese Dietary Guidelines (2022) (31). Moreover, the study highlighted a significant correlation between the sociodemographic characteristics of pregnant women and their OCM levels. Specifically, educational attainment and household income exhibited a positive association with the concentrations of maternal RBC folate, serum choline and vitamin B₂. This aligns with prevailing research indicating that women with higher levels of education and socioeconomic status (SES) tend to adhere more closely to dietary guidelines, thereby meeting essential nutrient requirements (32, 33). Conversely, lower educational attainment and SES may predispose individuals to unhealthy lifestyles characterized by dietary imbalances, obesity, and smoking (32, 33). Additionally, women belonging to lower educational level and SES often have infrequent antenatal visits, which diminishes their opportunities for dietary assessments, necessary adjustments, and adequate supplementation (32, 33). While some studies suggest that older women are more likely to adhere to healthier diets (32), animal research indicated that folate-modulated DNA methylation decreased with age (34, 35). This could explain the notably lower serum methionine levels observed in pregnant women aged 35 and above in this study. The negative correlation between multiparity and OCM levels might also be attributed to advanced age and postpartum recovery (32, 33). However, it is noteworthy that the number of pregnancies/deliveries did not exhibit a significant relationship with tHcy and SAM in multiple regression analysis. This observation highlights the need for increased attention to this demographic group.

Homocysteine serves as the principal functional biomarker of OCM, and undergoes catabolism via remethylation and transsulfuration pathways, where folate acts as a methyl donor and vitamins B₆ and B₁₂ function as enzyme cofactors. Maternal Hcy concentration decreased from early to mid-gestation and increased from mid-to-late gestation in a normal pregnancy (36). Clinical studies have highlighted the potential impact of elevated maternal homocysteine during pregnancy on fetal cognitive decline (20) and Down’s syndrome (22). Insights from the Ottawa and Kingston birth cohort suggest an independent effect of early to mid-pregnancy elevated maternal Hcy on placenta-mediated pregnancy complications, even on children’s cardiovascular-metabolic diseases after delivery (13, 36, 37). In this study, 1.8% of mid-to-late pregnant women exhibited HHcy, with adequate blood folate and vitamin B₆ identified as protective factors against HHcy. Other studies have similarly indicated that sufficient dietary intake and serum concentrations of folate and other B-vitamins are crucial to maintaining normal Hcy levels (38, 39). Moreover, genetic variations in metabolic enzyme-encoding genes could contribute to elevated homocysteine levels (38, 40, 41), though this aspect was not examined in the study. Nevertheless, to mitigate the risk of HHcy and promote maternal-offspring health, attention to folate and other B-vitamin status remains critical.

SAM provides methyl to DNA, proteins and lipids directly within OCM, as the single direct factor associated with most biomethylation reactions (42), while folate and choline are recognized as primary methyl donors. A decrease in SAM levels strongly correlates with reduced OCM function. The level of SAM is noteworthy, but studies focused on SAM are relatively scarce, and definitive cutoff levels are yet to be established. In this study, the median SAM of pregnant women was 52.50 (P₂₅ ~ P₇₅: 43.50 ~ 65.60) nmol/L, and a positive correlation between SAM and betaine levels was found. A case–control study in China showed that SAM concentration was 49.9(±3.0) nmol/L in women with neural tube defects-affected children, and 52.8(±3.2) nmol/L in women with healthy children (43). Interestingly, the study showed plasma SAM was negatively correlated with serum vitamin B₆, which is not directly involved in SAM or choline-betaine metabolism but rather in homocysteine catabolism. One possible explanation is that vitamin B₆ deficiency can lead to the accumulation of Hcy, resulting in upstream S-adenosylhomocysteine (SAH) elevation and subsequent SAM elevation through feedback regulation. An animal study showed that with age, folate deficiency alters the methylation index reflected by the SAM/SAH ratio, leading to a decrease in overall methylation (44). In this study, SAM concentration was measured while SAH was ignored. Accidental errors could be present in the results, and SAH would be explored in subsequent studies. Additionally, the cutoff level of SAM has not been harmonized and more attention should be paid to defining the cutoff levels of SAM to strengthen the study’s findings.

Furthermore, attention should be directed toward choline-betaine metabolism. This study noted a positive correlation between serum folate levels and serum choline and betaine, indicating a potential interaction between these two methyl donor pathways.

This study comprehensively illustrates the intricate interplay between folate and other nutrients in OCM, emphasizing that folate status alone is insufficient. It underscores the necessity of evaluating additional OCM markers, including vitamins B₂, B₆, B₁₂, as well as Hcy, SAM, and choline, during routine prenatal screenings. This is particularly crucial for women with lower educational backgrounds. When warranted, these B-vitamin levels can be optimized through a balanced diet and targeted supplementation. The alarming prevalence of OCM disturbances among women in the mid-to-late stages of pregnancy underscore the critical importance of maintaining consistent B-vitamin supplementation throughout the entire gestational period.

The study revealed an imbalance in blood OCM during mid-to-late pregnancy, characterized by lower folate, vitamin B₆, and B₁₂ levels alongside elevated tHcy levels. Adequate folate and vitamin B₆ emerged as significant predictors of lower tHcy levels, and a positive association was noted between serum betaine and plasma SAM.

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

The studies involving humans were approved by the Peking University Institutional Review Board (Beijing, China, approval number IRB00001052-19145), and conducted according to the Declaration of Helsinki. The studies were conducted in accordance with the local legislation and institutional requirements. Written informed consent for participation in this study was provided by the participants’ legal guardians/next of kin.

RZ: Data curation, Formal analysis, Investigation, Writing – original draft, Writing – review & editing. XW: Investigation, Writing – review & editing. LL: Investigation, Resources, Writing – review & editing. RH: Investigation, Resources, Writing – review & editing. YT: Investigation, Resources, Writing – review & editing. LP: Investigation, Resources, Supervision, Writing – review & editing. XZ: Investigation, Resources, Supervision, Writing – review & editing. WJ: Investigation, Resources, Writing – review & editing. WL: Investigation, Resources, Writing – review & editing. LD: Investigation, Resources, Writing – review & editing. WZ: Conceptualization, Investigation, Methodology, Project administration, Resources, Writing – review & editing.