STUDY QUESTION: To what extent is dietary folate intake and total folate intake (dietary and supplemental intakes) associated with fecundability, the per cycle probability of conception?
SUMMARY ANSWER: Preconception dietary folate intake was positively associated with fecundability in a monotonic pattern.
WHAT IS KNOWN ALREADY: Supplemental folic acid has been associated with improved fertility, but little is known about the relation between dietary folate and fecundability.
STUDY DESIGN, SIZE, DURATION: A prospective cohort study including 9559 women trying to conceive without fertility treatment and enrolled in the period 2013-2020.
PARTICIPANTS/MATERIALS, SETTING, METHODS: We used data from two internet-based prospective cohort studies of pregnancy planners from Denmark, where folic acid fortification is not performed (SnartForældre.dk (SF); n = 3755) and North America, where the food supply is fortified with folic acid (Pregnancy Study Online (PRESTO); n = 5804). Women contributed menstrual cycles at risk until they reported conception or experienced a censoring event. We used proportional probabilities regression models to compute fecundability ratios (FRs) and 95% CI, adjusting for potential confounders.
MAIN RESULTS AND THE ROLE OF CHANCE: Compared with a dietary folate intake ≥400 µg/day, the adjusted FRs for women in SF were 0.92 (95% CI: 0.85-0.99) for intake 250-399 µg/day, and 0.80 (95% CI: 0.68-0.94) for intake of <250 µg/day. The corresponding FRs in PRESTO were 0.95 (95% CI: 0.89-1.01) and 0.81 (95% CI: 0.65-1.00). Compared with the highest level of total folate intake (diet folate ≥400 µg/day plus folic acid supplementation), in both cohorts fecundability was lowest among women with the lowest dietary intake <250 µg/day dietary folate and no supplementation (FR: 0.76, 95% CI: 0.59-0.98 [SF] and 0.49, 95% CI: 0.31-0.77 [PRESTO]). Further, total intake dietary folate <250 µg/day plus supplementation was associated with reduced fecundability for SF participants (FR; 0.79, 95% CI: 0.65-0.98) and for PRESTO participants (FR; 0.92, 95% CI: 0.72-1.16).
LIMITATIONS, REASONS FOR CAUTION: It is unknown whether dietary folate and folic acid intake affect fecundability on its own or if there is an interaction with other micronutrients provided in healthy diet. Thus, the observed associations may not reflect dietary folate intake alone, but overall healthy diet.
WIDER IMPLICATIONS OF THE FINDINGS: Recommendations for preconception dietary folate intake and folic acid supplementation are of importance not only to prevent neural tube defects but also to enhance fecundability.
STUDY FUNDING/COMPETING INTEREST(S): The study was supported by the National Institute of Child Health and Human Development (R01-HD086742). The authors report no competing interests.
TRIAL REGISTRATION NUMBER: N/A.