Highlights from Fertility and Sterility: Danish Nationwide Study Shows Extra Vitamin D May Boost Infertile Women’s Chances of Having a Child

Results from a Danish nationwide cohort study suggest that consistent dietary vitamin D supplementation significantly increases the chances a woman diagnosed with infertility will have a live birth.

Danish researchers took advantage of a unique time period- a decade including five years of a nationwide dietary program to increase vitamin D levels- to examine the effect such supplementation may have on the likelihood that women diagnosed with infertility would deliver a child.

They examined the records of 16,212 women with a primary diagnosis of female infertility between June 1, 1980 and August 31, 1991. Data including each patient’s date of fertility evaluation, cause of infertility, and personal identification number were obtained from the Danish Infertility Cohort, the National Patient Register, and the Danish IVF Register. Patients’ i.d. numbers were linked to the Danish Medical Birth Register to determine whether they had a child. 

The first five years of the study’s date range fell within Denmark’s mandatory vitamin D fortification program for margarine.  From 1962 until May 31,1985, all margarine products in Denmark were fortified with vitamin D to an approximate level of 50 IU per 100 grams of margarine, amounting to 13% of the daily vitamin D intake for adults.

Patients in the study were separated into three groups depending on the date they received their primary infertility diagnosis: the vitamin D exposed group, diagnosed from study start to May 31, 1985; a “wash-out” period group, diagnosed from June 1, 1985 to August 31, 1986; and a vitamin D non-exposed group, diagnosed from September 1, 1986 to August 31, 1991. Statistical methods were used calculate odds ratios for the association between vitamin D exposure and the chance of live birth within a year of infertility diagnosis and the results were adjusted for age at time of diagnosis.

Women diagnosed with infertility during the vitamin D exposed period had an 87% higher chance of having a live birth in the 12 months following their diagnosis, compared to women diagnosed later, during the non-exposed period.  Women diagnosed during the transitional, wash-out period also had an increased (52% higher) chance of live birth compared to unexposed women.

Additional analyses showed that neither seasonal variation (summer versus winter conception) nor a longer follow-up period affected the association between vitamin D exposure and the chance of live birth.

ASRM President-elect Hugh Taylor, MD commented, “This large, nationwide study shows that small increases in vitamin D levels can have appreciable positive effects for infertility patients.  Vitamin D deficiency is common where exposure to sunlight is limited and it affects many of our patients.  While more research is needed on vitamin D and human reproductive function,  we should advise women who are attempting to conceive that  vitamin D supplementation may be helpful.” 

Jensen et al, Exposure to vitamin D fortified margarine in women with fertility problems and chance of live birth: results from a Danish population-based cohort study, Fertility and Sterility, in press.

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