Fertility cannot be assumed for everyone. While it may seem like a given, fertility is often a challenge including for healthy young adults, male and female alike.
Optimal fertility that is conducive to easy conception and high pregnancy success rates is a delicate balance of genetically and environmentally determined health. Aspects such as age and nutrition are largely involved in fertility status.
One aspect of fertility that is steadily gaining interest, and righteously so, is vitamin D status. Emerging research has linked low vitamin D levels to reduced female and male fertility and even infertility. There is actually extensive research on the effects of vitamin D for fertility, with studies showing a marked increase in fertility and pregnancy success among women and men with adequate vitamin D levels.
How does vitamin D help fertility?
1) Reproductive system tissues and organs need vitamin D
The reproductive system needs vitamin D to function optimally. Studies have shown that male and female reproductive system tissues and organs express vitamin D receptors, and enzymes needed for the metabolism of vitamin D. That is, vitamin D is needed at the level of the reproductive system, where it’s absorbed and processed into an active form.
The ovaries and ovarian tissue, uterus as well as endometrium, the mucous tissue lining the inside of the uterus, and male reproductive tissues and organs have receptors for vitamin D and vitamin D-metabolizing enzymes, according to research (source). This is a strong indicator of the crucial role of vitamin D in maintaining and advancing reproductive health and fertility in men and women.
2) Vitamin D modulates human reproductive capacity and activity
Vitamin D imparts seasonality to human reproductive capacity, actively modulating fertility. There is extensive research that shows natural vitamin D availability from sun exposure can determine fertility, and subsequently also pregnancy rates. More specifically, vitamin D availability from sun exposure imparts seasonality to ovulation rates, the quality of the ovum and receptivity of the endometrium to implantation (source).
3) Vitamin D deficiency reduces male and female fertility
Low vitamin D levels can lead to the underdevelopment of the reproductive system in both men and women (e.g. underdeveloped uterus, thin endometrium in women), reduce reproductive capacity (e.g. low count and poor motility in men) and cause dysregulations that impair fertility (e.g. impaired folliculogenesis, lack of ovulation, low implantation rates).
It has already been observed in animal studies that vitamin D deficiency reduces fertility, both male and female (study 1, study 2, study 3) by affecting the reproductive system function.
Animal studies have shown that fertility was reduced by a staggering 73% in litters from vitamin D-deficient male rats, for example. Reduced fertility was observed in vitamin D-deficient female rats as well.
In animal studies, vitamin D deficiency was identified as a cause of ‘uterine hypoplasia, impaired follicular development and anovulation’ in female rats fed a vitamin D-deficient diet and female rats with knockouts for vitamin D receptors (study).
Both the low levels of vitamin D and the inability to process vitamin D into its active form were the factors that caused the underdevelopment of the reproductive system and lack of ovulation that led to reduced fertility and a dysregulation of the reproductive function that further lowered fertility and pregnancy rates.
4) Good vitamin D levels increase conception rates and pregnancy outcomes
Women with the highest blood levels of vitamin D have been observed to have ‘the highest chances of pregnancy’ (source). It has also been proposed that adequate vitamin D levels can improve pregnancy outcomes such as the number of live births.
Human studies have further observed that women with a low vitamin D intake and low vitamin D levels show lower conception rates. At the same time, increasing vitamin D intake to the point it brings vitamin D levels within normal range could improve conception rates, studies note.
Animal studies also support the hypothesis that ‘vitamin D and its metabolites are necessary for normal reproductive functions’. For example, vitamin D-deficient rats that receive vitamin D supplementation exhibit a restoration of normal reproductive function, with good pregnancy outcomes, ‘giving rise to normal, healthy liters’ (source).
5) Vitamin D is needed for the physical development of the reproductive system
Studies have observed that adequate levels of vitamin D promote the normal physical development of the organs and components of the reproductive system in both men and women (source) which correlates with good fertility. In addition to this, vitamin D regulates the production of hormones such as estradiol, progesterone, androgen hormones such as testosterone, and others, actively supporting normal reproductive system functions and processes conducive to optimal fertility, high conception rates and good pregnancy outcomes.
6) Vitamin D regulates folliculogenesis and ovulation
There are many ways that vitamin D regulates fertility in women. One is by regulating the production of a hormone called the Anti-Müllerian hormone (AMH) which determines the maturation of the ovarian follicle in a process called folliculogenesis that is meant to culminate with ovulation.
Vitamin D is also involved in hormone production, including the production of female hormones such as estradiol and progesterone which are directly responsible for modulating female fertility, with important roles in determining conception rates and pregnancy outcomes. For example, female hormones are responsible for the growth of tissues in the reproductive system meant to support conception (implantation) and normal pregnancy development. Modulation of such key reproductive processes further emphasizes the importance of vitamin D plays for fertility.
7) Vitamin D modulates ovarian reserve and maintains fertility late age
The Anti-Müllerian hormone, or AMH, is a hormone directly and meaningfully involved in regulating fertility. The Anti-Müllerian hormone (AMH) is ‘one of the most reliable markers for ovarian reserve’. According to research, the ‘highest values of AMH in women are attained after puberty and subsequently decrease with age, likely reflecting the age-related decline in ovarian reserve’.
Studies note that vitamin D may contribute to the production of AMH whose regulatory actions on hormone production and ovarian follicular development could hold important benefits for female fertility such as improved late-age fertility.
Conversely, low vitamin D levels ‘might be associated with lower ovarian reserve in late-reproductive-aged women’ (source) and reduced or impaired fertility. It can be speculated that striving to achieve normal vitamin D levels throughout one’s lifetime, which is a prerequisite for good general health, may help extend fertility in women.
8) Vitamin D has a remodeling effect on the endometrium
There are receptors for vitamin D across most, if not all parts of the reproductive system. The presence of vitamin D receptors on the surface of the endometrium, the tissue lining the inside of the uterus, is purported to hold important benefits for female fertility and improve pregnancy outcomes.
The endometrium lines the inside of the uterus and serves an important functional purpose: to allow for implantation and pregnancy development. If pregnancy does not occur, the endometrium is shed. If pregnancy does occur, it develops into the placenta. But in order to support pregnancy, the endometrium cannot be too thin. An endometrium that is too thin cannot support normal pregnancy development.
Studies have observed that vitamin D exerts a regulatory action on the thickness of the endometrium, improving endometrium thickness in women with PCOS, and also endometrial receptivity (source 1, source 2). This remodeling effect is favorable to implantation and good pregnancy outcomes.
9) Vitamin D can improve IVF pregnancy rates and pregnancy outcomes
In one of the first studies on vitamin D and fertility after IVF, it was reported that pregnancy rates were almost four fold higher in women who were vitamin D sufficient compared to those who were vitamin D deficient (source). According to research, vitamin D increases endometrial receptivity which explains the increase in fertility success stories.
In another study, subgroup analyses showed that women with the highest serum levels of vitamin D (>30 ng/mL) had the highest chances of pregnancy. In another study on the effects of vitamin D levels on in vitro fertilization outcomes, it was observed that pregnancy rates and live births were higher among vitamin D-replete recipients (78% and 59%) vs vitamin D-deficient (38% and 31%).
10) Vitamin D holds benefits for benign uterine tumors
One of the causes of infertility in women is the presence of cysts, fibroids and tumors at the level of the reproductive system such as the uterus. Of the many causes and risk factors for uterine tumors such as fibroids, hormonal imbalances, family history, high BMI and age rank high, but also low vitamin D status according to more recent research (source 1, source 2).
It has been proposed then that maintaining good vitamin D levels can help reduce risks of fibroids (source). For one, vitamin D has been shown to improve metabolic disturbances that could be conducive to the development of various formations (source) via hormonal pathways.
In addition to this, ‘vitamin D takes part in cell cycle regulation and cell differentiation, and it also has anti-angiogenic activities’. Moreover, it exhibits immunomodulating effects with antiproliferative and apoptosis-inducing activities that may contribute to benefits such as reduced risks of tumors.