What is Vitamin D and What does it do?
Vitamin D is a steroid hormone that allows our bodies to absorb important minerals like calcium, magnesium, and zinc. It is produced by the body when skin is exposed to the sun. Like other hormones, vitamin D acts by binding to receptors throughout the body in order to accomplish its actions. These receptors are present in bone, intestine, parathyroid glands, the ovary, the uterine endometrium, and the pituitary gland, among other areas of the body. Vitamin D is critical in maintaining the integrity of bone, it acts as a modulator of cell growth, and it helps regulate the immune and neuromuscular systems.
Deficiency of vitamin D results in soft bones in children (rickets) and fragile and thin bones in adults (osteomalacia and osteoporosis) and may be linked to the development of several chronic diseases, including cardiovascular disease, diabetes, and cancer.
Receptors for this vitamin have also been identified in the placenta and it has been found that maternal levels of vitamin D have a profound influence on the health of a developing fetus.
What is a normal blood level of Vitamin D?
The blood test that is ordered by a physician is 25-hydroxyvitamin D. By the standards of the Endocrine Society of the US, a level of 30 ng/mL or higher is sufficient. A level between 20 and 30 ng/ml is considered insufficient, between 10 and 20 ng/mL is deficient, and less than 10 ng/mL is severely deficient.
Can a vitamin D level be too high?
Yes, levels over 100 ng/mL may be toxic and can increase chances of an abnormal heart rhythm and kidney stones due to an abnormally high blood calcium level. This could result from taking too much supplemental vitamin D. However, it would be very difficult to obtain too much of this vitamin just from food.
Are many people deficient in vitamin D?
Studies have shown that 42% of adults in the US are deficient in vitamin D, most likely since we live much of our lives clothed and indoors, do not live in a sun-drenched equatorial climate, and use sunscreen to help protect against skin cancer when we are outside, all of which severely limit our sun exposure. Deficiency rates are 82% in African Americans and 69% in Hispanic Americans due to decreased absorption of sunlight by their darker skin.
Why are adequate levels of vitamin D so important in pregnancy?
A 2013 report in the British Medical Journal reviewed 31 scientific studies published between 1980 and 2012 that looked at the association between serum vitamin D levels during pregnancy and potential adverse outcomes. The conclusion was that Vitamin D insufficiency was associated with an increased risk for three critical issues in pregnancy: gestational diabetes, pre-eclampsia, and small for gestational age infants.
A breastfeeding mother also needs to have adequate levels of vitamin D in her body to pass this important nutrient on to her baby.
Is there a link with Vitamin D and infertility? Possibly.
Since there are receptors for vitamin D in the ovary and in the uterine endometrium, it is possible that vitamin D is important for successful conception.
It is known that fibroids may decrease a woman’s chance for conceiving or increase the chance of miscarriage. There is some data indicating that a low vitamin D level may be a risk factor for development of fibroids.
Women with polycystic ovary syndrome (PCOS) tend to have difficulty conceiving due to infrequent ovulation. Vitamin D levels tend to be lower in women with PCOS. Vitamin D supplementation has been shown to decrease insulin resistance and increase the response to medication such as clomiphene citrate, as evidenced by the development of functional ovarian follicles. A very recent study in rhesus monkeys has shown that vitamin D has a direct positive effect on the growth of ovarian follicles.
Data from studies looking at vitamin D levels and outcomes of fertility treatments including IVF have suggested that higher levels might improve pregnancy rates, possibly through the beneficial effect on the endometrium, but reports have been conflicting. Studies looking at this are ongoing and more answers will likely be available in the future.
What are good food sources of vitamin D?
- Fish – salmon (especially wild-caught), mackerel, tuna canned in water, sardines canned in oil, and cod liver oil (but too much may provide too much vitamin A)
- Milk or yogurt fortified with vitamin D. It does not matter whether the milk is whole, reduced fat or non-fat. Many cereals and brands of orange juice are also fortified with vitamin D.
- Egg yolks
- Beef liver
How much Vitamin D do I need each day?
Adults generally require 600 to 1000 international units/day (IU/day) in their diet to maintain normal blood levels of vitamin D. However, a supplement may be required to achieve a normal level.
As indicated above, adequate levels of vitamin D are extremely important in pregnancy and during breastfeeding. Therefore, if your levels are found to be low prior to conception, your physician will recommend Vitamin D supplementation in order to bring your level into the normal range before you attempt to conceive. It may take several weeks to achieve normal levels, which is why it is important to have your level checked if you are planning to conceive.
Important: the maximum daily amount that should be taken during pregnancy or when breastfeeding is 4000 IU/day.
Role of vitamin D in ovarian physiology and its implication in reproduction: a systematic review. Fertility and Sterility, 2014: 102; 460-468.
Prevalence and correlates of Vitamin D deficiency in US adults. Nutr Res, 2010: 31; 48-54.
Association between maternal serum 25-hydroxyvitamin D level and pregnancy and neonatal outcomes: systematic review and meta-analysis of observational studies. British Medical Journal 2013:346. March 27, 2013.
Influence of vitamin D and transforming growth factor beta3 serum concentrations, obesity, and family history on the risk for uterine fibroids. Fertility and Sterility, 2016: Article in Press.
Direct vitamin D3 actions on rhesus macaque follicles in three-dimensional culture: assessment of follicle survival, growth, steroid, and antimullerian hormone production. Fertility and Sterility, 2016: Article in Press