Vitamin D is such a misnomer...it's not a vitamin at all. Technically, vitamin D is a pro-hormone. Once it's converted from one form in the skin into the active form D3, it gets to work!
It is so important in a multitude of bodily functions. Theoretically, you could get all you need from UVB sun exposure in about 30 minutes daily. However, as we get older, our ability to use UVB to produce vitamin D3 decreases. Experts estimate that a shocking 80% of the population is deficient, especially older people and darker-skinned people since they need a lot longer sun exposure to produce D3.
We know about vitamin D's importance in cardiovascular function, cancer prevention, calcium and phosphorus levels, and even immune function. It also helps with conception and sperm quality.
Some recent studies suggest a link between ovulatory function and vitamin D deficiency. PMS and PCOS have also been linked to low vitamin D levels. It is an interesting fact that in countries like Sweden that have dramatic differences in the amount of sunlight in the winter versus the summer, conception rates are highest in the summer, with higher birth rates in the following spring.
The link for vitamin D and female fertility extends to men as well. Some research has shown that vitamin D supplementation improves sperm quality and quantity. Since virtually all tissues in the body have receptors for vitamin D, its importance can't be ignored.
The recommended daily supplementation of vitamin D is considered to be outdated. The upper limit is listed as 2000 IU, but most experts would push that to 10,000 IU/day. If you plan to take that much, you probably ought to get a D3 blood level after a few months of supplementing. It takes a while to build up, so give it at least 2 months. It is possible to become toxic on D3 supplements, but since most of us start at a very deficient level, you would have to take a lot for a long time to cause problems. Maybe, if you've been trying to conceive, some extra vitamin D is all you and your partner need!