Vitamin D is produced in response to sunlight, but what is its role in fertility?
Over the years, assisted reproduction therapy (ART) — including in vitro fertilization (IVF) and fertility medication — has become much more widespread and its success rates have increased.
As an example, depending on the woman's age and the clinic involved, success rates of IVF in the U.S. range from 13–43 percent.
An initial uptick in ART success rates was thanks to improved methods of picking out embryos with the highest chances of survival. But more recently, success rates have started to stagnate.
Vitamin D and reproduction
Researchers believe that there is room for improvement in ART success rates. A range of potential factors are being explored, and some scientists have turned their attention to the potential role of vitamin D.
The vast majority of our vitamin D supply is generated in our skin after exposure to sunlight. This means that individuals who live in colder or darker environments are more susceptible to lower vitamin D levels, as are people with darker skin, those who regularly wear clothes covering the majority of their skin, and those who rarely go outside.
A link between vitamin D and fertility has been theorized based on a number of observations. For instance, vitamin D receptors and enzymes have been found in the endometrium. Also, in animal studies, vitamin D deficiency causes poorer fertility and reduced function of the reproductive organs.
Researchers from the University of Birmingham and Birmingham Women's and Children's National Health Service (NHS) Foundation Trust, both in the United Kingdom, decided to take a look at existing data to probe the links further.
They carried out a meta-analysis, reopening 11 studies including 2,700 women undergoing ART. Their findings are published this week in the journal Human Reproduction.
Vitamin D deficiency and lower success rates
The featured studies involved women undergoing IVF or intracytoplasmic sperm injection, frozen embryo transfer, or both.
All the participants' vitamin D levels were checked by blood test. Vitamin D concentrations of more than 75 nanomoles per liter of blood were considered as sufficient, under 75 nanomoles per liter of blood as insufficient, and under 50 nanomoles per liter of blood as deficient.
The analysis showed that procedures in women with adequate vitamin D levels were one third more likely to lead to live births than in women who were deficient.
When the researchers looked at positive pregnancy tests and clinical pregnancies — that is, where a heartbeat can be detected — rather than live births, the results were similar.
When compared with women who had insufficient vitamin D concentrations, those with sufficient amounts were 46 percent more likely to have a clinical pregnancy, and 34 percent more likely to have a positive pregnancy test result.
The analysis showed no associations between miscarriage and vitamin D concentrations.
"One startling finding was the high prevalence of vitamin D deficiency among these women. We found that only 26 percent of women in the studies had sufficient concentrations of vitamin D; 35 percent had deficient concentrations, and 45 percent had insufficient concentrations."
Study co-author Dr. Ioannis Gallos
The researchers are quick to explain the study's limitations. Team leader Dr. Justin Chu says, "Although an association has been identified, the beneficial effect of correction of vitamin D deficiency or insufficiency needs to be tested by performing a clinical trial."
He also adds an important note of caution. "In the meantime," he says, "women who want to achieve a successful pregnancy should not rush off to their local pharmacy to buy vitamin D supplements until we know more about its effects."
"It is possible to overdose on vitamin D and this can lead to too much calcium building up in the body, which can weaken bones and damage the heart and kidneys," he explains.
This current analysis backs up the theory that vitamin D plays an important role in fertilization and pregnancy. Its exact role is still not understood, and the optimum blood concentrations are not yet known. However, testing for vitamin D is relatively simple and cost-effective, so further studies are sure to follow.