“Vitamin D boosts sperm quality,” the Daily Mail has reported. The newspaper says that vitamin D made sperm “better at swimming towards the egg, have greater speed and be more penetrative.”
The research behind this news was a two-part study that first tested the blood vitamin D levels and sperm motility of 300 Danish men taken from the general population. The second part then looked at what happened when vitamin D was added to sperm samples from an additional 40 men. The researchers found vitamin D raised calcium levels in the sperm and that, in turn, increased their motility and the proportion of the sperm undergoing the changes needed to fuse with a female egg.
This study shows that vitamin D can have an effect on sperm, but further work is needed to see whether the amount of vitamin D used in the experimental phase of this study reflects the range of vitamin D that sperm would typically be exposed to in the human body.
The men in this study were recruited from the general population and although those with deficient vitamin D levels had less motile sperm, it is unclear whether this would be a sufficient impairment to cause problems for them to conceive with their partner. Further research is needed to see whether vitamin D levels are associated with clinical fertility problems.
Where did the story come from?
The study was carried out by researchers from The University of Copenhagen, Denmark and was funded by various research organisations, including the Danish Agency for Science, Technology and Innovation, and the Novo-Nordisk Foundation of the Novo-Nordisk pharmaceutical company. It was published in the peer-reviewed medical journal Human Reproduction.
In reporting this research newspapers tended to focus on the effect of vitamin D on fertility or virility. Although there was found to be an association between vitamin D levels and sperm motility, which may affect fertility if it is severe, the study did not directly look at fertility (the chance of a successful pregnancy). Nor did it look at whether increasing vitamin D levels boosted the chances of conceiving a baby.
What kind of research was this?
This was a cross-sectional study that looked at whether there was an association or link between vitamin D levels in 300 men and their sperm quality, measured in different ways.
The researchers say that there is a receptor for vitamin D on human sperm and genetically modified mice that do not have this receptor and normal rodents that are reared without sufficient vitamin D tend to have impaired fertility, low sperm counts and less motile sperm. The researchers wanted to see whether vitamin D levels in humans were associated with sperm characteristics.
A cross-sectional study can show an association between a factor (in this case vitamin D) and a condition (sperm quality) but it cannot show whether vitamin D levels cause differences in sperm quality. To establish a cause and effect relationship would require an experimental study design, like a randomised trial.
The researchers followed up their cross-sectional study with lab-based in vitro studies (test-tube studies) on the sperm from 40 men. They looked at the effect of adding vitamin D had on the characteristics of this sperm.
Importantly, this study did not look at infertile men or fertility rates and so it is not yet possible to say if increasing vitamin D levels is a useful treatment for couples having difficulty conceiving.
What did the research involve?
The researchers recruited 300 men from an ongoing monitoring study of semen quality of young men from the general Danish population. The men had participated in the study between January and December in 2007 and provided one semen sample and a blood sample. They also took part in a physical examination and answered a comprehensive questionnaire, including information on age and previous or current diseases. The researchers also asked for any known history of fertility and medication.
The researchers analysed the blood samples to measure the participants’ vitamin D levels, as well as levels of:
- follicle stimulating hormone (FSH): a hormone that regulates the maturation of sperm
- Inhibin-B: a protein that regulates FSH production
- parathyroid hormone: which regulates the amount of calcium in the blood)
They also measured the protein albumin, calcium levels, and levels of the enzyme alkaline phosphatase.
The researchers asked the participants to recall how long it had been since their last ejaculation prior to their semen sample. They measured the volume of the sample, the motility of the sperm, the sperm count and the shape of the sperm.
For the in vitro analysis of the effect of vitamin D on sperm, the researchers collected semen from 40 men from the general population between October 2009 and December 2010. The semen samples were analysed in the same manner as before. The researchers added a small amount of vitamin D to the sample, waited for 45 minutes and then looked at the effect of this on sperm motility, calcium signalling in the sperm and the sperms ‘acrosome reaction’. The acrosome reaction is a series of reactions that allows the sperm to fuse and penetrate the female egg.
What were the basic results?
In the cross-sectional phase of the study, vitamin D levels varied according to the season in which the samples had been taken, with samples taken in the winter having significantly lower vitamin D levels than samples taken in the spring, summer or autumn. Forty-four percent of the sample had vitamin D levels that were lower than optimum (defined by these researchers as less than 50 nanomoles per litre (nM).
They found that men who were classified as having a vitamin D deficiency (having vitamin D levels less than 25 nM) had a lower proportion of motile sperm, sperm that were motile and could propel forwards and sperm that were a normal shape, than men who had ‘high’ vitamin D levels (more than 75 nM).
They found that when they added vitamin D to sperm samples there was a rapid increase in their calcium concentration. If they used an inhibitor chemical that blocked the vitamin D receptor in these sperm then they did not see this rise in calcium when they added the vitamin D.
They found that adding vitamin D increased sperm motility by up to 7%, although increasing vitamin D beyond a certain concentration decreased sperm motility. Vitamin D increased the proportion of sperm that had undergone the acrosome reaction by on average 6%.
How did the researchers interpret the results?
The researchers say that there is a positive association between blood vitamin D levels and sperm motility. They add that their novel functional findings show that vitamin D activates the vitamin D receptor, causing an increase in calcium levels inside the sperm. This in turn induces greater sperm motility and the acrosome reaction. In this study researchers looked at a cross-section of young men. They said that follow up studies are warranted to see what the effect of vitamin D is in infertile men.
This cross-sectional study showed that there was an association between vitamin D levels and sperm motility in a sample of men taken from the general Danish population. As this was a cross-sectional study with measurements taken at one point in time it could not confirm whether vitamin D levels caused lower levels of sperm motility.
Studies looking at vitamin D also need to take into account confounding factors. For example, people with higher vitamin D levels might have achieved these through spending greater time outdoors and being active, which might in some way affect sperm motility. Also, they may have got more vitamin D from a diet that is richer in other vitamins, which might also play a role in sperm motility.
However the in vitro part of this study did show that vitamin D can have direct effects on sperm behaviour. Further work is now needed to see how blood vitamin D levels relate to the concentration of vitamin D that sperm would be exposed to in the body.
The researchers point out that further work is needed to see the effect of vitamin D in men with fertility problems. Although the researchers found that men with deficient vitamin D levels had less motile sperm, it was not assessed whether this decrease in motility would be severe enough to prevent the man and his partner from conceiving a child.
As men with fertility problems were not specifically assessed, further research is needed to see whether men with problematically immotile sperm have lower vitamin D levels and what other factors may contribute to this lack of motility.