The findings mean that some couples may be undergoing unnecessary and costly fertility treatment when spending time in the sun could be the answer.
For women, vitamin D helps boost levels of the female sex hormones progesterone and oestrogen by 13 per cent and 21 per cent respectively, regulating menstrual cycles and making conception more likely.
Fathers-to-be increase their fertility by going into the sun, too – because vitamin D is essential for the healthy development of each sperm’s nucleus.
It also increases levels of the male sex hormone testosterone, improving a man’s libido, according to the review of several studies, published this week in the European Journal of Endocrinology.
The vitamin’s effect on both male and female sex hormones may explain why conception rates fall in the winter and peak in the summer in Northern European countries, say the researchers at the Medical University of Graz in Austria.
In their own study of nearly 2,300 men, they also found that levels of testosterone and vitamin D peaked in August and were lowest in March, just after the winter.
Women have been found to ovulate less – and their eggs have a reduced chance of implanting in the womb – in the winter months.
Fertility problems affect one in seven couples in the UK. In four out of ten cases, the difficulty lies with the male partner.
Scientists found the fertility of men and women increased after spending time in the sun which could mean some couples would not need to resort to IVF to conceive
Although vitamin D can be obtained in small quantities by eating oily fish, eggs and liver, about 80 per cent of the amount the body needs is obtained via a chemical process that happens when the UVB rays in sunlight are absorbed by the skin.
Those living in Britain tend to be particularly prone to having lower levels because there is so much cloud cover, even in summer.
Lead author Dr Elisabeth Lerchbaum stressed that while sunshine appears to improve fertility, it is important couples don’t overdo it because of the risk of skin cancer from over-exposure.
The findings are the latest good news about vitamin D. Recent research has found that it may also play a part in reducing cases of sudden infant death syndrome – and also cutting mental health problems in children.