Would-be fathers should make the most of the next sunny day by taking off their shirt and topping up their tan.
A study of 340 men found that vitamin D, produced by the body when exposed to the sun, boosts the quality of sperm.
They become better at swimming towards the egg, have greater speed and are more penetrative.
Researchers at the University of Copenhagen tested the quality of sperm from 300 men chosen at random and carried out a detailed analysis of samples from a further 40 in the laboratory.
At the same time the level of vitamin D in their blood was measured. Almost half had an insufficient amount – linked to lack of exposure to natural sunlight or time in a solarium.
Sunlight is the major source of vitamin D, which helps regulate levels of calcium and phosphorus to generate healthy bones.
In addition, the number of healthy sperm in men with insufficient vitamin D was considerably lower than in those with normal levels. The ability to absorb calcium was also inhibited.
The findings, published in the journal Human Reproduction, back up previous research linking vitamin D to sperm production, and might explain why many couples seem to conceive on holiday.
Researchers discovered that almost half of the participants had an insufficient amount of vitamin D, below 50 nmol/l. The optimum level recommended by most experts is a minimum of 75 nmol/l.
The ability chemical process needed to fertilise the egg known as the acrosome reaction was also hindered. Tests conducted in the laboratory resulted in similar findings.
Dr Martin Blomberg Jensen said: 'Vitamin D levels were positively associated with sperm motility, suggesting a role for vitamin D in human sperm function.'