For couch potatoes everywhere, it could be a dream come true.
Vitamin D supplements may make people fitter without them having to move a muscle, research suggests.
Taking a daily dose for just two weeks significantly improved fitness, a study found. The pills were also found to lower blood pressure and levels of the stress hormone cortisol.
With vitamin D already credited with a host of health benefits, the team behind the research suggest we all take a supplement.
They particularly recommend doing so during the dark winter months because, while vitamin D is found in some foods such as oily fish, we get most of our supply when our skin is exposed to sunlight and the British climate means millions of us do not get enough of it.
But other experts warned people not to be too hasty to cancel their gym memberships, saying that the results seem too good to be true and that more research is needed.
The team, from the Queen Margaret University in Edinburgh, did a battery of medical tests on 13 healthy men and women before putting them through their paces on an exercise bike.
Some were then asked to take 50 micrograms of vitamin D a day for a fortnight - ten times the amount in multi-vitamin pills - while the others took a dummy pill.
After the two weeks, those who had taken the vitamin D pills did dramatically better on the bike. They went from being able to cycle 4.8 kilometres in 20 minutes to covering 6.4 kilometres. And despite cycling a third further, they found it easier than they had done before.
Those who took the placebo pill were no fitter or healthier than they had been at the start of the study, the Society for Endocrinology's annual conference in Edinburgh heard.
It is thought that vitamin D reduces the amount of the stress hormone cortisol made by the body.
Cortisol can raise blood pressure, which, in turn, raises the risk of potentially deadly heart attacks and strokes.
Vitamin D supplements have also been linked to a range of other health benefits, such as helping to ward off Alzheimer's, keeping prostate cancer under control, stopping multiple sclerosis from developing and boosting bone health.
However, it is unclear why vitamin D would improve exercise performance. One of the researchers, dietitian Dr Raquel Revuelta Iniesta, said that a large study is needed to confirm the vitamin's benefits.
But she added that there is no harm in people taking a supplement in the meantime. She recommends taking ten micrograms a day, especially between October and April.
Others said the number of people involved in the study was too small to reveal much. Dr Helen Simpson, a consultant endocrinologist at Addenbrooke's Hospital in Cambridge, warned that the results seem too good to be true.
She said that there is no reason for healthy people to take vitamin D supplements and that the dosage used in the study could be harmful if taken for long periods.
- Daily Mail