Vitamin D should be part of the Government's health measures to help prevent the severity of Covid-19 illnesses, health experts have said.
The group of professors and researchers say explicit guidelines should be issued by public health officials on the supplementation of vitamin D.
The experts, who are members of the Covit-D Consortium, said the advice should be part of the Government's Living with Covid-19 strategy.
The Oireachtas health committee was told research shows vitamin D reduces mortality among people who are diagnosed with coronavirus.
Dr Daniel McCartney, director of human nutrition and dietetics at TU Dublin, said there is an extensive body of international research showing the protective role of vitamin D against Covid-19.
Dr McCartney said:
"We think a recommendation and explicit guidelines for people in the Irish population, to supplement 20 to 25 micrograms per day, or 800 to 1,000 units per day, is absolutely essential.
"We also feel that higher supplementation may be required under the medical supervision in at-risk groups.
"Those would include people who are overweight or obese, which is 60% of our adult population, and also older people or people in nursing homes in particular who have notably low vitamin D levels."
He also said frontline healthcare workers would also benefit from vitamin D supplements.
He added: "These supplements are very cheap, they're readily available. So if the public got a clear message that they should be supplementing at 'x' dose, in the current context, I think they would want to follow on from that."
He said that while vitamin D deficiency is more prevalent in older adults, it was a wide societal problem.
Dr McCartney said about 50% of young people aged 18 to 39 have low vitamin D levels at this time of year.
"It's across the population and this is why it requires a central intervention from the statutory health authorities," he added.
Professor Rose Anne Kenny said a study from the Irish Longitudinal Study on Ageing (TILDA) showed 60% of middle-aged adults and older persons in Ireland have insufficient vitamin D in winter.
Prof Kenny also said the National Public Health Emergency Team (Nphet) should issue clear advice.
She added: "I think the bottom line is it would be very valuable if Nphet did more than recommend raising awareness of vitamin D.
Social Democrats co-leader Roisin Shortall said she is concerned about the lack of clear advice or recommendations from Nphet in relation to vitamin D supplementation.
"Nphet say they are repeating the advice and say that the recommendations for vitamin D are being incorporated into wider messaging in relation to Covid. I have to say I haven't heard that wider messaging from Nphet," Ms Shortall told the committee.
"It's really important for messaging to be made very clear."
Dr McCartney said large studies pooling data from dozens of individual trials have described an approximately 10% reduction in risk of respiratory infection amongst people taking vitamin D supplements.
The studies also revealed there was more than a 50% reduction in risk among those with low vitamin D levels to begin with, particularly when taking supplements daily.
Data from May last year also shows a direct relationship between low vitamin D status and increased risk of Covid-19 infection, severe disease and death, the committee heard.
Professor James Bernard Walsh, from the School of Medicine at Trinity College Dublin, said around 70% of people living in the Dublin metropolitan area as well as Kildare and Wicklow, are deficient in vitamin D during the wintertime.
"That's a huge amount," he added.
"We've seen the highest incidence in the BAME (black, Asian, and minority ethnic) communities who are at the highest level of deficiency - they have been so badly struck down with Covid."