People are being advised to take Vitamin D supplements throughout the winter up to St Patrick's Day in updated recommendations.
The Department of Health recommends people aged 65 and older should take Vitamin D all year round for bone and muscle health.
The body creates Vitamin D from direct sunlight on the skin when outdoors, but people do not get enough Vitamin D from sunlight between October and early March.
An FSAI report published last year marked the first time Vitamin D was examined in terms of how people aged five to 65 years can achieve "optimal intakes for long-term health."
Registered Dietitian Orla Walsh told Lunchtime Live the advice has always been to take Vitamin D.
"They've changed the recommendations and they've increased what they think we should be taking," she said.
"It's not new advice - we've always needed to take a Vitamin D supplement.
"We know that at least 90% of people have low levels of Vitamin D in their body.
"What they might experience is that they just get sick more often, but it's important for a multitude of things in the body".
Ms Walsh said Vitamin D can support the immune system along with healthy bones and muscles.
"There's loads of studies suggesting that Vitamin D, and low Vitamin D levels, are correlated to a whole heap of things including mental health, immune support and autoimmune diseases."
How to take Vitamin D
Ms Walsh said Vitamin D can be taken in several ways, including in a spray.
"They just keep it by the toothbrush, so after they've brushed their teeth, they just spray under their tongue and get on with the day," she said.
"Sprays can be handy if you're working with babies.
"So there is different methods of taking it and Vitamin D is of course in a lot of multivitamins".
Dosage
Ms Walsh said the dosage for babies depends on how much formula they are getting.
"If they're getting over 300mls they might not need to take a Vitamin D supplement.
"Breastfed babies, and babies getting less than 300mls of formula, might need five micrograms.
"You might need five micrograms up to the age of five, and then it increases to around 10 micrograms.
"When you become a teenager and adult it increases to 15 micrograms and it kind of stays at that for the rest of your life.
"They do recommend taking it every day from Halloween to Paddy's Day, but some people need to take it all year-round.
"That would include people living in Ireland with darker skin, as well as people who are indoors a lot of the time".
Vitamin D is also found in a small number of foods like oily fish, red meat, liver, egg yolks and fortified foods - such as some fat spreads and breakfast cereals.
In Ireland, studies have shown that adults have low levels of Vitamin D.
More information can be found here.
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