There are mounting questions about whether vitamin D can help fight coronavirus.
The Scientific Advisory Commission on Nutrition and the health watchdog the National Institute of Health and Care Excellence (NICE) have done a rapid review of the evidence.
What is the advice?
With more people staying indoors during the pandemic, some may have been deprived of vitamin D.
Normally, many of us get it by spending time outside. Our skin makes it when exposed to the sun.
The NHS says people should consider taking 10 micrograms of vitamin D a day if they are spending a lot of time indoors.
Scottish and Welsh governments and Northern Ireland's Public Health Agency issued similar advice during lockdown.
Before the pandemic, people in the UK were already advised to consider taking supplements from October to March.
Public Health England recommends vitamin D throughout the year if:
People with dark skin may also not be getting enough, even if they spend time outdoors, and should consider an all-year-round supplement.
There is evidence that black, Asian and minority ethnic (BAME) people have a higher risk of getting seriously ill with coronavirus.
Why do we need vitamin D?
Vitamin D is important for healthy bones, teeth and muscles. A lack of it can lead to a bone deformity illness called rickets in children, and a similar bone weakness condition called osteomalacia in adults.
There are also suggestions that vitamin D boosts the immune system and helps fight off infections.
Some studies suggest adequate vitamin D levels help when we have common colds and flu, for example. But evidence from research is inconsistent.
The Scientific Advisory Committee on Nutrition (SACN) says studies on using vitamin D for treating or preventing chest infections showed insufficient evidence to recommend it for this.
Can it stop coronavirus?
A review of research by NICE suggests there is no evidence to support taking vitamin D supplements to specifically prevent or treat coronavirus.
But experts think that it may have some broader health benefits during the pandemic to keep people as nutritionally fit as possible.
Some researchers have suggested that vitamin D deficiency might be linked with poorer outcomes if someone catches coronavirus. But other underlying risk factors, such as heart disease, are common in these patients too, making it hard to draw conclusions.
Prof Jon Rhodes, Emeritus Professor of Medicine at the University of Liverpool, says vitamin D has anti-inflammatory effects, and some research suggests it may dampen down the body's immune response to viruses.
This could be relevant in very ill coronavirus patients, where severe lung damage can result from an inflammatory "cytokine storm" in response to the virus, he says, although much more research is needed.
Should I take lots of it?
No. Although vitamin D supplements are very safe, taking more than the recommended amount every day can be dangerous in the long run.
If you choose to take vitamin D supplements:
Higher doses may sometimes be recommended by a doctor for patients with proven vitamin D deficiency.
Some people with certain medical conditions, such as kidney problems, cannot safely take vitamin D.
Where can I buy it?
Vitamin D supplements are widely available from supermarkets and chemists. They may be just vitamin D or part of a multivitamin tablet.
The ingredient listed on the label of most Vitamin D supplements is D3, the one made by your skin. Vitamin D2 is produced by plants.
Vitamin drops are available for babies.
What about diet?
Although eating a well-balanced diet can help ensure the normal functioning of the immune system, no individual nutrient, food or supplement is going to "boost" it beyond normal levels.
It's difficult to get enough vitamin D from food alone.
Eating a well-balanced diet is important for good health and is advisable even outside a pandemic.
It can include vitamin D-rich foods like oily fish and eggs. Some breakfast cereals, margarines and yoghurts are fortified with vitamin D.
Should I sunbathe?
Although you cannot overdose on vitamin D through exposure to sunlight, strong sun burns skin so you need to balance making vitamin D with being safe in the sun.
Cover up or protect your skin with sunscreen to prevent burning and damage.
What about children, babies and pregnant women?
The advice is:
The dose for adults (10 micrograms a day) applies to pregnant and breastfeeding women. A higher dose may be recommended for pregnant women with dark skin or with high-risk pregnancies.