Health officials say everyone in England and Wales should now consider taking vitamin D supplements in autumn and winter.
Previous guidelines were limited to pregnant and breastfeeding women, older people and those who don't get out in the sun enough.
So why is the advice changing?
What Is Vitamin D and Why Is It Important?
Vitamin D is important for healthy bones, teeth and muscles. In children, a lack of vitamin D can lead to the bone disease rickets. Adults who don't get enough vitamin D may develop the painful bone condition osteomalacia.
Where Does Vitamin D Come From?
The body's main natural source of vitamin D is safe exposure to the summer sun.
Safe sun exposure means a short time in the sun without sunscreen - but before burning. The time needed varies from person to person - but a balance is needed because too much sun exposure increases the risk of skin cancer. Sunbeds are not recommended.
Vitamin D also comes from eating certain foods, including oily fish, liver, egg yolks and specially fortified foods, such as breakfast cereals and spreads.
Why Aren't Many People in the UK Getting Enough Vitamin D?
Around 23% of adults aged 19 to 64, 21% of adults aged 65 years and over and 22% of children aged 11 to 18 in the UK have low levels of vitamin D in their blood. These levels aren't low enough to make anyone unwell, but they are at risk of going on to have vitamin D deficiency that could affect their health.
The British weather can restrict how much sun is available during the year - plus many adults and children don't get outside as much as people did in the past. Also the sun is not strong enough here between October and March.
People with darker skin are also at a higher risk of not getting enough vitamin D, as are people who cover up for cultural reasons, and people not well enough to go outdoors.
What's the New Recommendation?
Health officials from Public Health England and the Welsh Government are now recommending people get 10 micrograms of vitamin D a day though their diet or supplements.
The UK's Scientific Advisory Committee on Nutrition (SACN) made the recommendation after reviewing all the evidence on vitamin D and health. It couldn't determine how much vitamin D we get though skin being exposed to the sun - so instead focuses on diet.
It is hard to get the daily amount of vitamin D needed from food, so most adults and children aged 4 and over are asked to consider supplements containing 10 micrograms of vitamin D every day during autumn and winter.
Supplements are recommended all year round for people with dark skin, including people from African, African-Caribbean and South Asian backgrounds.
Children aged 1-4 years should have a 10 microgram vitamin D supplement every day all year round.
All babies under a year old should have a daily 8.5 to 10 microgram vitamin D supplement, even if they are breastfed exclusively up to around 6 months under official advice.
The guidelines are different for formula-fed children who have more than 500ml of infant formula a day. They don't need extra vitamin D because formula is already fortified with it.
What About Scotland?
In a statement, Scottish public health minister Aileen Campbell says: "We note SACN's report and recommendations. Our advice for everyone aged six months and over has been updated in line with the recommendations. We are assessing if our advice for infants under 6 months should be revised."
Is the NHS Providing the Vitamin D Supplements?
Not for most people. However, Vitamin D supplements are available free for low-income families under the Healthy Start scheme.
In Scotland, all pregnant women will be entitled to free vitamins from Spring 2017, which will include the recommended dose of vitamin D. Public health minister Aileen Campbell says: "This is part of a concerted effort to give every child in Scotland the best start in life."
Single vitamin D supplements are available in pharmacies, some health food shops and supermarkets.
What Are Experts Saying About the New Guidelines?
In a statement, Dr Louis Levy, head of nutrition science at Public Health England, says: "A healthy, balanced diet and short bursts of sunshine will mean most people get all the vitamin D they need in spring and summer. However, everyone will need to consider taking a supplement in the autumn and winter if you don't eat enough foods that naturally contain vitamin D or are fortified with it. And those who don't get out in the sun or always cover their skin when they do, should take a vitamin D supplement throughout the year."
Wales' minister for social services and public health, Rebecca Evans, says: "We will now act on the findings of the [Scientific Advisory Committee on Nutrition] report and will raise awareness and support people to achieve this intake as part of our wider approach to health and wellbeing."
In a statement, Susan Fairweather-Tait, professor of mineral metabolism at the University of East Anglia says the new guidelines take a conservative approach and it is possible that in future evidence will emerge for a higher daily intake of vitamin D, "and the Dietary Reference Values may need to be revised upwards."
The Scientific Advisory Committee on Nutrition (SACN) recommendations on vitamin D
Public Health England
Cancer Research UK: Vitamin D
NHS Choices: Vitamin D
Science Media Centre