You may have seen headlines saying that vitamin D supplements do not improve bone health or prevent fractures and falls, even though official advice says everyone should consider taking them - particularly during autumn and winter.
So what's the truth?
Why do we need vitamin D?
Vitamin D supplements have been a topic of hot debate for many years.
Some say they keep bones healthy and help ward off colds and flu, while others argue they are a waste of money.
Vitamin D helps to control the amount of calcium and phosphate in our bodies, which is needed for healthy bones, teeth and muscles.
Normally it's made when the skin is exposed to sunlight - so we get plenty in spring and summer.
But in autumn and winter, when sunlight is scarce, less of the vitamin is made and one in five people in the UK become deficient.
Although vitamin D is contained in some food, it's difficult to get the recommended amount this way.
That's why, in 2016, health officials said everyone should think about taking a daily vitamin D pill because it would be hard for some people to meet the daily 10 micrograms (400IU) recommended by experts.
They said the advice was to protect bone and muscle health.
That does not mean everyone has to buy the supplements - just consider whether they might need them.
The study behind the headlines is a large meta-analysis of 81 previous trials into vitamin D and bone health, but only four of those trials looked at people who were actually deficient in the vitamin.
Healthy people with healthy levels of vitamin D are unlikely to see any benefits.
Public Health England responded, saying its advice was still appropriate.
Stick to current advice
Other independent experts were critical too, pointing to the fact that taking supplements only works if you are low on the vitamin in the first place.
Prof Adrian Martineau, of Queen Mary University of London, said the findings "do not provide any reason to revisit or reconsider" current advice.
And Prof Martin Hewison, from the Society for Endocrinology, said: "Almost all trials for vitamin D supplementation have shown that supplementation is only effective if you are vitamin D-deficient to begin with.
"As such, the benefits of vitamin D supplements were difficult to determine from the study, even though it involved a large number of individuals."
The researchers, writing in The Lancet Diabetes and Endocrinology, say themselves that "more research might be needed".
Deficiency is defined as having levels of vitamin D below 25 nanomoles per litre of blood serum (nmol/L).
Who should take vitamin D?
- Everyone over the age of four should have a daily intake of 10 micrograms (400IU) of vitamin D, particularly from October to March, and consider taking a supplement
- Pregnant and breastfeeding women and at-risk groups (such as people from ethnic minority groups with dark skin, elderly people in care homes and those who wear clothing that cover most the skin) should take 10 micrograms of vitamin D per day all year round
- Children between the age of one and four should take 10 micrograms of vitamin D supplements all year round
- All babies from birth up to one year of age should take 8.5 to 10 micrograms of vitamin D per day (particularly those being breastfed)
Public Health England recommends that a healthy balanced diet and short bursts of sunshine will mean most people get all the vitamin D they need in the spring and summer.
Which foods contain vitamin D?
- oily fish
- red meat
- egg yolks
- fortified foods, such as most fat spreads and some breakfast cereals