Vitamin D is an important vitamin that regulates the amount of calcium and phosphate in the body.
Calcium and phosphate are needed to keep bones, teeth and muscles healthy.
A lack of vitamin D can cause bones to become soft and weak.
This can lead to bone deformities such as rickets in children and bone pain in adults.
In the spring and summer months, from about the end of March to the end of September, most people get the necessary vitamin D from sunlight.
This is because the body produces vitamin D from direct sunlight on the skin.
During the winter months of the UK from October to March, sunlight in winter does not provide enough UVB radiation for the skin to produce vitamin D.
To avoid a shortage, the UK Department of Health recommends everyone take vitamin D daily during those months.
Daily supplements should contain 10 mg of vitamin D.
It is important not to consume too many vitamin D supplements over a longer period of time, as this can cause too much calcium to accumulate in the body.
This condition is called hypercalcaemia and can weaken the bones and damage the kidneys and heart.
The NHS warns not to consume more than 100 mg of vitamin D per day as it may be harmful.
Children between 1 and 10 years old should not have 50mcg per day and infants under 12 months should have a maximum of 25mcg.
"If you decide to take vitamin D supplements, 10mcg per day is enough for most people," the NHS said.
It is also possible to obtain vitamin D from some foods, such as oily fish, red meat, liver, egg yolks, and fortified foods.
Too rich fish include salmon, sardines, herring and mackerel.
Fortified foods include most fat spreads and some breakfast cereals.
While sunlight in spring and summer provides enough vitamin D for most people, some people may still be at risk of being deficient at this time.
People who are still at risk during the summer include people who are not often outdoors, people living in care homes, and people who usually wear clothes that cover most of the skin outdoors.
People with dark skin from African, African-Caribbean and South Asian backgrounds may not get enough vitamin D from sunlight.
Therefore, people in these categories want to take vitamin D supplements throughout the year.