What is vitamin D and why is it important?
Vitamin D is produced by the body when it is exposed to sunlight. It is essential as your body needs it to absorb calcium and to promote healthy bones and teeth. There is also some evidence to show that vitamin D could help prevent some cancers, cardiovascular disease and multiple sclerosis.
How much vitamin D do you need?
According to the NHS, both adults and children, over the age of one year, need 10 micrograms of vitamin D per day. While it's possible to get this quantity from the sun between April and September, throughout the winter months you may need to supplement the amount.
How do you know if you're deficient?
"Deficiency can be assessed using a simple blood test – if you suspect you may be deficient ask your GP to test you or get tested privately," says the nutritionist Kim Pearson. "Common symptoms of deficiency include lower-back pain, muscle weakness, muscle aches and fatigue, but you can be deficient and experience no symptoms at all."
Does SPF affect your vitamin-D levels?
"We all need some UV for our bodies to make vitamin D – but most people can make enough vitamin D without spending a long time in strong sun or risking sunburn," says Sarah Williams, Cancer Research UK's health information manager. "The best way to get a good balance between making vitamin D and risking skin cancer is to regularly have short exposures to the sun without sun protection."
Can you eat your vitamin D?
"The majority of people get the bulk of vitamin D through their diet," says the skin expert Dr Lancer. Red meat, egg yolks, liver and oily fish, including salmon, sardines and mackerel, are the best food sources.