Vitamin D is used by the body to keep the muscles, teeth and bones healthy, said the NHS.
It uses the vitamin to regulate the amount of calcium or phosphate in the body.
Not getting enough vitamin D can lead to severe bone deformities, rickets, or even osteomalacia.
You could be at risk of a vitamin D deficiency if you suffer from hair loss, it’s been revealed.
Losing your hair could be a sign that you should eat more vitamin D-rich foods, scientists have claimed.
The deficiency was linked to alopecia areata – an autoimmune condition that’s caused by the immune system attacking the growing hair.
The link isn’t entirely understood, and more research is needed to find why a vitamin D deficiency causes hair loss.
“Alopecia areata is a common cause of non-scarring [does not cause scarring to the scalp] hair loss that can occur at any age,” said the British Skin Foundation.
“It usually causes small, coin-sized, round patches of baldness on the scalp, although hair elsewhere such as the beard, eyebrows, eyelashes, body and limbs can be affected.
“In some people larger areas are affected and occasionally it can involve the whole scalp [alopecia totalis] or even the entire body and scalp [alopecia universalis].”
You could also be at risk of a vitamin D deficiency if you have an excessively sweaty head, it’s been claimed.
It’s one of the classic signs of the condition, and is why doctors ask new mothers about head sweating in their babies.
Nutritionist and dentist Dr Steven Lin added: “Vitamin D deficiency is the most common and easiest indicator of your health you need to be familiar with.
“Because vitamin D is such an integral part of your immune system health, some of the first symptoms of low vitamin D levels are overall malaise and getting sick easily.”
You could also be at risk of a vitamin D deficiency if you have wounds that take longer to heal, or if you have a general feeling of weakness.
Other deficiency symptoms include joint pain and achy bones – usually in the knees or back.
You’re more likely to become deficient in vitamin D if you’re over 50 years old.
That’s because the skin doesn’t produce as much vitamin D as people get older.
You can add more vitamin D to your diet by eating more oily fish, red meat, liver, or egg yolks.
Alternatively, try taking a vitamin D supplement. A 10mcg supplement should be enough for most people.