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 osteomalacia.
You could be at risk of a vitamin D deficiency if you always have a cold, it’s been revealed.
Getting sick easily or often could be a warning sign of a vitamin D deficiency, warned dentist Dr Steven Lin.
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A lack of vitamin D could impact the immune system, which may lead to feeling sneezy relatively often.
But just because you always have runny nose and a fever, it doesn’t necessarily mean you have a vitamin D deficiency, he warned.
“Vitamin D deficiency is the most common and easiest indicator of your health you need to be familiar with,” said Dr Lin.
“It’s estimated that 50 per cent of the world is vitamin D deficient.
“Vitamin D deficiency is so prevalent and significant to your health, it’s important to know you’re getting a sufficient amount.
“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 be at risk of a vitamin D deficiency if you have chronic pain, have regular gut issues, or have wounds that take longer to heal, he added.
A general feeling of weakness, hair loss or having bones that break easily could also be warning signs of the condition.
He added: “Keep in mind, some people run a higher risk of having vitamin D deficiency than others.”
You’re more likely to develop a vitamin D deficiency if you are overweight, have darker skin, or don’t often go outside, said Lin.
People over 50 years old are also more likely to develop a vitamin D deficiency.
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.
But, taking too much vitamin D over a long period of time could cause too much calcium to build up in the body, and it may damage the heart and kidneys.