What is Vitamin D?

Vitamin D is a nutrient which is required by the body to ensure the good health of bones, teeth and muscles. Vitamin D is a group of fat soluble vitamins which are responsible for the absorption in the intestine of calcium and phosphate. Vitamin D can be acquired 3 ways via 2 major forms.

Vitamin D2

  • Naturally occurring in plants
  • Produced from the irradiation of UVB irradiation of ergocalciferol

Vitamin D3

  • Known as cholecalciferol
  • Product of UVB irradiation of 7-dehydrocholesterol
  • Vitamin D3 made in the skin of humans
  • Can be consumed in fortified foods or supplements

Vitamin D deficiency

Due to vitamin D being the product of UVB irradiation and as a result of it being made in the epidermis (the outer layer of the 2 layers which make up the inner layer) of human skin, it is essential for us to receive enough sunlight to be able to make adequate and healthy amounts of the nutrient.

Vitamin D deficiency is quickly becoming a public health issue with around 1 billion people having low levels of vitamin D worldwide.

In the UK, during the months of March and October we should be able to expose ourselves to enough daily sunlight (at least when the sun shines for the British summer). Thus, providing us with adequate levels of vitamin D.

The body can synthesise vitamin D from the sunlight when we are outdoors. By being in the sunshine for short periods of time daily should provide you with enough sunlight and in turn vitamin D.

However, during the Winter months, October through to March, there isn’t always enough sunlight to ensure good amounts of vitamin D. So, during the winter as there is not enough sunlight containing UVB radiation we need to get vitamin D from alternative sources. You can check for vitamin D deficiency with doing a blood test, either an at-home blood test or with your GP.

These sources may include:
Oily fish

  • Salmon, mackerel, herring

Red meat


Fortified food

  • Breakfast cereal
  • Fat spreads

Non-dairy milk alternatives

  • Soya
  • Almond
  • Coconut

Infant formula milk contains Vitamin D by law

Dietary supplements

Deficiency in vitamin D has been described as having a serum 25-hydroxyvitamin D ((25(OH)D)) concentration of <25 – <75 nmol/L.

Thus, a normal range for vitamin D is between 75 and 250nmol/L.

Signs and symptoms of vitamin D deficiency

Vitamin D is renowned for its anti-inflammatory and immune modulating benefits. Therefore, deficiency can cause many signs and symptoms which may include:

  • Aching bones and joints
  • Weak muscles
  • Depression
  • Feeling tired
  • Weight gain
  • Susceptible to getting ill
  • Rickets
  • Digestive issues
  • Sweaty scalp

Only 10% of our vitamin D comes from our diet which leaves 90% being acquired from sunlight exposure. Thus, it is important to look out for the signs and symptoms particularly if your lifestyle means you’re not out in the sun very often.

Who is at risk?

Anybody can become deficient in vitamin D. However, certain individuals are at a greater risk. For example, vegans because most foods which have naturally occurring vitamin D are animal sources, individuals with darker skin as the pigment reduces the ability to make vitamin D and those who are not often exposed to sunlight.

What are the causes of Vitamin D deficiency?

Sunlight Exposure

The biggest cause of vitamin D deficiency is a lack of sunlight exposure. This is mainly due to many people not really knowing or appreciating that the sun is a key feature in the human synthesis of vitamin D.

There are very few foods which naturally contain vitamin D and those that do are mostly animal sources so vegans and vegetarians are at a greater risk.

Melanin in our skin is very good at absorbing UVB radiation and so increased pigmentation in the skin can lower the synthesis of vitamin D.
Equally a sunscreen with a sun protection factor of 15 absorbs 99% of UVB radiation and so can block skin from absorbing and lower the synthesis of the vitamin.

Furthermore, the angle at which the sun’s rays reach the earth can also have a detrimental effect on our production of vitamin D. During winter, the zenith angle is increased as well as in the early morning and late afternoon. Therefore, at these times very little vitamin D production occurs.
In people who are institutionalised e.g. in a care home where sunlight exposure may also be reduced. This is also true for individuals who cover their bodies for example for religious reasons.

Deficiency has grown in countries where sunlight exposure has been limited. For example, in Australia where a huge increase in the cases of skin cancer occurred, the encouragement to completely cover up and wear high factor sun protection resulted in a higher number of Vitamin D deficiency cases.

On the other hand, vitamin D deficiency can also be a risk factor for developing cancer. Some studies have shown a link between the deficiency and the development of cancers such as breast and prostate.


As we get older our bodies don’t always work as they used to when we were younger. Things like walking up the stairs may become a little more challenging, but this is also true for the processes which occur within our body, including the production of vitamin D.

As we age the precursor to vitamin D3 in our skin, 7-dehydrocholesterol becomes less abundant in its concentration. Young people have around a 25% higher amount of 7-dehydrocholesterol than someone who is 70 years of age. Therefore, the 70-year olds capacity to produce vitamin D3 in their skin is 75% less than their younger counterparts.

Medical Conditions

Conditions such as kidney disease mean sufferers are less able to convert the precursor to vitamin D into the hormone itself. As a result, this can lead to the development of other conditions such as secondary hyperparathyroidism and metabolic bone disease.

Furthermore, children and adults who have musculoskeletal problems such as muscle weakness, myalgia and bone pain should also have their Vitamin D levels routinely tested. A lack of vitamin D can be a marker for poor nutrition and low levels of exercise.


Supplementation of vitamin D is both safe and relatively inexpensive. Research has shown that supplements of D2 and D3 are sufficient for treating and preventing vitamin D deficiency.
However, at-risk groups are recommended to supplement their vitamin D intake. For example:

  • Breastfed babies up to 1 year should be given 8.5-10mcg of vitamin D supplements to ensure they get an adequate amount
  • Children aged 1-4 years should take a daily supplement of 10mcg of Vitamin D
  • Everybody should consider supplementing vitamin D particularly in the winter months of 10mcg daily
  • Daily supplements of 10mcg should be given to at-risk adults daily (the elderly, those that don’t go outdoors ofter and those who wear clothes to cover their skin most of the time.)


Vitamin D is an important nutrient for musculoskeletal and indeed non-skeletal health. However, due to a lack of foods with naturally occurring vitamin D it can be hard to acquire it, particularly during the winter.

Humans need UVB radiation from the sun to synthesise vitamin D in their skin. Little or no exposure to sunlight or wearing sunscreen, dark skin pigmentation or not spending long outdoors can all be potential risk factors for developing Vitamin D deficiency.

Over the last two decades, the cases of Vitamin D deficiency have increased dramatically. In cases of deficiency dietary intake alone is rarely beneficial. Instead, supplements should be administered daily to ensure levels of Vitamin D in the body remain within an optimal range.

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Having always been intrigued by how body’s work, both in their day to day functions and also when things go wrong, Leanne is pursuing her passion for health. By combining her undergraduate degree in Biomedical Science and a Masters in Science communication with her love for writing, she is passing on her knowledge to Health Hub readers. Leanne strongly believes that science and health are something that should be talked about more and she hopes her articles will enable such conversations.