The two main routes by which the body obtains vitamin D are from dermal synthesis - making it in the skin following exposure to the sun - and from a person’s diet.1 Vitamin D deficiency arises in patients who do not produce or consume enough vitamin D over a period of time, which can be the result of a number of reasons including prolonged lack of exposure to natural sunlight, a diet lacking in animal products, or high levels of melanin in the skin, which reduce its ability to produce vitamin D.2

Despite these two routes to production - and the important role vitamin D plays in keeping people healthy - deficiency is common and can be found across all groups of people, in all age ranges, across all parts of the world. During winter in the UK, for example, when exposure to natural sunlight is often limited, between 30 and 40% of people in the general population above one year old are classed as having vitamin D deficiency.1 Even towards the end of summer, 8% of adults and 13% of adolescents remain deficient, according to data from Public Health England and the Food Standards Agency.1

Causes of vitamin D deficiency

Vitamin D deficiency is prevalent across all ethnic groups in all locations, but there are a number of factors - including a lack of exposure to natural sunlight - that increase people’s risk of having inadequate levels of vitamin D in their bodies. Other risk factors and potential causes include lifestyle, the use of sunscreen or sunblock, geographical location, the tone of someone’s skin, their age and their body weight.2

Time outdoors

Some people won’t be able to produce the right amount of vitamin D because they spend less time outdoors, or they cover up with clothing while they are outdoors, and so get less exposure to natural sunlight.3 The Department of Health recommends people take a daily vitamin D supplement if they spend a lot of time indoors or otherwise do not get a lot of sunlight. This may include people who work long office hours or are housebound or in a care home, or if they cover up most of their skin when they do go outdoors.

Geographical location

Linked to the issue of sunlight exposure is the geographical location of someone’s home country. In the autumn and winter months, light of the critical wavelength needed to stimulate dermal synthesis of vitamin D cannot reach people living at certain latitudes, because the sun is at too low an angle in the sky. In the UK, people cannot make enough vitamin D from the sun for six months out of the year, between October and March. Even in the summer months, there is a difference in the amount of critical wavelength sunlight available in the north of the UK versus the south.

Sunscreen use

For people who do spend a lot of time outdoors in sunlight, sun creams or other sun protections are vital in blocking the harmful ultraviolet B rays that cause cancer, but using them also blocks most of the skin’s production of vitamin D, so heavy users of sunscreen are often more likely to be at risk from vitamin D deficiency.3 How much time in the sun it takes to produce enough vitamin D differs from person to person, as does the time it takes skin to burn, so the advice from the NHS remains that if someone plans to be outside for a long time, they should cover up, wear sunglasses, seek shade and apply at least SPF15 sunscreen.

Skin colour

Melanin is present in higher levels in people with darker skin tones, and this pigment acts as a natural sunscreen, slowing down levels of vitamin D production. Because people with darker colour skin have to spend more time in sunlight to produce the same amount of vitamin D as people with light skin4, the risk of vitamin D deficiency is often greater in those of African, African-Caribbean or south Asian ethnic origin.4

Body weight

Being overweight or obese can be a cause of vitamin D deficiency, because people with excess body fat generally have lower levels of vitamin D. This is because vitamin D is sequestered in fat cells, meaning overweight people store more of it and have less of it circulating in their blood, decreasing its availability for hormonal actions.5,6


As people get older, their ability to effectively produce vitamin D in the skin decreases, making ageing a common cause of vitamin D deficiency.7 Older people are also less likely to spend time outdoors and more likely to be in the shade or have their skin covered when they do.


In the UK, a healthy diet alone does not provide sufficient levels of vitamin D, since few foods naturally contain it.1 Oily fish such as salmon, mackerel, herring and sardines are a significant source, while red meat, eggs and a small number of fortified foods provide smaller amounts. Examples of fortified foods include margarine, formula and fortified milks, a few breakfast cereals and some brands of yoghurts. As most of the food-based sources of vitamin D are animal products, it is harder for people to get vitamin D from food if they are vegetarian or vegan.

Ultimately, the causes of vitamin D deficiency differ from person to person and low levels of the vitamin can be found in all kinds of ethnic groups and age ranges, around the world, for a variety of reasons. The Department of Health recommends people take supplements if they rarely spend time outdoors, or cover their skin up when they do, or have darker skin tones, as these groups may be more at risk of vitamin D deficiency.8 Additionally, where there are indications that might suggest the need, practitioners may want to consider testing for vitamin D deficiency to determine whether patients may need prescribed medication to bring their vitamin D levels up to sufficient levels.



3. Holick MF. Vitamin D deficiency. N Engl J Med. 2007; 357:266-81.







Job Code: FUL-519 Date of Preparation: June 2019 Date of Revision: January 2020