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Why do people become vitamin D deficient?

Written by | 2 Dec 2022 | 'In Discussion With'

Vitamin D deficiency occurs even in sunny parts of the world and one way to tackle it could be fortification of flour, a foodstuff that is used by most communities, suggests Professor of Molecular Endocrinology, Martin Hewison.

One reason for low levels of vitamin D is limited exposure to sunlight and countries in the far North or South of the globe receive considerably less sunlight during the winter months. “You can only really be assured of getting a lot of sunlight access if you live … around the Equator, so, yes, it could be simple geography”, says Professor Hewison.

Another important factor is skin colour. “You need more photons of sunlight to create the same amount of vitamin D if you have dark darker skin pigmentation. So, if somebody has darker skin pigmentation living in a country like the UK there’s always going to be a greater risk of vitamin D deficiency whereas if they lived in the tropics that probably wouldn’t apply”, he says.

“You do need to have your arms and face exposed to sunlight in order to make adequate amounts of vitamin D in any country”, explains Professor Hewison. Factors that prevent this level of skin exposure such as a dress code that requires complete skin coverage, using a large amount of sunscreen or simply living and working indoors all the time can lead to vitamin D deficiency.  Even in a sunny place this can happen.

Professor Hewison recalls:

“I worked for 10 years in California in Los Angeles. When I was checked for my vitamin D levels – and I’d been there for a few months …. I was vitamin D deficient and that was principally because I was working so hard I would get into the car, drive to work, to a covered parking lot ….. and I was rarely outside and so I wasn’t making as much vitamin D as you might suppose living in a very sunny place.”

A similar pattern is seen in Saudi Arabia where low vitamin D levels are common because people routinely cover their skin.

In 2015 the Science Advisory Council on Nutrition admitted that the UK was vitamin D deficient as a nation and that it could not be guaranteed that everyone would maintain adequate levels of vitamin D on their own. The period between October and April is when there is least sunlight – often easily remembered as the period between when the clocks go back (to GMT) and when the clocks go forward (to British Summer Time).  “That’s really the time when you should be actively supplementing with vitamin D.  … During the winter months, absolutely, I think everybody should be given vitamin D supplements”, he says.

Striking a balance between sufficient skin exposure to make vitamin D and too much skin exposure leading to the risk of skin cancer is another important issue. Professor Hewison says, “It’s important to recognize that we are aware of potential damage that can be done to our skin by being in in sunshine and that’s really why the supplement story has become quite prominent”.

Vitamin D deficiency is less common North America and that is partly because much of the continent is further south than the UK but also because milk and orange juice are routinely fortified with vitamin D. In addition, there is greater awareness of the risks of vitamin D deficiency and so many people also take supplements, especially the elderly. “But in North America there is still a high prevalence of vitamin D deficiency in African-American communities where darker skin pigmentation means that it’s it is more difficult to make vitamin D. So, it’s not the whole of the North America that is vitamin D replete – there are communities where there is still a significant problem”, he says.

One approach to this has been food fortification – an approach that has worked well in Finland where several common foodstuffs were fortified.

Professor Hewison says: “We’re making, I think, good inroads in the UK in trying to approach this issue.  I think it’ll become more prominent in the next few years principally because the technology to put vitamin D into foodstuffs has improved. So previously, like in North America, it used to be in orange juice and milk, for example, but not all communities use those products so something that may be more accessible is flour. If you can put vitamin D into flour, most communities use flour regularly and that could be a better way of getting a more even supplementation ….. of vitamin D in the general population.”

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