woman sunbathing

Vitamin D – the sunshine vitamin

There has been a question which a large proportion of my patients have been asking with an ever-growing frequency, and that is “How can my vitamin D be low if I am living in such a sunny climate?”

A bit of a conundrum, yes? After all, it is the vitamin that we get from the sun, is it not? Well, actually, no, not exactly.

Just like so many things in life, the version we are aware of is not exactly the whole truth of the matter. So, let’s start with a bit of history and biochemistry …

Vitamin D was first described in 1645 by an English physician with regards to its role in the development of rickets, a childhood disease characterised by abnormally developed, soft, weak long bones which start to bow under the weight of a child as it starts to walk.

Since vitamin D is responsible for the absorption of calcium, magnesium and phosphate from the gut (the main building blocks of bone), low circulating levels of this vitamin will lead to bone weakening and other related conditions in adults such as osteomalacia and osteoporosis.

Vitamin D is actually a composite of fat-soluble compounds which come in several forms, the main being D2 (ergocalciferol, found in fungi and plants) and D3 (cholecalciferol, found in animal products). Once ingested, they are actively metabolied by the kidneys and liver to 25OHD, the principal circulating form of vitamin D.

The kidneys then further metabolize 25OHD to 1,25(OH)2D, although other tissues including various skin cells, cells of the immune system, and the parathyroid gland also have this capability. This final active metabolite (otherwise known as calcitriol) is the principal hormonal form of vitamin D, responsible for most of its biologic actions.

So, where does the sun factor come into this?

UVB rays activate the provitamin D3, a completely different element named 7-dehydrocholesterol (7-DHC), which appears to be a by-product of our endogenous cholesterol-forming system and is mainly stored in the epidermal layer of our skin.

This irradiation process transforms 7-DHC into the active metabolite D3, which then follows the same breakdown process in the liver and kidneys as the ingested varietals, until the final active element of calcitriol has been formed.

What affects this production/transformation?

  • Lack of sun exposure: whether due to actual geographical position limiting UV exposure (such as northern European countries in the winter months), or excessive sunblock/sun screening usage. Some of the populations with the highest level of vitamin D deficiency actually live in hot climates, where covering up from the sun is part of their traditions. A good example of this was the high rate of rickets in children from the Saharan Tuareg peoples. Remember that our sunblocks have also improved in terms of the degree of barrier protection against both UVA and UVB rays, allowing for less chance of photosynthesis. The same goes for windows in houses and cars … better UV protection equals less photosynthesis.
  • High melanin levels in the skin: darker skin tones exhibit a photoprotective function not only protecting the skin from UV rays but also reducing the photosynthesis of D3. Hence the more tanned we are, the less vitamin D3 we produce.
  • Low fat diets: this is a rather controversial one. Although vitamin D is one of the fat-soluble vitamins, there is no clear evidence that a lack of dietary fats affects absorption of the vitamin D3. However, it makes sense that if there is no ingestion of external sources of D3 (such as fatty fish, egg yolks, dairy, etc.), there will also be minimal absorption resulting in reduced total circulating levels thereof.
  • Age and obesity: some research seems to point to a slower skin synthesis with age and increased sequestration of D3 in fat cells in individuals with more abundant adipose tissues.

So, what are the benefits of vitamin D?

Vitamin D supplementation has increased dramatically in the last few years, especially during the COVID pandemic, when research found a link between low vitamin D levels and poorer covid outcomes. This is in keeping with some of the other functions of D3:

  • Immune support and anti-inflammatory function
  • Improved muscle strength, recovery and balance, thus reducing the risk of falling and fractures especially in the older population
  • Potential role in type 1 and 2 diabetes prevention through possible reduction of insulin resistance
  • Potential improvement in cardiovascular disease outcomes, although research is still very ambiguous on this point, with a recent five-year study showing very minimal benefits of vitamin D supplementation in major cardiovascular events

So, all in all, it looks like we simply need to revert to the good old days of “sun’s out, guns out”, rather than the Aussie-styled “slip, slop, slap” campaign, even if it is for a few minutes a day while we munch on this year’s catch of sardines on the beach!

By Dr Jo

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Dr Joanna Karamon is a General Practitioner with over 20 years’ experience. She is Clinical Director of Luzdoc International Medical Services Network