All you need to know about vitamin D

Analysis: we hear a lot about vitamin D and a healty lifestyle, but what are the actual facts and figures about it?

By Marina Navas, Trinity College Dublin

Such is the coverage it gets, you would be forgiven for thinking that vitamin D is the key to unlocking a healthy lifestyle. While vitamin D is important for many health outcomes such as bone formation, calcium absorption and immune function, its promotion is always counterbalanced with arguments around its elusive nature and difficulty to obtain and maintain in the body. So what are the facts and figures about vitamin D?

47% of the Irish adults aged 18 to 39 are deficient in vitamin D and it rises to 93% in dark-skinned Irish Black, Asian and Minority Ethnic communities. Why is this the case? First of all, food sources don't supply enough of it. We can obtain vitamin D from oily fish like salmon, tuna and mackerel; seafood; cod liver oil; irradiated mushrooms (which are more difficult to find and more expensive than normal mushrooms) and fortified foods such as dairy.

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From RTÉ Radio1's Ray D'Arcy Show, director of human nutrition and dietetics at TU Dublin Daniel McCartney on the benefits of Vitamin D, particularly in response to Covid-19

Only 3.1 micrograms of the 15 micrograms of vitamin D adults should be consuming daily are obtained through a typical Irish nutritional intake. This could be due to a lack of fish in Irish diets, which is four times lower than in other countries such as Iceland. Despite fish being one of the best sources of vitamin D, it only provides us with 12% of our daily intake. Some commenters note that it has to do with people not knowing how to cook fish or due to its price. This is reinforced by the Food Safety Authority of Ireland (FSAI), which concludes that most daily Irish food intake patterns do not provide enough vitamin D.

This is why we always hear about the sun. Even though the idea of vitamin D jumping into a rocket to come all the way from the sun to be absorbed by us humans is appealing, this is not what happens. Instead, we have its precursor naturally present in the skin. When we receive sunlight, our bodies transform this precursor into another form of the molecule named vitamin D3. Vitamin D3 travels to the liver and the kidneys, where it is further metabolized to the active form which is responsible for all health outcomes.

But the step where the precursor is transformed into vitamin D3 in our skin is the problematic one, as we don’t get a lot of sunlight in Ireland. Another problem is that the UVB sunrays have to be of a specific intensity to be successful in allowing us to produce vitamin D, which doesn’t happen in Ireland from October to March. One could think of sunbeds as an alternative for obtaining vitamin D during those months as they use the same type of radiation. However, neither sunbathing nor sunbeds are safe as the International Agency for Research on Cancer has determined that exposure to UV rays can lead to skin cancer, regardless of their source.

From RTÉ Radio 1's Drivetime, how a lack of Vitamin D has long been seen as a leading cause of a number of bone diseases.

However, there is one alternative that we haven’t explored yet: supplements. Not without controversy, supplements can aid your health if used properly. We should take into account that our bodies love the middle ground: they complain if we give them too little of a micronutrient, but they aren’t happy either if we give them too much.

Most people have heard of vitamin deficiency, but hypervitaminosis, or the excessive consumption of vitamins, is on the other end of the scale. In the case of vitamin D, it can result in damage to the bones, heart and kidneys.

The current recommendation for an adult’s supplementation is 20 to 25 micrograms per day. This is higher than the recommended daily intake of 15 micrograms, which is the minimum amount needed to maintain bone structure but not enough to achieve additional health benefits, according to the Endocrine Society.

From RTÉ Six One News, Oireachtas Health Committee told of Vitamin D deficiency in population

In case of vulnerable adults with a greater likelihood of deficiency, supplementation can be higher than 20 to 25 micrograms, but this should take place under medical supervision. Children should take 10 micrograms per day, while it recommended that babies should be given 5 micrograms per day. This is because their sensitive skin shouldn’t be exposed to the sun and their food sources may be lacking in vitamin D.

Even though these supplements in the given concentration are safe, there is controversy about their efficacy. In a recent review, scientists looked at studies linking vitamin D supplements and disease prevention and they found that the current evidence didn’t back them up as a way of preventing cancer, cardiovascular diseases, bone fractures or infections. Although, it is important to note that the quality of the evidence is still low, meaning that more studies are necessary.

Taking all of this into consideration, one could ask if vitamin D should be a measure in standard blood tests? Currently, the HSE advises against it, unless you are in a high-risk group. There are a number of reasons for this. First of all, official organisations don’t agree on what to label as deficient. For example, the Institute of Medicine recommend blood levels above 20 ng/ml, but the Endocrine Society says it should be 30 ng/ml. This can lead to misclassifications of people who aren’t vitamin D deficient as if they were.

Even though the idea of vitamin D jumping into a rocket to come all the way from the sun to be absorbed by us humans is appealing, this is not what happens

Then, the form of vitamin D that is measured is that made by the liver, not the active form made by the kidney. This is because the former is easier to measure and more stable, but there is a debate on whether this is the best marker of vitamin D health status. Adding to all of this uncertainty, depending on the laboratory technique used to measure the vitamin, the results can vary.

So, who should get tested? Patients with bone disorders like osteoporosis or patients with other relevant clinical conditions that could be attributed to vitamin D deficiency such as clinically significant muscle weakness, history of falls or liver and kidney disease.

For the rest of the population, HSE recommends enforcing a stricter food fortification system and the Oireachtas Health Committee recommends all adults to take 20-25 micrograms/day in the form of supplements. It is important to note that everybody is different - for example, there are some cases where vitamin D supplements might counteract some medicines, like cholesterol pills, that can interact with vitamin D. If you are in doubt on whether you should take a supplement or not, your doctor is the one to ask.

Marina Navas is a PhD candidate with SSPC, the SFI Pharmaceutical Research Centre, at Trinity College Dublin.

The views expressed here are those of the author and do not represent or reflect the views of RTÉ

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