Why do I need vitamin D?

The reason why most of us associate vitamin D with teeth and bones is because, just over 100 years ago, it was found that deficiencies in this vitamin could result in children having rickets, a condition that results in soft, weak bones and bone deformities, such as bowed legs. 

“Vitamin D helps the body absorb calcium and phosphorous from the gut,” explains Dr Milli Raizada, a GP and senior clinical lecturer in primary care at Lancaster University’s Medical School. 

“That calcium, in turn, mineralises bones and strengthens them.” For a long time, this was thought to be the only benefit of vitamin D. However, as Prof Martin Hewison, director of the Institute of Metabolism and Systems Research at the University of Birmingham, explains, in recent decades, the science has moved on. 

“Around 25 years ago, people found that the vitamin D receptor – the molecule that transmits the effects of vitamin D – is present in many more tissues than just the intestine. It’s in cancer cells, in muscles, in the immune system – all sorts of places that are not related to the prevention of rickets. 

So people started looking at other roles it might play and some studies have shown that it seems to slow the growth of cancer cells, it can regulate immune system cells, it can act as an anti-inflammatory agent and promote antibacterial and antiviral effects in immune cells.” 

But most of these functions have been observed in animals or cell cultures, which is why there is still not conclusive evidence that the same is true in humans. 

“A number of studies have shown that people with autoimmune diseases, such as type 1 diabetes, rheumatoid arthritis and multiple sclerosis are more likely to be vitamin D deficient. And there are other diseases, such as Parkinson’s and Alzheimer’s where patients generally have lower levels of vitamin D than the healthy population.” 

But, cautions Prof Hewison, these studies show association, not necessarily a causative link. “The only way to find out if higher levels of vitamin D will prevent disease is to supplement people with vitamin D over a very long period of time and that’s a very difficult, and expensive thing to do.” Hence the controversy over the true benefits of vitamin D.

What is vitamin D deficiency?

Vitamin D deficiency in children can cause bowed legs and rickets, but in adults it’s more likely to manifest as general tiredness, bone pain and muscle pain. And, as Dr Raizada flags, it can also make older people more susceptible to falls

A blood test carried out by your GP can establish your levels of vitamin D, but according to Nice guidelines, you would be unlikely to be tested unless you were showing symptoms associated with a deficiency, such as those described above – even if you are in one of the at-risk groups. 

“People susceptible to lack of vitamin D include children, those over 65, women who are pregnant or breastfeeding, and anyone who has problems absorbing the vitamin, or issues with the kidney or liver, which help metabolise vitamin D,” says Dr Raizada.

“Because much of the vitamin D in the body is synthesised by the body through exposing the skin to the sun [see below], darker-skinned people, who have more melanin in their skin, may also be deficient.” 

However, what is meant by deficient varies around the world. Here in the UK, vitamin D levels are measured by looking at the number of nano moles (nmol – a very small unit of weight measurement) in a litre of blood – less than 25, and you’re considered deficient, 25-50 is insufficient and over 50 is sufficient. 

Prof Hewison believes that the UK should be more aspirational about vitamin D levels in the population. “In North America, they say that you should be targeting 50nmol/l and above,” he says. 

“But over there they add it to milk, and orange juice. Here in the UK, not only do we not do that, but when the Science Advisory Committee on Nutrition set out their advice in 2016, they advocated aiming not to be below 25nmol/l, which seems a very timid approach. Their argument is that the only proven link is between lack of vitamin D and rickets, and so if you keep people above the threshold for that, that’s acceptable. However, there is a lot of data, including data on mortality that shows that mortality is much higher when levels are below 50nmol/l, which is why I and many of my colleagues in the UK have been arguing that we should be aiming to be above 50nmol/l, not just not below 25nmol/l.”

What are good sources of vitamin D?

While diet can be a source of vitamin D – you’re looking at oily fish like herring, mackerel and salmon, as well as egg yolks – it’s not a very good source, and so even here in the UK, where there’s not much sun, we still get 90 per cent of our vitamin D from exposure to the sun. 

“Our skin naturally contains a precursor to vitamin D,” explains Dr Raizada. “When the UVB rays in sunlight hit the skin, they start the process of converting this precursor of vitamin D to a form that the body can use. Further changes take place in first the liver and then the kidneys, until the activated form of vitamin D is produced.” 

According to consultant dermatologist Dr Anastasia Therianou, someone with Caucasian skin needs around nine minutes of sunlight between midday and 3pm. 

Those with darker skin, closer to 25 minutes. These figures are based on not wearing sun protection and 35 per cent of the skin area being exposed – roughly what you’d be showing off if wearing a modest pair of shorts or a skirt with a T-shirt.

 A little daily sun exposure is better than going out once a week without sun cream for an hour as it balances your vitamin D needs with the skin cancer risks associated with UV. 

However, Dr Therianou points out that if someone has a sun-sensitive condition, such as lupus or skin cancer, it’s best to get your vitamin D from supplements. 

This is echoed in a 2021 paper entitled Vitamin D and Skin Cancer: An Epidemiological, Patient-Centered Update and Review which was published in the journal Nutrients. 

The authors concluded that: “Sun protection recommendations among people at risk of skin cancer or patients with a personal history of skin cancer should be kept, and … neither natural nor artificial sun exposure should be encouraged as the main source of vitamin D. Given that dietary and supplemental vitamin D is functionally identical to that produced after UV exposure (and is also more reliable and quantifiable), it should be the preferred source of this vitamin.” 

Of course here in the UK we don’t really have a lot of choice in the matter. In fact, because of the lack of sunlight in this country, the NHS recommends that between September and March adults take a daily supplement of 10mcg (micrograms), or 400IU (international units). 

The research suggests that this will be enough to ensure that 97.5 per cent of the population have a level that is equal to, or greater than, 25nmol/l. 

And those at risk of deficiency – people with darker skin, housebound adults, or people who cover up a lot when they are outside – should continue to take it all year round. Other groups at risk of deficiency include patients who are taking drugs such as orlistat (aka Alli or Xenical) which stop fat absorption. And people with a high BMI. “It’s very common for people with a high proportion of adipose tissue, or fat, to have lower circulating levels of vitamin D,” says Prof Hewison. 

“Vitamin D gets deposited in adipose tissue which means it’s stored in a way that isn’t usable. There are currently some studies being done at the  University of Bath to find out whether exercise can help to move the vitamin D from the adipose stores into circulation.”


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