Why vitamin D is so important - and how to tell if you’re not getting enough

Recent research suggests that taking vitamin D supplementally is just as effective as metabolising it through exposure to sunlight. Photo / 123RF

Most of us have a vague awareness that the “sunshine vitamin” – vitamin D – is associated with good health. But an ever-increasing volume of medical research is suggesting specific ways in which vitamin D can help to protect against serious conditions, and it is also becoming clear that taking it supplementally is just as effective as metabolising it through exposure to sunlight.

A recent landmark study, published in the British Medical Journal, concluded that vitamin D supplements could help to reduce the risk of heart attacks if routinely taken by the over-60s. Previous research has suggested taking vitamin D supplements in old age can help prevent dementia.

So should we all be taking them? Vitamin D is the only supplement that the UK government broadly recommends for all adults – and even then only during the autumn and winter months – and yet its direct benefits remain hotly debated within medical circles. While most of us know it to be “good for healthy teeth and bones”, in recent years it has been associated with far more wide-ranging health claims.

During the Covid pandemic, it came under the spotlight after it was reported that the people who had the most severe cases of Covid-19 also had low levels of vitamin D.

However, subsequent research suggests that this might be because the disease itself reduces the body’s levels of vitamin D, and that having low levels of vitamin D isn’t a predictor of how badly you will get Covid.

Moreover, several trials treating people with vitamin D to either prevent Covid-19 or reduce its severity have proved, at best, inconclusive and, at worst, ineffective.

So what’s the truth about the so-called sunshine vitamin?

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 Professor Martin Hewison, director of the Institute of Metabolism and Systems Research at the University of Birmingham, explains, the science has since moved on.

Vitamin D supplements come in all forms – there are pills, capsules and sprays. Photo / 123RF

“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 Professor 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 UK health 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.

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.

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

However, what is meant by deficient varies around the world. 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.

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 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.”

While diet can be a source of vitamin D, it’s not a very good source, so we still get 90 per cent of our vitamin D from exposure to the sun.

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.”

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 has 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 semaglutide or Ozempic) 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 Professor 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.”

How should you take your vitamin D supplement?

You’ll have undoubtedly seen that vitamin D supplements come in all forms – there are pills, capsules, sprays – but what’s the best way to take it? One study done in rats looked at the bioavailability of different forms of vitamin D supplement, including a microencapsulated version, a version where the vitamin D was in tiny fat bubbles in a liquid (a bit like the sprays you get), and an oil-based vitamin D, similar to the capsules.

They found that the microencapsulated version was significantly more bioavailable than the oil-based version, and that the oil-based version was better than the spray-style preparation. So does that mean we should all be looking for microencapsulated vitamin D?

It’s not quite as simple as that. To get optimum absorption, you want to make sure you’re taking your supplement at the right time.

“While most people take their vitamin D in the morning, you’re actually better off taking it with your main meal of the day which has a decent amount of fat in it to help absorption,” says Dr Raizada.

And, while the rats were swallowing the spray-style version of vitamin D, Emily Royle, who is currently doing her PhD research into vitamin D at Ulster University, says that actually spray versions of vitamin D can be better. “For children or people who can’t easily swallow pills, a spray is a good idea,” she says.

“But, if used properly – you have to spray it onto the inside of the cheek so it gets absorbed directly into the bloodstream, rather than into your mouth and swallowing it – it also circumvents any undiagnosed disorders as you’re bypassing the gut entirely.”

Basically, even if you have any issues with absorption – such as Crohn’s disease – if you use a spray, it doesn’t matter.

And, if you’re diagnosed by a blood test at your doctor’s as being deficient, they can prescribe vitamin D for you. Depending on how deficient you are, they might prescribe a very high once-a-week dose for six weeks, and then a lower maintenance dose, then subsequently test you a month after your last high dose. But don’t expect things to happen overnight.

“If you’re deficient and start taking 2000IU/day, it will take three months to move your levels to a healthy level,” says Hewison.

Can you take too much vitamin D?

While the NHS recommends supplements of 10mcg or 400IU a day, the recommended upper limit for adults is actually 10 times this – 100mcg or 4,000IU – meaning most healthy adults can take this dose every day for the whole of their lives without doing themselves any harm, and in fact many experts in the field think it could be highly beneficial for most of us to be on this higher dose.

However, because vitamin D is a fat-soluble vitamin, the body has no way of excreting it so it can build up in the body – unlike water-soluble vitamins such as vitamin C which, if you have “too much”, your body just excretes in urine. So yes, in theory, you can overdose on vitamin D, but it is, as Dr Raizada puts it’s “very rare – I’ve never seen a case of it – and it would usually be the result of supplementation, you’re not going to overdose on vitamin D in your diet or from the sun”.

According to the Scientific Advisory Committee on Nutrition, the biggest concern when it comes to high levels of vitamin D is something called hypercalcaemia, when calcium is deposited in the soft tissues of the body. However, in their 2016 report, they point out that: “Case reports of vitamin D toxicity are associated with [vitamin D concentrations] greater than 300 nmol/l and more usually 600-1,000 nmol/l.”

And they conclude that: “In adults, a single dose of 7,500mcg (300,000IU) vitamin D at three-month or longer intervals would not be expected to cause any adverse effects.”

Can vitamin D deficiency cause hair loss?

“We know that vitamin D plays an important role in the hair follicle and affects the hair cycle,” says Dr Ben Esdaile, consultant dermatologist at Skin & Me. “There are a number of studies that show a correlation between low vitamin D levels and hair loss.”

However, that doesn’t mean that taking a load of vitamin D will make your hair grow back. “Conclusive studies to show the benefits of vitamin D in correcting hair loss are limited,” Esdaile says.

Dr Therianou agrees. “There has been a link between three hair-loss diseases and low levels of vitamin D, namely chronic telogen effluvium, alopecia areata and androgenetic alopecia. However, there is still much more research required before we can fully understand the link between vitamin D and hair loss.

“We know that vitamin D has a role to play in hair regrowth, and we aim to keep the levels within a normal range. However, supplementation to correct the vitamin D deficiency doesn’t necessarily mean that the hair issue will be resolved and you will get regrowth. So yes, in patients with any type of hair loss, we will tackle vitamin D deficiency if needed, but this is not considered the main treatment.”