In 2024, researchers in Egypt published a review of 58 studies looking at vitamin D and COVID-19. They found that people with sufficient vitamin D levels were less likely to experience a severe bout of the virus and less likely to develop long COVID.
Also in 2024, across the Mediterranean Sea, a team of researchers in Greece reported that vitamin D could help diabetic patients manage their glucose levels. Meanwhile, here in the UK, scientists found that vitamin D could alter the gut microbiome of mice and increase their immune response to cancer.
All of that just in one year.
Research is also mounting that vitamin D protects against autoimmune disease. It seems to improve a person’s exercise tolerance. It may even reduce our long-term risk of cognitive decline and brain disorders like dementia. And that’s to say nothing of the vital role vitamin D plays in healthy calcium levels, bone health and immune function.
Medical scientists hate the word ‘panacea’, meaning a remedy for everything. But 100 years after its discovery, waves of research showing the stark, interconnected ways that vitamin D keeps us healthy… well, they just keep on coming.
Which makes it strange and slightly disturbing that so many of us remain vitamin D deficient. In fact, globally, there may be as many as one billion people who don’t have enough in their blood – including one in four Americans.
“In the UK we have over 20 per cent of people who are vitamin D deficient,” says Prof Susan Lanham-New, head of the nutritional science department at the University of Surrey.
She has been researching vitamin D for 20 years and served on the government panel that decided what the recommended daily intake for vitamin D should be back in 2016 (10 micrograms per day, though your individual requirements may vary significantly – more on this later).
Lanham-New says the true scale of deficiency in the UK may even be higher. “Ethnic groups, including South Asians and Black Afro-Caribbeans are not particularly well represented in the National Diet and Nutrition Survey. But these populations are extremely deficient in vitamin D. About 50 per cent of them are vitamin D deficient.”
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Sun kissed
Now, many researchers believe it’s past time that we take vitamin D supplementation seriously, as individuals and as a society. And it’s especially important during the winter because our primary source of vitamin D – sometimes known as the sunshine vitamin – is UVB from the Sun.
When UVB rays from the Sun hit the skin, they trigger a chemical reaction that produces vitamin D3. This form of vitamin D is then processed by the liver and kidneys, transforming it into its active form, which the body can use.
The trouble in winter is that UVB is in short supply at UK, Canadian and northern US latitudes. The angle of the Sun needs to be at least 50° above the horizon for our bodies to get enough.
“October to March, you haven't got the right wavelength to make vitamin D,” says Lanham-New. “The best way of knowing that is that your shadow has to be shorter than your height. April to September, on a sunny day your shadow will be shorter than your height. October to March, a sunny winter's day your shadow will be double your height.”
Recommendations for exactly how much Sun you need for optimum vitamin D levels in the summer can be a little hazy. They depend on where you live, the colour of your skin and how much of it is exposed to the sunlight.
Recommendations tend to hover around 10-15 minutes of midday Sun for pale-skinned people, and up to 25 minutes for dark-skinned people. In both cases, recommendations usually assume that your forearms and lower legs are exposed.
Feel it in your bones
Whatever the weather, we’ve known for decades that vitamin D is vital for the human body. It’s necessary for healthy bones because it helps us absorb calcium from food; within 20 years of its discovery, it was being used to fortify foods, especially milk, and cases of the bone condition rickets plummeted.
More recently, it’s become clear that it has other critical roles in the body, including cell growth and the immune system.
In fact, vitamin D is so multifunctional that it’s sometimes described as behaving more like a hormone than other vitamins. “We have over 200 vitamin D receptors in the body, all over the place,” Lanham-New says. “Whether it's the brain, whether it's immune cells, whether it's heart cells. It impinges on so many health outcomes beyond just musculoskeletal health.”
Vitamin D’s role in immune function is likely a factor in how it covers so much ground. Research has shown associations with cancer protection and autoimmune disease. It also seems to be involved in multiple aspects of healthy ageing. So perhaps it should be no surprise that we see so much research into vitamin D.
In one review in 2022, researchers using data from the UK Biobank estimated that people with a vitamin D deficiency have a 25-per-cent greater chance of all-cause mortality.
Another growing area of interest is how vitamin D interacts with the microbiome, the friendly neighbourhood microorganisms that live on us and inside us. The microbiome is itself a white-hot research topic, with researchers investigating its role in everything from brain health to cardiovascular disease, depression to kidney function.
Now, there’s tantalising evidence to suggest that vitamin D can influence how the microbiome affects our health. Multiple studies have shown that vitamin D can regulate and modify the gut microbiota and promote the diversity of microbes living inside us.
The relationship between the two is complex and researchers are cautious – they’re always cautious – about how critical vitamin D is. However, studies are beginning to piece together mechanisms for how the vitamin, through its impact on the microbiome, might lead to improvements in cancer protection and treatment.
The big D for the big C
Earlier this year, researchers found that vitamin D could improve the gut microbiome of mice and increase their immune response to cancer. It began with speculation that increased availability of vitamin D could somehow improve immunity to cancer, based on how vitamin D binds to a protein in the blood.
“We find that mice lacking a protein known as Gc globulin show enhanced immunity to cancer and increased responses to immunotherapy,” says Dr Evangelos Giampazolias, a researcher at the Cancer Research UK Manchester Unit, who led the study.
“What we knew from previous studies in the literature was that Gc globulin binds to vitamin D in the blood and keeps it away from tissues. This led us to speculate that increased availability of vitamin D in tissues could somehow enhance immunity to cancer.”
Giampazolias and his team found that mice fed with a vitamin D-rich diet had enhanced resistance to transplantable tumours. “They were able to fight cancer better,” he says.
But there was more. Those mice were able to transmit immunity to cancer through their faecal matter to other mice. (A faecal microbiota transplant is a procedure that transfers healthy bacteria from a donor’s stool into a recipient’s gastrointestinal tract.)
“We were really intrigued,” Giampazolias says. “We wanted to find this magic trait of cancer resistance that can be passed from one mouse to the other. One of the main things that came to our minds is that faecal matter is a big source of communities of friendly microorganisms that live inside the gut.”
That’s exactly what the team found. It wasn’t vitamin D itself that protected mice against cancer but its role in altering the friendly microorganisms in the gut of mice, allowing them to boost immune responses to tumours.
The results could tell us something about the complex, interconnected ways that vitamin D affects your health. But of course, caution is advised: this was not a human trial so we have to be careful about whether it tells us anything about the human toll of cancer.
“There were previous attempts to try to connect vitamin D with cancer either in mice or humans, and most of them were inconclusive,” Giampazolias says.
Still, he and his team are working with clinicians to see if their findings could be relevant to humans. “We have analysed some human datasets retrospectively,” he says. “We examined a dataset from 1.5 million people in Denmark and found a correlation between lower vitamin D levels in the blood and a higher risk of developing cancer.
“In another cohort of cancer patients, there are indications that higher vitamin D levels correlated with a better response to immune-based cancer treatments. But it is still early days and further research needs to be done.”
D for definite?
Scientists always say that: “Further research needs to be done.” In the case of vitamin D, hundreds, probably thousands, of papers have already been published, but there’s still much that researchers want to know. Not least because some studies have shown conflicting results about various aspects of vitamin D and our health.
“A number of randomised controlled trials don't show a benefit of vitamin D for different health outcomes,” says Lanham-New. These include some looking into whether it shortens the length of hospital stays, or improves a person’s risk of coronary heart disease.
There are also studies that show vitamin D supplementation has no effect on cancer incidence – or even that too much vitamin D increases your risk (although toxic levels are way above what most of us have due to sunlight or diet).
Part of the reason for the ambiguity is how the studies are designed, and the difference between what’s a low level of vitamin D, and what’s enough.
“In some studies, many of the subjects were vitamin D replete when a randomised control trial starts. In other words, they have enough. So getting more vitamin D to somebody who's already got enough doesn't seem to have an added health benefit,” says Lanham-New.
“What we need to do is to really work out exactly what that threshold is: where the vitamin D will be of the most benefit.”
The future is personalised
Do you know what your vitamin D levels are? Are you deficient? The interesting thing about that magic number – the threshold where it becomes beneficial – is that it will be different for everyone.
Some researchers think that we should all know and track vitamin D like we do our blood pressure, heart rate or waist circumference. Like those, it’s a metric that can tell us a lot about our underlying health.
The future might also lead us to more tailored or even personalised recommendations about how much vitamin D we all need, either from sunlight, supplements or food.
Current UK guidelines for vitamin D levels vary only by skin colour. As mentioned above, if you have dark skin, you need more sunlight than your pale-skinned neighbours. You might also see a greater benefit from supplementation, especially in the winter.
“Obviously people who have lighter skin have less pigment, which kind of means they are more effective in making vitamin D from the Sun,” says Prof Lina Zgaga, from Trinity College Dublin, in Ireland. “There's a good degree of awareness that people of African ancestry are at higher risk of vitamin D deficiency if they live in places like the UK.
“What is more recent is that people of Asian ancestry seem to be even worse off, which intuitively sounds less clear because you don't see as much pigment.”
More than skin deep
Earlier this year, Zgaga and her colleagues published a paper that called for a step away from the one-size-fits-all approach to vitamin D supplementation.
The team analysed vitamin D data from half a million people in the UK. For each of them, researchers also calculated an individualised estimate of ambient UVB, so they could tell the extent to which sunlight was the cause of their own vitamin D levels.
The research concluded that ambient UVB is the most important factor for people in the UK, but it’s not the only one. An individual’s vitamin D is determined also by their ethnicity, age, gender, body mass index (BMI) and other factors.
Age is a factor because the enzymes involved in producing vitamin D under the skin become less efficient as we get older. BMI is a factor because vitamin D is fat-soluble. That means it can be stored – inactive – in fat tissue instead of circulating in the blood.
“If you have somebody who has twice as much fat as another person, their concentration of vitamin D will be half,” Zgaga says. “Those two people may have an equal absolute amount of vitamin D, but it's going to be more diluted in those who have higher BMI. So what that basically means is that if you're overweight or obese, you need a higher dose of vitamin D supplementation.”
Zgaga believes that factors like BMI can and should be taken into account when recommending how much vitamin D a person needs. “I am frustrated because I feel that the knowledge we have now about vitamin D is fully sufficient for us to create better, tailored guidelines,” she says.
In time, she thinks these could even incorporate lifestyle factors such as whether a person works outside, or whether they’re allergic to fish (oily fish is the best nutritional source of vitamin D).
This would mean everyone who is deficient in the vitamin – and remember, that’s millions of people in the US alone – gets a dose recommendation based on their unique needs.
If you think you’re deficient and want to begin increasing your vitamin D levels immediately, the advice is quite straightforward: more sunlight, more supplements, more fish. “Eating fish more than twice a week, or taking supplements, is highly likely to prevent more severe forms of vitamin D deficiency,” Zgaga says.
While the salmon cooks, we can expect researchers to keep producing studies on vitamin D and the multiple ways it affects our health. “There’s still so much we don’t know,” Lanham-New says. “I think the number one would be that we need to move to a scenario where we do not have a population group who are vitamin D deficient.”
About our experts
Prof Susan Lanham-New is the head of the nutritional science department at the University of Surrey, England. Her work has been published in Public Health Nutrition, Proceedings of The Nutrition Society and Nutrition Bulletin to name a few journals.
Dr Evangelos Giampazolias is a researcher at the Cancer Research UK Manchester Unit and cancer immunity expert. His work has been published in Science, Journal for Immunotherapy of Cancer and Cell.
Prof Lina Zgaga is an Associate Professor of Public Health and Primary Care at Trinity College Dublin, Ireland. Her work has been published in the International Journal of Molecular Sciences and the British Journal of Cancer.
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