The benefits of vitamin D may depend on the diversity of bacteria in your gut.

How effective is vitamin D against diseases like Covid-19? From the early days of the pandemic, low levels of vitamin D were associated with worse Covid-19 outcomes.

Then in July, a small Spanish pilot study concluded that treatment vitamin D reduced the severity of the disease.

Out of 50 patients treated with vitamin D, one required admission to ICU, whilst out of 26 who were not treated with vitamin D, 13 required admission to ICU.

However, a 2020 study from researchers at the University of California San Diego (UCSD), USA, and their collaborators, suggests that there is more to adequate vitamin D than soaking up sunshine or popping a pill.


greater diversity in gut bacteria was associated with higher levels of active vitamin D — suggesting that our gut bacteria play a role in helping the body convert inactive vitamin D to the active form


To get the most out of vitamin D it appears that we may also need to feed our gut bacteria with a healthy diet; and that high levels of ‘inactive’ vitamin D stored in the body do not always mean there will be high levels of ‘active’ vitamin D doing its job.

For the study, researchers measured levels of vitamin D in blood samples, and gut bacteria in stool samples obtained from 567 men.

Typically, standard blood tests measure one type of vitamin D: an inactive precursor that is stored by the body. However, another active form is also present in the blood, so the researchers measured both types.

They discovered that volunteers who had high levels of stored, inactive vitamin D did not always have high levels of active vitamin D.

Because we get vitamin D from the sun, the researchers were not surprised to find that subjects who lived in sunny California had the most precursor (inactive) form of vitamin D in their blood.

Yet there was no correlation between where the men lived and their levels of active vitamin D.

Instead, they found that greater diversity in gut bacteria was associated with higher levels of active vitamin D — suggesting that our gut bacteria play a role in helping the body convert inactive vitamin D to the active form.

Senior author of the study Deborah Kado, MD, director of the Osteoporosis Clinic at UC San Diego Health, spoke to Optimum Nutrition.

Why do tests usually measure the inactive form of vitamin D?

“The inactive form, 25-hydroxy vitamin D, represents bodily stores of vitamin D and is considered a more stable measure than the active form, 1,25-dihydroxy vitamin D.

“Precisely because it is the active hormone, 1,25-dihydroxy vitamin D is more tightly regulated by our body’s physiology, may fluctuate daily, and because of this variability is not considered to be as stable. Hence, it is a less reliable measure of overall vitamin D status.”

Can you be vitamin ‘deficient’ even when there are high levels of stores in the body?

Yes, if one is not able to adequately convert the inactive form to the active form, then there is a problem.

“We see this clinically in those with chronic kidney disease because the main enzyme responsible for converting the precursor form to the active hormone is thought to be primarily located in the kidney.

“Patients with chronic kidney disease are unable to generate active vitamin D as efficiently as someone with normal kidney function.”

Should you take vitamin D supplements if you don’t have good gut bacteria?

Theoretically, if the kidneys are working well, then one should be able to metabolise the inactive form to the active form in sufficient quantity.

“However, our study results do suggest that the gut microbiome may nevertheless play a key role in optimising vitamin D metabolism in humans — or at least, in predominantly white older men [the demographic for the study].”

Do vitamin D supplements help support the immune system against Covid-19?

“This is an intriguing question. There is an underlying biological rationale for how vitamin D signalling may be important for optimal immune function.

“Thus this scientific background, taken in the context of the ongoing Covid pandemic, has led to some recommending vitamin D supplementation to prevent or lessen the effects of Covid infection.

“Given that vitamin D3 is not expensive and there are not many associated harms with low levels of supplementation, I think most physicians would not argue against it.

“If persons do wish to take supplements, my recommendation would be to remember that more is not necessarily better, especially as our research suggests that our biology is smart and our body will use only what it needs.

“Maybe a good approach would be to try and optimise gut microbial health by eating daily fresh fruits and vegetables and limiting processed foods, so that the survival of beneficial commensal bacteria are optimised to better metabolise vitamin D.”

How does magnesium help metabolise and regulate vitamin D?

“There seem to be a few studies to suggest that having sufficient magnesium is important for optimal vitamin D signalling, but the data are not definitive and to my knowledge, an exact mechanism is not understood.

“That being said, there are dietary sources of magnesium…that are associated with optimal gut microbial health, so following a healthy diet of daily fresh fruits and vegetables is probably the best approach in 2021.”


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