Does vitamin D improve resistance against viruses such as influenza or the SARS-CoV-2 virus that causes COVID-19? Grandmothers across northern Europe have long relied upon cod liver oil, which is rich in vitamin D, to maintain immunity during winter months when sunlight is scarce. Research suggests that the old wives might have been right with this approach to avoid infection.
Vitamin D Helps People with Diabetes Avoid Infection:
A new study of nearly 20,000 participants in the UK Biobank compared blood levels of vitamin D and the risk of infection among people with type 2 diabetes (American Journal of Clinical Nutrition, Aug. 2024). The researchers report that those with low circulating levels of vitamin D, known as 25-hydroxyvitamin D, were significantly more likely to suffer infections in the lungs or digestive tract. They were also more vulnerable to sepsis. Type 2 diabetes increases the risk of infection.
The authors conclude that
“these findings highlighted the importance of maintaining adequate vitamin D in reducing the risk of infection in patients with type 2 diabetes.”
These results agree with those of previous studies that people with low levels of vitamin D are less likely to avoid infection. We don’t know whether vitamin D supplements are as effective as sun exposure to keep immunity up. Previous research suggested that sunlight exposure lowers the risk of infection and death from COVID-19 (Tropical Biomedicine, March 1, 2024). Presumably that’s because humans make vitamin D when sunlight strikes the skin.
SARS-CoV-2 is not the only virus worrying people, however. One reader wonders whether vitamin D with a probiotic could help against several different respiratory tract infections.
What Is the Story on Vitamin D with a Probiotic?
Q. Somewhere I read that vitamin D paired with a probiotic could help prevent infections. Does it? I’m worried about respiratory problems this winter.
A. There isn’t a great deal of research on this combination. Individually though, each supplement does appear to have immune boosting capability.
A review of the medical literature published in the journal Nutrients (Sept. 2023) concludes that “Vitamin D adequacy is critical to overcoming infections and autoimmunity.” The author suggests that keeping 25-hydroxyvitamin D above 40 to 50 ng/mL is a cost-effective way to reduce chronic diseases and overcome viral epidemics.
A review of randomized controlled trials found that probiotics “were better than placebo or no treatment in preventing URTIs (upper respiratory tract infections). This analysis was published in the Cochrane Database of Systematic Reviews (Aug. 24, 2022), a source of rigorous objective evaluation.
Do Probiotics Protect People from COVID-19?
With respect to COVID-19 specifically, probiotics did not prevent infection (Frontiers in Nutrition, Oct. 27, 2023). However, clinical trials showed that people who took probiotics recovered more quickly and were less likely to die from COVID.
In one randomized controlled trial, investigators assigned healthy people with no previous COVID-19 infections or vaccinations to either of two groups. People in both groups took 10 micrograms of vitamin twice daily throughout the six-month study (Gut Microbes, Jan-Dec. 2023). Those in the intervention arm also got the probiotic Limosilactobacillus reuteri DSM 17938 in their tablets. By the end of the study, many of the 159 participants had been exposed to COVID-19 or a vaccine. Those who took the probiotic mounted more robust antibody responses.
Why Might Vitamin D Help Someone Avoid Infection?
The British investigators we mentioned above reviewed average blood levels of vitamin D in various European countries. As a consequence, they found an inverse relationship with mortality rates from COVID-19. That means that where vitamin D levels are lower, mortality rates are higher (Aging Clinical and Experimental Research, May 6, 2020).
The authors pointed out that vitamin D plays a crucial role in keeping the innate immune system functioning well. As they noted, older people have the lowest levels of vitamin D and are at the greatest risk of dying from COVID-19. Consequently, if older people could boost their vitamin D levels, they might avoid infection. They would, of course, also have to practice other preventive strategies. This research was conducted before vaccination became widely available.
Another Study Finds Vitamin D Might Help People Avoid Infection:
This is not the only study to examine these data. Another group of researchers came to similar conclusions from their analysis of vitamin D levels and COVID-19 infection and mortality rates.
Writing in the Irish Medical Journal, these scientists conclude:
“Vitamin D levels are low in countries in Europe which have high infection and mortality rates.” (Irish Medical Journal, May 2020)
As they point out, important immune system cells have vitamin D receptors. They recommend that vitamin D intake be optimized to potentially improve immune response against COVID-19. People who are likely to have low levels of the vitamin–the elderly, overweight and those with chronic conditions–are also most at risk from the coronavirus.
More Evidence for Vitamin D:
Previous studies have shown that vitamin D supplements may reduce respiratory tract infections (Health Technology Assessment, Jan. 2019). In particular, the benefits are clearest among people with inadequate blood levels at the outset. Daily rather than monthly or quarterly doses also appear more effective.
No one really knows if vitamin D would help you avoid infection with COVID-19. However, another recent study offers even more evidence of the link (Nutrients, May 9, 2020). The researchers analyzed blood samples from patients who had tested positive for SARS-CoV-2, the virus that causes COVID-19. They found that these people had significantly lower levels of circulating vitamin D than those who had tested negative. In summary, they recommend randomized controlled trials of vitamin D supplements to see if they will help you avoid infection.
