Certain supplements may reduce risk of testing positive for SARS-CoV-2 for women, according to a new study published inBMJ Nutrition Prevention & Health.

Researchers drew on data from the COVID-19 Symptom Study app, which was launched in the U.K., the U.S., and Sweden in March 2020 to capture self-reported information on the evolution of the pandemic. While it initially only recorded location, age, and risk factors of its users, as time went on subscribers were asked to provide daily updates on symptoms, healthcare, and coronavirus test results. People without obvious symptoms were also encouraged to use it.

The researchers were able to analyze information supplied by 372,720 adult U.K. subscribers to the app about their regular use of dietary supplements throughout May, June, and July 2020, as well as coronavirus test results.

The numbers: Between May and July, 175,652 U.K. subscribers regularly took supplements, while 197,068 didn’t. 67% were women, and over half were overweight. 23,521 people tested positive, and 349,199 tested negative.

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After accounting for influential factors including underlying conditions and usual diet, the researchers found that taking probiotics, omega-3 fatty acids, multivitamins, or vitamin D was associated with a lower risk of infection by 14%, 12%, 13%, and 9%, respectively. No such effects were observed among those taking vitamin C, zinc, or garlic supplements. Furthermore, while these effects were observed in women of all ages and weights, no association was seen in men.

Overall patterns were mirrored in the U.S. and Sweden, which had 45,757 and 27,373 subscribers respectively.

The researchers note several limitations of this study—it relied on self-reported data from a self-selected group, no information was collected on supplement doses or ingredients, and it is an observational study, which cannot establish cause. That said, the effects were significant, and the researchers are calling for large clinical trials.

Sumantra Ray, Executive Director, NNEdPro Global Centre for Nutrition and Health, which co-owns the journal, commented in a press release: "We know that a range of micronutrients, including vitamin D, are essential for a healthy functioning immune system. This, in turn, is key to prevention of, and recovery from, infections. But to date, there is little convincing evidence that taking nutritional supplements has any therapeutic value beyond maintaining the body's normal immune response. What's more, this study wasn't primarily designed to answer questions about the role of nutritional supplements in COVID-19. This is still an emerging area of research that warrants further rigorous study before firm conclusions can be drawn about whether specific nutritional supplements might lessen the risk of COVID-19 infection.”