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Women who take multivitamins, probiotics or vitamin D are less likely to catch the coronavirus, study suggests.
Nutritional supplements have skyrocketed in popularity amid the pandemic. Leading up to the UK's first lockdown, when the infection dominated the headlines, vitamin C sales alone had risen by 110%.
Certain nutritional supplements have been linked to a stronger immune response and reduced rate of respiratory infections, but their role in preventing the coronavirus specifically has been unclear.
Results reveal taking probiotics, omega-3 fatty acids, multivitamins or vitamin D had a "modest but significant" effect when it came to warding off the coronavirus among the female participants only.
Vitamin C, zinc and garlic supplements were found to have no effect, despite some unsubstantiated claims on social media.
"Prominent medical entertainment personalities on television and social media" have promoted dietary supplements to prevent and treat the coronavirus, the King's scientists wrote in the BMJ Nutrition Prevention & Health.
Multivitamin sales alone rose by 93% in the lead-up to the UK's first lockdown, implemented on 23 March, 2020. In the US, zinc supplements were up 415% "at the height" of COVID-19 (the disease caused by the coronavirus) concern.
"A biologically plausible role exists for certain vitamins and minerals in immune pathways," wrote the scientists.
Vitamin D specifically is thought to enhance antiviral immunity, as well as mitigating the so-called cytokine storm that can arise in severe coronavirus cases.
Nevertheless, "robust evidence to support a role for vitamins and minerals in preventing infection" with the coronavirus was not available.
"We know a range of micronutrients, including vitamin D, are essential for a healthy functioning immune system," said Professor Sumantra Ray, from the NNEdPro Global Centre for Nutrition and Health, which co-owns the BMJ journal.
"This, in turn, is key to prevention of, and recovery from, infections.
"To date, there is little convincing evidence that taking nutritional supplements has any therapeutic value beyond maintaining the body's normal immune response."
The scientists analysed participants of the COVID-19 Symptom Study app, which allows users to self-report "information related to" the coronavirus.
Of more than 370,000 participants, over 175,000 claimed to "constantly" take dietary supplements in the three months up to 31 July, 2020, during the UK's first coronavirus wave.
The results suggest taking probiotics, multivitamins, omega-3 fatty acids – "fish oil", or vitamin D reduces the risk of catching the coronavirus by 14%, 13%, 12% and 9%, respectively.
This remained true after adjusting for other factors that influence a person's risk of testing positive, like their underlying health.
When homing in on the participants' sex, age and weight, the scientists found the results only applied to the women, regardless of how old they were or their body mass index (BMI).
Similar findings were also observed among US and Swedish users of the app.
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A "discordant immune system" between the sexes could explain why the male and female participants may react differently to the supplements, according to the scientists.
"Indeed, a sexual dimorphism in nutrient metabolism has been previously reported, with females having a more robust immune response than men," they wrote.
Females "typically possess a more resilient immune system", including higher levels of infection-fighting B cells.
Variations in BMI and "composition" may mean "supplement dosing on a per body weight basis" is higher in females.
The male and female participants may also have had "differences in health-related behaviours" amid the pandemic.
"Polling reveals a greater percentage of females versus males are anxious for the health of themselves or their family, and therefore are more precautionary, cancelling plans and staying home more often," wrote the scientists.
"Females who purchase vitamins may also be more health conscious than males, such as having greater use of wearing face masks and hand washing."
When it comes to the benefits of multivitamins, the supplements are often rich in antioxidants, which may support the immune system.
Omega-3 oils are known to be anti-inflammatory, while probiotics could "interact with the host's gut-associated immune system".
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The scientists have stressed their study was observational, and therefore does not prove cause and effect.
The participants also self-reported their supplement use and coronavirus test results, potentially leaving room for errors.
Nevertheless, the significant results should encourage further research, added the scientists.
"This study wasn't primarily designed to answer questions about the role of nutritional supplements in COVID-19," said Professor Ray.
"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."