Vitamin D, known as the "sunshine" vitamin and essential for bone health, is also known for promoting immune response, much needed to fight COVID-19, experts said on 11 February.

Several studies have also shown how Vitamin D deficiencies can reduce vaccine efficacy and also increase the severity of COVID-19 infection in individuals. In a study led by Purdue University, researchers found that Vitamin D functions to reduce inflammation caused by immune cells, relevant to responses during severe COVID infection.

"Because the Vitamin D receptor is expressed on immune cells and because these cells can synthesise the active Vitamin D metabolite. Vitamin D also has the potential to modulate adaptive immune responses," Dr Sanjay Kumar Gogia, Director, Internal Medicine, Fortis Hospital, Shalimar Bagh, told IANS. Further, COVID has been particularly worse to older adults, people with obesity and hypertension -- again a group with a deficiency in Vitamin D.

In another study led by Bar-Ilan University in Safed, Israel, Vitamin D levels, prior to COVID infection, may increase the severity of the disease as well the risk of mortality. The study found that patients with Vitamin D deficiency (less than 20 ng/mL) were 14 times more likely to have severe or critical cases of COVID than those with more than 40 ng/mL.

The US Centers for Disease Control and Prevention, recently stated that over 75 per cent of people who died due to COVID had comorbidities like uncontrolled hypertension, diabetes, renal diseases and obesity. "Low Vitamin D levels have been associated with an increased risk of community-acquired pneumonia in older adults and children," Gogia said.

He cited that in a meta-analysis of randomised clinical trials, vitamin D supplementation was shown to protect against acute respiratory tract infection. However, in a few clinical trials, administering high doses of Vitamin D to critically ill patients with vitamin D deficiency (but not COVID-19) did not reduce the length of the hospital stay or the mortality rate when compared to placebos.

"The rationale for using Vitamin D is based largely on immunomodulatory effects that could potentially protect against COVID-19 infection or decrease the severity of illness," Gogia noted. Vitamin D is most often recognised for its role in bone health, but low levels of the supplement have been associated with a range of autoimmune, cardiovascular, and infectious diseases.

According to Dr Amitav Banerjee, Prof & Head of Community Medicines, D.Y. Patil Medical College, Pune, cultural and social changes have led to the condition that today "young adults are about four times more likely to have Vitamin D deficiencies as compared to older adults". "Lifestyle changes do not only consist of diet and exercise but stepping outdoors for getting your mandatory dose of Vitamin D," Banerjee said. "Your body needs adequate amounts of Vitamin D to maintain and support the immune system," he said.