SINGAPORE - A study published on Sept 3 in medical journal JAMA Network Open suggested that vitamin D deficiency may be associated with a higher risk of Covid-19.
Vitamin D is mainly produced in the skin when it is exposed to the ultraviolet (UV) B rays of the sun. But despite Singapore's reputation as a "sunny island", many here are deficient in this vitamin.
The 2010 National Health Survey found that 40 per cent of Singaporeans were deficient in vitamin D, while another 8 per cent were very deficient.
And the figures do not seem to have improved much over the past decade. Assistant Professor Verena Tan, an expert in dietetics and nutrition at the Singapore Institute of Technology, cited a 2019 study which found that the prevalence of vitamin D deficiency among indoor workers here was 32.9 per cent.
Office workers, those who work in workshops and those on night shift were also found to be at a greater risk of this condition.
Dr Tan told The Straits Times on Tuesday (Sept 15) that the condition is prevalent not only in Singapore but also elsewhere in South-east Asia - despite the presence of sunlight all year round.
"Sun-protection behaviours such as use of sunscreen, staying in the shade or indoors, the wearing of long sleeves and using umbrellas are common practices in this region," she explained.
Some of those who engage in such behaviours may do so to protect themselves from skin cancer, which is also caused by UV rays. But blocking the rays also means blocking a source of vitamin D production, she said.
The body needs vitamin D to absorb calcium and maintain its bone structure. Together with calcium, vitamin D is effective in preventing osteoporosis and bone fractures, she noted.
Dr Tan added that emerging evidence also shows that vitamin D is critical for one's immune function. "A deficiency may compromise immune response and increase risk of infection and disease," she said.
To get an adequate amount of vitamin D, she said, people are recommended to get sun exposure on their arms and legs for about five to 30 minutes twice a week.
The best time to do so is between 10am and 3pm each day, she said, adding that people with paler skin require less sun exposure than those with darker skin.
Vitamin D can also be found in eggs, liver and oily fish. Other sources include fortified food products such as milk, soya milk, yogurt, orange juice, breakfast cereals and margarine.
The daily recommended dietary allowance of vitamin D for those aged 18 and up is 2.5 mcg per day. One large egg yolk contains 1.02mcg of vitamin D, and one 250ml cup of fortified milk contains 2.9 to 3.1 mcg.
"Hence, it is easy to achieve adequate vitamin D from food and sun exposure," said Dr Tan.
She noted, however, that more vitamin D is not always better, as too much may result in negative side-effects such as a build-up of calcium in the blood, which can cause nausea and vomiting, weakness, and frequent urination.
Too much vitamin D may also lead to bone pain and kidney problems, such as the formation of calcium stones.
"Always check with a doctor or dietitian before starting a vitamin D supplement or any high-dosage supplement," said Dr Tan.