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 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 that found the prevalence of vitamin D deficiency among indoor workers here was 32.9 per cent.
Prof Tan told The Straits Times yesterday 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 noted.
The body needs vitamin D to absorb calcium and maintain its bone structure, said Prof Tan.
She added that emerging evidence also shows that vitamin D is critical for one's immune function.
To get an adequate amount of vitamin D, said Prof Tan, people are recommended to get sun exposure on their arms and legs for about five minutes to 30 minutes twice a week.
The best time to do so is between 10am and 3pm each day.
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 above is 2.5mcg a day.
One large egg yolk has 1.02mcg of vitamin D, and one 250ml cup of fortified milk has 2.9mcg to 3.1 mcg.
"Thus, it is easy to achieve adequate vitamin D from food and sun exposure," said Prof 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.
"Always check with a doctor or dietitian before starting a vitamin D supplement or any high-dosage supplement," she said.