Singapore may be known for its year-round sunshine, but a substantial proportion of its population does not get enough Vitamin D. According to data from the Ministry of Health’s 2010 National Health Survey, about 40 per cent of Singaporeans were found to be Vitamin D deficient, while around 8 per cent had very low levels.

Vitamin D is produced naturally when ultraviolet B (UVB) radiation from sunlight reaches exposed skin. The process produces a precursor that is eventually converted by the liver and kidneys into 25-hydroxyvitamin D, or 25(OH)D, which is the form generally measured through blood tests.

Why can Vitamin D be low despite abundant sunshine?

One explanation is modern urban life. People can spend much of their day travelling between homes, cars, offices, shopping centres and other enclosed spaces, limiting their exposure to direct sunlight.

“Everyone blames sunblock,” said Dr Stephen Chu, founder and medical director of Aquila Medical Center. “It plays a part but I don’t think it’s the main culprit in Singapore. The bigger issue is urban planning and how we live in it.”

According to Chu, “you can go from your flat to the car park to the office to the mall and home again without ever being properly in the sun”. He pointed to sheltered walkways, underground routes, widespread air-conditioning and working hours as factors that can reduce exposure to useful UVB radiation.

Indoor workers can therefore face a particular risk. A 2019 study involving 213 indoor workers found that nearly one-third had Vitamin D deficiency. Office workers, workshop workers and night-shift employees were among those at greater risk, despite similar consumption of Vitamin-D-rich foods.

Simply sitting near a sunny window may not provide the same benefit as being outdoors. Dr Sueziani Binte Zainudin, a senior consultant endocrinologist at Ascensus Health, explained that “Ordinary glass blocks most of the UVB needed to make vitamin D. Being in the shade also reduces UV exposure, although some scattered UV may still reach the skin.”

Skin tone and other risk factors

Skin pigmentation can also affect how efficiently the body produces Vitamin D from sunlight. Higher levels of melanin absorb more UVB radiation, reducing the amount available for Vitamin D production.

A 2013 study involving women in Singapore found particularly high rates of Vitamin D deficiency among Malay and Indian participants compared with Chinese participants. Local studies have also reported higher deficiency rates among women than men, with about 54 per cent of females affected compared with roughly 30 per cent of males.

Body weight may also play a role. Because Vitamin D is fat-soluble, it can become stored in fatty tissue, potentially reducing the amount available for the body to use. One study cited in the report found that obese participants had Vitamin D levels 57 per cent lower than leaner participants.

Older adults are another group at risk. A Singapore study involving people aged over 65 found that 44 per cent were Vitamin D deficient, while almost 86 per cent had low levels.

Why Vitamin D matters

Vitamin D plays an important role in calcium and phosphate metabolism, making it essential for healthy bones and muscles. It is particularly important during pregnancy because these nutrients are required for the developing baby’s skeleton.

A small 2024 study of 93 women in their first trimester found that only 2.2 per cent had sufficient Vitamin D. All of the Malay and Indian pregnant participants in the study were deficient.

Vitamin D deficiency is also associated with osteoporosis. By helping the body absorb calcium, Vitamin D contributes to bone strength and supports muscle function. This can be particularly important for older people and post-menopausal women.

Research has also examined possible links between Vitamin D and conditions including Type 2 diabetes, cardiovascular disease and certain cancers, although associations between Vitamin D levels and these conditions should not automatically be interpreted as proof that supplementation prevents them.

What Vitamin D level is considered too low?

There is no universal agreement among medical experts about the precise Vitamin D level that should be considered optimal.

The Endocrine Society has identified a blood 25(OH)D concentration below 20 ng/ml as deficient and has suggested levels of 30 ng/ml or higher for certain groups at increased risk. However, the US Institute of Medicine concluded that approximately 20 ng/ml is sufficient to meet the needs of most people in the general population.

Many doctors therefore use 20 ng/ml as a practical threshold, while 30 ng/ml or higher may be considered for some older people at risk of falls or individuals with osteoporosis.

“Vitamin D supplementation is recommended in people who have a confirmed deficiency, that is, less than 30ng/ml,” said Dr Crystal Phuan, an associate consultant dermatologist at the National Skin Centre.

However, Dr Sueziani stressed that “experts’ consensus is that there is no single ‘perfect’ Vitamin D level that applies to everyone”.

Dr Chu also cautioned against interpreting an individual blood-test result in isolation. “But it was only ever meant as a population figure for bone health, not for personal optimum,” he said.

He added that testing itself can produce differences between laboratories. “One study compared labs and found results differing by around 12nmol/l (about 5ng/ml). They included labs using mass spectrometry, which is supposed to be the gold standard.”

For patients with borderline results, Chu advised that “a borderline result needs interpreting and looking at the entire picture of the patient’s situation – not panicking or acting on immediately”.

“And if you’re going to repeat the test, use the same lab. One number on one day isn’t a diagnosis,” he said.

Is more sunlight always the answer?

Getting more sunlight is not necessarily the safest or most effective solution for everyone with low Vitamin D.

Dr Phuan advised against deliberately exposing unprotected skin to excessive UV radiation because of the risk of skin cancer. “It is prudent to limit the time in the sun without sun protection due to the increased risk of skin cancer,” she said.

She noted that even a relatively small number of serious sunburns over a person’s lifetime can increase the risk of skin cancer. Deliberate unprotected UV exposure may also be less effective for people who are older or have darker skin.

For those who are able to obtain some Vitamin D through sunlight, Dr Chu suggested spending brief, regular periods outdoors rather than deliberately seeking prolonged exposure.

“I’d say go out in the mid-morning or late afternoon, make it short and regular, with a decent bit of skin uncovered.”

He also emphasised that direct sunlight is more useful for UVB exposure than being in the shade, while still recommending appropriate sunscreen use.

For people with fair skin or a personal or family history of skin cancer, Chu said supplements may be preferable. “You can replenish Vitamin D with a tablet safely and cheaply. You cannot undo photodamage.”

Can food provide enough Vitamin D?

Diet can contribute to Vitamin D intake, particularly for people who limit their sun exposure.

Fatty fish such as salmon and trout are among the food sources naturally rich in Vitamin D. Fortified foods can also provide the vitamin, including some types of milk, yoghurt, plant-based milk, cereals and orange juice. Certain mushrooms exposed to ultraviolet light can also contain increased levels of Vitamin D.

For healthy adults, Dr Sueziani said the general recommendation is around 600 IU of Vitamin D per day, rising to approximately 800 IU for people over 70.

“Those clearly deficient may need higher doses for a period of time, but this should be guided by a doctor,” she said.

Taking excessive amounts of Vitamin D supplements can also be harmful. Very high intake can cause hypercalcaemia, a condition in which calcium levels in the blood become abnormally high. Severe cases can result in kidney problems, abnormal heart rhythms, confusion and problems with balance.

For that reason, people concerned about Vitamin D deficiency should not automatically take high-dose supplements without appropriate medical advice. A blood test and assessment of individual circumstances can help determine whether supplementation is necessary and what amount may be appropriate.