Why so many people in sunny Singapore have Vitamin D deficiency – and what you should know

Here’s one of life’s biggest ironies: Despite living in the tropics and being blasted by the sun’s rays all year round, about 40 per cent of Singaporeans are deficient in Vitamin D and 8 per cent are very deficient, according to the 2010 National Health Survey by the Ministry of Health. 

If you’ve been living under a rock and are wondering what’s the connection (and are probably deficient in Vitamin D, too), sunlight – more specifically, UVB – triggers a chemical reaction in your skin to naturally produce Vitamin D.

“UVB” is short for ultraviolet B and is the part of the sun’s rays that causes sunburn and skin cancer. UVB rays are also the only rays that trigger the chemical reaction in your skin that creates Vitamin D. 

Think of your skin as a Vitamin D factory. When sunlight hits bare skin, UVB starts a chemical reaction that results in pre-Vitamin D. But it is not used immediately; instead, it circulates for weeks in the body via your blood. When your body needs to tap on the pre-Vitamin D (it can also be derived from food and supplements), your liver and kidneys process it into 25 hydroxyvitamin D or 25(OH)D, a form that your body can use. 

You can think of 25(OH)D as your body's Vitamin D “storage tank”, which is what blood tests measure when assessing your Vitamin D level. It is usually measured in micrograms per litre (µg/l), nanograms per millilitre (ng/ml) or nanomoles per litre (nmol/l).

WHAT’S CAUSING THE DEFICIENCY?

“Everyone blames sunblock,” said Dr Stephen Chu, the 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.”

He explained that “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”. “Sheltered walkways, underground walkways, air-conditioning everywhere, and office hours happen to cover exactly the part of the day when the UVB is useful,” said Dr Chu.

Your skin gets even less sunlight if you work in an office. In a 2019 study involving 213 participants who were indoor workers, Vitamin D deficiency was found in nearly 33 per cent of the participants. Office workers, workshop workers and night-shift workers were also found to be at a greater risk of this condition, despite a similar intake of Vitamin-D-rich foods across the job types. 

Sitting next to a sunny window doesn’t quite cut it either, said Dr Sueziani Binte Zainudin, a senior consultant endocrinologist from Ascensus Health. “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 certainly plays a big part. Having a darker complexion and hence, more skin pigmentation or melanin, means that your skin absorbs the UVB radiation before it can trigger the chemical reaction needed to synthesise Vitamin D. This was shown in a 2013 study on 197 women in Singapore, which found that the deficiency was more prevalent in Malay women (89 per cent) and Indian women (82 per cent) compared to Chinese women (56 per cent). 

And if you’re wondering, yes, more women than men in Singapore are affected. In fact, local studies reported that about 54 per cent of females are Vitamin D deficient, compared to roughly 30 per cent of males. Women tend to cover up more or stay out of the sun more than men, owing to cultural practices (dressing for modesty) and social preferences (avoiding tanning because being fair is considered beautiful). 

You may be surprised to know that your weight matters as well. Obesity has even been suggested as a risk factor for Vitamin D deficiency because the fat-soluble vitamin is readily stored in body tissue, preventing the body from using it. In a study that looked at Vitamin D deficiency, obese subjects had 57 per cent lower Vitamin D levels than leaner subjects. 

The elderly is another group that is often lacking in Vitamin D. In a local study that looked at 134 subjects aged above 65 in a Singapore rehabilitation centre, 44 per cent were found to be deficient, while nearly 86 per cent had low levels of the vitamin. Malay and Indian patients fared significantly worse than Chinese, which again, fits with the skin pigmentation reasoning.

WHY IS VITAMIN D IMPORTANT?

The problem with starting out low on Vitamin D is that it becomes even more worrisome when you’re growing a baby; Vitamin D supports calcium and phosphate metabolism needed for the foetus’ skeletal development. As the pregnancy progresses, the calcium transfer from mother to foetus increases substantially. 

It could explain the statistics among pregnant women in a small 2024 study; it found that out of the 93 participants in their first trimester, only 2.2 per cent of mums-to-be have sufficient Vitamin D. Among the Malay and Indian pregnant women who participated in the study, all of them were deficient.

Vitamin D matters to non-pregnant individuals, too. Its deficiency has been associated with a range of conditions, the most known of all is osteoporosis. The vitamin prevents it by helping the body absorb calcium to build bone and support muscle strength – two functions that make the vitamin highly beneficial for menopausal women and seniors who have sarcopenia or age-related muscle loss. 

The sunshine vitamin is also linked to lower cancer incidence (most notably, colorectal cancer, breast cancer and prostate cancer) and a reduced risk of developing Type 2 diabetes, along with lowering blood pressure, which reduces the risk of heart attacks and strokes. 

A dose of Vitamin UVB has also been shown to fend off sleepiness. The next time you have difficulty dragging yourself out of bed, try drawing the curtains and letting the sun in for a while. Morning sunlight contains blue light wavelengths that increase cortisol and suppress the sleepy hormone melatonin.

WHAT LEVELS ARE CONSIDERED TO BE DEFICIENT?

Interestingly, experts can’t agree on what qualifies as Vitamin D deficient and how high levels need to be. The guidelines of the global medical community, Endocrine Society, noted that people with 25(OH)D below 20ng/ml should be considered deficient, and suggested 30ng/ml or above as a desirable level in various at-risk groups.

However, the US Institute Of Medicine (IOM) argued that the evidence doesn't support setting the bar that high for everybody. Their review, published in 2012, suggested that around 20ng/ml is enough to meet the needs of almost everyone in the general population. In fact, they noted that around 16ng/ml would already suffice for roughly half the general population. 

And while IOM acknowledged that preventing nutrient deficiencies during pregnancy is important, there wasn't sufficient evidence to conclude that pregnant women should routinely aim for 30ng/ml or more, or that higher Vitamin D levels would prevent outcomes such as pre-eclampsia.

So, what's the accepted number then? Many doctors would recommend maintaining your serum 25(OH)D concentration at 20ng/ml or higher; 30ng/ml or higher for elderly persons at risk of falls or people diagnosed 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 from National Skin Centre.

However, “experts' consensus is that there is no single 'perfect’ Vitamin D level that applies to everyone”, said Dr Sueziani. The recommendation of 20ng/ml is still the most widely used cut-off and it's what the Singapore studies apply, said Dr Chu. “But it was only ever meant as a population figure for bone health, not for personal optimum.”

Moreover, continued Dr Chu, the test itself isn't consistent. “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.”

Dr Chu’s advice to patients is 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.

WHY IS WALKING UNDER THE SUN TO GET LUNCH NOT ENOUGH?

Should you be heading outside without sunscreen once in a while? “It is prudent to limit the time in the sun without sun protection due to the increased risk of skin cancer,” said Dr Phuan. And for good reason. “As few as six sunburns in a lifetime have been shown to increase an individual’s risk for non-melanoma and melanoma skin cancer,” she said.

While it has been proposed that sun exposure may maintain your Vitamin D level or bring it up to the desired level, Dr Phuan said that doing so can be akin to recommending a carcinogen in moderation when there is an equally effective non-carcinogenic alternative. Furthermore, deliberately exposing unprotected skin to UV rays may not be efficient in those with deficiency, such as elderly and dark-skinned persons, she added. 

Instead, the protected exposure of the face and backs of the hands to “0.25 per cent minimum erythema dose (MED) of UVB radiation” three times weekly generates enough Vitamin D, said Dr Phuan. To break it down, think of MED as your personal "sunburn threshold”; it refers to the amount of UV your skin can take before it reddens. So, 0.25 per cent of your MED is a tiny fraction of that sunburn threshold. 

What does that look like in real life? Instead of providing a “prescription” of UV exposure, simply be less avoidant of the sun, said Dr Chu. “I'd say go out in the mid-morning or late afternoon, make it short and regular, with a decent bit of skin uncovered.” 

And you’ll need direct sun; being in the shade cuts down UVB a lot, he said. This isn’t an excuse to forgo sunblock. You should still use sunscreen properly and top up if you sweat a lot – rather than hoping incidental exposure covers you, said Dr Chu.

But scratch all that if you’re fair-skinned, or have a personal or family history of skin cancer; you’re better off taking supplements, said Dr Chu. “You can replenish Vitamin D with a tablet safely and cheaply. You cannot undo photodamage.”

WHAT ABOUT FOOD SOURCES OF VITAMIN D?

Yes, your diet can also contribute to your Vitamin D need, said Dr Phuan. If you’re sun shy, you may be better off supplementing your diet with Vitamin D-fortified foods, such as drinking two eight-ounce glasses (about 240ml per glass) of fortified milk or orange juice each day. 

“Foods high in Vitamin D include fatty fish, such as salmon or trout. Foods fortified with added Vitamin D are another source,” said Dr Phuan. “These include milk, yoghurt, some plant milks such as soy, almond and oat milk, and some cereals. Some mushrooms are treated with ultraviolet light, which gives them more Vitamin D (look for the 'Vitamin D' label).”

If you’re thinking of taking supplements, Dr Suezaini said that most healthy adults need about 600IU a day, while those above 70 may need about 800IU daily. “Those clearly deficient may need higher doses for a period of time, but this should be guided by a doctor,” she said. 

Over-dosing with over-the-counter Vitamin D is no light matter. Excess Vitamin D can lead to hypercalcaemia, which means the calcium levels in your blood are higher than normal. Severe cases, though rare, can cause issues such as kidney failure, abnormal heart rhythm (arrhythmia), unsteady gait and/or confusion.