You Live on the Equator and You're Still Vitamin D Deficient

I check vitamin D levels on almost every patient who walks into my clinic. And I'm going to tell you something that still catches me off guard, even after years of doing this.

About 4 out of every 10 come back deficient.

We live in Singapore. It's 1 degree north of the equator. We get sunshine 365 days a year. The UV index here is regularly above 10. And yet, almost half my patients don't have enough of a vitamin that your body is literally designed to make from sunlight.

If that doesn't stop you in your tracks, maybe this will: a study published just this month in Neurology found that people with higher vitamin D levels in their 30s and 40s had significantly less tau protein in their brains 16 years later. Tau is one of the key proteins behind Alzheimer's disease. And a separate trial out of Harvard showed that vitamin D supplementation slowed the shortening of telomeres, the biological countdown clocks at the ends of your chromosomes, by the equivalent of roughly three years of ageing.

Your vitamin D level isn't just about bones anymore. It might be one of the most underappreciated numbers in longevity medicine.

The Singapore Sunshine Paradox

Let's talk about why this happens, because it makes no sense on paper.

Singapore has some of the strongest UV radiation on the planet. In theory, you'd need about 10 to 15 minutes of midday sun exposure on your arms and face to produce a decent amount of vitamin D. So why are we deficient?

Because we don't go outside.

Think about your typical day. You wake up in your aircon HDB or condo. You walk to the MRT through a sheltered corridor. You sit in an office from 9 to 6 (or later). You grab lunch at a food court that's indoors. You take the MRT home. Maybe you go to an indoor gym. Then you go home and spend the evening on your phone.

How much actual sunlight hit your skin today? Probably close to zero.

A study of indoor workers in Singapore published in the International Journal of Environmental Research and Public Health found that 32.9% were vitamin D deficient, defined as levels below 20 ng/mL. A separate study in PLOS ONE found an even higher figure: 42.1% of participants in Singapore were at risk of deficiency. Women were hit harder than men (54.5% vs 30.5%), likely due to more sunscreen use and higher body fat percentage, since vitamin D gets sequestered in fat tissue.

Night shift workers were at even higher risk. And the researchers were clear: this high prevalence of vitamin D deficiency in a sunny tropical region reinforces the fact that sunlight alone, without actual exposure to it, does nothing for you.

It's like living next to a gym and never going in. The proximity doesn't give you the benefit.

What Vitamin D Actually Does (Beyond Bones)

Most people, if they think about vitamin D at all, think about bones. And yes, vitamin D is critical for calcium absorption and bone mineral density. Severe deficiency causes rickets in children and osteomalacia in adults. That part is well-established.

But the past five years of research have blown the door wide open on what vitamin D does across nearly every system in your body. And for those of us in the longevity space, the findings are hard to ignore.

Your Brain

This is the big one. A study published on 1 April 2026 in Neurology Open Access (an official journal of the American Academy of Neurology) looked at participants from the Framingham Heart Study. Researchers measured vitamin D levels in dementia-free adults between 2002 and 2005, then performed tau PET brain imaging on the same participants between 2016 and 2019.

The gap between blood draw and brain scan? An average of 16 years.

What they found: participants with higher serum 25-hydroxyvitamin D in early midlife had significantly lower tau deposition in their brains over a decade and a half later. Tau is one of the two hallmark proteins of Alzheimer's disease (the other being amyloid beta). More tau accumulation means more neuronal damage, more cognitive decline, and a higher likelihood of dementia.

Now, this is an observational study. It doesn't prove that taking vitamin D prevents Alzheimer's. The researchers are careful to say that. But the signal is strong enough that they described low vitamin D in midlife as "a potentially modifiable target to mitigate the risk of neuroimaging signs of preclinical dementia."

Translation? Your vitamin D levels in your 30s and 40s might be quietly shaping your brain health in your 60s and 70s. And by the time you notice symptoms, you've lost 16 years of opportunity.

Your Biological Clock

Here's where it gets even more interesting for anyone who cares about ageing (which, if you're reading this blog, I'm guessing you do).

The VITAL trial is one of the largest randomised controlled trials ever conducted on vitamin D supplementation. Over 25,000 US adults aged 50 and over. The trial ran for five years. Nested within it, a sub-study of 1,054 participants measured something called leukocyte telomere length (LTL) at baseline, year 2, and year 4.

Telomeres are the protective caps at the ends of your chromosomes. Every time a cell divides, they get a little shorter. When they get too short, the cell can no longer replicate properly. It either dies or becomes dysfunctional. Telomere shortening is one of the most well-established hallmarks of biological ageing.

The results, published in 2025 in The American Journal of Clinical Nutrition by Haidong Zhu and colleagues: participants who took 2,000 IU of vitamin D3 daily lost 140 fewer base pairs of telomere DNA over four years compared to placebo. That's roughly equivalent to slowing biological ageing by about three years, as measured in immune cells.

140 base pairs might sound abstract, but put it this way: your cells in the vitamin D group were ageing slower. Not by a little bit. By a measurable, significant amount. Over just four years.

Other research backs this up. A separate analysis found that vitamin D-sufficient individuals had a 1.4-year lower DNA methylation age acceleration and longer relative telomere length compared to deficient participants.

Your Muscles

This one matters more than most people realise, especially as you get older.

Vitamin D deficiency has been consistently linked to muscle weakness, reduced physical function, and an increased risk of sarcopenia (age-related muscle loss). Low serum vitamin D leads to type II muscle fibre atrophy, increased fat infiltration within muscle tissue, and impaired muscle protein synthesis.

A randomised controlled trial published in The Lancet Healthy Longevity in 2024 found that active vitamin D treatment helped prevent sarcopenia in adults with prediabetes. Muscle mass and function were better preserved in the treatment group.

For anyone in the longevity world, preserving muscle is non-negotiable. Sarcopenia is one of the strongest predictors of frailty, falls, fractures, and loss of independence in later life. If your vitamin D is tanked, your muscles are quietly deteriorating even if you're hitting the gym.

Your Heart (With a Caveat)

I want to be balanced here because the cardiovascular data is nuanced. Observational studies consistently show that low vitamin D is associated with higher cardiovascular mortality. A meta-analysis found that people with low vs high vitamin D levels had a cardiovascular mortality risk ratio of 1.54.

But here's the thing: randomised controlled trials of vitamin D supplementation in the general population have not shown a significant reduction in heart attacks, strokes, or cardiovascular death. A large meta-analysis of 21 trials involving over 83,000 participants, published in JAMA Cardiology, found no benefit.

The likely explanation? Most of these trials enrolled people who weren't deficient to begin with. If your levels are already adequate, adding more vitamin D probably won't help your heart. But if you're deficient, the observational data suggests you're carrying extra cardiovascular risk that correcting the deficiency might address.

The evidence is strong enough that I think checking and correcting deficiency is worthwhile. But I won't tell you vitamin D is a miracle pill for your heart. It isn't.

How Much Do You Actually Need?

This is where things got a bit awkward in the medical world recently.

In 2024, the Endocrine Society updated its vitamin D guidelines and made a fairly radical move: they stopped recommending specific target blood levels for healthy adults. The 2011 guidelines had defined deficiency as below 20 ng/mL and sufficiency as above 30 ng/mL. The 2024 version basically said "we don't have enough clinical trial evidence to set a specific number."

That doesn't mean vitamin D doesn't matter. It means the data on what exact blood level is "optimal" is messier than we'd like. For some outcomes (bone health, falls prevention), the evidence is clear. For others (cancer prevention, heart disease), we're still working it out.

Here's what I tell my patients in practice:

Get tested. A simple blood test for serum 25-hydroxyvitamin D will tell you where you stand. If you're in Singapore, you can get this done at any lab. It's inexpensive and it takes the guesswork out entirely.

If you're deficient (below 20 ng/mL), correct it. This is not controversial. Every guideline agrees on this. Your doctor can advise on dosing, but 1,000 to 2,000 IU daily is a common maintenance dose for adults. Some people need more depending on their baseline, body weight, and absorption.

Don't mega-dose. More is not better. High-dose supplementation above 4,000 IU daily has paradoxically been linked to increased fall and fracture risk in some studies. And vitamin D toxicity, while rare, is real. Stay within recommended ranges and recheck your levels after 3 months.

Get some actual sunlight. 10 to 15 minutes of midday sun on your arms and face, two to three times a week, can make a meaningful difference. I know it's hot. I know you'll sweat. But you don't need to sunbathe for an hour. A walk to the hawker centre at lunch with your sleeves rolled up counts. Sitting near a window doesn't count. UVB rays (the ones that trigger vitamin D synthesis) don't pass through glass.

Eat vitamin D-rich foods. Fatty fish (salmon, mackerel, sardines), egg yolks, mushrooms exposed to sunlight, and fortified foods like milk can help. But diet alone rarely corrects a deficiency in Singapore. The sun and supplements need to do the heavy lifting.

Who's Most at Risk?

Based on the Singapore data and global research, certain groups should be especially vigilant:

Office workers who rarely go outdoors. The Singapore indoor worker study found the highest deficiency rates in people with minimal outdoor time, regardless of how close they lived to the equator.

Night shift workers. If you're sleeping during the day and working at night, you're missing the sun entirely. This group had some of the lowest vitamin D levels in the Singapore studies.

Women, particularly those who use sunscreen daily. SPF 30 blocks about 97% of UVB radiation. That's great for skin cancer prevention but terrible for vitamin D synthesis. The Singapore data showed women had nearly double the deficiency rate of men (54.5% vs 30.5%).

People with darker skin. Melanin acts as a natural sunscreen, which means darker-skinned individuals need more sun exposure to produce the same amount of vitamin D. In a multi-ethnic country like Singapore, this matters.

Older adults. Your skin becomes less efficient at synthesising vitamin D as you age. A 70-year-old makes about 25% of the vitamin D that a 20-year-old makes from the same sun exposure. The 2024 Endocrine Society guidelines specifically recommend empiric vitamin D supplementation for adults over 75.

People with higher body fat. Vitamin D is fat-soluble and gets stored in adipose tissue. Higher body fat can lower circulating vitamin D levels, making it less available where your body needs it.

The Bigger Picture: What Your Vitamin D Level Tells Me

When I see a patient with low vitamin D, I don't just see a number on a lab report. I see a signal.

It usually tells me that this person spends most of their life indoors. That they're probably not getting enough natural light exposure, which also affects their circadian rhythm and sleep quality. That they may not be eating enough whole foods. That they're probably not exercising outdoors.

Low vitamin D isn't just a vitamin problem. It's often a lifestyle red flag. And when I look at their biological age markers, the telomere data, the DNA methylation clocks, the inflammatory markers, the picture tends to be consistent. The lifestyle that creates vitamin D deficiency also creates accelerated ageing across the board.

The good news? It's fixable. Unlike some of the more complex drivers of biological ageing, vitamin D deficiency is one of the most straightforward things you can address. Get tested. Supplement if needed. Get outside more. It won't cost you much and the evidence says the returns, for your brain, your muscles, your cells, are significant.

What I'd Do If I Were You

If you've never had your vitamin D checked, get it done. Seriously. It's a single blood test that costs under $30 at most labs in Singapore.

If your levels come back below 20 ng/mL, talk to your doctor about supplementation. 1,000 to 2,000 IU daily is a reasonable starting dose for most adults, but your doctor may recommend more depending on how low you are.

If you want a more complete picture of how your vitamin D status fits into your overall biological ageing profile, that's something we do as part of a comprehensive longevity assessment. We look at vitamin D alongside your epigenetic clock, inflammatory markers, metabolic health, cardiovascular risk, and more. Because no single number tells the whole story. But vitamin D is a very good place to start.

Stop taking the sunshine for granted. You're surrounded by it, but you're not using it. And the research is telling us, louder every year, that this matters more than we thought.

References

1. Miao Y, Wang J, Li X, Guo J, Ekblom MM, Sindi S, Zhang Q, Dove A. Association of Circulating Vitamin D in Midlife With Increased Tau-PET Burden in Dementia-Free Adults. Neurology Open Access. 2026. DOI: 10.1212/WN9.0000000000000057
2. Zhu H, et al. Vitamin D3 and marine ω-3 fatty acids supplementation and leukocyte telomere length: 4-year findings from the VITamin D and OmegA-3 TriaL (VITAL) randomized controlled trial. The American Journal of Clinical Nutrition. 2025. DOI: 10.1016/S0002-9165(25)00255-2
3. Bi X, Tey SL, Leong C, Quek R, Henry CJ. Prevalence of Vitamin D Deficiency in Singapore: Its Implications to Cardiovascular Risk Factors. PLOS ONE. 2016;11(1):e0147616. DOI: 10.1371/journal.pone.0147616
4. Lai JS, et al. Prevalence of Vitamin D Deficiency and Its Associated Work-Related Factors among Indoor Workers in a Multi-Ethnic Southeast Asian Country. International Journal of Environmental Research and Public Health. 2020;17(1):164. DOI: 10.3390/ijerph17010164
5. Jiang H, et al. Active vitamin D treatment in the prevention of sarcopenia in adults with prediabetes (DPVD ancillary study): a randomised controlled trial. The Lancet Healthy Longevity. 2024;5(2). DOI: 10.1016/S2666-7568(24)00009-6
6. Barbarawi M, et al. Vitamin D Supplementation and Cardiovascular Disease Risks in More Than 83,000 Individuals in 21 Randomized Clinical Trials: A Meta-analysis. JAMA Cardiology. 2019;4(8):765-776. DOI: 10.1001/jamacardio.2019.1870
7. Demay MB, et al. Vitamin D for the Prevention of Disease: An Endocrine Society Clinical Practice Guideline. The Journal of Clinical Endocrinology & Metabolism. 2024;109(8):1907-1947. DOI: 10.1210/clinem/dgae290