Why Is My Vitamin D Low Even in the Sun?
- , by SANUSq Research team
- 8 min reading time
You spend time in the sun, so surely your vitamin D is fine? For a surprising number of people, the answer is no — and understanding why is the first step to fixing it.
The sunshine vitamin paradox
Vitamin D keeps your bones and muscles healthy, and its role in immunity and overall health is increasingly recognised. We make it in our skin when it's exposed to sunlight, which is why it's nicknamed the "sunshine vitamin". So it seems logical that anyone getting regular sun should have plenty — yet vitamin D deficiency is remarkably common worldwide, even in sunny countries and even among people who feel they get outdoors enough. This is one of nutrition's genuine puzzles, and the explanation is that a whole series of factors sit between "being in the sun" and "having healthy vitamin D levels". Understanding these factors explains why so many people are low despite the sunshine — and what to do about it.
Why sun exposure often isn't enough
Some of the most common reasons for vitamin D deficiency are simply everyday things that block or reduce the vitamin D your skin actually makes.
Wearing a sunscreen with SPF 30 reduces vitamin D synthesis in the skin by more than 95%, people with naturally darker skin need three to five times longer sun exposure to make the same amount, and higher body weight is associated with lower vitamin D levels (Holick, vitamin D review).
Add to this the many people who spend most daylight hours indoors, cover their skin, or sit behind glass (which blocks the UVB rays needed), and it's clear how easily sun exposure falls short. Season and latitude matter hugely too: in autumn and winter, and further from the equator, the sun is too weak for much of the year to make vitamin D at all. And as we age, our skin becomes less efficient at producing it. So "getting some sun" doesn't guarantee adequate levels.
The deficiency that hides in sunny places
Perhaps the most striking evidence that sun alone isn't enough comes from the sunniest regions on earth.
Vitamin D deficiency remains highly prevalent even in sunlight-rich countries, driven by factors such as clothing that covers the skin, air pollution that reduces UV penetration, dietary habits and indoor lifestyles (hypovitaminosis D review).
There are also internal reasons someone can be low despite ample sunlight, and vitamin D absorption issues are among the most easily missed of them. Because vitamin D is fat-soluble, conditions affecting fat absorption — coeliac disease, Crohn's, or after weight-loss surgery — can limit how much you take up from food. Higher body fat can sequester vitamin D, keeping it out of circulation. And problems with the liver or kidneys can impair the steps that convert vitamin D into its active form. Even magnesium matters, as it's needed to activate vitamin D, so a magnesium shortfall can leave supplementation less effective.
Why being low actually matters
It's worth pausing on why any of this matters, because vitamin D is far more than a bone vitamin. Its best-known role is helping the body absorb calcium and keep bones and teeth strong — long-term deficiency causes soft, weak bones (rickets in children, osteomalacia in adults) and contributes to the fragile bones of osteoporosis. But receptors for vitamin D are found throughout the body, and adequate levels support healthy muscle function (low vitamin D is linked with weakness and falls in older people), a well-regulated immune system, and mood. Deficiency has been associated in research with a range of problems from frequent infections to low mood and fatigue. The tricky part is that mild to moderate deficiency often causes no obvious symptoms at all, or vague ones like tiredness and aches that are easily blamed on other things, so it quietly goes unnoticed. This combination — important roles throughout the body, common deficiency, and often no clear warning signs — is exactly why it's worth taking seriously and checking rather than assuming your sunshine exposure has it covered.
How to actually get your levels up
The good news is that low vitamin D is very fixable once you know the causes. A sensible approach combines a few strands. Get some sensible sun exposure when you can (short, regular, without burning), bearing in mind its limits in winter or for your skin type. Include vitamin-D-rich foods — oily fish, egg yolks, fortified products — though it's hard to get enough from diet alone. For most people who are low, especially in less sunny climates or higher-risk groups, a good vitamin D supplement is the reliable answer, and pairing D3 with K2 supports where the calcium it helps absorb is directed — our Liposomal Vitamin D3 + K2 is formulated for this. Making sure your magnesium intake is adequate helps too. Above all, if you suspect you're low — or have symptoms like fatigue, frequent illness or low mood — ask your doctor for a simple blood test, which takes the guesswork out and guides the right dose. We look at the wider importance of this nutrient in why vitamin D is so important.
Frequently asked questions
Why is my vitamin D low even though I get sun?
Many factors sit between sun and healthy levels: sunscreen (SPF 30 blocks over 95% of vitamin D synthesis), darker skin needing longer exposure, time indoors or behind glass, season and latitude, ageing skin, higher body weight, and absorption issues. Sun exposure alone often isn't enough.
Can you be vitamin D deficient in a sunny country?
Yes — deficiency is common even in the sunniest regions, driven by skin-covering clothing, air pollution reducing UV, indoor lifestyles and dietary habits. Living somewhere sunny is no guarantee of adequate vitamin D, which is why so many people are surprised to find they're low.
What stops the body absorbing vitamin D?
Because vitamin D is fat-soluble, conditions affecting fat absorption (coeliac, Crohn's, after weight-loss surgery) can limit uptake. Higher body fat can sequester it, and liver or kidney problems impair its activation. Low magnesium, needed to activate vitamin D, can also blunt supplementation.
How can I raise my vitamin D levels?
Combine sensible sun exposure, vitamin-D-rich foods (oily fish, eggs, fortified products) and, for most people who are low, a good supplement — ideally D3 with K2. Ensure adequate magnesium too. A blood test guides the right dose, especially in less sunny climates or higher-risk groups.
Does sunscreen cause vitamin D deficiency?
Sunscreen significantly reduces vitamin D production — SPF 30 by more than 95% in theory. In practice, most people don't apply it perfectly, so some vitamin D is still made. Don't skip sun protection for skin-cancer reasons; instead ensure vitamin D through diet or a supplement.
Should I get my vitamin D tested?
If you have risk factors (darker skin, little sun, covering up, older age, higher body weight, gut conditions) or symptoms like fatigue, frequent illness or low mood, a simple blood test is worthwhile. It confirms whether you're low and guides the right supplement dose — ask your doctor.
Reliably top up your vitamin D
Our Liposomal Vitamin D3 + K2 offers a well-absorbed way to correct a common deficiency and support bones and immunity.
References
- Nair R, Maseeh A. Vitamin D: The "sunshine" vitamin. J Pharmacol Pharmacother. 2012;3(2):118–26. PMC3356951
- Qureshi G, Khemissa M, Amr G, et al. The Non-conventional Effects of Hypovitaminosis D: A Pandemic Even in Sunlight-Rich Countries. Cureus. 2024;16(4):e59267. PMC11135140
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The team at SANUSq.
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