Most people assume a deficiency in this nutrient is a problem for people who never see the sun.
They picture someone living in a cold northern country, indoors all winter, going weeks without daylight. They do not picture themselves.
But the numbers tell a different story. An estimated one billion people worldwide have insufficient levels. Not just in Scandinavia. In sunny countries. In people who go outside regularly. In people who eat a varied diet and consider themselves healthy. The Global Burden of Disease study identified vitamin D deficiency as one of the most widespread nutritional problems on the planet, cutting across income levels, climates, and age groups.
The assumption that you are probably fine is, statistically speaking, not a safe one.
What Vitamin D Is and Why It Is Different From Other Vitamins
Calling vitamin D a vitamin is technically accurate but slightly misleading. It functions less like a vitamin and more like a hormone.
When your skin is exposed to UVB radiation from sunlight, it synthesises a precursor molecule that your liver converts to 25-hydroxyvitamin D, the form measured in blood tests. Your kidneys then convert this into the active hormone form, calcitriol, which binds to receptors found in virtually every tissue in the body.
That last part is worth sitting with. Receptors for this nutrient exist in your brain, heart, immune cells, gut lining, bones, muscles, pancreas, and more. This distribution tells you something important: it is not doing one specific job. It is involved in the regulation of hundreds of biological processes simultaneously.
When adequate amounts are present, those processes run quietly in the background. When levels fall low, the effects distribute so widely across the body that the resulting picture looks scattered and hard to attribute to any single cause. Which is precisely why deficiency is so commonly missed.
Why Sun Exposure Is Not Enough for Most People
The standard advice is to get fifteen to twenty minutes of sun exposure daily and you will be fine. For many people, this is simply not how the biology works out in practice.
Several factors determine how much your skin produces from sun exposure, and most of them work against adequate synthesis in the modern world.
The angle of the sun matters enormously. UVB radiation only reaches the earth’s surface when the sun is above approximately 35 degrees above the horizon. In most parts of the world, this means that from October through March or April, almost no synthesis is possible from sunlight regardless of how much time you spend outdoors. Even in summer, before 10am and after 3pm, UVB levels are too low to trigger meaningful production.
Skin pigmentation also plays a significant role. Melanin, the pigment that gives skin its colour, absorbs UVB radiation. This is protective against sunburn, but it also reduces synthesis efficiency. People with darker skin tones require significantly longer sun exposure to produce equivalent amounts.
Sunscreen blocks UVB. An SPF of 30 reduces synthesis by approximately 95 percent. Age reduces efficiency further: skin becomes less capable of producing the precursor molecules as we get older, with some research suggesting people over 70 produce roughly four times less from the same sun exposure as a young adult.
Indoor living is the biggest factor of all. Glass blocks UVB completely, so sitting next to a sunny window does nothing for synthesis. If you spend the majority of your daylight hours indoors, which most people in desk-based work do, your skin exposure to UVB is negligible regardless of the weather outside.
What Low Vitamin D Does to the Body
I want to be specific here rather than vague, because the connections between low levels and particular symptoms are more concrete than most people realise. The NIH Office of Dietary Supplements provides a thorough overview of the evidence for each body system.
Bone and muscle function. This is the most widely known role. The nutrient is essential for calcium absorption in the gut. Without adequate levels, the body cannot absorb calcium efficiently regardless of how much dairy or calcium-rich food you consume. Over time, this impairs bone density. In the shorter term, low levels are associated with muscle weakness and a specific kind of deep bone aching that people often attribute to ageing or overexertion.
Immune regulation. Vitamin D modulates immune function, supporting the system’s ability to mount an appropriate response to infection while also preventing the excessive inflammatory responses that underlie many autoimmune conditions. Low levels are consistently associated in the research with higher rates of respiratory infections, and several large studies conducted during the pandemic identified this deficiency as one of the most significant nutritional risk factors for severe illness.
Mood and brain function. Receptors for this nutrient are present throughout the brain, including in regions involved in mood regulation. Multiple large studies have found significant associations between low levels and depression, with some intervention trials showing meaningful improvement in depressive symptoms with supplementation. The mechanism is not fully understood but appears to involve roles in serotonin synthesis and the regulation of inflammatory cytokines in the brain.
Cardiovascular health. Receptors are also present in heart muscle cells and the cells lining blood vessels. Low levels are associated with higher blood pressure, increased arterial stiffness, and elevated cardiovascular risk in observational studies. The mechanisms involve roles in renin regulation, the system that governs blood pressure, and anti-inflammatory effects in vascular tissue.
Blood sugar and metabolic function. Pancreatic beta cells, which produce insulin, carry receptors for this nutrient and require it for optimal function. Low levels are associated with impaired insulin secretion and increased insulin resistance. This connection has led researchers to investigate whether correcting deficiency might reduce type 2 diabetes risk, and several large trials are currently underway.
The Testing Problem and What the Numbers Mean
Getting tested is straightforward. The test measures 25-hydroxyvitamin D in your blood and is available through most doctors and private labs. MedlinePlus offers a clear patient-friendly guide to testing and interpreting results.
What is less straightforward is interpreting the result, because the definition of sufficient varies considerably depending on who you ask.
The conventional medical threshold for deficiency is typically below 50 nmol/L (20 ng/mL). Below this level, bone health is clearly compromised. But many researchers and clinicians working in this area argue that this threshold reflects the minimum needed to prevent rickets, not the level needed for optimal function across the many other systems involved.
The range most frequently cited in the functional medicine and research literature for broad health optimisation is between 100 and 150 nmol/L (40 to 60 ng/mL). People whose levels fall between 50 and 100 nmol/L are technically not deficient by conventional standards but may still experience the effects of suboptimal status across immune, mood, and metabolic function.
If you have not tested recently and you spend most of your time indoors, live anywhere with seasonal variation in sunlight, have darker skin, or are over 50, the likelihood that your levels are below the optimal range is high enough to make testing worthwhile.
Supplementation: What the Research Supports
For most people in most parts of the world, supplementation is the most reliable way to maintain adequate levels year-round.
The form matters. Vitamin D3 (cholecalciferol) is the form your skin produces and the form most efficiently used by the body. Vitamin D2 (ergocalciferol), found in some supplements and fortified foods, is less effective at raising and maintaining blood levels. If you are supplementing, D3 is the appropriate choice.
The cofactor that most people overlook is vitamin K2. Higher calcium absorption is only beneficial if that calcium reaches your bones and teeth rather than depositing in soft tissue and arteries. K2 handles that directing role, and the combination of D3 with K2 is better supported by current evidence than D3 alone, particularly at higher doses.
Magnesium is also required to convert the nutrient into its active form. This means magnesium deficiency can impair the effectiveness of supplementation even when the dose itself is adequate. I covered this in a previous post on magnesium.
For general maintenance in an adult without a confirmed deficiency, 1,000 to 2,000 IU of D3 daily is a conservative and well-tolerated starting point. For those with confirmed deficiency, higher therapeutic doses are sometimes used under medical supervision to restore levels before dropping to a maintenance dose.
Vitamin D is fat soluble, meaning it accumulates in the body rather than being excreted like water-soluble vitamins. Very high doses taken over extended periods can cause toxicity. Supplementing at sensible doses and retesting periodically is the responsible approach. If you are on medications that affect calcium metabolism or have a condition affecting your kidneys or parathyroid glands, please discuss supplementation with your doctor before starting. You can also review the full medical disclaimer for this site.
What I Changed and What Shifted
I tested my own levels after reading the research on immune function and mood. My result came back at 52 nmol/L. Technically not deficient by the conventional threshold, but well below the range associated with optimal function across the systems I cared about.
I started supplementing with 2,000 IU of D3 combined with K2 daily, taken with my largest meal since D3 is fat-soluble and absorbs better alongside dietary fat. I retested after three months. My level had risen to 118 nmol/L.
The changes I noticed over that period were not dramatic or sudden. The low-grade susceptibility to every passing cold that had characterised the previous winter improved noticeably. The afternoon mood dip that I had been addressing through dietary changes improved further once levels were restored. My sleep quality, which I had been working on through other changes described in previous posts, consolidated in a way that felt like a missing piece had been added.
I cannot attribute any single change to this alone because I was not controlling variables in a clinical trial. What I can say is that restoring a measurable deficiency to an optimal range, at a cost of a few pence per day, seems like one of the higher-value interventions available given how many systems it touches. If you want to know more about the approach I take to these topics, you can read a little more about me here.
Have you had your levels tested? And if so, where did they sit? A surprising number of people who feel broadly well discover they are operating in the suboptimal range when they test for the first time… Share your experience in the comment
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