Persistently low vitamin D usually results from a combination of factors rather than a single culprit. Limited sun exposure is the most common driver, but your body weight, gut health, genetics, kidney and liver function, certain medications, and even your age all influence how much vitamin D circulates in your blood. Understanding which of these factors applies to you explains why supplementation alone sometimes fails to move the needle and why two people living in the same city can have very different levels.
How Your Body Makes and Uses Vitamin D
Most of your vitamin D starts in your skin. When UVB rays hit the outer layer, a cholesterol compound called 7-dehydrocholesterol converts into a precursor molecule, which then rearranges into vitamin D3. That raw vitamin D3 travels to the liver, where it becomes 25-hydroxyvitamin D, the form doctors measure on a blood test. From there it moves to the kidneys for one more conversion into the active hormone your cells actually use.1PubMed Central. Sunlight and Vitamin D: A global perspective for health This chain of events means a problem at any step, from skin exposure all the way through liver and kidney processing, can leave your levels chronically low.
Sunlight, Latitude, and Season
The single biggest reason for widespread low vitamin D is that modern life keeps people indoors. Office work, commuting in enclosed vehicles, and leisure time spent in front of screens all reduce the amount of UVB your skin actually receives. But even if you do go outside, geography matters enormously. At latitudes above about 40 degrees (roughly the line from New York to Madrid), the sun sits too low in the sky during winter months for the atmosphere to let enough UVB through, effectively shutting down skin production for part of the year.2PubMed Central. Globally Estimated UVB Exposure Times Required to Maintain Sufficiency in Vitamin D Levels Research tracking vitamin D3 production across tropical and subtropical sites found that even at 23 degrees south latitude, winter conversion dropped by more than half compared with an equatorial site.3PubMed. Sun-induced production of vitamin D3 throughout 1 year in tropical and subtropical regions: relationship with latitude, cloudiness, UV-B exposure and solar zenith angle
If you live in a northern climate, you may be running a vitamin D deficit for four to six months each year without realizing it. Cloud cover, air pollution, and the simple fact of shorter days compound the problem. The body does store some vitamin D in fat and muscle, but those reserves deplete over a long winter, which is why many people’s levels bottom out by late February or March.
Does Sunscreen Block Vitamin D Production?
This is one of the most persistent myths. Under tightly controlled lab conditions, sunscreen can reduce UVB penetration substantially. But in real-world use, people miss spots, apply too little, reapply inconsistently, and still get plenty of incidental UV exposure. A panel review of the evidence concluded that sunscreen use for daily and recreational purposes does not compromise vitamin D synthesis, even when applied under optimal conditions.4PubMed Central. Sunscreen photoprotection and vitamin D status A separate review reached the same conclusion, finding that normal sunscreen usage does not generally result in vitamin D insufficiency.5PubMed. Does chronic sunscreen use reduce vitamin D production to insufficient levels? So if your doctor tells you your levels are low, ditching the sunscreen is not the answer. Supplementation or targeted brief unprotected exposure is safer than increasing your skin-cancer risk.
Skin Pigmentation
Melanin, the pigment responsible for darker skin tones, absorbs some of the UVB radiation that would otherwise trigger vitamin D synthesis. The question has always been how much that actually matters. A study comparing the most extreme skin types found that melanin’s inhibitory factor was only about 1.3 to 1.4, meaning darker skin reduces vitamin D production modestly rather than dramatically.6PubMed. Melanin has a Small Inhibitory Effect on Cutaneous Vitamin D Synthesis: A Comparison of Extreme Phenotypes That sounds small on paper, but even a modest daily reduction adds up over weeks and months, especially if you also live at a high latitude, spend most of your time indoors, or have other risk factors stacking on top. Modeling of sun-exposure times bears this out: at the equator under clear skies, estimated noontime exposure needed to maintain adequate vitamin D ranges from about 3 minutes for lighter skin to about 15 minutes for darker skin.2PubMed Central. Globally Estimated UVB Exposure Times Required to Maintain Sufficiency in Vitamin D Levels Move to London or Toronto and that gap widens considerably, because the UVB available is already limited.
Aging and Skin Changes
Your skin’s ability to produce vitamin D declines as you get older. Research on surgically obtained skin samples spanning ages 8 to 92 found that aging can reduce the skin’s capacity to make the vitamin D precursor by more than twofold.7PubMed Central. Aging decreases the capacity of human skin to produce vitamin D3 The reason is straightforward: the concentration of the raw material (7-dehydrocholesterol) in the outer skin layer drops with age. An older adult sitting in the same sunshine as a teenager simply manufactures less vitamin D. This is on top of the fact that older adults tend to spend more time indoors, may have reduced dietary intake, and are more likely to have kidney function changes that further impair vitamin D activation.
Body Weight and Fat Tissue
If you carry excess body fat, your vitamin D levels are likely to be lower than someone of a similar age and lifestyle who does not, even if you take the same supplement dose. The reason is that vitamin D is fat-soluble. It dissolves into adipose tissue, and a larger fat mass acts as a bigger sponge. Research confirms that people with obesity have greater total stores of vitamin D locked up in fat, but that enlarged reservoir means more vitamin D is needed to bring blood levels to the same point as in a leaner person.8PubMed Central. Vitamin D Storage in Adipose Tissue of Obese and Normal Weight Women
The problem goes beyond simple dilution. Fat tissue in people with obesity appears to release vitamin D less readily. When researchers stimulated fat breakdown in tissue samples, the decrease in vitamin D concentration was significantly more pronounced in lean tissue than in obese tissue, suggesting a blunted release mechanism.9The Journal of Clinical Endocrinology & Metabolism. Impaired Release of Vitamin D in Dysfunctional Adipose Tissue: New Cues on Vitamin D Supplementation in Obesity In practical terms, vitamin D goes into fat easily but comes back out sluggishly, making it harder for your body to access what it has stored. This is why people with obesity often need higher supplement doses to reach the same blood level, and why weight loss itself can sometimes improve vitamin D status without changing supplement intake.
Gut Problems and Malabsorption
Because vitamin D is fat-soluble, it relies on your digestive system working well enough to absorb dietary fat. Conditions that impair fat absorption hit vitamin D hard. Celiac disease, Crohn’s disease, ulcerative colitis, and the aftereffects of bariatric surgery all severely impair vitamin D status.10PubMed Central. Vitamin D and malabsorptive gastrointestinal conditions: A bidirectional relationship? The relationship is bidirectional: these gut conditions reduce vitamin D absorption, but low vitamin D itself may worsen the inflammation driving these diseases.
If you have been diagnosed with a malabsorptive condition and your vitamin D never seems to budge despite supplementation, the problem may be that standard oral doses simply are not getting through. Some people in this situation benefit from higher doses, liquid formulations that are easier to absorb, or in severe cases, injections that bypass the gut entirely. This is a conversation to have with a doctor who knows your gastrointestinal history.
Kidney and Liver Disease
Since the liver performs the first conversion of vitamin D into its measurable form, and the kidneys handle the second conversion into the active hormone, damage to either organ disrupts the entire chain. Vitamin D deficiency is extremely common in people with chronic liver disease, with up to 93% showing some degree of insufficiency.11PubMed Central. Impact of Vitamin D Status in Chronic Liver Disease On the kidney side, chronic kidney disease carries an exceptionally high rate of severe deficiency, made worse by the kidneys’ declining ability to perform that final activation step.12PubMed Central. Vitamin D and chronic kidney disease In advanced kidney disease, standard vitamin D supplements may bring up the 25-hydroxyvitamin D number on a blood test but still leave the body short on active hormone, because the kidneys cannot finish the job.13PubMed Central. Vitamin D and the kidney That is why nephrologists often prescribe the already-activated form (calcitriol) rather than regular vitamin D3.
Medications That Lower Vitamin D
Several widely used medications accelerate the breakdown of vitamin D in the liver. Certain anti-seizure drugs, including phenytoin, phenobarbital, and carbamazepine, ramp up the activity of a liver enzyme that converts vitamin D into inactive breakdown products.14Annals of Clinical Nutrition and Metabolism. Drug-Induced Vitamin Deficiency – Section: VITAMIN D DEPLETION Glucocorticoids (like prednisone), some HIV medications, and certain cholesterol-lowering drugs have also been linked to lower vitamin D levels through various mechanisms. If you take any of these long-term and your levels stay stubbornly low, the medication could be working against your supplements. Your prescriber may recommend a higher dose or more frequent monitoring rather than stopping a drug you need.
Genetics and the Vitamin D Binding Protein
Some people are genetically wired to have lower circulating vitamin D. The gene that codes for the vitamin D binding protein (called GC) comes in several common variants, and these variants meaningfully affect how much vitamin D shows up on a blood test. Two well-studied variants in this gene alter the protein’s structure enough to create population-wide differences in vitamin D status.15PubMed Central. Common variants of the vitamin D binding protein gene and adverse health outcomes In a large genetic study, the six possible combinations of these variants explained over half of the variation in binding-protein levels, though they accounted for less than 1% of variation in 25-hydroxyvitamin D itself.16Nature Communications. Genetic correlates of vitamin D-binding protein and 25-hydroxyvitamin D in neonatal dried blood spots
That last number is worth pausing on: genetics strongly shape the binding protein, but the binding protein is just one part of the picture for your total circulating level. Still, at an individual level, carrying a particular GC variant can mean you need more sun or more supplementation than average to hit the same blood number. On the rarer end, mutations in the vitamin D receptor itself cause hereditary vitamin D-resistant rickets, a condition where vitamin D levels may actually be very high but the body cannot respond to the hormone.17PubMed Central. Mutations in the vitamin D receptor and hereditary vitamin D-resistant rickets This is uncommon but illustrates that sometimes the issue is not a deficiency in vitamin D itself but a breakdown in how cells detect and use it.
The Magnesium Connection
Vitamin D does not work in isolation. Magnesium serves as a cofactor for the enzymes that metabolize vitamin D in both the liver and kidneys, meaning you need adequate magnesium for those conversions to happen efficiently.18PubMed. Role of Magnesium in Vitamin D Activation and Function If you are supplementing vitamin D and seeing minimal improvement, a magnesium shortfall could be part of the explanation. Magnesium deficiency is common in its own right, particularly among people who eat few nuts, seeds, legumes, and leafy greens. Correcting a magnesium deficit sometimes helps vitamin D supplements work better, though this is not a substitute for addressing the other causes on this list.
D2 Versus D3 Supplements
Not all vitamin D supplements are equal. Vitamin D2 (ergocalciferol, derived from plants and fungi) and vitamin D3 (cholecalciferol, the same form your skin makes) are both used in supplements and food fortification, but they do not raise blood levels by the same amount. A systematic review and meta-analysis found that D2 is less potent at raising and maintaining total 25-hydroxyvitamin D, partly because it binds less tightly to the transport protein and has a shorter half-life in the blood.19PubMed Central. Relative Efficacy of Vitamin D2 and Vitamin D3 in Improving Vitamin D Status: Systematic Review and Meta-Analysis If your supplement says “vitamin D2” or “ergocalciferol” on the label, switching to a D3 formulation could make a noticeable difference in your numbers. Prescription vitamin D is often D2 (the 50,000 IU capsules commonly given for repletion), so if your levels rise slowly on those, the form of the supplement may be part of the reason.
Testing Variability and What “Low” Actually Means
Before assuming your vitamin D is genuinely and persistently low, it is worth knowing that the test itself carries some uncertainty. Different laboratories use different methods to measure 25-hydroxyvitamin D, and despite standardization efforts, significant differences remain among methods and labs.20PubMed. 25-Hydroxyvitamin D Testing: Immunoassays Versus Tandem Mass Spectrometry A result of 28 ng/mL at one lab might come back as 24 ng/mL at another, which could flip you from “sufficient” to “insufficient” on paper without anything actually changing in your body.
The threshold for “low” is also debated. Many labs flag anything below 30 ng/mL as insufficient, and some clinical guidelines recommend aiming for 40 to 60 ng/mL to capture potential benefits beyond bone health.21PubMed Central. Revisiting Vitamin D Guidelines: A Critical Appraisal of the Literature Other expert bodies set the bar lower, arguing that 20 ng/mL is sufficient for most people and that levels above that do not confer proven extra benefit. Where you fall in that debate can change whether your result is “low” or “fine.” If your number hovers in the mid-20s and your doctor calls it low, it is worth asking which guideline they are using and whether your clinical picture actually demands a higher target.
Why Human Skin Color Evolved Around Vitamin D
The fact that vitamin D deficiency is so common is partly a consequence of evolutionary mismatch. Human skin pigmentation evolved under two competing pressures: protection from intense UV near the equator, which favored darker melanin-rich skin, and the need for UVB to penetrate the skin for vitamin D synthesis in low-UV environments, which favored lighter skin as populations moved to higher latitudes.22PubMed Central. Human skin pigmentation as an adaptation to UV radiation This has been formalized as the vitamin D-folate hypothesis, which proposes that skin color evolved as a balancing act between maintaining vitamin D production and protecting folate from UV-driven breakdown.23PubMed Central. The Vitamin D⁻Folate Hypothesis as an Evolutionary Model for Skin Pigmentation: An Update and Integration of Current Ideas
The trouble is that modern humans do not live the way those evolutionary pressures assumed. Migration, indoor lifestyles, clothing norms, and urbanization have all decoupled where people live from the skin pigmentation that once matched their UV environment. A dark-skinned person living in Scandinavia and a light-skinned person working night shifts in a windowless building are both fighting the same mismatch: their biology expects more UVB than their lifestyle provides. Understanding this helps explain why vitamin D deficiency is not a personal failing or a sign that something is medically wrong with you. For many people, it is simply the predictable result of living in a way that the human body did not evolve for.