Vitamin D deficiency almost always results from too little ultraviolet B (UVB) sunlight hitting your skin, compounded by one or more factors that make the shortfall worse: where you live, how much time you spend indoors, your body composition, your age, how well your gut absorbs fat, and even how your liver and kidneys process the vitamin once you have it. Most people assume the fix is simple, eat more vitamin D or take a supplement, but the causes run deeper than diet and can stack on top of each other in ways that make deficiency stubbornly persistent.
How Your Body Actually Makes Vitamin D
Your skin is a vitamin D factory, but it only runs when UVB light is available. A compound already sitting in your skin cells, called 7-dehydrocholesterol, absorbs UVB radiation in the 295-to-315-nanometer range and converts into previtamin D3, which then rearranges itself into vitamin D3.1PubMed Central. Sunlight and Vitamin D: A global perspective for health2PubMed Central. A revised action spectrum for vitamin D synthesis by suberythemal UV radiation exposure in humans in vivo From there, your liver converts vitamin D3 into its circulating form (25-hydroxyvitamin D), and your kidneys activate it into the hormone your cells actually use. A breakdown at any step along this chain, from the initial sun exposure down through the liver and kidneys, can leave you deficient even if the other steps are working fine.
Where You Live and What Season It Is
Geography is one of the biggest determinants of your vitamin D status, but the relationship is more seasonal than most people realize. During roughly March through October, UVB levels across a wide range of latitudes in the United States are similar enough that location alone does not dramatically alter your skin’s ability to produce vitamin D.3PubMed. Location and vitamin D synthesis: is the hypothesis validated by geophysical data? The problem kicks in during the four cooler months, November through February, when latitude makes a huge difference. The farther north you are, the more steeply the available UVB drops, and at high latitudes there may not be enough UVB reaching the ground to trigger any meaningful vitamin D production in your skin at all.3PubMed. Location and vitamin D synthesis: is the hypothesis validated by geophysical data?
This is why deficiency rates spike in late winter and early spring. You have been running on stored vitamin D for months, and if you did not build up enough during the summer, your levels have been slowly declining since October. People in places like Seattle, Minneapolis, or London are especially vulnerable, but even someone in Atlanta can run low if they spent the summer mostly indoors.
Your Job and How You Spend Your Days
The single most practical cause of vitamin D deficiency for working-age adults in developed countries is simply not being outside during daylight hours. If you commute to an office before the sun is fully up and leave after most of the strong UVB is gone, your skin gets almost no opportunity to make vitamin D on a weekday. Systematic reviews have consistently found that indoor workers have lower vitamin D levels than outdoor workers, and shift workers and healthcare workers are at particularly high risk.4PubMed Central. Vitamin D levels and deficiency with different occupations: a systematic review5PubMed Central. Are Shiftwork and Indoor Work Related to D3 Vitamin Deficiency? A Systematic Review of Current Evidences
Night-shift workers get a double hit. They sleep during the day, so even if UVB is plentiful outside, they are not in it. And the lifestyle changes that come with shift work, irregular meal times, fatigue that discourages outdoor activity, tend to make things worse. If you work nights or spend your entire workday under fluorescent lights, your body is essentially in permanent “winter mode” as far as vitamin D production goes, regardless of the season.
What Your Clothes Block
This one seems obvious, but the degree of the effect surprises people. Clothing does not just reduce UVB reaching your skin; it can eliminate vitamin D production entirely. Research testing common fabrics, cotton, wool, and polyester in both black and white, found that none of them allowed any measurable previtamin D3 formation even after extended UVB exposure. Even at UVB doses several times higher than what would cause sunburn on bare skin, subjects wearing normal clothing showed no rise in blood vitamin D3 levels.6The Journal of Clinical Endocrinology & Metabolism. Clothing prevents ultraviolet-B radiation-dependent photosynthesis of vitamin D3 Regular seasonal street clothing was just as effective at blocking production.
This matters most for people who cover the majority of their body for religious, cultural, or personal reasons, and for anyone living in a cold climate where bundling up is the norm for half the year. Even rolling your sleeves up on a lunch break might not expose enough skin area to move the needle if the rest of your body is covered.
The Sunscreen Question
Given how effectively clothing blocks vitamin D synthesis, you might expect sunscreen to do the same. The worry is logical, and it has circulated for decades. But the evidence tells a different story. A panel review published in the British Journal of Dermatology concluded that sunscreen use for daily and recreational sun protection does not compromise vitamin D synthesis, even when applied under optimal conditions.7PubMed Central. Sunscreen photoprotection and vitamin D status The reason is that in real life, people rarely apply sunscreen as thickly or as uniformly as lab conditions require, and enough UVB gets through gaps and missed spots to keep some production going. Skipping sunscreen out of vitamin D anxiety is not a good trade-off; there are safer ways to maintain your levels.
Skin Pigmentation
Darker skin has long been cited as a major risk factor for vitamin D deficiency, and the epidemiological data does show that people with more melanin tend to have lower blood levels. The evolutionary explanation is straightforward: lighter skin appears to have evolved at higher latitudes to allow more efficient vitamin D production under weaker UVB conditions.8PubMed. Vitamin D: in the evolution of human skin colour More melanin means more UVB gets absorbed before it reaches the deeper skin layers where vitamin D3 is actually made.9JAMA Dermatology. Racial Pigmentation and the Cutaneous Synthesis of Vitamin D
That said, the actual size of the effect is smaller than many people assume. A study comparing extreme skin types, the lightest and darkest on the standard scale, found that melanin’s inhibition of vitamin D3 synthesis was only about 1.3 to 1.4 times, depending on the UV source. The researchers described the inhibitory effect as “small compared with erythema,” meaning the difference in how quickly each skin type burns is much larger than the difference in how much vitamin D each produces.10PubMed. Melanin has a Small Inhibitory Effect on Cutaneous Vitamin D Synthesis: A Comparison of Extreme Phenotypes That small difference can be enough to explain population-level disparities in vitamin D status, especially when combined with other risk factors, but skin color alone is not as powerful a barrier as it is sometimes made out to be.
Getting Older
Your skin becomes less efficient at producing vitamin D as you age. The reason is that the raw material, 7-dehydrocholesterol, becomes less abundant in your skin over the decades. Research comparing skin samples from people aged 8 to 92 found that aging can decrease the skin’s capacity to produce previtamin D3 by more than twofold. The epidermis was the primary production site in both young and older subjects, accounting for over 80 percent of previtamin D3 formation, but the older skin simply had less starting material to work with.11JCI Insight. Aging decreases the capacity of human skin to produce vitamin D3
This means an older adult getting the exact same amount of sun as a teenager will make roughly half the vitamin D. Add in the fact that older adults tend to spend more time indoors, wear more clothing, and may have kidney function that has declined with age, and the reasons for deficiency stack up fast. For adults over 65, supplementation is not optional; it is practically a necessity.
Body Weight and Fat Tissue
Obesity is one of the strongest and most consistent risk factors for low vitamin D. Multiple reviews have converged on the same set of explanations for why this happens. First, vitamin D is fat-soluble, so a larger volume of body fat dilutes the vitamin across a bigger space, lowering the concentration in your blood. Second, adipose tissue can actively sequester vitamin D, locking it away in fat stores where it is not available for the rest of the body. Third, people with obesity may get less sun exposure due to reduced outdoor physical activity. And there is evidence that obesity itself impairs some of the metabolic steps involved in processing vitamin D in both fat tissue and the liver.12PubMed. Vitamin D and Obesity: Current Evidence and Controversies13Obesity Medicine. Vitamin D deficiency in obesity: Epidemiological evidence, biological mechanisms, and clinical considerations
The practical consequence is that standard supplement doses may not be enough. A person with a BMI over 30 often needs higher doses to reach the same blood levels as someone at a healthy weight, because a larger fraction of whatever they take gets pulled into fat storage before it can circulate. If you carry significant excess weight and your levels stay stubbornly low despite supplementing, this is likely why.
Gut Conditions That Block Absorption
Vitamin D is fat-soluble, which means absorbing it from food or supplements requires a healthy gut that can handle dietary fats. Conditions that disrupt fat absorption can severely impair vitamin D status. Celiac disease, Crohn’s disease, ulcerative colitis, and the aftermath of bariatric surgery all fall into this category.14PubMed Central. Vitamin D and malabsorptive gastrointestinal conditions: A bidirectional relationship? The relationship may be bidirectional: the gut condition lowers your vitamin D, and low vitamin D may worsen the gut condition, creating a cycle that is hard to break without targeted intervention.
If you have been diagnosed with any inflammatory bowel disease, had weight-loss surgery, or struggle with chronic diarrhea or fatty stools, your gut’s ability to absorb vitamin D from food and supplements may be compromised. Blood testing and potentially higher or more absorbable forms of supplementation are worth discussing with a doctor.
When Your Liver or Kidneys Are Struggling
Even if your skin makes plenty of vitamin D and your gut absorbs it fine, your body still needs to process it through two organ systems before it becomes useful. The liver handles the first conversion step, turning vitamin D3 into 25-hydroxyvitamin D, which is the form measured in standard blood tests. Research has shown that liver injury impairs this step directly, reducing circulating levels of the usable form.15PubMed Central. Liver Injury Impaired 25-Hydroxylation of Vitamin D Suppresses Intestinal Paneth Cell defensins, leading to Gut Dysbiosis and Liver Fibrogenesis Patients with chronic liver disease, regardless of the underlying cause, tend to have reduced vitamin D levels.16PubMed Central. Role of Vitamin D in Liver Disease and Complications of Advanced Chronic Liver Disease
Kidney disease affects the second conversion step. Healthy kidneys activate 25-hydroxyvitamin D into its final, hormonally active form. As kidney function declines, this activation drops. The mechanism involves not just loss of the activating enzyme but also reduced uptake of the precursor from filtered blood, along with interference from substances that accumulate when kidneys are not clearing waste properly.17Nature / Kidney International Supplements. Kidney disease and vitamin D levels: 25-hydroxyvitamin D, 1,25-dihydroxyvitamin D, and VDR activation – Section: CRITICAL ROLE OF THE KIDNEY IN AUTOCRINE/PARACRINE VDR ACTIVATION Anyone with chronic kidney disease or significant liver disease should have vitamin D monitored routinely, because the deficiency is almost built into the condition.
Diet Is Rarely the Whole Problem
Very few foods naturally contain meaningful amounts of vitamin D. Fatty fish like salmon and mackerel, egg yolks, and certain mushrooms exposed to UV light are among the best natural sources, but you would need to eat them in impractical quantities to meet your daily needs through food alone. Fortified foods like milk, orange juice, and some cereals help, but population-level intake data consistently shows that most people do not get enough vitamin D from diet to compensate for limited sun exposure. Food fortification has been proposed as a broader public health strategy precisely because individual dietary habits alone are not solving the problem.18PubMed Central. Vitamin D: dietary requirements and food fortification as a means of helping achieve adequate vitamin D status
This is an important point for people who think they are eating well enough that they should not be deficient. Your body was designed to get most of its vitamin D from sunlight, with food as a minor supplement. In modern life, where sun exposure is limited, food alone almost never closes the gap.
Genetic Variation
Some people are genetically predisposed to lower vitamin D levels regardless of their lifestyle. Variations in genes involved in vitamin D transport and metabolism can affect how efficiently your body handles the vitamin at every step, from how it travels through the blood to how it gets activated. Research has identified specific genetic variants in genes like GC (which encodes the protein that carries vitamin D in the blood), CYP2R1 (involved in the liver’s conversion step), and CYP27B1 (involved in the kidney’s activation step) that are associated with vitamin D deficiency.19PubMed Central. Identification and Association of CYP2R1, CYP27B1, and GC Gene Polymorphisms with Vitamin D Deficiency in Apparently Healthy Population and in Silico Analysis of the Binding Pocket of Vitamin D3
You cannot change your genetics, but knowing that a hereditary component exists helps explain why some people remain deficient despite doing everything right. If your parents or siblings have chronically low vitamin D despite adequate sun and supplement use, there may be a genetic contribution at work. Genetic testing for vitamin D metabolism is not standard practice, but it is increasingly available and may eventually guide more personalized dosing.
Air Pollution as a Hidden Factor
You can be at the right latitude, outside at the right time of day, and still not get the UVB your skin needs if the air above you is heavily polluted. Research analyzing four years of weather-station data found that several common pollutants, including benzene, ozone, nitric oxide, sulfur dioxide, and particulate matter, were negatively associated with the intensity of UVB radiation reaching the ground.20Scientific Reports. Air pollutants are negatively associated with vitamin D-synthesizing UVB radiation intensity on the ground These pollutants scatter and absorb UVB in the atmosphere before it ever reaches your skin.
This factor is easy to overlook because you cannot see UVB, and a hazy day might still feel bright. Cities with heavy traffic pollution and industrial emissions effectively reduce the vitamin D-producing potential of their sunlight. It is one more reason why urban populations tend to have lower vitamin D than rural ones, even when they are at the same latitude and spend similar amounts of time outdoors.
Magnesium and Vitamin D Work Together
Magnesium does not make vitamin D, but your body needs it to activate the vitamin D you already have. Magnesium is a cofactor in the enzymatic steps that convert stored vitamin D into its usable forms.21PubMed Central. Magnesium and Vitamin D Deficiency as a Potential Cause of Immune Dysfunction, Cytokine Storm and Disseminated Intravascular Coagulation in covid-19 patients If you are low in magnesium, supplementing with vitamin D alone may not fully correct a deficiency because the conversion machinery is not running at full capacity. A substantial portion of people in Western countries do not meet the recommended magnesium intake, often without knowing it. If your vitamin D levels are not responding to supplementation the way you would expect, checking your magnesium status is a reasonable step.
Breastfed Infants
Breast milk is nutritionally excellent in most respects, but it contains very little vitamin D. This makes exclusively breastfed infants particularly vulnerable to deficiency, especially if they also have limited sun exposure, live in polluted areas, or have mothers who are themselves deficient.22PubMed Central. Breastfeeding and vitamin D The re-emergence of rickets in some populations has been tied directly to this combination. Most pediatric guidelines recommend vitamin D drops for breastfed infants starting shortly after birth. Some research has explored whether mothers taking high-dose supplements (up to 6,400 IU per day) can boost the vitamin D content of their milk enough to protect the infant, offering an alternative to giving drops directly to the baby.22PubMed Central. Breastfeeding and vitamin D
When Multiple Causes Overlap
What makes vitamin D deficiency so common is that these causes rarely exist in isolation. Consider a person working night shifts in a northern city: they sleep through the daylight hours, live at a latitude where winter UVB is nearly nonexistent for four months, and the urban air pollution further reduces whatever UVB does get through. Or consider an older adult with kidney disease and excess body weight living in a region with limited food fortification. Each individual factor might produce mildly low levels on its own; together, they can drive a severe deficiency that resists casual supplementation.
Understanding which causes apply to you is more useful than just taking a generic supplement dose and hoping for the best. Someone whose deficiency stems mainly from limited sun exposure may respond well to moderate supplementation. Someone whose gut does not absorb fat properly may need a different form of vitamin D. Someone with kidney disease may need the pre-activated form prescribed by a physician. And someone with several overlapping risk factors may need a higher dose, better magnesium intake, and a deliberate effort to get outdoors, all at once. A blood test measuring 25-hydroxyvitamin D is cheap, widely available, and the single most useful step for figuring out where you stand and whether what you are doing is working.