Vitamin D deficiency usually comes down to one or more of three things: not enough sun exposure, not enough dietary intake, or something in your body that blocks absorption or speeds up breakdown. A blood level below 12 ng/mL is classified as deficient, while anything between 12 and 20 ng/mL is considered inadequate for bone and overall health. Levels at or above 20 ng/mL are what most people need. Understanding which factors apply to you makes it much easier to fix the problem.
How Your Body Makes Vitamin D
Your skin is your primary vitamin D factory. When UVB rays from the sun hit your skin, they convert a cholesterol compound already sitting in your outer skin layers into a precursor form of vitamin D. That precursor then transforms into cholecalciferol (vitamin D3), enters your bloodstream, and travels to your liver and kidneys, where it gets activated into the hormone your cells actually use.
This process is remarkably efficient under the right conditions, but it’s also fragile. Disrupt any step, from the sunlight hitting your skin to the final activation in your kidneys, and your levels drop.
You May Not Be Getting Enough UVB Light
The most common reason for deficiency is simply not getting enough of the right kind of sunlight. UVB radiation, the wavelengths between roughly 295 and 315 nanometers, is the only part of the sun’s spectrum that triggers vitamin D production. Several everyday factors block it.
Where you live matters more than you think. If you live north of about the 37th parallel (roughly the latitude of Los Angeles, Seville, or Seoul), UVB rays are too weak from November through March to produce meaningful vitamin D. This period is sometimes called “vitamin D winter.” People in northern cities like London, Seattle, Toronto, or Stockholm face an even longer gap. During these months, your body runs entirely on stored vitamin D, dietary sources, and supplements.
Indoor lifestyles are a major driver. Office workers, night-shift employees, homebound older adults, and anyone who spends most daylight hours inside simply don’t get enough UVB exposure. Window glass blocks nearly all UVB light, so sitting by a sunny window doesn’t help. Sunscreen with SPF 15 or higher also dramatically reduces vitamin D synthesis, though dermatologists still recommend it for skin cancer prevention.
Darker skin tones need more time. Melanin, the pigment that gives skin its color, acts as a natural sunscreen. People with more melanin absorb UVB less efficiently and need longer sun exposure to produce the same amount of vitamin D as someone with lighter skin. This is a significant factor for people with darker complexions living at higher latitudes, where UVB is already limited.
Diet Alone Rarely Provides Enough
Very few foods naturally contain vitamin D. Fatty fish like salmon, mackerel, and sardines are the best natural sources, but even a generous serving only provides a fraction of what your skin can make in a short sun session. Egg yolks, beef liver, and certain mushrooms exposed to UV light contain small amounts. Fortified foods like milk, orange juice, and some cereals help fill the gap, but most people don’t consume enough of them to fully compensate for low sun exposure.
If you follow a vegan or dairy-free diet, your options narrow further. Plant-based milks are often fortified, but not always to the same levels as cow’s milk. Without deliberate planning or supplementation, dietary intake alone is unlikely to keep your blood levels in the adequate range.
Your Gut May Not Be Absorbing It
Vitamin D is fat-soluble, meaning it needs dietary fat and a healthy digestive tract to be absorbed properly. Several gastrointestinal conditions interfere with this process. Crohn’s disease, ulcerative colitis, and celiac disease all damage or inflame the intestinal lining in ways that reduce nutrient absorption. Even if you’re eating the right foods or taking supplements, your body may not be pulling enough vitamin D from them.
Gastric bypass surgery creates a similar problem from a different angle. Because the procedure reroutes food past a section of the small intestine where vitamin D is normally absorbed, people who’ve had this surgery are at ongoing risk for deficiency and typically need higher supplement doses or more frequent monitoring.
Body Fat Can Trap Vitamin D
Carrying extra body weight is one of the most underrecognized causes of low vitamin D. Because vitamin D is fat-soluble, it gets pulled into and stored by fat tissue. In people with higher amounts of body fat, larger quantities of vitamin D become sequestered in adipose tissue instead of circulating freely in the blood. The vitamin D is released very slowly from fat stores, and mainly comes back into circulation when fat breaks down during weight loss.
This means that two people with identical sun exposure and diets can have very different blood levels if one carries significantly more body fat. It also means standard supplement doses may not be sufficient for someone with obesity, since more of each dose gets absorbed into fat rather than reaching the bloodstream.
Certain Medications Lower Your Levels
A surprisingly long list of medications can interfere with vitamin D. Some speed up its breakdown in the liver, others block its absorption, and still others alter the enzymes involved in activating it.
- Anti-seizure medications like carbamazepine, phenobarbital, and phenytoin increase the rate at which your liver breaks vitamin D down into inactive byproducts. People on these drugs long-term often develop deficiency even with adequate sun and diet.
- Weight-loss drugs that block fat absorption (like orlistat) also block the absorption of fat-soluble vitamins, including D. If you’re taking one of these, you’re losing a portion of the vitamin D from both food and supplements.
- Corticosteroids, certain antidepressants, proton pump inhibitors (used for acid reflux), and some HIV medications can also affect vitamin D status through various pathways involving liver enzymes.
If you take any medication daily and your levels are persistently low, the drug itself could be part of the equation.
Age and Kidney Function Play a Role
As you age, your skin becomes less efficient at producing vitamin D. The amount of the cholesterol precursor in your skin declines, so even with the same sun exposure you got at 25, your body produces less vitamin D at 65. Older adults also tend to spend less time outdoors and may eat less overall, compounding the problem.
Your kidneys handle the final step of activating vitamin D. If kidney function is reduced, whether from chronic kidney disease or simply from aging, that activation step slows down. You can have a reasonable amount of vitamin D in your blood but not enough of the active form your cells need.
Genetics Can Affect How You Process It
Your genes influence how well your body handles vitamin D at every stage, from production to transport to cellular use. Variations in the vitamin D receptor gene can change how efficiently your cells respond to the vitamin once it arrives. Other genetic variants affect the proteins that carry vitamin D through your bloodstream or the enzymes that activate it.
That said, genetics alone rarely cause deficiency. Research on common vitamin D receptor gene variants has found they don’t consistently correlate with lower blood levels. They’re more likely a contributing factor that tips the balance when combined with limited sun, poor diet, or other risks.
What a Blood Test Actually Tells You
The standard test measures 25-hydroxyvitamin D in your blood, which reflects your total vitamin D from sun, food, and supplements combined. Here’s how the NIH classifies the results:
- Below 12 ng/mL: Deficient. Associated with bone-softening conditions in both children and adults.
- 12 to 19 ng/mL: Inadequate for bone and overall health.
- 20 ng/mL or above: Adequate for most people.
- Above 50 ng/mL: Potentially harmful, particularly above 60 ng/mL.
If your result falls in the deficient or inadequate range, look at the factors above and consider which ones overlap in your life. Most people with low vitamin D have at least two or three contributing causes working together: maybe limited sun plus a medication, or darker skin plus a northern latitude plus an indoor job. Identifying the combination that applies to you is the first step toward getting your levels back up effectively.