Low vitamin D most often comes down to not enough sun exposure, not enough in your diet, or something in your body preventing it from being absorbed or activated. Most adults need 600 IU (15 mcg) of vitamin D daily, rising to 800 IU after age 70, and falling short is surprisingly easy because very few foods naturally contain it. Understanding the specific causes helps you figure out which ones apply to you.
How Your Body Makes Vitamin D
Vitamin D is unusual among nutrients because your body can manufacture it. When UVB rays from the sun (in the 295 to 315 nanometer wavelength range) hit your skin, they convert a cholesterol compound called 7-dehydrocholesterol into previtamin D3. That molecule then transforms into vitamin D3 (cholecalciferol), which enters your bloodstream. But it’s not usable yet. Your liver converts it once, then your kidneys convert it a second time into the active hormone your cells actually use, called calcitriol.
Any disruption along this chain, from sun exposure to skin chemistry to liver and kidney processing, can leave you deficient.
Limited Sun Exposure
This is the single most common reason people run low. Your skin needs direct UVB light to produce vitamin D, and several everyday factors block that from happening. Living at higher latitudes (roughly above 37°N, which includes most of the U.S. north of Richmond, Virginia) means UVB rays are too weak for vitamin D production during winter months. Spending most of the day indoors, wearing clothing that covers most of your skin, and applying sunscreen all reduce the amount of UVB that reaches the cholesterol precursor in your epidermis.
Shift workers, office workers, and people who are homebound or institutionalized are especially likely to have low levels simply because their skin rarely sees midday sun.
Darker Skin Tone
Melanin, the pigment that gives skin its color, acts as a natural sunscreen. It absorbs UVB radiation before it can trigger vitamin D production. Research comparing the lightest and darkest skin types (Fitzpatrick types II and VI) found that melanin reduces the skin’s vitamin D output by a factor of roughly 1.3 to 1.4 under the same UV conditions. That may sound modest, but it compounds over time, especially for people with darker skin living at northern latitudes where UVB is already limited. Immigration studies consistently show that people who move from equatorial regions to higher-latitude countries experience significant drops in vitamin D levels, driven by the combination of less intense sunlight and slower physiological adaptation to the new environment.
Few Natural Food Sources
Unlike most vitamins, vitamin D is found naturally in very few foods. Fatty fish like salmon, mackerel, and sardines are the best sources, but even they provide only a few hundred IU per serving. Egg yolks and beef liver contain small amounts. Fortified milk, orange juice, and some cereals help, but they typically add only 100 to 150 IU per serving. Reaching 600 IU a day through food alone requires deliberate effort, and most people don’t come close without fortified products or supplements.
If you follow a vegan or dairy-free diet, your options narrow further since the richest natural sources are all animal-based. Fortified plant milks exist, but the amounts vary by brand.
Digestive and Absorption Problems
Vitamin D is fat-soluble, meaning your gut needs to absorb it alongside dietary fat. Several gastrointestinal conditions interfere with this process, each in a slightly different way.
Celiac disease damages the lining of the small intestine, destroying the cells that absorb nutrients and shrinking the surface area available for absorption. People with untreated celiac disease absorb calcium and vitamin D poorly as a result. Crohn’s disease and other forms of inflammatory bowel disease cause chronic inflammation and epithelial damage in the gut wall, which similarly impairs absorption. Cystic fibrosis takes a different route: the pancreas can’t release enough digestive enzymes, so dietary fat (and the vitamin D dissolved in it) passes through unabsorbed.
Gastric bypass surgery can also reduce absorption by rerouting food past the sections of intestine where fat-soluble vitamins are normally taken up.
Obesity
Carrying significant excess body fat is strongly linked to lower vitamin D levels. The reason is partly mechanical: vitamin D is fat-soluble, so it gets pulled into and trapped within large stores of adipose tissue instead of circulating freely in the blood. On top of this sequestration effect, vitamin D metabolism itself appears to be disrupted in obesity. The result is that people with a higher BMI often need larger doses of vitamin D to reach the same blood levels as leaner individuals.
Aging
As you get older, your skin becomes less efficient at making vitamin D. The concentration of the precursor molecule (7-dehydrocholesterol) in the skin drops by roughly 50% between age 20 and age 80. At the same time, the body breaks down active vitamin D faster. This is one reason the recommended daily intake rises from 600 IU to 800 IU after age 70. Older adults also tend to spend less time outdoors and may eat less overall, compounding the problem.
Kidney and Liver Disease
Because both your liver and kidneys play a role in converting vitamin D into its active form, damage to either organ can leave you deficient even if you’re getting plenty of sun and dietary intake. Chronic kidney disease is the more common culprit. By stage 4 or 5, the kidneys often can’t complete that final conversion step, so the vitamin D circulating in your blood never becomes the active hormone your body needs. People in advanced kidney disease or on dialysis typically require a pre-activated form of vitamin D prescribed by their care team.
Severe liver disease, including cirrhosis, can impair the first conversion step, though this tends to become significant only when liver function is substantially compromised.
Medications That Lower Vitamin D
Several common medications can deplete vitamin D or interfere with how your body processes it:
- Anti-seizure drugs like carbamazepine, phenobarbital, and phenytoin speed up the liver’s breakdown of vitamin D into inactive byproducts, which also reduces calcium absorption over time.
- Orlistat, a weight-loss drug that blocks fat absorption, also blocks absorption of fat-soluble vitamins including D. Studies show measurable drops in blood levels of vitamin D in people taking it.
- Corticosteroids used for conditions like asthma, autoimmune diseases, and inflammatory disorders can negatively affect vitamin D status, particularly with long-term use.
If you take any of these medications regularly, your provider may monitor your vitamin D levels or recommend supplementation.
How Deficiency Is Measured
A simple blood test measuring 25-hydroxyvitamin D (the form your liver produces) tells you where you stand. Most labs and guidelines use these thresholds: below 20 ng/mL (50 nmol/L) is considered deficient, 20 to 29 ng/mL is insufficient, and 30 ng/mL or above is generally considered adequate. Some experts argue the cutoff for sufficiency should be higher, but these are the most widely used benchmarks.
Symptoms of deficiency develop slowly and can be vague: fatigue, muscle weakness, bone pain, and frequent illness. Severe, prolonged deficiency can lead to bone-softening conditions like osteomalacia in adults or rickets in children. Many people with moderately low levels, though, feel no obvious symptoms at all, which is why the condition often goes unnoticed until a blood test reveals it.