Low vitamin D most often results from not getting enough sunlight, but a surprising number of other factors can drive levels down, from body weight to kidney function to where you live. Your body produces the vast majority of its vitamin D through skin exposed to UVB rays, so anything that interrupts that process, limits absorption from food, or impairs how your organs activate the vitamin can leave you deficient. A blood level below 12 ng/mL is considered deficient, while 12 to 20 ng/mL is generally inadequate for bone and overall health.
How Your Body Makes Vitamin D
Understanding why levels drop starts with understanding the supply chain. When UVB radiation (wavelengths between roughly 295 and 315 nanometers) hits your skin, it converts a cholesterol compound called 7-dehydrocholesterol into a precursor form of vitamin D. That precursor travels through your bloodstream to the liver, where it’s converted into the circulating form that doctors measure on blood tests (25-hydroxyvitamin D). From there, the kidneys convert it again into the fully active hormone your body actually uses.
Each step in this chain is a potential bottleneck. If your skin doesn’t get enough UVB, the process never starts. If your liver or kidneys aren’t functioning well, the raw material can’t be activated. If your gut can’t absorb dietary vitamin D, that backup source fails too.
Limited Sun Exposure
This is the single biggest driver of low vitamin D worldwide. People who work indoors, cover most of their skin, or consistently wear sunscreen reduce the UVB reaching their skin. Window glass blocks UVB, so sitting in a sunny office does nothing for your vitamin D levels.
Season and geography matter enormously. During the four cooler months from November through February, the amount of UVB capable of triggering vitamin D production drops dramatically as latitude increases. If you live in the northern half of the United States or anywhere in Canada, the winter sun sits too low in the sky to deliver meaningful UVB for several months. During the rest of the year (roughly March through October), researchers have found no significant latitude difference in vitamin D-producing UV across the U.S., from the southernmost to the northernmost measurement sites. The practical takeaway: winter is when your stores are most at risk, especially if you live far from the equator.
Body Weight and Fat Tissue
People with obesity consistently have lower circulating vitamin D levels than people at a normal weight, and the gap is significant. Several mechanisms work together to cause this. Fat tissue absorbs and holds onto vitamin D, effectively pulling it out of circulation. The larger your total body volume, the more any given amount of vitamin D gets diluted across a bigger pool. There’s also evidence that obesity impairs the liver’s ability to perform its conversion step. On top of the biology, lifestyle factors associated with higher body weight, like less outdoor activity, can compound the problem.
Gut Conditions That Block Absorption
Vitamin D is fat-soluble, meaning your intestines need to absorb it alongside dietary fat. Any condition that damages the gut lining or disrupts fat absorption can limit how much vitamin D you take in from food or supplements. Celiac disease is a well-established cause: the intestinal damage it creates leads to broad nutrient malabsorption, and vitamin D is one of the casualties. Crohn’s disease, particularly when it affects the small intestine, poses a similar risk. Other conditions like cystic fibrosis, chronic pancreatitis, and surgical removal of part of the stomach or intestines can also impair absorption.
This matters even if you take supplements, because the vitamin D in a pill still has to be absorbed through the gut. People with these conditions often need higher doses or specific formulations to compensate.
Kidney Disease
Your kidneys handle the final activation step, converting the circulating form of vitamin D into the hormone that actually works in your body. When kidney function declines, this conversion slows. Research on patients with varying levels of kidney impairment found a clear, graded relationship: as kidney filtration rates dropped, active vitamin D levels dropped with them. Patients with significantly impaired kidney function had measurably lower active vitamin D than those with normal function. This is why chronic kidney disease is one of the more common medical causes of persistent deficiency that doesn’t respond well to simple supplementation.
Skin Color
Melanin, the pigment that gives skin its color, absorbs some UVB before it can trigger vitamin D production. For decades, this was assumed to be a major barrier for people with darker skin. More recent research has refined the picture. A study comparing vitamin D synthesis across skin types (from very fair to very dark) found that melanin’s inhibitory effect is real but modest, with a factor of roughly 1.3 to 1.4 between the lightest and darkest skin types tested. In other words, very dark skin reduces UVB-driven vitamin D production by about 30 to 40 percent compared to very fair skin, not the dramatic difference once assumed. Still, in northern latitudes with limited winter sun, that 30 to 40 percent reduction on top of already low UVB can meaningfully contribute to deficiency.
Aging
It was long believed that older adults produce far less vitamin D because their skin contains less of the precursor compound. A more recent controlled study challenged this, finding that baseline levels of 7-dehydrocholesterol in the skin were essentially the same in healthy older and younger adults. The older group did show a somewhat smaller response to UV exposure (a 67% increase in precursor activity versus 107% in the younger group), but the difference was less dramatic than the field had assumed for roughly 40 years. The bigger factors for age-related deficiency are likely behavioral: older adults tend to spend less time outdoors, are more likely to have kidney disease, and often take medications that interfere with vitamin D metabolism.
Medications That Lower Vitamin D
Several common drug classes accelerate the breakdown of vitamin D in your body. Certain antiepileptic medications, including phenytoin, phenobarbital, and carbamazepine, rev up a liver enzyme that breaks vitamin D into its inactive form. Corticosteroids like dexamethasone can also lower levels by increasing the activity of a kidney enzyme that degrades vitamin D metabolites. If you take any of these long-term, your doctor may monitor your vitamin D or recommend supplementation to offset the effect.
Very Few Foods Contain Vitamin D
Diet alone is a poor source of vitamin D for most people. Fatty fish like salmon, mackerel, and sardines contain meaningful amounts, and egg yolks and beef liver provide small quantities. Many countries fortify milk, orange juice, and breakfast cereals, which helps but rarely closes the gap entirely. Without fortified foods, most diets provide well under the recommended daily intake. This is why sun exposure and supplementation carry the bulk of the responsibility, and why dietary shortfall alone can push someone from adequate to insufficient, especially during winter months.
What the Blood Test Numbers Mean
Vitamin D status is measured through a blood test for 25-hydroxyvitamin D. The NIH defines the thresholds as follows:
- Below 12 ng/mL: Deficient. Associated with rickets in children and softening of bones in adults.
- 12 to 20 ng/mL: Inadequate for bone and overall health.
- 20 ng/mL or above: Sufficient for most people.
- Above 50 ng/mL: Potentially harmful, with risk increasing above 60 ng/mL.
If your levels fall in the deficient or inadequate range, the cause is almost always one or a combination of the factors above. Identifying which ones apply to you is what determines whether the fix is more outdoor time, a supplement, a higher dose due to absorption issues, or treatment of an underlying condition.