Low vitamin D usually comes down to one or more of three things: not enough sun exposure, not enough absorption from food, or a body composition or medical condition that alters how your body processes the vitamin. A blood level below 20 ng/mL is classified as deficient, while 21 to 29 ng/mL is considered insufficient. Understanding what drives levels down can help you figure out which factors apply to you.
Limited Sun Exposure
Your skin produces vitamin D when ultraviolet B (UVB) rays hit it, making sunlight the single largest natural source. Anything that reduces your UVB exposure reduces your production. Spending most of the day indoors, wearing clothing that covers most skin, and consistent sunscreen use all limit the raw material your skin needs.
Where you live matters just as much as your habits. At higher latitudes, the sun sits too low in the sky during winter months for UVB rays to reach the ground effectively. In Boston (about 42°N), skin exposed to sunlight from November through February produces essentially zero vitamin D. In Edmonton, Canada (52°N), that dead zone stretches from October through March. If you live in the northern half of the United States, most of Canada, the UK, or northern Europe, you simply cannot make vitamin D from sunlight for a significant chunk of the year, no matter how long you spend outside.
Darker Skin Pigmentation
Melanin, the pigment that gives skin its color, absorbs some UVB radiation before it can trigger vitamin D production. The effect is real but smaller than many people assume. Controlled studies comparing very fair skin to very dark skin found that melanin reduces vitamin D synthesis by a factor of roughly 1.3 to 1.4. That’s a meaningful gap over time, especially when combined with limited sun exposure or living at a high latitude, but it’s not the dramatic difference older estimates suggested. For people with darker skin, the practical takeaway is that you may need somewhat more sun or more dietary vitamin D to maintain the same blood levels as someone with lighter skin.
Obesity
Vitamin D is fat-soluble, and body fat acts like a sponge for it. In people with a higher body mass index, vitamin D from both sunlight and food gets deposited into fat tissue instead of circulating freely in the bloodstream. The vitamin isn’t destroyed; it’s just locked away where the body can’t easily use it. This means that even with adequate sun exposure and a reasonable diet, people carrying significant excess weight often show lower blood levels of vitamin D. Higher supplemental doses are sometimes needed to compensate.
Gastrointestinal and Malabsorption Conditions
Because vitamin D is absorbed along with dietary fat in the small intestine, any condition that disrupts fat absorption can leave you deficient. Several common conditions fall into this category, and the numbers are striking:
- Celiac disease. Damage to the intestinal lining impairs absorption of fat-soluble vitamins broadly. In studies of celiac patients, vitamin D deficiency appears in 64% of men and 71% of women. Many of these patients also develop weakened bones.
- Crohn’s disease. Chronic inflammation and damage to the gut wall interfere with absorption. Depending on the study, between 27% and 68% of people with Crohn’s disease are vitamin D deficient.
- Gastric bypass surgery. Rerouting the digestive tract reduces the surface area available for nutrient absorption. Four years after surgery, roughly 63% of patients in one cohort were vitamin D deficient.
- Cystic fibrosis and pancreatic insufficiency. Without enough digestive enzymes from the pancreas, fat passes through undigested, and vitamin D goes with it. Patients with cystic fibrosis absorb less than half the vitamin D that healthy people absorb from the same oral dose.
If you have any of these conditions, routine blood monitoring of vitamin D is standard practice, and supplementation often needs to be more aggressive than general guidelines suggest.
Aging
Older adults are among the most commonly deficient groups. Several factors stack against them. They tend to spend less time outdoors. Their dietary intake of vitamin D often drops. And for those in assisted living or with limited mobility, sun exposure can be nearly zero.
Earlier research suggested that aging skin contains about half the vitamin D precursor (a cholesterol compound in the outer skin layer) compared to younger skin, which would mean older skin is simply less efficient at making the vitamin. More recent work under carefully controlled conditions, however, found no significant difference in precursor levels between younger and older adults. This suggests the real driver is behavioral: less time in the sun rather than less capable skin. Either way, the result is the same. Older adults are at high risk and benefit from supplementation.
Dietary Shortfalls
Very few foods naturally contain meaningful amounts of vitamin D. Fatty fish like salmon and mackerel, egg yolks, and beef liver provide some, but not enough for most people to meet their needs through diet alone. Fortified foods like milk, orange juice, and certain cereals help close the gap, but only if you consume them regularly.
Breastfed infants face a specific version of this problem. Human breast milk contains very little vitamin D, which is why the American Academy of Pediatrics recommends that breastfed and partially breastfed infants receive 400 IU of supplemental vitamin D daily, starting in the first few days of life. Children aged 12 to 24 months need 600 IU per day. Without supplementation, exclusively breastfed babies are at real risk of deficiency.
People following strict vegan diets also face a narrower range of dietary sources, since most naturally rich foods are animal-based. Fortified plant milks and mushrooms exposed to UV light are two of the better plant-based options.
Kidney and Liver Disease
Your body doesn’t use vitamin D in the form it arrives from sunlight or food. It has to be converted first in the liver, then again in the kidneys, into its active form. Chronic kidney disease and severe liver disease can both impair these conversion steps. You might have adequate vitamin D intake but still end up functionally deficient because your body can’t activate it. This is particularly common in advanced kidney disease, where the second conversion step slows dramatically.
Certain Medications
Some drugs speed up the breakdown of vitamin D in the body. Antiseizure medications, including carbamazepine, phenobarbital, and phenytoin, are among the most well-documented offenders. They ramp up liver enzymes that metabolize vitamin D, clearing it from the body faster than normal. Long-term use of glucocorticoids (steroids prescribed for inflammation) and certain other medications can also interfere with vitamin D metabolism or calcium absorption. If you take any of these regularly, your provider may check your vitamin D levels more frequently.
Multiple Causes Often Overlap
In practice, low vitamin D rarely comes from a single factor in isolation. A person living at a northern latitude who works indoors, has darker skin, and carries extra weight faces four compounding risks at once. An older adult with Crohn’s disease on antiseizure medication faces three. The most useful thing you can do is look at the full picture of your lifestyle, geography, body type, and medical history rather than attributing low levels to any one cause. A simple blood test measuring 25-hydroxyvitamin D gives a clear snapshot of where you stand and whether your current intake, from sunlight, food, or supplements, is actually enough.