Low vitamin D usually comes down to one or more of three things: not enough sun exposure, not enough in your diet, or something in your body that prevents proper absorption or activation. Most people have a combination of these factors working against them, which is why deficiency is so common. Here’s a closer look at each cause.
Limited Sun Exposure
Your skin is your primary vitamin D factory. When UVB rays (wavelengths between 290 and 315 nanometers) hit exposed skin, they trigger a chemical reaction that converts a cholesterol compound into previtamin D3. That molecule then transforms into vitamin D3 and enters your bloodstream. This single process accounts for the majority of vitamin D in most people’s bodies.
The problem is that many aspects of modern life cut off this supply. Working indoors during daylight hours, living at higher latitudes where UVB intensity drops in fall and winter, wearing clothing that covers most skin, and consistently applying sunscreen all reduce the amount of UVB that reaches your skin. If you live north of about 37 degrees latitude (roughly the line from San Francisco to Richmond, Virginia), your skin produces little to no vitamin D from sunlight during the winter months, regardless of how much time you spend outside.
Very Few Foods Contain Vitamin D
Unlike most other vitamins, vitamin D is naturally present in only a handful of foods, and most of them aren’t things people eat every day. Fatty fish is the best natural source: a 3-ounce serving of cooked sockeye salmon provides about 570 IU, and rainbow trout delivers around 645 IU. Cod liver oil packs 1,360 IU per tablespoon, but it’s not exactly a staple. After fish, the numbers drop fast. A large scrambled egg has just 44 IU (and it’s all in the yolk). Cheddar cheese has 17 IU per serving. Chicken breast has 4 IU.
To compensate, many countries fortify milk, plant milks, and cereals with vitamin D. A cup of fortified cow’s milk contains about 120 IU, and fortified plant milks range from 100 to 144 IU per cup. These amounts help, but they still fall well short of the 600 to 800 IU daily intake most guidelines recommend. If your diet leans away from fish and fortified dairy, it’s hard to get meaningful amounts of vitamin D from food alone. White mushrooms exposed to UV light are a notable exception at 366 IU per half cup, but standard grocery-store mushrooms that haven’t been UV-treated contain almost none.
Aging Reduces Your Skin’s Capacity
As you get older, your skin becomes less efficient at making vitamin D. Research published in The Journal of Clinical Investigation compared skin samples from young subjects (ages 8 and 18) with elderly subjects (ages 77 and 82) and found that aging decreases the skin’s capacity to produce vitamin D3 by more than twofold. This means an older adult spending the same amount of time in the sun as a younger person generates roughly half the vitamin D or less. Combined with the fact that older adults often spend less time outdoors, this creates a compounding effect.
Obesity and Body Fat
Vitamin D is fat-soluble, which means it gets absorbed into fat tissue. In people with obesity, a significant portion of newly made or ingested vitamin D gets pulled into fat stores instead of circulating in the blood where the body can use it. Research in The American Journal of Clinical Nutrition found that after whole-body UV exposure, obese individuals had a 57% lower increase in blood vitamin D3 levels compared to non-obese individuals. BMI was inversely correlated with vitamin D levels after both UV exposure and oral supplementation, meaning this isn’t just a sunlight issue. Even taking vitamin D supplements, people with higher body fat may need larger doses to reach the same blood levels.
Digestive Conditions That Block Absorption
Vitamin D from food and supplements is absorbed through the intestines, so any condition that impairs gut function can lead to deficiency. Three of the most common culprits are celiac disease, Crohn’s disease, and cystic fibrosis. Each of these damages or disrupts the intestinal lining in ways that prevent adequate vitamin D uptake, especially when the condition is untreated or poorly managed. People who have had gastric bypass surgery face a similar challenge because the procedure physically bypasses parts of the small intestine where fat-soluble vitamins are normally absorbed.
Kidney and Liver Problems
The vitamin D your skin makes or you swallow isn’t immediately usable. It has to be converted into its active form through a two-step process: first in the liver, then in the kidneys. If either organ is compromised, this activation stalls. Kidney disease is the more common bottleneck. People with stage 4 or stage 5 chronic kidney disease often can’t complete this final conversion step, leaving them deficient even if they get plenty of sun or take standard supplements. In those cases, doctors typically prescribe a pre-activated form of vitamin D that bypasses the kidneys entirely.
Darker Skin Pigmentation
Melanin, the pigment that gives skin its color, absorbs some UVB radiation before it can trigger vitamin D production. For years, this was thought to be a major driver of deficiency in people with darker skin. More recent research has refined this picture. A study comparing people across the full range of skin tones (Fitzpatrick types II through VI, from very fair to very dark) found that melanin’s inhibitory effect is real but smaller than previously assumed, with a factor of about 1.3 to 1.4 between the lightest and darkest skin types. That’s meaningful over time, especially at higher latitudes with less intense sunlight, but it’s not the dramatic difference older studies suggested. Other factors, like time spent outdoors and latitude, likely matter more.
Medications That Deplete Vitamin D
Several common drug classes can lower your vitamin D levels by speeding up how quickly your body breaks it down or by interfering with absorption. The main offenders include:
- Corticosteroids (oral, inhaled, or topical), used for inflammation and autoimmune conditions
- Anticonvulsants and barbiturates, used for seizure disorders
- Bile acid sequestrants, a type of cholesterol-lowering medication
- Certain ulcer medications, including H2 blockers
- Mineral oil laxatives, which can carry fat-soluble vitamins out of the body before they’re absorbed
If you take any of these long-term, your doctor may monitor your vitamin D levels more closely or recommend a higher supplementation dose to compensate.
Why Multiple Causes Often Overlap
What makes vitamin D deficiency so widespread is that these causes rarely exist in isolation. An older adult with darker skin living in a northern city, spending most of the day indoors, and taking a corticosteroid for arthritis is being hit from every direction at once. Someone with Crohn’s disease who also has a BMI over 35 faces both poor absorption and increased sequestration in fat tissue. Understanding which factors apply to you helps explain not just why your levels are low, but how aggressively you may need to address it through supplementation, dietary changes, or safe sun exposure.