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 process that interferes with how vitamin D gets activated. A blood level below 30 ng/mL is generally considered insufficient, and below 10 ng/mL is severely deficient. Understanding the specific cause matters because the fix looks different depending on what’s driving the problem.
How Your Body Makes Vitamin D
Most of your vitamin D doesn’t come from food. It’s manufactured in your skin when UVB rays (wavelengths between roughly 295 and 315 nanometers) hit a cholesterol compound sitting in your outer skin layer. That compound converts into a precursor molecule, which then transforms into vitamin D3 and enters your bloodstream. From there, your liver processes it into a storage form, and your kidneys convert that into the active hormone your body actually uses.
This chain has multiple links, and a problem at any point, from sunlight hitting your skin to your kidneys performing the final conversion, can leave you deficient.
Limited Sun Exposure
This is the most common cause worldwide. If you live above about 37 degrees latitude (roughly the line from San Francisco to Richmond, Virginia), the sun sits too low in the sky during winter months for UVB rays to reach your skin effectively. Spending most of your day indoors, wearing clothing that covers most of your body, or consistently applying sunscreen all reduce the UVB exposure your skin needs to start the production process.
Skin tone plays a significant role. Melanin, the pigment that gives skin its color, acts as a natural sunscreen. People with darker skin may need up to ten times longer in the sun to produce the same amount of vitamin D as someone with fair skin. This doesn’t mean darker skin is a disadvantage in sunny climates where UVB is abundant year-round, but it becomes a major factor at higher latitudes or for people who spend limited time outdoors.
Very Few Foods Contain Enough
The short list of foods naturally rich in vitamin D is surprisingly thin. A 3-ounce serving of farmed rainbow trout provides about 645 IU, and sockeye salmon delivers around 570 IU per serving. After that, the numbers drop sharply. A large scrambled egg has just 44 IU. Two canned sardines offer 46 IU. Canned tuna provides about 40 IU per serving.
Fortified foods help but rarely close the gap on their own. A cup of fortified milk contains about 120 IU, fortified plant milks range from 100 to 144 IU per cup, and a serving of fortified cereal adds around 80 IU. Most adults need at least 600 IU daily, with people over 70 needing 800 IU. Unless you’re eating fatty fish several times a week, it’s difficult to reach adequate levels through diet alone, which is why sunlight and supplements carry most of the load.
Obesity and Body Fat
Carrying excess body fat is one of the strongest predictors of low vitamin D. Vitamin D is fat-soluble, so it gets pulled into fat tissue and essentially locked away there instead of circulating in your blood where your body can use it. Researchers describe this as “adipose sequestration,” and it’s compounded by a dilution effect: the more tissue mass you have, the more spread out your vitamin D becomes. People with obesity also tend to show impairments in the liver’s ability to process vitamin D into its usable forms. The combined result is that someone with a BMI over 30 often needs significantly more vitamin D intake to reach the same blood levels as someone at a healthy weight.
Digestive Conditions That Block Absorption
Vitamin D gets absorbed primarily in the middle and lower portions of the small intestine. Any condition that damages or shortens this area can dramatically reduce how much vitamin D makes it into your bloodstream, even if you’re getting plenty from food or supplements.
Between 40% and 60% of people with digestive disorders like celiac disease, Crohn’s disease, cystic fibrosis, short bowel syndrome, and ulcerative colitis are vitamin D deficient. The mechanisms vary by condition. In Crohn’s disease, inflammation and damage to the intestinal lining reduce the surface area available for absorption. In cystic fibrosis and chronic pancreatitis, the problem is fat malabsorption: vitamin D dissolves in dietary fat, so if your body can’t properly digest fat, the vitamin D passes through unabsorbed. Celiac disease creates a more complex situation where calcium loss triggers a hormonal chain reaction that burns through vitamin D stores faster than normal.
Ulcerative colitis is an interesting exception. Because it typically affects the large intestine rather than the small intestine, vitamin D absorption capacity often remains intact. Deficiency in these patients tends to stem from other factors rather than malabsorption itself.
Liver and Kidney Problems
Your liver and kidneys perform the two chemical conversions that turn raw vitamin D into the active hormone your cells respond to. The liver handles the first step, creating the storage form that doctors measure in blood tests. The kidneys handle the second, producing the active form through a tightly regulated enzyme system controlled by parathyroid hormone and other signals.
Chronic liver disease can slow or impair that first conversion, leaving you with low blood levels even when sun exposure and intake seem adequate. Chronic kidney disease disrupts the second conversion, which means the active hormone your body depends on for calcium regulation, bone health, and immune function drops even if the storage form in your blood looks acceptable. This is why people with advanced kidney disease often need a special form of vitamin D that bypasses the kidney step entirely.
Medications That Deplete Vitamin D
Several common drug classes speed up the breakdown of vitamin D in your body or interfere with its absorption. Anti-seizure medications like phenytoin and phenobarbital are among the most well-known offenders, activating liver enzymes that chew through vitamin D faster than your body can replace it. Corticosteroids, frequently prescribed for conditions like asthma, arthritis, and autoimmune disorders, also reduce vitamin D levels over time.
Other medications linked to lower vitamin D include certain antipsychotic drugs, the cholesterol-lowering medication cholestyramine (which binds fat and fat-soluble vitamins in the gut), the antibiotic rifampin, and long-term use of mineral oil as a laxative. If you take any of these regularly, your vitamin D needs are likely higher than average.
Age and Aging Skin
As you get older, your skin produces less vitamin D from the same amount of sun exposure. The concentration of the cholesterol precursor in your skin declines with age, so even someone who spends plenty of time outdoors may produce meaningfully less vitamin D at 70 than they did at 30. At the same time, kidney function gradually declines with age, reducing the final activation step. This double hit is a major reason why vitamin D deficiency is so common in older adults and why recommended intake increases from 600 IU to 800 IU daily after age 70.
Putting It Together
In practice, most people with low vitamin D have more than one contributing factor. Someone who works indoors, has darker skin, lives in a northern city, and carries extra weight is stacking four separate risk factors. A person with Crohn’s disease who also takes corticosteroids faces both an absorption problem and accelerated breakdown. Identifying which factors apply to you helps determine whether the solution is more sunlight, dietary changes, standard supplements, or higher-dose supplementation guided by blood testing.