Vitamin D levels drop when your body can’t make enough from sunlight, absorb enough from food, or properly convert what it gets into usable form. Most people with low levels have more than one factor working against them at the same time. Clinically, a blood level below 20 ng/mL is considered deficient, while levels between 20 and 30 ng/mL are often classified as insufficient.
How Your Body Makes Vitamin D
Understanding what lowers vitamin D starts with understanding how you get it in the first place. Your skin produces vitamin D when a narrow band of ultraviolet B light (wavelengths between 295 and 315 nanometers, peaking at 297 nm) hits a cholesterol compound sitting in the outer layers of your skin. This compound converts into a precursor of vitamin D3, which then travels to your liver, where it’s converted into the form that circulates in your blood (what your doctor measures). From there, your kidneys convert it again into the fully active hormone your cells actually use.
Any disruption along this chain, from sunlight hitting your skin all the way through liver and kidney processing, can result in low levels. That’s why the causes of vitamin D deficiency are so varied.
Not Enough Sunlight Exposure
This is the single most common driver. Your skin needs direct UVB exposure to produce vitamin D, and several everyday realities block that from happening. Sunscreen, clothing, and spending most of the day indoors all reduce the UVB that reaches your skin. Office workers, night-shift employees, and people who are homebound or institutionalized are especially vulnerable.
Where you live matters enormously. At latitudes above roughly 35 degrees north, the sun sits too low in the sky during winter months for UVB rays to trigger any vitamin D production at all, no matter how long you stay outside. In Boston (42°N), skin exposed to sunlight from November through February produces zero vitamin D. In Edmonton, Canada (52°N), this dead zone stretches from October through March. If you live in the northern United States, Canada, the UK, or northern Europe, you simply cannot make vitamin D from sunlight for a significant chunk of the year.
Skin Pigmentation and Aging
Melanin, the pigment that gives skin its color, absorbs the same UVB wavelengths your skin needs to produce vitamin D. The more melanin you have, the longer you need in the sun to make the same amount. Research comparing people with darker brown skin (Fitzpatrick type V) to white-skinned individuals in the UK found that darker-skinned individuals needed about 25 minutes of midday sun exposure to meet their vitamin D needs, compared to just 9 minutes for lighter-skinned people. That’s nearly three times as long. This is a major reason why vitamin D deficiency rates are substantially higher in Black and South Asian populations living at northern latitudes.
Age is another factor your body can’t control. The concentration of the cholesterol precursor in your skin declines as you get older. Comparing skin samples from people in their teens with those from people in their late 70s and 80s, researchers found that aging reduces the skin’s capacity to produce vitamin D by more than half. This decline is gradual but significant, which is why older adults are one of the groups most likely to be deficient even if they spend reasonable time outdoors.
Higher Body Weight
Vitamin D is fat-soluble, meaning it dissolves in and gets stored in body fat. In people with a higher percentage of body fat, vitamin D gets pulled into fat tissue and essentially locked away, reducing the amount available in the bloodstream. A meta-analysis found that the prevalence of vitamin D deficiency was 35% higher in people with obesity and 24% higher in those who were overweight, compared to normal-weight individuals.
This isn’t just about storage. When researchers gave people with obesity the same dose of vitamin D (either through UVB exposure or a 50,000 IU supplement), their blood levels rose less than in normal-weight participants. Some scientists attribute this to the vitamin being sequestered in fat cells. Others argue it’s a simple dilution effect: the same amount of vitamin D is spread across a larger body volume. Either way, the practical result is the same. People carrying more body fat typically need higher intake to reach the same blood levels.
Digestive and Absorption Problems
Because vitamin D is fat-soluble, your intestines need to absorb fat effectively to absorb vitamin D along with it. Several medical conditions interfere with this process:
- Celiac disease damages the lining of the small intestine, reducing its ability to absorb fat-soluble vitamins, particularly when the disease is undiagnosed or untreated.
- Crohn’s disease causes inflammation in the digestive tract that impairs nutrient absorption, especially when it affects the small intestine.
- Cystic fibrosis disrupts the release of digestive enzymes needed to break down and absorb fats.
- Weight-loss surgeries such as gastric bypass physically reroute or reduce the small intestine, cutting down the surface area available for absorption. People who’ve had these procedures often need higher supplemental doses and lifelong monitoring.
Even without a diagnosed condition, chronic digestive issues that cause frequent diarrhea or fatty stools can signal poor fat absorption and, by extension, poor vitamin D uptake.
Liver and Kidney Disease
Your liver and kidneys each play a critical role in activating vitamin D. The liver performs the first conversion, turning the raw vitamin D from your skin or diet into the circulating form measured by blood tests. The kidneys handle the second conversion, producing the fully active hormone that your body’s cells respond to. Chronic liver disease can impair the first step, and chronic kidney disease can impair the second. People with advanced kidney disease are particularly prone to deficiency because the kidney is the primary source of the active hormone in your bloodstream.
Medications That Lower Vitamin D
Certain medications speed up the breakdown of vitamin D in your body by activating liver enzymes that metabolize it faster. The most well-documented culprits are anti-seizure medications, including phenobarbital, phenytoin, carbamazepine, and primidone. These drugs ramp up the same enzyme pathways that clear vitamin D from your system, leading to lower blood levels over time.
Some glucocorticoids (steroids used for inflammation) also activate these pathways. If you take any of these medications long-term, your doctor may check your vitamin D levels periodically and recommend supplementation.
Diet Alone Rarely Provides Enough
Very few foods naturally contain meaningful amounts of vitamin D. Fatty fish like salmon, mackerel, and sardines are the best dietary sources, but you’d need to eat them frequently to make a real dent. Egg yolks, beef liver, and cod liver oil contain smaller amounts. In many countries, milk, orange juice, and cereals are fortified with vitamin D, but the amounts added are modest.
For most people, diet contributes only a fraction of what the body needs. This means that if any of the other factors on this list are also present, relying on food alone virtually guarantees insufficient levels. Supplements (typically vitamin D3) are the most reliable way to compensate, especially during winter months, for people with darker skin, for older adults, and for those with absorption issues. Taking vitamin D with a meal that contains some fat improves absorption, since it’s a fat-soluble vitamin.
Multiple Factors Stack Up
What makes vitamin D deficiency so widespread is that these causes overlap. A person with darker skin living in a northern city, working indoors, carrying extra weight, and eating a typical diet low in fatty fish has four or five factors simultaneously dragging their levels down. Data from the CDC illustrates the scope: the proportion of adults in the U.S. with sufficient vitamin D (at least 30 ng/mL) dropped from about 60% in the late 1980s to roughly 30% by the early 2000s among white adults, and from about 10% to approximately 5% among Black adults over the same period. Severe deficiency (below 10 ng/mL) has also become more common.
A simple blood test measuring your circulating vitamin D level is the only way to know where you stand. If your level comes back low, the cause is almost always some combination of the factors above, and identifying which ones apply to you makes it easier to correct the problem effectively.