Vitamin D deficiency is overwhelmingly common, and it usually comes down to a combination of factors rather than a single cause. Most people are deficient because they don’t get enough sun exposure to produce vitamin D in their skin, they don’t eat enough of the few foods that contain it, or something about their body makes it harder to use the vitamin D they do get. Understanding which factors apply to you is the first step toward fixing the problem.
Where You Live May Be the Biggest Factor
Your body produces vitamin D when ultraviolet B (UVB) rays from the sun hit your skin. But UVB rays don’t reach the earth’s surface equally everywhere. If you live north of the 37th parallel, roughly anywhere north of Los Angeles, the sun sits too low in the sky from November through March to trigger meaningful vitamin D production in your skin. That covers the vast majority of the continental United States, all of Canada, the UK, and most of Europe.
Even during summer months, the window for effective synthesis is limited to midday hours when the sun is high enough. If you work indoors during those hours, wear sunscreen (which blocks UVB), or cover most of your skin with clothing, you’re cutting off your primary source. Glass also blocks UVB, so sitting by a sunny window doesn’t help.
Your Skin Tone Plays a Role
Melanin, the pigment that gives skin its color, acts as a natural sunscreen. People with darker skin need more sun exposure to produce the same amount of vitamin D as people with lighter skin. Research published in the Journal of Investigative Dermatology comparing the lightest and darkest skin types found that melanin reduces vitamin D synthesis efficiency by a factor of about 1.3 to 1.4. That means someone with very dark skin produces roughly 30 to 40 percent less vitamin D per minute of sun exposure than someone with very fair skin.
This difference becomes especially significant at higher latitudes or during winter, when UVB exposure is already limited. It’s one reason vitamin D deficiency rates are disproportionately high among Black and South Asian populations living in northern climates.
Very Few Foods Contain Vitamin D
Diet alone rarely provides enough vitamin D, because the vitamin exists naturally in only a handful of foods. The richest sources are fatty fish: a 3-ounce serving of cooked rainbow trout delivers about 645 IU, and the same amount of sockeye salmon provides around 570 IU. After that, the numbers drop sharply. A large scrambled egg contains just 44 IU (and only if you eat the yolk). Two canned sardines provide 46 IU. Three ounces of canned tuna gives you about 40 IU.
White mushrooms exposed to UV light are a surprisingly decent source at 366 IU per half cup, but regular mushrooms that haven’t been UV-treated contain almost nothing. Most people aren’t eating fatty fish daily, so they fall well short of the 600 to 800 IU recommended for adults. Fortified milk, orange juice, and cereals help bridge the gap, but many people still don’t reach adequate levels through food alone.
Body Weight Affects Your Levels
Vitamin D is fat-soluble, which means it gets absorbed into fat tissue. In people carrying excess body fat, vitamin D gets pulled into adipose tissue and essentially trapped there, making less of it available in the bloodstream where your body can use it. This sequestration effect is well documented: numerous studies consistently find lower blood levels of vitamin D in people with obesity compared to people at a healthy weight, even when sun exposure and diet are similar.
The relationship is proportional. The more body fat you carry, the more vitamin D gets locked away. This is one reason why people with obesity often need higher supplementation doses to reach the same blood levels as leaner individuals.
Aging Reduces Your Skin’s Production Capacity
As you get older, your skin becomes less efficient at producing vitamin D. The reason is straightforward: aging skin contains lower concentrations of the precursor molecule that UVB converts into vitamin D. An older adult exposed to the same amount of sunlight as a younger person will simply produce less vitamin D from that exposure. Combined with the fact that older adults tend to spend more time indoors, this creates a compounding deficit that makes supplementation increasingly important with age.
Gut Conditions Can Block Absorption
Because vitamin D is fat-soluble, your body absorbs it through the same pathway it uses for dietary fats, primarily in the small intestine. Any condition that damages the intestinal lining or impairs fat absorption can reduce how much vitamin D you actually take in from food or supplements.
Celiac disease is one of the most well-studied examples. The immune reaction triggered by gluten damages the intestinal lining, leading to malabsorption of calcium and vitamin D. Research shows vitamin D deficiency is common among people with celiac disease, and the resulting deficiency can weaken bones on top of the intestinal damage. Crohn’s disease, ulcerative colitis, and other inflammatory bowel conditions cause similar absorption problems. People who have had gastric bypass surgery also absorb less vitamin D because the procedure bypasses or shortens the part of the intestine where absorption occurs.
Certain Medications Speed Up Vitamin D Breakdown
Some medications cause your body to break down vitamin D faster than normal, effectively draining your stores. These drugs activate an enzyme pathway in the liver that degrades both the storage form and the active form of vitamin D.
The most common culprits include:
- Anti-seizure medications like phenytoin, carbamazepine, phenobarbital, and primidone
- Certain steroids like dexamethasone and other glucocorticoids
- Antiretroviral drugs used to treat HIV, including efavirenz and ritonavir
- Some antibiotics like rifampicin
- Certain blood pressure medications like nifedipine and spironolactone
If you take any of these long-term, your vitamin D levels may drop even if your diet and sun exposure haven’t changed. People on anti-seizure medications are particularly at risk because these drugs are typically taken for years or decades.
Indoor Lifestyles Compound Everything
Modern life works against vitamin D production in almost every way. Most people commute to indoor jobs, exercise in gyms, and spend evenings inside. Even on weekends, errands and screen time often win out over extended time outdoors. Add sunscreen use (important for skin cancer prevention, but a barrier to UVB), and the average person’s sun exposure is a fraction of what it was a few generations ago.
The result is that deficiency isn’t really an anomaly. It’s the expected outcome for anyone living in a northern climate, spending most of the day indoors, and not deliberately eating vitamin D-rich foods or taking a supplement. For most people, the fix is simple: a daily vitamin D supplement, ideally taken with a meal containing some fat to improve absorption. A blood test can tell you exactly where your levels stand, which helps determine how much supplementation you actually need.