Vitamin D deficiency is remarkably common, affecting nearly half the world’s population at insufficient levels. A pooled analysis of 7.9 million participants across 79 countries found that 47.9% of people have blood levels below the sufficiency threshold, with 15.7% falling into outright deficiency. The reasons come down to a combination of biology, geography, lifestyle, and health conditions that make this nutrient uniquely difficult to maintain.
How Your Body Makes Vitamin D
Understanding why deficiency is so widespread starts with how vitamin D works. Unlike most nutrients, you don’t primarily get it from food. Your skin manufactures it when ultraviolet B radiation from sunlight hits a cholesterol compound sitting in your skin cells. That compound converts into a precursor form of vitamin D, which then travels to your liver, where it’s converted into the main circulating form that shows up on blood tests. From there, your kidneys convert it again into the fully active hormone your body actually uses.
This three-step process (skin, then liver, then kidneys) means a breakdown at any point in the chain can leave you deficient. Problems with liver or kidney function directly impair your ability to activate vitamin D, even if your skin is producing plenty of the raw material. Immune cells, intestinal tissue, and other organs can also perform that final activation step for their own local needs, but the kidneys remain the primary source for your bloodstream.
Where You Live Matters More Than You Think
Geography is one of the biggest drivers of deficiency. The further you live from the equator, the less UVB radiation reaches the ground, especially in winter. At latitudes above 40 degrees (roughly the line running through New York, Madrid, and Beijing), there are months when the sun sits so low in the sky that virtually no vitamin D synthesis occurs, no matter how long you stay outside. Researchers call this the “vitamin D winter.”
At 50 degrees latitude (London, Vancouver, Prague), vitamin D winter lasts from roughly October through March. At 60 degrees (Helsinki, Anchorage), it stretches from September through April. Near the equator, by contrast, a person with light skin needs only about 3 minutes of noontime sun exposure to maintain adequate levels year-round. At high latitudes during summer months, that same person needs 8 to 15 minutes. Cloud cover, air pollution, and window glass all filter out UVB as well, further reducing what reaches your skin.
Skin Tone and Sun Exposure
Melanin, the pigment that gives skin its color, absorbs UVB radiation. That’s protective against sun damage, but it also means darker-skinned individuals need significantly more sun exposure to produce the same amount of vitamin D. Research at UK latitudes found that people with dark brown skin (Fitzpatrick skin type V) need about 25 minutes of daily lunchtime sun exposure from March through September to maintain sufficient levels, compared to roughly 9 minutes for people with lighter skin. Put another way, they need 2.5 to 3 times the UVB dose to raise their blood levels by the same amount.
This disparity has real consequences. People with darker skin who live at higher latitudes face a compounding disadvantage: less UVB available in the environment and less efficient conversion of what does reach them. Deficiency rates in these populations are consistently higher.
Indoor Lifestyles and Sunscreen Use
Even people living in sunny climates can be deficient if they spend most of their time indoors. Office workers, shift workers, and anyone whose daily routine keeps them inside during peak sun hours (roughly 10 a.m. to 3 p.m.) miss the window when UVB is strongest. Clothing covering most of the body has the same effect, blocking UVB from reaching skin.
Sunscreen also reduces vitamin D synthesis. SPF 15 filters out 93% of UVB rays, SPF 30 blocks 97%, and SPF 50 blocks 98%. In practice, most people don’t apply sunscreen thickly or consistently enough to achieve those numbers, so some synthesis still occurs. But for someone who applies sunscreen diligently before any outdoor time, the reduction is substantial.
Very Few Foods Contain Vitamin D
Diet alone rarely provides enough vitamin D, because so few foods naturally contain it in meaningful amounts. The richest natural source is cod liver oil, with 1,360 IU per tablespoon. Fatty fish like trout (645 IU per 3-ounce serving) and sockeye salmon (570 IU) are the next best options. After that, the numbers drop sharply: a whole scrambled egg provides just 44 IU, and an ounce and a half of cheddar cheese has only 17 IU.
For context, the recommended daily intake for most adults is 600 to 800 IU. You’d need to eat salmon nearly every day or consume unrealistic quantities of eggs and cheese to hit that through food alone. Fortified foods like milk, orange juice, and cereals help close the gap in some countries, but fortification policies vary widely around the world, leaving many populations without this safety net.
Body Weight and Vitamin D Dilution
Obesity is a well-established risk factor for vitamin D deficiency. When researchers gave obese and normal-weight people the same dose of vitamin D, either through UV exposure or supplements, the obese group consistently showed smaller increases in blood levels. For years, the leading explanation was that fat tissue “traps” vitamin D, locking it away where the body can’t use it. But more recent research points to a simpler explanation: volumetric dilution.
Studies measuring vitamin D concentrations in fat tissue found that obese and normal-weight people had similar concentrations per gram of fat. The total amount stored was higher in obese individuals simply because they had more fat tissue overall. The vitamin D gets spread across a larger body volume, diluting the amount available in the bloodstream. This means people with higher body weight generally need more vitamin D, whether from sun or supplements, to reach the same blood levels as someone who weighs less.
Gut Conditions That Block Absorption
Vitamin D is fat-soluble, meaning your intestines need to absorb it along with dietary fat. Any condition that disrupts fat absorption can cause deficiency, even when intake seems adequate. Celiac disease damages the intestinal lining and reduces its surface area, directly impairing absorption of vitamin D and calcium. Crohn’s disease and other forms of inflammatory bowel disease cause similar problems through chronic inflammation and, in some cases, surgical removal of portions of the small intestine.
Cystic fibrosis and chronic pancreatic insufficiency work through a different mechanism. The pancreas doesn’t produce enough digestive enzymes, so fat passes through the gut undigested, taking fat-soluble vitamins with it. Gastric bypass surgery, which reroutes or removes sections of the digestive tract, also significantly increases the risk. In one study of patients with bone disease caused by gastrointestinal conditions, the most common underlying causes were gastrectomy (43%), celiac disease (26%), and intestinal bypass (15%).
Age and Vitamin D Production
Older adults have long been considered at higher risk for deficiency, and they are, but the reason may not be what was previously assumed. Earlier research suggested that aging skin loses its ability to produce vitamin D because levels of the precursor compound decline with age. However, a 2024 comparative study found no significant difference in skin precursor concentrations between younger and older adults, and both groups showed similar increases in vitamin D production after UV exposure.
This suggests the elevated deficiency rates in older adults are driven more by behavioral and health factors than by skin biology alone. Older people tend to spend less time outdoors, are more likely to have kidney impairment that reduces activation of vitamin D, more often take medications that interfere with vitamin D metabolism, and may eat less overall. Reduced mobility and institutionalization in care facilities further limit sun exposure.
What Deficiency Levels Look Like on a Blood Test
Vitamin D status is measured through a blood test for 25-hydroxyvitamin D, the form produced by the liver. The most widely used thresholds define deficiency as below 20 ng/mL (50 nmol/L) and insufficiency as 20 to 29 ng/mL. Levels at or above 30 ng/mL are generally considered sufficient, though some experts set the bar at 20 ng/mL for bone health. The optimal range is typically reported as 25 to 80 ng/mL.
Because the causes of deficiency stack on top of each other, many people carry multiple risk factors simultaneously. A dark-skinned person living at a northern latitude who works indoors, wears sunscreen, and eats little fish faces a convergence of factors that makes deficiency almost inevitable without supplementation. Recognizing which of these factors apply to you is the first step toward addressing the gap.