The Nutrient That Requires Two Organs to Activate
Most vitamins arrive in the body and go to work. Vitamin D is different - and that difference explains a lot about why deficiency is so common, and so frequently missed.
Vitamin D is a fat-soluble vitamin naturally present in a small number of foods, and it is also produced in the skin when ultraviolet rays from sunlight trigger its synthesis. But the form that arrives - whether from food, supplements, or the sun - is biologically inert. It must undergo two separate hydroxylation reactions before it becomes active. The first occurs in the liver, which converts vitamin D into 25-hydroxyvitamin D (calcidiol). The second occurs primarily in the kidney, which produces calcitriol - the physiologically active form. Any condition that impairs either the liver or the kidney can therefore interfere with vitamin D status even when dietary intake and sun exposure appear adequate.
Once activated, vitamin D promotes calcium absorption in the gut and helps maintain adequate calcium and phosphate concentrations for normal bone mineralization.
It also plays a role in the nervous system, musculoskeletal system, and immune system.
Who Is Deficient, and Why the Numbers Are Striking
Subclinical vitamin D deficiency remains prevalent worldwide, affecting an estimated up to 1 billion people in both developed and developing countries. In the United States, data from national surveys has suggested that a substantial proportion of adults may have vitamin D deficiency, with higher rates observed in Hispanic and African American adults.
The size of that disparity has a biological explanation. People with darker skin are better protected from UV rays but may need to spend more time in the sun than people with lighter skin to produce the same amount of vitamin D - and the darker the skin, the less vitamin D may be made from a given sunlight exposure.
Geography and body composition add further layers. People in higher latitudes may not receive adequate vitamin D through sunlight depending on their geographical location.
Obesity increases the risk of vitamin D deficiency because once vitamin D is synthesized in the skin or ingested, it can be sequestered in body fat stores, making it less bioavailable to people with higher body fat mass.
Some supplementation studies have found that people with obesity reached lower serum 25-hydroxyvitamin D concentrations than normal-weight participants given equivalent oral dosages.
The Absorption Problem Most People Don’t Know About
Because vitamin D is fat-soluble, its absorption depends on the presence of dietary fat. Dietary vitamin D is absorbed with other dietary fats in the small intestine, and efficient absorption depends on the presence of fat in the lumen, which triggers the release of bile acids. This means that conditions affecting fat digestion can significantly impair vitamin D uptake.
Malabsorption syndromes - including celiac disease, short bowel syndrome, gastric bypass, inflammatory bowel disease, chronic pancreatic insufficiency, and cystic fibrosis - may all contribute to vitamin D deficiency.
Vitamin D deficiency is also common among people with cystic fibrosis and both cholestatic and non-cholestatic liver diseases, due to decreased absorption of dietary vitamin D and impaired conversion to 25-hydroxyvitamin D.
What Deficiency Actually Does to the Body
Vitamin D deficiency impairs bone mineralization, causing rickets in children and osteomalacia in adults. In adults, the bones - particularly the spine, pelvis, and leg bones - may weaken, affected areas may be painful to touch, and in older adults, fractures may result from only slight jarring or a minor fall.
Beyond bone, the picture is more nuanced. If someone becomes sick frequently, low vitamin D levels may be a contributing factor - research has found a link between vitamin D deficiency and respiratory tract infections. Some reviews of studies have found that vitamin D supplementation was associated with a reduced risk of respiratory tract infections, with a stronger association in those who had very low baseline levels. That said, the mechanisms connecting vitamin D to immune function are still an active area of research, and low vitamin D is one of many factors that may affect immune resilience - not a singular cause.
The deficiency can also present as muscle aches and weakness alongside bone pain
- symptoms that are non-specific enough to be attributed to overwork, aging, or stress for months or years before anyone checks a blood level.
Why It Goes Undetected
The symptoms of vitamin D deficiency - fatigue, diffuse musculoskeletal pain, low mood - overlap substantially with dozens of other conditions. There is no symptom cluster that reliably points to low vitamin D without a blood test.
Screening blood tests for vitamin D status in adults without symptoms are no longer recommended as a routine measure and have not shown clear benefit at the population level. Instead, clinicians typically assess individual risk factors before testing. Serum concentrations of 25-hydroxyvitamin D reflect the cumulative effects of both sunlight exposure and dietary vitamin D intake , making it the standard measure used for diagnosis.
What You Can Eat - and Its Limitations
Dietary sources of vitamin D are genuinely limited. Natural food sources include some fish, fish liver oils, mushrooms, fatty fish, and egg yolks, as well as fortified dairy products and orange juice.
Vitamin D3 is found in fish, cheese, and egg yolks, while vitamin D2 comes primarily from mushrooms and yeast.
The dietary route alone is rarely sufficient for people with significant deficiency. In the 1930s, North America began fortifying milk with vitamin D specifically to address dietary rickets in children
- a sign that food alone was understood even then to be insufficient for many people. The recommended daily intake for adults varies by age and individual circumstance; guidelines generally reference a range of 400 to 800 international units (IU) , though clinical requirements can be considerably higher when correcting an established deficiency.
On the toxicity end, it is worth noting that overdose risk comes almost exclusively from supplements, not sun exposure. Hypervitaminosis D typically results from excessive oral intake rather than excessive sunlight exposure, and toxicity has been reported at elevated serum 25-hydroxyvitamin D levels in some studies.
Acute intoxication can lead to hypercalcemia, which may produce symptoms such as confusion, anorexia, vomiting, polyuria, and muscle weakness.
The Practical Gap Between Knowing and Correcting
Understanding why vitamin D deficiency is so prevalent requires holding several mechanisms together at once: limited dietary sources, geography-dependent sun exposure, skin pigmentation, fat malabsorption, sequestration in body fat, and a two-step activation process that can be disrupted by liver or kidney dysfunction. No single factor explains all cases.
For people who fall into higher-risk groups - those with darker skin pigmentation, inflammatory bowel disease, obesity, or conditions affecting fat absorption - a conversation with a clinician about blood testing and, if indicated, supplementation is more informative than adjusting diet alone. Fatty fish eaten alongside other fat-containing foods may modestly improve dietary absorption, since bile release is involved in the uptake process, but this is unlikely to resolve a clinically significant deficiency on its own.
The most telling detail: a deficiency this common, in a nutrient the body can manufacture itself from sunlight, points less to personal failure and more to the mismatch between modern indoor lifestyles, higher latitudes, and a metabolism that evolved under far more consistent sun exposure than most people currently get.
