vitamin d

Vitamin D is a fat‑soluble micronutrient essential for human health, distinct among vitamins because it can be synthesized endogenously in the skin upon exposure to ultraviolet B (UVB) radiation. It exists in two primary forms: vitamin D2 (ergocalciferol), typically obtained from plant or fungal sources, and vitamin D3 (cholecalciferol), derived from animal sources and synthesized in human skin from 7‑dehydrocholesterol. Chemically, these forms differ only slightly in their side chains but undergo similar metabolic activation pathways in the liver and kidneys to become the biologically active hormone, calcitriol, which binds to vitamin D receptors in various tissues. The discovery of vitamin D dates back to the early 20th century when research into rickets, a bone‑softening disease in children, revealed that dietary and sunlight sources of this nutrient could prevent and reverse skeletal abnormalities. The physiological roles of vitamin D are diverse. Classically, it facilitates intestinal absorption of calcium and phosphorus, thereby influencing bone mineralization and skeletal integrity. Without adequate vitamin D, calcium absorption is impaired, leading to compensatory elevations in parathyroid hormone and subsequent bone resorption. Vitamin D receptors are present in nearly every organ system, implicating the vitamin in muscle function, neuromuscular signaling, immune regulation, and cellular proliferation. While dietary sources alone rarely meet vitamin D needs, fortified foods and supplements help bridge the gap, particularly in populations with limited sun exposure. Serum 25‑hydroxyvitamin D [25(OH)D] is the accepted clinical marker of vitamin D status because it reflects both cutaneous synthesis and dietary intake. Levels above 50 nmol/L (20 ng/mL) are generally considered sufficient for bone health, whereas levels below 30 nmol/L (12 ng/mL) indicate deficiency. Because of its fat‑soluble nature, vitamin D can accumulate in adipose tissue, and excessive intake, mainly from supplements, can lead to toxicity.

The most well‑established function of vitamin D is the regulation of calcium and phosphorus metabolism, which is fundamental for bone mineralization. Calcitriol, the active form of vitamin D, enhances intestinal absorption of calcium by upregulating transport proteins, ensuring adequate mineral availability for the formation and maintenance of hydroxyapatite crystals in bone. Inadequate vitamin D disrupts this balance, leading to conditions such as rickets in children and osteomalacia in adults, characterized by defective mineralization and bone softness. Beyond skeletal health, vitamin D interacts with the immune system. Immune cells, including macrophages and dendritic cells, express vitamin D receptors, and calcitriol modulates innate and adaptive immune responses. Although the evidence on vitamin D’s role in reducing the risk of respiratory infections and autoimmune conditions is mixed, some meta‑analyses suggest modest reductions in incidence of acute respiratory infections with supplementation in individuals with low baseline levels. Additionally, observational studies have linked low vitamin D status with increased risk of chronic conditions including type 2 diabetes, cardiovascular disease, and certain cancers, although causality remains under investigation. Vitamin D also influences muscle function. Deficiency is associated with proximal muscle weakness and increased risk of falls in older adults, possibly due to impaired calcium handling in muscle cells and altered neuromuscular signaling. Some clinical trials indicate improvements in muscle performance with supplementation in deficient individuals. Emerging research explores vitamin D’s role in mood regulation and cognitive function, with observational associations between low 25(OH)D levels and depression; however, randomized controlled trials yield mixed results. The broad distribution of vitamin D receptors across tissues underscores the nutrient’s potential systemic effects, although strong evidence supports primarily skeletal benefits.

Recommended intake values for vitamin D vary by age and life stage. The Food and Nutrition Board at the National Academies of Sciences, Engineering, and Medicine has set the Recommended Dietary Allowances (RDAs) primarily based on bone health outcomes. For most adults up to age 70, the RDA is 15 mcg (600 IU) daily. Adults over 70 have an increased RDA of 20 mcg (800 IU) per day due to diminished skin synthesis and potential reduction in gastrointestinal absorption. Infants up to age 12 months have an Adequate Intake (AI) of 10 mcg (400 IU), while children and teens have an RDA of 15 mcg (600 IU). Pregnant and lactating individuals have similar requirements to non‑pregnant adults. Several factors influence individual vitamin D needs. Sun exposure greatly impacts endogenous synthesis; geographic latitude, season, time outdoors, clothing, and sunscreen use all affect UVB exposure and subsequent vitamin D production. Individuals with darker skin synthesize less vitamin D for the same sun exposure due to higher melanin content. Obesity may sequester vitamin D in adipose tissue, reducing its bioavailability. Conditions that impair fat absorption, such as celiac disease, Crohn’s disease, and cystic fibrosis, can decrease vitamin D absorption from the gut. Recommendations consider minimal sun exposure, as reliance on tanning or unprotected sun exposure carries skin cancer risk. Optimal vitamin D status is assessed via serum 25‑hydroxyvitamin D [25(OH)D], the primary circulating form. Most experts consider levels ≥50 nmol/L (≥20 ng/mL) sufficient for bone health, while some advocate for higher targets (≥75 nmol/L or ≥30 ng/mL) in at‑risk populations. Routine screening in asymptomatic individuals is not universally recommended, but testing is important in those with conditions affecting bone health or malabsorption.

Vitamin D deficiency can be clinically silent initially, especially in mild cases, and may only be detected through blood testing. When deficiency becomes significant, symptoms emerge due to impaired calcium and phosphorus metabolism. The hallmark skeletal manifestations are rickets in children and osteomalacia in adults. Rickets presents with bowed legs, delayed growth, and skeletal deformities due to inadequate mineralization of growing bones. In adults, osteomalacia causes diffuse bone pain, tenderness, and weakness, particularly in weight‑bearing bones. Secondary hyperparathyroidism, resulting from low 25(OH)D, increases bone turnover and may contribute to osteopenia or osteoporosis over time. Muscle weakness is a common non‑skeletal symptom of deficiency, particularly proximal muscle weakness, which increases fall risk in older adults. Patients may report difficulty rising from chairs or climbing stairs. Chronic deficiency also associates with fatigue and generalized aches, though these symptoms are non‑specific. Psychological symptoms, including low mood and depressive symptoms, have been observed in individuals with low 25(OH)D levels, but the directionality and causality remain under investigation. Risk factors for deficiency include limited sun exposure, use of sunscreen, aging, dark skin pigmentation, malabsorption syndromes, obesity, and certain medications such as anticonvulsants that accelerate vitamin D metabolism. Breastfed infants without supplementation may develop deficiency, as breast milk alone provides insufficient vitamin D. Prevalence of vitamin D deficiency varies, with tens of percent of some adult populations in North America showing serum 25(OH)D levels below sufficiency thresholds. Diagnosis relies on measuring serum 25(OH)D, with levels <30 nmol/L (<12 ng/mL) indicating deficiency, 30–50 nmol/L (12–20 ng/mL) considered inadequate for many individuals, and ≥50 nmol/L (≥20 ng/mL) generally adequate for bone health.

Dietary sources of vitamin D are limited compared to other nutrients, and many foods contain little or no vitamin D unless fortified. Fatty fish are among the richest natural sources. Cooked sockeye salmon provides approximately 28 mcg (142% Daily Value) per 6‑ounce fillet, while rainbow trout and mackerel also supply substantial amounts. Canned pink salmon and herring are convenient options with high vitamin D content. Fish liver oils, such as cod liver oil, are particularly concentrated sources, often providing doses comparable to supplements. UV‑exposed mushrooms produce vitamin D2 similarly to human skin, making them a valuable plant‑based source; a cup of UV‑treated cremini mushrooms can provide over 25 mcg per serving. Fortified foods contribute most dietary vitamin D in many populations. Fortified cow’s milk and plant milks (soy, almond, oat) typically carry added vitamin D, often contributing significant percentages of daily needs per cup. Fortified breakfast cereals, orange juice, and fortified tofu expand options, especially for individuals following plant‑based diets. Eggs contain vitamin D primarily in the yolk, and although amounts per egg are modest, incorporating multiple eggs can contribute meaningfully. Organ meats like beef liver and meats like pork offer smaller quantities but diversify dietary intake. Bioavailability of vitamin D varies with food matrix and fat content; because vitamin D is fat‑soluble, co‑consuming with dietary fats enhances absorption. Given the paucity of natural sources, combining dietary strategies with appropriate sun exposure or supplementation helps achieve adequate intake. Because fortified foods are common in many diets, reviewing nutrition labels helps identify products with meaningful vitamin D levels.

Vitamin D absorption occurs in the small intestine and is enhanced by dietary fat due to its fat‑soluble nature. While some absorption occurs without fat, the presence of dietary lipids promotes micelle formation and improves uptake. Both vitamin D2 and D3 are absorbed similarly through passive diffusion and carrier proteins. Obesity may sequester vitamin D in adipose tissue, potentially lowering circulating 25(OH)D levels. Age, gastrointestinal disorders that affect fat absorption (such as celiac disease or Crohn’s disease), and certain medications can impair absorption. Comparisons between D2 and D3 suggest that D3 raises and maintains serum 25(OH)D levels more effectively, though both forms contribute to status. Sun exposure induces endogenous D3 production, which enters circulation bound to vitamin D binding protein. Once absorbed or synthesized, vitamin D undergoes hepatic and renal hydroxylation to become 25(OH)D and ultimately the active form, calcitriol. Factors such as liver or kidney disease can impair conversion, affecting functional status. Dietary inhibitors of fat absorption, such as bile acid sequestrants or orlistat, may reduce vitamin D uptake. Seasonal variations influence UVB availability and subsequent endogenous synthesis, particularly at latitudes farther from the equator. Because of these dynamics, clinicians consider both environmental and physiological factors when evaluating vitamin D status and recommending supplementation.

Supplementation with vitamin D is common, particularly in individuals with limited sun exposure, dark skin, older adults, people with obesity, and those with conditions impairing fat absorption. Supplements come in D2 (ergocalciferol) and D3 (cholecalciferol) forms; evidence suggests D3 may be more effective at raising serum 25(OH)D levels. Many multivitamins contain vitamin D, but standalone supplements allow tailored dosing. Typical doses range from 600–2000 IU daily, with higher therapeutic doses under medical supervision for deficiency correction. Healthcare providers base dosing on baseline serum 25(OH)D, underlying health conditions, and concurrent medications. Supplement benefits include improved bone mineral density and decreased risk of fractures in deficient individuals. Some studies indicate modest benefits in immune function and reduced risk of acute respiratory infections, particularly in those with low baseline levels. Supplements may also improve muscle strength in older adults. However, evidence for broad extraskeletal benefits remains mixed, and supplementation should be individualized. Factors such as body weight, baseline status, season, and latitude influence response. Third‑party tested products ensure ingredient accuracy and reduce contamination risk. Individuals taking certain medications (e.g., bile acid sequestrants or orlistat) may require adjusted timing or dosing to optimize absorption.

Vitamin D toxicity, or hypervitaminosis D, is rare and almost always the result of excessive supplemental intake rather than sunlight exposure, as the skin self‑regulates cutaneous production. Toxicity leads to hypercalcemia, with symptoms such as nausea, vomiting, weakness, polyuria, dehydration, and kidney stones. Prolonged high serum 25(OH)D levels (>150 ng/mL) can cause soft tissue calcification, including vascular and renal calcification, potentially resulting in kidney failure or cardiovascular complications. Case reports include severe hypercalcemia leading to fatal outcomes in individuals consuming very high supplement doses over prolonged periods. The tolerable upper intake level (UL) for adults is set at 100 mcg (4000 IU) daily to minimize the risk of adverse effects. Some therapeutic regimens for deficiency exceed this UL for short periods under medical supervision, with monitoring of serum calcium and 25(OH)D levels. Because vitamin D is fat‑soluble and stored in adipose tissue, excessive long‑term intake can accumulate and increase toxicity risk. Individuals with granulomatous diseases (e.g., sarcoidosis) or certain lymphomas may have increased sensitivity to vitamin D and develop hypercalcemia at lower intakes. Regular monitoring and individualized dosing help prevent toxicity while achieving adequate vitamin D status.

Vitamin D interacts with a range of medications that can affect its absorption, metabolism, or outcomes. Bile acid sequestrants, such as cholestyramine, bind fat‑soluble substances in the gut and reduce vitamin D absorption, necessitating timing adjustments of supplements. Orlistat, a lipase inhibitor used for weight loss, decreases fat absorption and may impair vitamin D uptake, which is why a multivitamin containing fat‑soluble vitamins is often recommended in conjunction. Anticonvulsants such as phenytoin, phenobarbital, and carbamazepine accelerate vitamin D metabolism via hepatic enzymes, potentially lowering serum 25(OH)D levels and requiring higher supplement doses. Certain antibiotics, notably rifampin, modulate cytochrome P450 enzymes and alter vitamin D metabolism, although clinical significance varies. Thiazide diuretics, like hydrochlorothiazide, reduce calcium excretion; combined with vitamin D supplementation, they increase the risk of hypercalcemia. Digoxin’s toxicity risk may elevate with high calcium levels driven by vitamin D supplementation. Statins metabolized by CYP3A4 may have altered levels in the presence of vitamin D, though evidence is mixed. Corticosteroids don’t directly interact but are associated with vitamin D deficiency and bone loss, prompting concurrent calcium and vitamin D supplementation in long‑term users. Clinicians should review medication regimens when recommending vitamin D supplements and monitor serum levels as needed.

Common Forms: vitamin D3 (cholecalciferol), vitamin D2 (ergocalciferol)

Typical Doses: 600–2000 IU daily; therapeutic doses under supervision

When to Take: with a meal containing fat for best absorption

Best Form: vitamin D3

⚠️ Interactions: cholestyramine, orlistat, phenytoin, hydrochlorothiazide