Vitamin D Glossary

Terminology used across the vitamin D literature, with abbreviations expanded and short definitions. Sorted alphabetically.

7-dehydrocholesterol (7-DHC)

The provitamin D3 precursor stored in the epidermal cell membrane. UVB radiation (280–315 nm) opens its B-ring to form previtamin D3, which thermally isomerises to cholecalciferol.

1,25-dihydroxyvitamin D (calcitriol; 1,25(OH)â‚‚D)

The biologically active hormonal form of vitamin D. Produced in the kidney by CYP27B1 hydroxylation of 25(OH)D. Regulates calcium, phosphorus, and bone mineralisation; tightly controlled by PTH.

25-hydroxyvitamin D (25(OH)D; calcidiol)

The main circulating form of vitamin D and the standard clinical measure of vitamin D status. Produced in the liver by CYP2R1 hydroxylation of cholecalciferol or ergocalciferol. Half-life ~15–25 days.

Adequate Intake (AI)

A Dietary Reference Intake set when insufficient evidence exists to establish an RDA. Used for infants; 400 IU/day is the AI for vitamin D in infants 0–12 months.

Adipose sequestration

The storage of lipophilic cholecalciferol in body fat, from which it is slowly released to the circulation. Explains the ~60–90 day terminal half-life of 25(OH)D in humans and the lower serum 25(OH)D observed in obese individuals at equivalent UVB dose.

Cholecalciferol (vitamin D₃)

The animal form of vitamin D. Produced in the skin from 7-dehydrocholesterol under UVB, or ingested from fatty fish, egg yolk, and fortified foods. Preferred supplement form in most guidelines.

Ergocalciferol (vitamin Dâ‚‚)

The plant/fungal form of vitamin D. Produced by UV irradiation of ergosterol in yeasts and fungi. Raises serum 25(OH)D less effectively than D₃ at equivalent doses.

Fitzpatrick skin phototype

A six-category classification (I–VI) of skin colour and UV response introduced by Thomas Fitzpatrick in 1975. Type I always burns and never tans; Type VI is darkly pigmented and rarely burns. Determines minimal erythemal dose and vitamin D synthesis efficiency.

Institute of Medicine (IOM)

The US body (now the National Academy of Medicine) that established the current Dietary Reference Intakes for vitamin D (2011). Defines deficiency as < 20 ng/mL 25(OH)D.

International Unit (IU)

A unit used for vitamin dosing. For vitamin D, 1 IU = 0.025 µg cholecalciferol; conversely, 1 µg = 40 IU.

Minimal Erythemal Dose (MED)

The lowest UV dose that produces perceptible skin reddening 24 hours after exposure. Varies by skin type: ~200 J/m² for type I, ~1000+ J/m² for type VI.

Parathyroid hormone (PTH)

The hormone that regulates serum calcium. Elevated PTH is a sensitive marker of vitamin D insufficiency; secondary hyperparathyroidism develops when 25(OH)D falls below ~20 ng/mL.

Photoequilibrium

The steady-state balance between previtamin D3 production and its UV-driven degradation. Prevents unlimited vitamin D3 synthesis from sunlight alone — the reason vitamin D toxicity cannot occur from sun exposure.

Recommended Dietary Allowance (RDA)

The intake sufficient to meet the needs of 97.5% of healthy individuals. Vitamin D RDA is 600 IU/day for ages 1–70 and 800 IU/day for 71+.

Rickets

The clinical disease of impaired bone mineralisation in growing children caused by vitamin D or calcium deficiency. Presents with bowed legs, growth failure, and delayed dentition. Serum 25(OH)D typically below 12 ng/mL.

Osteomalacia

The adult analogue of rickets: bone pain, muscle weakness, and reduced mineralisation. Caused by prolonged severe vitamin D or calcium deficiency.

Solar zenith angle (SZA)

The angle between the sun and the vertical (zenith). Determines the atmospheric path length of sunlight and therefore UVB attenuation. Vitamin D synthesis effectively stops when SZA > 60°.

Standard Erythemal Dose (SED)

A weighted UV exposure unit; 1 SED = 100 J/m² erythemally-weighted UV. Approximates the MED of type I skin.

Tolerable Upper Intake Level (UL)

The highest daily intake likely to pose no risk of adverse effects in the general population. IOM sets vitamin D UL at 4,000 IU/day for adults.

UVB

Ultraviolet B radiation, wavelengths 280–315 nm. The narrow band responsible for vitamin D3 photosynthesis in skin. Blocked by window glass, ozone, and sunscreen.

Vitamin D-binding protein (DBP; GC-globulin)

The principal serum carrier of vitamin D metabolites. Genetic polymorphisms (rs4588, rs7041) affect free 25(OH)D and may modulate individual vitamin D response.

Vitamin D winter

The months during which UVB is insufficient to drive cutaneous vitamin D synthesis. Above ~40° latitude, roughly November through February; above ~50° latitude, October through March.