Vitamin D

A hormone, not really a vitamin. It controls how much calcium and phosphate you absorb.

Mechanisms, studies and evidence grades for each claim above.

Core mechanism

Calcitriol binds the vitamin D receptor (VDR), altering gene transcription to regulate calcium/phosphate transport proteins.² Key targets include TRPV6 (intestinal Ca²⁺ uptake), calbindin (intracellular Ca²⁺ transport), and Na⁺/Pi cotransporters (intestinal phosphate absorption) — together controlling mineral flux across gut, kidney, and bone.

Key functions

• Intestinal calcium absorption — increases TRPV6 and calbindin expression²
• Phosphate regulation — increases intestinal phosphate absorption, supporting hydroxyapatite formation and ATP/phosphorylation capacity²
• Bone remodeling control — regulates osteoblast/osteoclast signaling and works with PTH to mobilize or deposit calcium³
• Renal reabsorption — increases Ca²⁺ reabsorption in the kidney²
• Cellular signaling — the Ca²⁺ availability it controls is itself a core signal for muscle contraction, neurotransmission, and enzyme activity¹

Secondary effects (gene regulation layer)

Immune modulation. Immune cells express VDR and can activate vitamin D locally; this increases antimicrobial peptides (cathelicidin) and modulates T-cell balance toward less inflammatory, more regulatory activity.²

Cell growth regulation. Controls proliferation, differentiation, and apoptosis — a context-dependent influence on tumor biology.²

System interactions

• Calcium system — primary regulator of Ca²⁺ absorption and availability. See Calcium.
• Electrolyte system — magnesium is required for vitamin D's own activation (hepatic and renal hydroxylases). See Magnesium.
• Bone/structural system — controls mineral deposition, working alongside vitamin K2 for calcium placement. See Vitamin K2.

Deficiency effects

Reduced calcitriol impairs calcium/phosphate transport,¹ with downstream effects: reduced bone mineralization (osteomalacia/rickets), increased PTH driving bone resorption, and reduced muscle function from impaired Ca²⁺ signaling.

Intake

Roughly 600–800 IU/day — though as the Metabolic Health hub notes, this is a seasonal call above ~37°N where cutaneous synthesis stops in winter; sunlight is the summer replacement, not a year-round supplement need.

Top foods

• Salmon
• Eggs

Key insight

Vitamin D regulates mineral availability — it does not directly build tissue, but enables mineral-dependent systems to function. It controls calcium and phosphate flux via gene regulation, determining mineral availability for structure and signaling.

References