Half‑life (t½) is the time it takes for the concentration of a substance in your body to decrease by 50%. Understanding half‑life helps determine:

• Dosing frequency — supplements with short half‑lives (like Vitamin C at ~1 hour) need multiple daily doses for sustained levels, while long half‑life supplements (like Vitamin D3 at 15–25 days) can be taken weekly or even monthly.
• How long effects last — a supplement with a 6‑hour half‑life will be mostly cleared within 24–30 hours (4–5 half‑lives).
• Toxicity risk — substances with long half‑lives can accumulate if taken too frequently, increasing the risk of adverse effects.

Vitamin D3's long half‑life is due to several factors:

• Fat solubility — D3 dissolves in fat and is stored in adipose tissue, creating a slow‑release reservoir.
• Strong protein binding — 25(OH)D is tightly bound to vitamin D‑binding protein (DBP), protecting it from rapid clearance.
• Low renal clearance — unlike water‑soluble vitamins, D3 metabolites are not rapidly filtered by the kidneys.

This is why a single large dose of vitamin D can sustain levels for weeks, and why it takes 2–3 months of consistent supplementation to reach a new steady state.

The 6‑day plasma half‑life represents how quickly B12 is cleared from circulation, but the body has a sophisticated conservation system:

• Liver storage — 2–5 mg of B12 is stored in the liver, enough for 3–5 years of normal metabolic needs.
• Enterohepatic recirculation — B12 is excreted into bile (5–10 µg/day) and then over 65–75% is reabsorbed in the ileum. This recycling dramatically extends the body's B12 supply.
• Slow depletion — even if absorption stops entirely (e.g., pernicious anemia), it takes 2–5 years for clinical deficiency to develop.

• Serum/Plasma half‑life — measures how quickly a substance disappears from the bloodstream. This is what most half‑life values in our table refer to.
• Biological/Functional half‑life — measures how long the physiological effects of a substance last in the body. For example, B12 has a plasma half‑life of ~6 days, but its biological effect persists for years due to liver storage.
• Terminal half‑life — the final elimination phase after distribution equilibrium, often much longer than the initial plasma half‑life for fat‑soluble compounds.

For practical supplementation purposes, the biological half‑life and storage capacity are often more relevant than the serum half‑life alone.

A general rule of thumb:

• After 1 half‑life: 50% remains in the body
• After 2 half‑lives: 25% remains
• After 3 half‑lives: 12.5% remains
• After 4 half‑lives: 6.25% remains
• After 5 half‑lives: ~3% remains (considered "cleared")

For optimal steady levels, dosing intervals should generally be shorter than one half‑life. For Vitamin C (HL ~1 hour), frequent dosing throughout the day is ideal. For Vitamin D3 (HL 15–25 days), daily, weekly, or even monthly dosing all work well.

Fat‑soluble vitamins (A, D, E, K) have the highest accumulation risk because they are stored in body fat and the liver, rather than being rapidly excreted in urine:

• Vitamin A — Hypervitaminosis A can cause liver damage, bone pain, and neurological symptoms. Tolerable upper limit: 3,000 µg/day.
• Vitamin D — Toxicity causes hypercalcemia. Upper limit: 4,000 IU/day (100 µg) for adults, though many experts consider 10,000 IU/day the true threshold.
• Vitamin E — Excess can interfere with blood clotting. Upper limit: 1,000 mg/day.
• Iron — Though not fat‑soluble, iron overload (hemochromatosis) is dangerous as the body has limited mechanisms to excrete excess iron.

Water‑soluble vitamins (B complex, C) are generally safer because excess is excreted in urine, though extremely high doses can still cause side effects.

Yes, the chemical form can significantly impact half‑life:

• Vitamin B12 — Cyanocobalamin (synthetic) vs. methylcobalamin vs. hydroxocobalamin. Hydroxocobalamin has a longer retention time and is preferred for injections in some countries.
• Vitamin K — K1 (phylloquinone, HL ~1–2h) vs. K2‑MK7 (menaquinone‑7, HL ~3 days). MK7's much longer half‑life makes it more effective for once‑daily dosing.
• Melatonin — Immediate‑release (HL 35–50 min) vs. extended‑release formulations (effective HL 4–6h).
• Magnesium — Different salts (citrate, glycinate, oxide) affect absorption rate, which influences the apparent half‑life in plasma.

Vitamin C has one of the shortest half‑lives among common supplements — just 30 minutes to 2 hours in plasma. This means:

• A single 500 mg dose is largely cleared within 4–6 hours.
• The body tightly regulates vitamin C levels, and absorption decreases as doses increase (saturable transport).
• For sustained tissue saturation, it's better to take smaller doses (100–250 mg) 2–3 times per day than one large dose.
• Liposomal vitamin C formulations can extend the effective half‑life by protecting the vitamin from rapid renal clearance.