What it is
Regulatory status: dietary supplement
At a glance
What it is, why people use it, the evidence, and what to know before trying it.
Why people use it
- Maintains plasma ascorbate for many hours vs an IR bolus (“continuous supply”).
- Greater bioavailability (higher AUC) than the same milligram dose of IR C.
- Exceeds the homeostatic saturation ceiling / renal threshold that limits ordinary oral C.
- Fewer common colds, or shorter / milder colds (Pauling-type immune claim, attached to the SR form).
- Less cardiovascular disease / slower atherosclerosis.
- Cancer prevention or treatment (Pauling / Cameron claim).
- Better skin (collagen, wrinkles, photoprotection) because of a continuous cofactor supply.
Evidence at a glance
Timed-release vitamin C is ordinary L-ascorbic acid in a slower tablet. The only human head-to-head PK trial with modern assays (Viscovich 2004, 250 mg BID, smokers) found **identical AUC and Cmax** and a **6.9 µmol/L** smaller peak–trough span — the authors called it clinically unimportant. One 1977 paper reported higher relative bioavailability; a 1982 paper reported **half** the bioavailability; a 1998 bioequivalence study found SR = IR. The “continuous supply because humans lack GULO and ordinary C is gone in 2–4 hours” story is a mechanism-as-outcome error: Tmax is 2–3 hours, terminal t½ is about a day, and plasma and leukocytes are already saturated at 200–400 mg/day of any oral form. SR does not beat the renal threshold or reach pharmacological plasma levels.
Forms and variants
This review does not automatically apply to:
- Ordinary IR ascorbic acid 250 mg BID (separate item)