In the NIH in-hospital depletion-repletion studies, neutrophils, monocytes, and lymphocytes saturated at 100 mg/day in men, plasma and circulating cells saturated at 400 mg/day in women, complete plasma saturation in men occurred at 1,000 mg/day, and bioavailability declined at single doses of 500 mg and higher with the absorbed excess excreted in urine.
Those authors concluded the RDA should be raised to 200 mg/day.
Intravenous vitamin C is a different pharmacological entity because it bypasses the tightly controlled absorption of oral dosing: at a matched 1.25 g dose it produced about 6.6-fold higher peak plasma vitamin C than oral, and pharmacokinetic modeling predicted about 61-fold higher for a single 50 g intravenous dose versus the maximum tolerated oral regimen.
That pharmacokinetic gap is the stated rationale for intravenous use in integrative oncology, and it is a rationale rather than outcome evidence.
Liposomal vitamin C produced a plasma area under the curve about 1.4-fold that of unencapsulated oral in the single small crossover trial retrieved here, below the 2-fold to 3-fold advantage commonly quoted.