Vitamin C

Vitamin C and immunity: much more than the common cold

Immune cells accumulate vitamin C concentrations 50 to 100 times higher than those in plasma, where it acts as a potent antioxidant and facilitates their phagocytic function; vitamin C stimulates the production of white blood cells, especially neutrophils, lymphocytes, and phagocytes, promotes their ability to detect and destroy pathogens, and protects the immune cells themselves from the oxidative damage they generate during the immune response, creating a virtuous circle of protection and efficacy that few other molecules are able to sustain.

In endurance athletes, who are at higher risk of upper respiratory tract infections because of the oxidative stress induced by intense exercise, six trials with more than 600 marathon runners, skiers, and soldiers found 50 percent fewer colds among those who supplemented with vitamin C, an effect far greater than that observed in the general population, suggesting that people who perform high-intensity exercise are especially sensitive to the benefit of supplementation with this vitamin.

Dose, bioavailability, and the problem with most supplements

The bioavailability of oral vitamin C depends strongly on dose and frequency: at 200 milligrams daily, plasma concentration averages about 70 micromoles per liter, which is the level considered optimal for preventing oxidation of low-density lipoproteins; however, supplementing 1 to 3 grams of vitamin C at once produces peak plasma concentrations two to three times higher than those at 200 milligrams, although those peaks normalize within the following 24 hours, which makes a single dose less effective than multiple doses spread throughout the day.

Liposomal vitamin C shows superior bioavailability to free vitamin C when taken orally: in one study, 10 grams of liposomal vitamin C produced a peak plasma concentration of 300 micromoles per liter versus 180 micromoles for the free form, a difference of 70 percent, with a half-life two hours longer; but even liposomal vitamin C falls far short of intravenous administration, which can reach plasma concentrations 30 to 70 times higher than the same oral dose, opening up an entirely different activity profile with therapeutic applications that the oral form simply cannot replicate.

Intravenous vitamin C: cancer, infections, and sepsis

Intravenous vitamin C has been investigated as an adjuvant treatment in multiple types of cancer with results ranging from promising to preliminary: in a phase 1 trial of 13 glioblastoma patients who received chemotherapy and radiotherapy along with 15 to 125 grams of intravenous vitamin C three times a week, median survival was 18.2 months versus the historical 14 months, a difference that does not reach definitive conclusions but justifies continued research in this direction.

In sepsis, where patients typically have plasma vitamin C levels below 25 percent of those in healthy people, a study of more than 160 ICU patients with sepsis-induced respiratory failure found that those who received intravenous vitamin C had a 28-day mortality of 30 percent versus 46 percent in the placebo group, along with more ventilator-free days and earlier transfers out of the ICU; these are not definitive data, but they are solid enough that intravenous vitamin C is one of the most actively investigated adjuvant treatments in contemporary critical care medicine.

Vitamin C and the brain

Vitamin C is found in high concentrations in the brain, especially in the hippocampus and frontal cortex, where it acts as a cofactor in neurotransmitter regulation by preventing excessive accumulation of dopamine in neurons that generate reactive oxygen species; for every 54-milligram increase in vitamin C intake, a Dutch study of nearly 5,400 participants over 55 years of age found an 18 percent reduction in the risk of Alzheimer's disease, with an even more pronounced effect in smokers, where the reduction reached 35 percent.

The exercise and vitamin C paradox

The relationship between vitamin C and exercise has a complexity that the popular narrative oversimplifies: doses between 0.2 and 1 gram daily appear to reduce oxidative stress without compromising the beneficial adaptations to training, but doses above 1 gram per day, especially when combined with vitamin E, may blunt mitochondrial biogenesis, exercise-induced insulin sensitivity, and other cellular adaptations that are precisely the mechanisms through which exercise improves metabolic health in the long term.

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