Vitamin C

Vitamin C absorption plateaus at a couple hundred milligrams a day, which is the whole reason most people don't need to supplement it at all and the big bottles marketed for fighting off colds are mostly wasted. Your gut can only absorb so much. Uptake plateaus around 200-400mg/day, and above 1,000mg less than half of what you swallow gets in - the rest leaves in your urine. The Cochrane review of 29 trials (11,306 participants) found that taking it regularly doesn't stop colds in the general population, though it did cut colds by 52% in people under heavy physical stress like marathon runners and soldiers. Vitamin C is one of the most thoroughly studied supplements there is, and the research is less flattering than most supplement sites let on.

The reason megadosing falls flat is plumbing. The transporters that pull vitamin C into your blood (called SVCT1) can only move so much at once - they're saturable. Plasma levels plateau at 200-400mg/day from oral intake, and below that threshold you absorb more than 80-90% of the dose. At 1,000mg, less than half gets in. Anything past that your kidneys flush out. So for a well-nourished adult, taking 1,000mg or more is largely redundant - you're paying for vitamin C you'll pee out.

Where vitamin C does real work is inside your immune cells. It concentrates in white blood cells (neutrophils and macrophages) at 50-100 times the level in your blood. There it helps those cells move toward, engulf, and chemically destroy pathogens (chemotaxis, phagocytosis, and the oxidative burst), while protecting your own tissue from the collateral damage. It also supports the differentiation and proliferation of T-cells and B-cells, so it has a hand in both the fast, general immune response and the slower, targeted one. When you're fighting an infection, vitamin C levels in your blood and white cells drop fast, which is why having enough on board to begin with matters for immune readiness (Carr & Maggini 2017, PMID: 29099763).

Now the cold question, which is what most people are really here for. The definitive Cochrane review pooled 29 trials and 11,306 participants. The headline finding: taking vitamin C regularly does NOT stop you from catching colds in the first place if you're an average person (pooled RR 0.97, 95% CI 0.94-1.00). What it does do is shorten a cold once you have one - by about 8% in adults and 14% in children. There's one striking exception. In people under heavy physical stress - marathon runners, soldiers, skiers - vitamin C cut the number of colds by 52%. And waiting until you feel a cold coming on to start taking it? Dosing after symptoms appear showed no consistent, significant, or reproducible effect on either how long or how bad the cold was.

Here's what vitamin C will not do, no matter the dose: protect your heart or prevent cancer. The Physicians' Health Study II followed 14,641 men for 8 years on 500mg/day and found zero cardiovascular benefit (HR 0.99, 95% CI 0.89-1.10 for major events, with every secondary endpoint null - heart attack, stroke, and cardiovascular death) and zero cancer benefit (HR 1.02, 95% CI 0.94-1.10 for total cancer; HR 1.03, 95% CI 0.93-1.15 for prostate cancer, across 1,943 total cancers). A meta-analysis of 7 RCTs landed in the same place (RR 1.00, 95% CI 0.95-1.05 for cancer incidence), and a Mendelian randomization study found no causal link between naturally higher vitamin C and cancer. The likely reason: these were well-fed men already maxed out from their diets, so adding a pill changed nothing.

One thing vitamin C does reliably help with is iron. If you eat plant-based iron (from beans, spinach, fortified grains), pairing it with vitamin C makes your body absorb more of it. As little as 50-100mg taken with the meal meaningfully boosts that uptake.

A safety note worth reading if you're a man: doses above 1,000mg/day raise your kidney stone risk. The chain is direct - your body breaks ascorbic acid down to dehydroascorbic acid, then 2,3-diketogulonic acid, which splits off into oxalate. Calcium oxalate makes up roughly 80% of all human kidney stones, and the amount of oxalate in your urine climbs steeply above 1,000mg/day. The HPFS/NHS cohorts (40,500 men and 156,000 women tracked for over a decade) found men taking 1,000mg+ had a 19% higher risk (HR 1.19, 95% CI 1.01-1.40), and a meta-analysis put the odds 62% higher (OR 1.62). Women don't appear to carry this risk, probably because of estrogen's protective effect on how the kidneys handle calcium, differences in how the gut absorbs calcium and binds free oxalate, and possible differences in liver oxalate production. If you have chronic kidney disease, skip supplemental vitamin C entirely - your kidneys can't clear the oxalate well. For most adults the sweet spot is 200-400mg/day: enough to fill your tank without the diminishing returns and the stone risk that come with megadosing.