Vitamin C's main jobs have little to do with colds. Your body uses it to make collagen, heal wounds and absorb iron from plant foods, and about 75 mg a day covers that for most women.
Maybe there's a tub of 1,000 mg tablets in your cabinet that only comes out when your throat starts to scratch. If so, the timing and the dose are both off. Starting vitamin C once a cold begins doesn't do much. And past a certain point, your body absorbs a smaller and smaller share of a bigger tablet.
What Does Vitamin C Do in Your Body?
Its biggest job is building collagen. The NIH says vitamin C is needed to make collagen, an essential part of your connective tissue. That tissue is central to wound healing, so cuts heal slowly when vitamin C runs short.
It also helps your body make L-carnitine and some brain chemical messengers. It works as an antioxidant too, which is real but not something you'll ever feel.
The job that matters most day to day for many women is iron. Vitamin C improves how well you absorb nonheme iron, the kind in beans, lentils, greens and fortified cereal. That one gets its own section below, because it works less neatly than most advice suggests.
Collagen in a tub is a separate question. Whether collagen supplements work depends a lot on who paid for the studies.
Does Vitamin C Help With Colds at All?
For most people, taking it every day doesn't stop colds. A Cochrane review pooled 29 trial comparisons with 11,306 participants and found regular vitamin C made no real difference to how often people caught one.
The exception is narrow. In five trials of 598 marathon runners, skiers and soldiers on subarctic exercises, daily vitamin C roughly halved the risk of a cold. Those were people under short bursts of extreme physical stress, not someone with a busy week.
Regular use did shorten colds a little: by about 8% in adults and 14% in children. For a cold that would last a week, 8% is roughly half a day. And taking it only once symptoms started showed no consistent effect, which is exactly how most people use it.
How Much Vitamin C Do Women Need a Day?
Women 19 and older need 75 mg a day. The NIH raises that to 85 mg in pregnancy and 120 mg while breastfeeding. If you smoke, add 35 mg to whichever figure applies, so 110 mg for a woman who isn't pregnant.
Food gets there without effort. These amounts come from the NIH's table, which draws on USDA food data:
- Raw sweet red pepper, half a cup: 95 mg
- One medium orange: 70 mg
- One medium kiwi: 64 mg
- Cooked broccoli, half a cup: 51 mg
- Sliced strawberries, half a cup: 49 mg
- Cooked Brussels sprouts, half a cup: 48 mg
- One baked potato: 17 mg
So an orange plus a handful of strawberries already beats the target. Vitamin C breaks down with heat and long storage. Steaming or microwaving vegetables loses less than boiling them.
Why Taking More Vitamin C Doesn't Do More
Your body handles vitamin C on a sliding scale, and the useful part of that scale ends well below a 1,000 mg tablet. Put the numbers side by side and the pattern is clear:
- About 10 mg a day prevents scurvy, according to the Linus Pauling Institute at Oregon State University. Below that, the NIH says signs can show up within a month.
- 75 mg a day is the recommended amount for women.
- The Linus Pauling Institute says single doses up to 200 mg are absorbed very efficiently. Blood levels plateau at 200 to 400 mg a day in healthy young adults.
- Above 1,000 mg a day, the NIH says absorption falls to less than 50%.
- 2,000 mg a day is the upper limit for adults, counting food and supplements. Too much can cause diarrhea, nausea and stomach cramps.
The NIH puts absorption at 70% to 90% at usual intakes of 30 to 180 mg a day. So a pepper or an orange sits at the efficient end of the scale. Three of the foods above add up to more than 200 mg, which is where blood levels start to plateau.
Does Vitamin C Help You Absorb Iron?
Yes, at a meal, but less than most advice implies over a whole diet. The NIH's iron fact sheet says vitamin C enhances the bioavailability of nonheme iron. It also says vegetarians need 1.8 times as much iron as meat eaters, because plant iron is harder to absorb.
Phytate in grains and beans, and certain polyphenols, pull iron absorption down. Vitamin C at the same meal is one way to push back. Lentils with tomatoes, beans with peppers, or fortified cereal with strawberries all make sense.
The catch shows up when researchers look at whole diets instead of single meals. A small study tracked 12 people on complete diets over three set periods. Over a whole diet, the boost from vitamin C was far less pronounced than that from single meals. Across a whole diet, the rest of your plate matters too.
Iron pills are another case. A trial in China enrolled 440 adults with iron deficiency anemia, almost 97% of them women. Iron alone was equivalent to iron plus 200 mg of vitamin C. Hemoglobin and iron stores rose about the same either way.
So if you've been prescribed iron, this trial suggests a separate vitamin C supplement isn't needed. Follow the instructions you were given, though.
If you eat little or no meat, protein for women who eat little meat covers foods that bring iron along too. And if tiredness is the reason you're reading this, what iron deficiency feels like goes through the signs to take to your doctor.
