“Vitamin C is absorbed in the small intestine where there’s a group of different proteins there that can take it up,” explains Michels. That’s easy enough–so what’s all the fuss about vitamin C absorption, then?
Well, the vitamin C in foods (and many supplements) is a form called ascorbic acid. According to Michels, the potential challenge with absorption of ascorbic acid (at high doses) is that it’s easily saturated–take a concentrated amount of vitamin C in this form at one time and you’re unlikely to absorb the excess.
However, here’s the important distinction: When the intestine is faced with lower levels of ascorbic acid (i.e., less than about 400 milligrams), an active transport system absorbs the vitamin C (i.e., moves the nutrient through the gut and into the bloodstream, to where it’s needed in the body).
As soon as these active transports become overwhelmed, passive diffusion takes over to absorb the rest of the vitamin C (which is a fairly inefficient process). The truth is that absorption isn’t as easy as it sounds, and the fact that ascorbic acid appears to have a ceiling for absorption, that can make it difficult to glean all the benefits of higher potency formulas (if they feature the ascorbic acid form).
Plus, vitamin C is water-soluble, meaning it dissolves in water (as the name suggests), but this type of vitamin can’t be stored in your body for later use.
You may be more familiar with the process of fat-soluble vitamins–including D, E, A, and K–which are absorbed and stored in tissue. Alternatively, excess water-soluble vitamins are discarded, leaving you to start fresh every day.
Your body can handle around 300 to 400 milligrams of the purely ascorbic acid form of vitamin C at a time according to Michels (which is a genuinely useful, lower dose found in foods, multivitamins, and other multi-ingredient complexes), but with higher doses, less is absorbed.
“This is more than enough capacity to absorb most of the vitamin C you get from food, but supplements can be a different story,” he explains. Because of the saturation phenomenon, the rest of that valuable vitamin C from an ascorbic acid supplement, for example, will theoretically be flushed down the toilet (i.e., you’ll just pee it out).