When we swallow 15 mg of vitamin C, the amount we’d get eating about a quarter of an orange, our body absorbs nearly 90 percent of it. If we instead take a supplement containing 1,250 mg of vitamin C, our body seems to realize that’s too much and clamps down on absorption at the intestinal lining level, and we end up absorbing less than half. By doing experiments where the level of intake is ratcheted up slowly, we can see when the body starts to say, “Okay. That’s enough.”
That magic level of intake appears to be about 200 mg a day. When we take up to 200 mg daily, our body absorbs it all. Above that level, however, the body tries to block further absorption, suggesting that our “intestinal vitamin C transport mechanisms… have evolved to fully absorb up to about 200 mg of vitamin C” a day.
In addition, vitamin C is reabsorbed in our kidneys back into our bloodstream to maintain our vitamin C blood levels around 70 or 80 micromoles per liter, which is what we reach at a vitamin C intake of about 200 mg a day. Even if we take ten times as much in vitamin C supplements, 2,000 mg a day, our body will just pee and poop out the excess to keep our blood levels in that narrow range of 70 to 80 micromoles per liter. Based on these kinds of data, one might “propose that 200 mg is the optimal daily intake of vitamin C…”