We’re entering the cold season, not cold as in temperature but cold as in common, the runny nose one. Although the former probably contributes to the latter.
People take supplemental vitamin C in the winter to fend off colds. I have learned it is not a good idea, e.g. Vitamin C Supplements Reduce Endurance, Fatigue Muscles Sooner. That doesn’t mean we shouldn’t pay attention to our vitamin C intake. Greger’s 200 mg seems credible, and a good goal. An orange and a baked potato will take you halfway there.
For many years, the RDA (the recommended daily allowance) for vitamins was just based on preventing deficiency with a margin of safety. But, the minuscule amount of vitamin C, for example, needed to avoid scurvy is not necessarily the ideal intake for optimal health. So, what might be the optimal intake of vitamin C? Let’s ask the body. How might we do that? By seeing how much the body absorbs and excretes.
If you swallow 15 milligrams of vitamin C, which is what you would get eating about one-quarter of an orange, your body sucks up nearly 90% of it. But, if you take a supplement containing 1,250 milligrams, your body seems to realize that’s too much—and so, clamps down on absorption at the intestinal lining level, and you end up absorbing less than half.
So, by doing experiments where you slowly ratchet up the level of intake, you can see when the body starts to say, “Okay, that’s enough.” And, that magic level of intake appears to be about 200 milligrams a day. When you take up to 200 a day, your body sucks it all up. But, above that, the body tries to block further absorption—suggesting 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 up at around 70 or 80 micromoles per liter. That’s the shaded region here, which is what you reach at a vitamin C intake of about 200 milligrams a day. So, even if you take ten times as much in vitamin C supplements—2,000 milligrams a day – your body will just pee and poop it out to keep your blood levels in that narrow range. So, based on these kinds of data, one might propose that 200 milligrams is the optimal daily intake of vitamin C.
You can confirm, using disease data. At what daily intake of vitamin C is there the lowest stroke risk? Apparently, at about 200 milligrams a day. While dietary vitamin C intake was associated with lower stroke risk, vitamin C supplements were not – which is consistent with the overall body of evidence showing that antioxidant supplements in general don’t seem to protect against heart attacks or strokes.
But, wait, can you get up to an intake of 200 milligrams a day without taking supplements? No problem. Single servings of fruits and vegetables may have about 50 milligrams each; so, a measly five servings of fruits and veggies a day could get you to ideal blood levels.