Can you get too much Vitamin C? News headlines suggested as much following a presentation at an American Heart Association conference earlier this year. The research that started the fuss surveyed 573 men and women about their Vitamin C intake before and after an 18-month period. Those who said they took 20-190 mg per day showed little change, those who reported taking 192-479 mg daily had more than twice the normal rate of artery wall thickening, and those taking 480-3,355 mg per day had 2.7 times the rate of thickening. Strangely enough, only supplemental, not dietary Vitamin C was associated with faster artery thickening.
This new report is considered scary because its the first trial testing Vitamin C's effects on arteries in the same people over time the first longitudinal study. Even so, the study is contradicted by an even bigger study done in 2004. While not longitudinal, it found that individuals with high Vitamin C intake actually have thinner arteries than those with low intake (after many adjustments due to variables), although for some reason the result only held for subjects over the age of 55. A third study found no change in artery wall thickness due to Vitamin C supplementation.
With so much other evidence to suggest that Vitamin C fights heart disease it relaxes blood vessels, reduces LDL cholesterol oxidation, etc., we're left to wonder how the negative result was obtained. Remember, however, that the report is still preliminary. A more detailed description of what the scientists actually saw, combined with the criticisms and insights of other scientists (a process called peer review) may unearth more clues.
Furthermore, the trial measured artery wall thickness, not incidence of heart attacks. This begs the question, "Is thickening of artery walls necessarily a bad thing?" If it's due to cholesterol buildup, sure. If it's due to more connective tissue, though, it may actually be good, it depends on which layer of the carotid artery wall is involved: outer, middle or inner. If the middle layer is thickened, that's probably good; it's rich in stretchy connective tissue. Indeed, fortification of artery walls with connective tissue is one reason why estrogen replacement therapy (ERT) may fight heart disease in postmenopausal women; ERT thickens the middle carotid artery wall and reduces heart disease at the same time. If Vitamin C also fortifies artery walls by promoting collagen synthesis, for example, it may be doing us good.
On the other hand, if Vitamin C thickens the inner carotid artery wall, it could indeed promote the growth of atherosclerotic plaques, bad news. Since the researchers lumped together both inner and middle artery wall thicknesses as one big measurement, however, we don't know whether Vitamin C may cause good artery wall thickening or bad. Along with Vitamin C's proven benefits and the National Research Council's recent establishment of 2,000 mg as an upper daily limit for the vitamin, this makes it inadvisable to flush your Vitamin C pills down the toilet.