How much Vitamin C (ascorbic acid) is optimal for health? To answer this question, I’ll examine the association between circulating levels of Vitamin C with all-cause mortality risk. Then, which dietary Vitamin C intake corresponds to optimal plasma levels? Let’s have a look!
A variety of studies have investigated associations between plasma (or serum) Vitamin C with all-cause mortality risk:
Studies that show weaker or no association between the plasma Vitamin C concentration with all-cause mortality risk include Loria et al. (2000) and Jia et al. (2007). In Loria et al. (2000), 9,450 middle aged adults (~48y) were followed for 12-16 years. Men in the highest Vitamin C quartile (> 74 uM) had significantly reduced all-cause mortality risk, when compared with men in the low plasma Vitamin C group (< 28 uM). Although a similar association was identified for women, significance was lost after multivariable adjustment. In Jia et al. (2007), although plasma Vitamin C values less than 61 uM were associated with increased all-cause mortality risk in older adults (median age, ~80y) that were studied for ~7.5 years, these data were not statistically significant (p-value = 0.18). However, the study sample size (398 subjects) may have been too small to detect significant effects.
Collectively these studies show that low circulating levels of Vitamin C may be related to an increased mortality risk, whereas plasma values greater than ~50 uM are consistently associated with reduced all-cause mortality risk. How much dietary vitamin C is required to attain 50 uM+?
As shown below, the RDA for dietary Vitamin C is 90 mg for males and 75 mg for females older than 19 years (Institute of Medicine 2000).
If you consume the RDA for Vitamin C, what plasma Vitamin C concentration will that yield? Shown below is how the plasma Vitamin C concentration varies according to ingested dose (Levine et al. 1996). Consuming the RDA value for Vitamin C yields a plasma Vitamin C value of 20-30 uM. From the studies listed above, that would put you in the increased all-cause mortality risk group! How much dietary Vitamin C would be needed to achieve plasma values greater than 50 uM? From the plot, we see that a dietary Vitamin C intake at double the RDA would be necessary. Furthermore, because 2 studies have reported decreased all-cause mortality risk at plasma Vitamin C values greater than 66 uM, dietary intakes intake between 500-1000+ mg/day may be necessary:
Which foods are Vitamin C-rich? As shown below, sweet peppers (yellow, red, and green) are the All-Stars for Vitamin C content per 100 calories:
What’s my average daily Vitamin C intake? Shown below is my average daily Vitamin C intake, 971 mg/day (red line) over the past 400+ days:
Note that I don’t supplement with Vitamin C, and that amount is completely from food. Also, I generally split my Vitamin C intake throughout the day, with about 2/3 in the morning and the remaining at night, to try to keep my circulating Vitamin C levels relatively saturated at > 70 uM. I haven’t measured my circulating C levels (yet), but that’s on the to do list!
With the goal of optimizing plasma levels of Vitamin C, it is also important to monitor dietary sodium intake. Intestinal absorption of Vitamin C requires dietary sodium (Friedman and Zeidel 1999). As shown below, 1 ascorbate ion (asc-) is absorbed from the intestinal lumen into intestinal epithelial cells in the presence of 2 sodium (Na+) ions. Vitamin C can then diffuse into the blood as Asc- or as dehydroascorbate (DHA):
Accordingly, based on my average dietary Vitamin C intake of 971 mg/day, to maximize absorption, a corresponding dietary sodium intake of 1940 mg would also be necessary.
These data are now in video format!
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