"The good news is that breastmilk always maintains a certain nutritional quality, even if your diet isn’t perfect. But the levels of some vitamins, such as vitamins B6 and C, and minerals, like chromium, in your breastmilk is directly affected by your dietary intake. That’s why a nutritious diet is so important and why supplements can be a real asset."

"Vitamin C (ascorbic acid) is excreted into human breast milk. Reported concentrations in milk vary from 24 to 158 g/mL (30,31,32,33,34,35,36,37 and 38). In lactating women with low nutritional status, milk vitamin C is directly proportional to intake (31,32). Supplementation with 4200 mg/day of vitamin C produced milk levels of 2461 g/mL (31). Similarly, in another group of women with poor vitamin C intake, supplementation with 34103 mg/day resulted in levels of 3455 g/mL (32). In contrast, studies in well-nourished women consuming the RDA or more of vitamin C in their diets indicate that ingestion of greater amounts does not significantly increase levels of the vitamin in their milk (33 and 34). Even consumption of total vitamin C exceeding 1000 mg/day, 10 times the RDA, did not significantly increase milk concentrations or vitamin C intake of the infants (36). However, maternal urinary excretion of the vitamin did increase significantly. ***These studies indicate that vitamin C excretion in human milk is regulated to prevent exceeding a saturation level (36).***"

http://drugsafetysite.com/vitamin-c/

they say the recommended daily allowance for vitamin c while nursing is 95mg. so if you took more, you’d pee it out to avoid exceeding the amount.