Why high intake of vitamin c cause diarrhea?

Unclear: I'm not sure we really know what the mechanism is, but diarrhea is one of the first symptoms people experience when they take too much vitamin C. Too much vitamin C seems to affect the entire GI tract as stomach cramps, nausea and vomiting are also reported. Usually these symptoms do not appear at doses less than 2 grams per day.

Possibly: It may not necessarily be how much Vitamin C, but the type of Vitamin C. There are many varieties of Vit. C and some do tend to cause digestive problems. Possibly do research or talk to a nutrition expert.

?: I am not aware that high dose of vitamin C would cause diarrhea. The other question is what do you mean by high intake of vitamin C an for what reason are you taking a high intake..

High doses: of Vitamin C can cause diarrhea, and some people are more sensitive to Vitamin C than others. Generally only high doses >2000mg can cause side effects like upset stomach and diarrhea. The diarrhea may be due to an osmotic effect from the high levels of Vitamin C. This draws more water back into the intestines to help dilute the Vitamin C, which then leads to diarrhea. Hope this helps!

Vitamin C: In large doses, vitamin C causes diarrhea in healthy subjects. In one trial in 1936, doses up to 6 grams of ascorbic acid were given to 29 infants, 93 children, and 20 adults for more than 1400 days. With the higher doses, toxic manifestations were observed in five adults and four infants. The signs and symptoms in adults were nausea, vomiting, diarrhea, & flushing of the face.