Total, Dietary, and Supplemental Vitamin C Intake and Risk of Incident Kidney Stones

Ferraro PM, Curhan GC, Gambaro G, Taylor EN · American Journal of Kidney Diseases

The largest prospective look at the question, and the answer is sex-specific. Among men, total intake of 1,000 mg/day or more carried a hazard ratio of 1.43 (95% CI 1.15-1.79) for incident stones, with a graded trend (p=0.005); supplemental intake at that level gave HR 1.19 (95% CI 1.01-1.40). Among the 156,735 women, neither total nor supplemental vitamin C was significantly associated with stones. Dietary vitamin C was not associated with stones in either sex — the risk attaches to supplements, not to fruit.

Population

156,735 women in Nurses' Health Study I and II and 40,536 men in the Health Professionals Follow-up Study, median follow-up 11.3-11.7 years

Intervention

Total, dietary and supplemental vitamin C intake

Comparator

Under 90 mg/day

Limitations

Observational, with intake from food frequency questionnaires and no stone composition for most cases, so the mechanism is assumed rather than shown. 6,245 stones across 197,271 people over more than a decade: the absolute risk increase behind HR 1.43 is small. Confounding by fluid intake and by whoever chooses to take gram-level supplements is not fully removable.