Vitamin C and citrate improve the absorption of iron. In urine, the same two substances increase the solubility of calcium salts, some of which (like calcium oxalate and calcium phosphate) are particularly insoluble in urine when these two substances are too low in concentration.
Roy Walford suggested that vitamin C should be consumed as a supplement between meals to avoid the danger of iron overload.
I do not think that is sound advice.
The body needs to absorb as much dietary iron as it can in case of an emergency. Secondarily, the more chemically reactive iron species (potent oxidizers) need to be absorbed to reduce harm to the downstream gut tissues. Iron species of low reactivity, typically well-chelated iron from plants, can be passed through the digestive tract without too much harm.
Enterocytes optimally take up as much iron as they can, and citrate and vitamin C help this, and they sequester the iron, in the absence of any real need. The enterocytes die in about 3 days, and this iron is harmlessly passed in stool, provided the enterocytes are not degraded. If there is a real shortfall of iron, as after heavy bleeding, properly nourished enterocytes release the iron to the liver for processing. Obviously, the enterocytes cannot release iron they failed to take up in the first place.
So: vitamin C and citrate with meals (optimally in the form of citrus fruit?)! Vitamin C between meals! Vitamin C in the urine stream to keep calcium and the other sparingly soluble cations soluble! High vitamin C in urine is not “expensive urine” – vitamin C is the cheapest insurance of a soluble urine stream one can buy! So is vitamin C’s constant companion, citric acid.