The bowel itself and the flora that inhabit the intestines have separate demands for nutrients and they take them from the input according to their pre-programmed priorities. When the bowel and the bowel flora are deprived of energy and nutrients, they secrete chemicals that affect our appetites and our health, both physical and mental. Unless unused nutrients can come back to the system from the intestines through the enterohepatic circulation or some other mechanism, unused nutrients become waste, but they are hardly “pure waste,” and more than components of the urine stream can be pure waste.
The circulatory system itself has demands for nutrients.
Organs and tissues and even individual cells have demands that they make from the circulatory system’s supply of nutrients.
Finally, the urinary system has basic needs that must be met or we suffer the consequences, and in that sense the urinary system makes nutrient demands on the circulatory system. Everything that ends up in the urine stream is not “pure waste” – it is doing things all the way from the very formation of a properly formulated urine to sweeping fimbriated bacteria out of the urinary tract or pressuring their very survival in that tract.
I can’t say it enough for the unimaginative person to get it or write it with block letters big enough for the blind to see, but vitamin C, magnesium, and citrate and so many other substances are not just pure waste products in urine. A urine stream cannot be properly formulated without sufficient quantities of these substances. To an improperly formulated urine stream, add a vitamin B6 deficiency, which, almost regardless of dietary oxalate, will almost certainly lead to a higher than normal plasma oxalate concentration, and thus higher urinary oxalate concentration, and there will be problems on top of problems with stones of various sizes and compositions from one end of the urinary tract to the other.