It’s not one specific ingredient... It’s the combination of ingredients and nutrients that accomplish this. Remember; an ingredient is a vehicle to provide nutrients to the body, example, chicken provides protein, vitamin B3, phosphorus, tryptophan, selenium. Corn provides protein (rich in sulfur amino acids... good for skin and coat), carbohydrates, fibre, vitamin C, vitamin B3, manganese, leutine, zeaxanthine, beta-carotene, etc..... By combining different ingredients, it is possible to produce diets with very specific nutrient profiles, and in this way manage and treat diseases, predispositions, etc......
Basically, researchers know the parameters involved in the formation of common crystals/stones; pH, urine volume, urine specific gravity, sodium, potassium, calcium, phosphate, citrate, ammonium, sulphate, urate, oxalate, magnesium. The formulators create a ‘recipe’ with a desired nutritional profile. They make the diet, to see if it physically ‘works’ (like making a cake recipe... it can look good on paper, but if it doesn’t rise you have to go back and try something new). Once they have a diet that looks good on paper and can be formed into a kibble or can or loaf, then they do testing on it for digestibility, palatability, etc...... Once they’ve done that, they do RSS testing to see if the diet performs as desired that way. If it fails RSS, they go back to the drawing board, adjust the nutrient levels, and try again. Having RSS doesn’t make the diet unnatural or unhealthy, it simply means it’s passed stringent testing to ensure it performs the way it should.
Urate formers generally have one of two issues: 1) an inborn error in protein (specifically purine) metabolism which leads to excess uric acid in the bladder, or 2) liver failure, which by a different pathway leads to the same thing.
These are metabolic stones which means the body is constantly trying to form them, and will do so regardless of the diet that is fed. A low purine diet decreases the amount uric acid formation as an end product, which decreases the precursor in the urine for stone formation. Combined with increased water intake to dilute the urine (and sometimes the drug allopurinol), this is the best approach to managing this disease. There are very few low purine diets available, but you’re right, they’re not all prescription. The question is, what makes the non-prescription diets higher quality and therefore a better choice than the prescription ones (I’m not saying they’re better or worse.... but if they are better (or worse), I want to know why)? (as an aside, urinary SO would be a completely inappropriate diet option for a urate or other metabolic stone former.... I don’t want to be giving the wrong impression here).