Cause: Crystallization of substances in urine.

Useful Supplements: Calcium, Chondroitin sulfate, Glucosamine Sulfate, Magnesium, Vitamin B6.

Supplements to Avoid: Vitamin C.

Further information: Kidney stones form when urine becomes highly concentrated and substances in it crystallize. These "stones" try to pass through the body and out the ureter with urine. However, as they move along, they can cause pain as they go through narrow passages (like the ureter).
Kidney stones are extremely common, affecting men twice as often as women. Recurrence is also common, especially when people have more than two kidney stone episodes.
Kidney stones may be influenced by hereditary factors, as well as Irritable Bowel Syndrome and renal tube defects.
Symptoms of kidney stones include back pain, difficulty urinating, nausea, blood in urine, groin pain, increased urinary urges, and fever.
In most cases, the pain associated with kidney stones is treated while waiting for the stone to pass. In some situations, the stone may be broken up using ultrasound treatment.
To prevent kidney stones, the best method is drinking large amounts of water daily. Six to eight glasses per day usually are enough to keep urine dilute and prevent crystallization. Kidney stone sufferers in hot climates should be especially certain to drink enough water.¹
Since calcium oxalate is responsible for most kidney stones, avoiding foods containing oxalate may be helpful.² Foods to be avoided include spinach, beet greens, nuts, chocolate, peanuts, bran, and strawberries.³
Caffeine should be avoided by anyone with kidney stones, as it appears to increase urinary calcium.⁴
Animal proteins are linked to increased kidney stones.⁵ A vegetarian diet will reduce the risk of stone formation.⁶
Salt increases urinary calcium excretion,⁷ and should be limited.
Both Potassium⁸ and Bran Fiber⁹ reduce urinary calcium levels. They are best taken in natural form, rather than as supplements.

The following supplements may be effective in preventing and treating kidney stones:

Calcium supplements cause oxalate to bind while in the stomach, before it can be absorbed into the urinary tract. This causes a decrease in urinary oxalate, which reduces the risk of kidney stones.¹⁰ So while the increased calcium in the urine may cause some kidney stones, overall higher calcium intake appears to lower kidney stone formation risk.¹¹ Calcium should not be taken with Vitamin D, as this can increase calcium absorption and increase the risk of stone formation.¹²

Both Chondroitin sulfate and Glucosamine Sulfate can help reduce urinary oxalate levels.¹³

Magnesium helps the body to convert oxalate into other substances. Supplementation with Magnesium can reduce urine levels of both calcium and oxalate.¹⁴

Vitamin B6, like Magnesium, is used to convert oxalate, and may be helpful in reducing the risk of kidney stone formation.¹⁵

Supplements to Avoid: Vitamin C should be avoided by those who suffer from kidney stones, as it can convert to oxalate.¹⁶ However, some research suggests that Vitamin C only undergoes this transformation in urine after the urine has left the body.¹⁷ In either case, it is best to speak to a health professional before supplementing with Vitamin C if there is any risk of kidney stones.

References:

¹Robertson WG, Peacock M, Heyburn PJ, Hanes FA. Epidemiological risk factors in calcium stone disease. Scand J Urol Nephrol Supplement 1980;53:15–30.
²Massey LK, Roman-Smith H, Sutton RAL. Effect of dietary oxalate and calcium on urinary oxalate and risk of formation of calcium oxalate kidney stones. J Am Dietet Assoc 1993; 93:901–6.
³Brinkley L, McGuire J, Gregory J, Pak CYC, et al. Bioavailability of oxalate in foods. Urol 1981;17:534.
⁴Hollingbery PW, Massey LK. Effect of dietary caffeine and sucrose on urinary calcium excretion in adolescents. Fed Proc 1986;45:375 (abstr #1280).
⁵Curhan GC, Willett WC, Rimm EB, Stampfer MJ. A Prospective study of dietary calcium and other nutrients and the risk of symptomatic kidney stones. N Engl J Med 1993;328:833–8.
⁶Robertson WG, Peacock M, Marshall DH. Prevalence of urinary stone disease in vegetarians. Eur Urol 1982;8:334–9.
⁷Silver J, Rubinger D, Friedlaender MM, Popovitzer MM. Sodium- dependent idiopathic hypercalciuria in renal-stone formers. Lancet 1983;ii:484–6.
⁸Curhan GC, Willett WC, Rimm EB, Stampfer MJ. A Prospective study of dietary calcium and other nutrients and the risk of symptomatic kidney stones. N Engl J Med 1993;328:833–8.
⁹Shah PJR. Unprocessed bran and its effect on urinary calcium excretion in idiopathic hypercalciuria. BMJ 1980;281:426.
¹⁰Marshall RW, Cochran M, Hodkginson A. Relationship between calcium and oxalic acid intake in the diet and their excretion in the urine of normal and renal-stone forming subjects. Clin Sci 1972;43:91–9.
¹¹Curhan GC, Willett WC, Rimm EB, Stampfer MJ. A Prospective study of dietary calcium and other nutrients and the risk of symptomatic kidney stones. N Engl J Med 1993;328:833–8.
¹²Rao PN, Blacklock NJ: Hypercalciuria. Lancet 1983:ii:747, [letter].
¹³Baggio B, Gambaro G, Marchini F, et al. Correction of erythrocyte abnormalities in idiopathic calcium-oxalate nephrolithiasis and reduction of urinary oxalate by oral glycosaminoglycans. Lancet 1991;338:403–5.
¹⁴Lindberg J, Harvey J, Pak CYC. Effect of magnesium citrate and magnesium oxide on the crystallization of calcium salts in urine: changes produced by food-magnesium interaction. J Urol 1990;143:248–51.
¹⁵Ettiniger B, Citron JT, Livermore B, Dolman LI. Chlorthalidone reduces calcium oxalate calculus recurrence but magnesium hydroxide does not. J Urol 1988;139:679–84.
¹⁶Piesse JW. Nutritional factors in calcium containing kidney stones with particular emphasis on vitamin C. Int Clin Nutr Rev 1985;5(3):110–129, [review].
¹⁷Wandzilak TR, D’Andre SD, Davis PA, Williams HE. Effect of high dose vitamin C on urinary oxalate levels. J Urol 1994;151:834–37.