Diet for Kidney Stones

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The latest research shows that the best diet for treating most patients with kidney stones is basically the same diet as that recommended to prevent osteoporosis, heart disease, hypertension, obesity, type 2 diabetes mellitus, many types of cancer, and stroke.

Kidney stones are responsible for about 1.3 million medical consults each year, costing about $2 billion annually in medical treatment.1 Most people who experience one episode of kidney stones will experience another, and many go on to have multiple recurrences. Aside from the extreme pain involved in passing a kidney stone, there is the danger of permanent damage to the kidney.

The most common stones are composed largely of calcium oxalate crystals. Calcium oxalate is poorly soluble in water (and urine). Dehydration, which results in greater concentration of the kidney distillate, increases the tendency for crystals of calcium salts to precipitate. In genetically susceptible people, these crystals can grow large enough to impair the flow of urine to the bladder.

Should Dietary Calcium Be Restricted?
Most patients with stones excrete more calcium in their urine than do non-stone formers. In the past, this observation led to the common recommendation of reducing dietary calcium intake in people with recurrent kidney stones. However, recent studies have shown that a low calcium intake is often associated with a greater risk of stone formation.2

A prospective study of more than 45,000 men found that those with a high dietary intake of calcium (more than 1000 mg/day) were 34% less likely to form kidney stones compared to those with a much lower calcium intake.3 In a 12-year prospective study of more than 90,000 nurses, this same group of researchers found that women who consumed more calcium from food also experienced a reduced risk of kidney stone formation. However, among women who took calcium supplements, there was about a 20% increased risk of kidney stones.4

It seems prudent to discourage the use of high-dose calcium supplements in most patients with recurrent kidney stones. Calcium-containing antacids should also be avoided; those containing magnesium (e.g. Maalox) should be used instead. However, there appears to be no reason to discourage the consumption of most calcium-rich foods. This is because calcium in foods prevents the absorption of oxalic acid. A calcium intake from foods of 800 to 1200 mg/day is probably best for most patients with a history of calcium oxalate kidney stones.

Should Dietary Oxalate Be Reduced?
Oxalic acid occurs naturally in many plant foods. Spinach, rhubarb, Swiss chard, cocoa, beets, sweet potatoes, strawberries, tea, okra, peanuts, pecans, wheat germ and bran contain sufficient oxalic acid to increase urinary oxalate excretion.5 Higher amounts of oxalic acid in the kidney distillate will increase stone formation. It is therefore prudent to encourage people with a history of recurrent kidney stones to limit their intake of oxalate-rich foods.

Urinary oxalic acid can also be derived from the breakdown of dietary protein as well as high doses of vitamin C.6 Vitamin C can be oxidized to oxalic acid in the human body. Large doses of vitamin C have been shown to increase oxalic acid excretion in the urine.7 Since there is no proven benefit to taking large doses of vitamin C, it seems reasonable to discourage Vitamin C supplements in patients with a history of kidney stones.

Should Dietary Protein Be Restricted?
Dietary protein is known to increase calcium excretion in the urine.8 9 The risk seems greater from animal than vegetable proteins.10 Lean meat, fish and poultry can also increase uric acid production and oxalic production, both of which promote kidney stones. Lean meat, fish or poultry should be limited to no more than about 3-4 oz daily for those trying to prevent stone formation.

Is It Necessary To Limit Salt Intake?
Dietary salt intake has long been known to increase calcium loss in the urine.11 12 13 14 Excessive salt intake increases the risk of kidney stones, osteoporosis, hypertension, stroke, kidney failure and heart disease.15 Because the dangers of excessive salt intake are many and the risk of reducing dietary salt intake are extremely uncommon, it seems wise to encourage all patients with a history of kidney stones to reduce their salt intake to no more than 1000 to 1500 mg sodium daily.16 The one caveat for patients greatly reducing their salt intake is to encourage them to keep their fluid high because they might not get as thirsty.

The Bottom Line:
Kidney stones result from various metabolic and nutritional factors in genetically susceptible individuals. To a large extent, the typical Western diet that is low in fiber, potassium, citrate, and magnesium and high in animal protein, salt and often supplemented with high doses of calcium and vitamin C promotes kidney stone formation. By contrast, a more vegetarian diet that is rich in potassium, magnesium, citrate, fiber and restricted in oxalic acid-rich foods and salt coupled with an increased fluid intake should prevent or at least greatly reduce the recurrences of kidney stones in most patients. This diet is also beneficial to prevent heart disease, type 2 diabetes mellitus, hypertension, stroke, osteoporosis and many cancers.

By Dr. James J. Kenney, PhD, RD, FACN.

1. National Institute of Diabetes & Kidney Diseases, 2001.
2. Am J Med 1968;45:700-14
3. N Engl J Med 1993;328:833-8
4. Ann Intern Med 1997;126:497
5. J Am Dietet Assoc 1993;93:901-6
6. J Critical Illness 2001;16:446-59
7. Miner Electrolyte Metab 1994;20:352-60
8. Am J Clin Nutr 1979;32:741-9
9. J Nutr 1981;111:545-52
10. Br J Urology 1979;51:427
11. Proc Soc Exp Biol Med 1964;115:29-32
12. Lancet 1967;1:1374-7
13. Kidney Int 1982;22:292-6
14. Lancet 1983;2:484-6
16. J Urology 1993;150:310-2

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