Kidney Stones, Calcium, Oxalates & Uric Acid

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Kidney Stones, Calcium, Oxalates & Uric Acid

JoAnn Guest

Nov 02, 2006 09:10 PST

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Nutritional steps important for preventing stones

 

About 12 percent of Americans develop a kidney stone at some point

intheir lives. Stones usually result from the crystallization of

calcium(which originally came in foods or supplements) and oxalate,

a

part of many plant foods.

Some people have a tendency to lose excessive amounts of calcium or

oxalate through their kidneys, and they have a greater

likelihood of a stone. Kidney stones can also form from uric

acid, which is a breakdown product of protein, or from struvite

(ammoniomagnesium phosphate) or cystine.

 

The prevalence of kidney stones is three times higher in men than

women,and is higher among Caucasians than Asians or African

Americans,

for reasons that are not clear. They are especially likely to strike

between the ages of 40 and 60.

 

Nutritional steps are important in preventing stones and can also

help prevent recurrences, which is important given that 30-50

percent of

people diagnosed with a renal stone have a recurrence within five

years.

 

 

Preventing stones is like keeping a salt crystal from forming in a

glass of salty water. You can either reduce the concentration of

salt or

add more water.

 

Epidemiologic studies have shown that certain parts of the diet help

" reduce the amount " of calcium that filters into the urine.

It is a simple matter to put these factors to work clinically.

 

WHAT'S IN A STONE?

 

Calcium oxalate 72%

Uric acid 23%

Ammoniomagnesium phosphate (struvite) 5%

Cystine <1%

 

Protective Foods

Certain parts of the diet clearly help reduce the risk. The first is

no surprise.

 

Water. Water dilutes the urine and keeps calcium, oxalates, and uric

acid in solution. In research studies, those subjects whose total

fluidintake (from all sources) over 24 hours was roughly 2.5 liters,

therisk of a stone was about one-third less than that of subjects

drinking only half that much.7 (They do not need to drink 2.5 liters

of

water per day; rather this is the total fluid consumption, including

juices, soups,etc.) Patients need to understand that their thirst

sense

can lag behind their hydration status, and they may need to develop

a

routine for extra water consumption.

 

High-Potassium Foods. A study of 46,000 men conducted by Harvard

University researchers found that a high potassium intake can cut

the

risk of kidney stones in half. Potassium helps the kidneys retain

calcium, rather than sending it out into the urine. Potassium

supplements are not generally necessary. Rather, a diet including

regular servings of fruits, vegetables, and beans supplies plenty of

potassium.

 

Calcium. Although most stones contain calcium, the calcium in foods

does not necessarily contribute to stones. Calcium supplements taken

between meals may increase the risk of stones, because about 8

percent

of any extra dietary calcium passes into the urine. On the other

hand,

calcium consumed with meals has the opposite effect, reducing the

risk

of stones.

 

The reason, apparently, is that calcium binds to oxalates in foods

and

holds them in the digestive tract, rather than allowing

them to be absorbed.

 

Problem Foods

 

Animal Protein. Animal proteins cause calcium to be leached from the

bones and excreted in the urine where it can form stones. Diets rich

inanimal proteins also increase uric acid excretion. In a controlled

research study, published in the American Journal of Clinical

Nutrition,research subjects on a diet eliminating animal protein had

less thanhalf the calcium loss that they had on their baseline diet.

 

The Harvard study mentioned earlier found that even a modest

increase in animal protein, from less than 50 grams to 77 grams per

day,

wasassociated with a 33 percent increased risk of stones in men. The

same is true for women. The Nurses' Health Study, a long-term study

of

health

factors in a large group of women, revealed an even greater risk of

stones from animal protein than was found in previous studies in

men.

 

The association between animal proteins and stones probably relates

both to the amount of protein they contain and to their content of

the

sulfur-containing amino acids.

 

In particular, the sulfur in cystine and methionine is converted to

sulfate, which tends to acidify the blood. As

a part of the process of neutralizing this acid, bone is dissolved,

and bone calcium ends up in the urine.

 

Meat and eggs contain two to five times more of these sulfur-

containing

amino acids than are found in grains and beans.

 

Between 1958 and the late 1960s, there was a sharp increase in the

incidence of kidney stones in Great Britain. During that period,

there was no substantial change in the amount of calcium or oxalate-

containing foods consumed.

 

However, the consumption of vegetables decreased,and

the use of poultry, fish, and red meat increased. Statistical

analysesshowed a strong relationship between the incidence of stones

andanimal protein consumption.

 

Sodium. Sodium increases the passage of calcium through the kidney

andincreases the risk of stones. When people cut their salt (sodium

chloride) intake in half, they reduce their daily need for calcium

by about 160 milligrams.15

 

Plants of any kind—grains, vegetables, legumes, and fruits—contain

almost no sodium at all unless it is added during canning or other

processing.

 

Dairy products and meats contain more salt than plant

products, and table salt, frozen meals, and canned and snack foods

arethe highest-sodium food products.

 

Sugar. Sugar accelerates calcium losses through the kidney. In the

Nurses' Health Study, those who consumed, on average, 60 grams or

more of sugar (sucrose) per day had a 50 percent higher risk of

stones

than those who consumed only about 20 grams.9

 

Climate. Kidney stones are also more common in warm climates,

presumablybecause perspiration leads to dehydration and a more

concentratedurine,and because sunlight increases the production of

vitamin D in the skin which, in turn, increases calcium absorption

from

the digestive tract.

 

 

Surprisingly, oxalate-rich foods, such as nuts, tea, and

spinach, are not associated with a higher risk of renal stones, nor

is vitamin C, even though it can be " converted " to oxalate.

A large study of

men taking vitamin C supplements found that they had no more kidney

stones than men who do not take them.

 

Helping Patients Avoid Kidney Stones

Here are simple steps to help you avoid kidney stones.

Drink plenty of water or other fluids, staying

ahead of their thirst.

 

Diets including generous amounts of vegetables, fruits, and beans

arerich in potassium and very low in sodium.

If you prescribe calcium supplements, encourage patients to take

them with meals, rather than between meals.

 

Encourage patients to avoid animal products. Their proteins and

sodiumcontent increase the risk of stones.

Patients should keep salt and sugar use modest.

 

 

 

References

1. Mahan LK, Arlin M. Krause's Food, Nutrition, and Diet Therapy.

W.B.

Saunders, Philadelphia, 1992.

2. The American Dietetic Association. Handbook of Clinical

Dietetics,

second edition. Yale University Press, 1992.

3. D'Amico G, Gentile MG, Manna G, et al. Effect of vegetarian soy

diet

on hyperlipidemia in nephrotic syndrome. Lancet 1992;339:1131-4.

4. The American Dietetic Association. Manual of Clinical Dietetics,

fifth edition. American Dietetic Association, Chicago, 1996.

5. Gretz N, Meisinger M, Strauch M. Does a low protein diet really

slow

down the rate of progression of chronic renal failure? Blood Purif

1989;7:33:33-8.

6. Barsotti G, Morelli E, Cupisti A, Meola M, Dani L, Giovannetti S.

A

low-nitrogen, low-phosphorus vegan diet for patients with chronic

renal

failure. Nephron 1996;74:390-4.

7. Curhan 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.

8. Curhan GC, Willett WC, Rimm EB, Spiegelman D, Stampfer MJ.

Prospective study of beverage use and the risk of kidney stones. Am

J

Epidemiol 1996;143:240-7.

9. Curhan GC, Willett WC, Speizer FE, Spiegelman D, Stampfer MJ.

Comparison of dietary calcium with supplemental calcium and other

nutrients as factors affecting the risk for kidney stones in women.

Ann

Int Med 1997;126:497-504.

10. Soucie JM, Thun MJ, Coates RJ, McClellan W, Austin H.

Demographic

and geographic variability of kidney stones in the United States.

Kidney

Int 1994;46:893-9.

11. Lemann J. Composition of the diet and calcium kidney stones. N

Engl

J Med 1993;328:880-2.

12. Remer T, Manz F. Estimation of the renal net acid excretion by

adults consuming diets containing variable amounts of protein. Am J

Clin

Nutr 1994;59:1356-61.

13. Breslau NA, Brinkley L, Hill KD, Pak CYC. Relationship of animal

protein-rich diet to kidney stone formation and calcium metabolism.

J

Clin Endocrinol 1988;66:140-6.

14. Robertson WG, Peacock M, Hodgkinson A. Dietary changes and the

incidence of urinary calculi in the U.K. between 1958 and 1976. J

Chron

Dis 1979;32:469-76.

15. Nordin BEC, Need AG, Morris HA, Horowitz M. The nature and

significance of the relationship between urinary sodium and urinary

calcium in women. J Nutr 1993;123:1615-22.

16. Lemann J Jr, Adams ND, Gray RW. Urinary calcium excretion in

human

beings. N Engl J Med 1979;301:535-41.

17. Soucie JM, Coates RJ, McClellan W, Austin H, Thun MJ. Relation

between geographic variability in kidney stones prevalence and risk

factors for stones. Am J Epidemiol 1996;143:487-95.

18. Avorn J, Monane M, Gurwitz JH, Glynn RJ, Choodnovskiy I, Lipsitz

LA.

Reduction of bacteriuria and pyuria after ingestion of cranberry

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JAMA 1994;271:751-4.

 

JoAnn Guest

mrsjo-

www.geocities.com/mrsjoguest/Diets