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Update on
Kidney Stones
Connie Crawley, MS,
RD, LD
Extension Nutrition & Health Specialist,
Department of Food and Nutrition
Kidney stones occur in
approximately 12% of the U.S. population. The incidence is rising in many
industrialized nations. Kidney stones are caused by infection, metabolic
disturbance, hormonal imbalance, poor fluid intake, immobilization, or lesions
or obstructions in the bladder or kidney that slow urine flow.
Kidney stones can be made of
uric acid, cystine, xanthine, magnesium phosphate, magnesium carbonate calcium
phosphate, calcium carbonate, or calcium oxalate. Approximately 85%-95% of
kidney stones contain calcium in some form and 70-80% are made of calcium
oxalate.
Since calcium is found in so
many stones, restricting calcium intake has been commonly recommended. But
recent studies have begun to question whether the general restriction of
calcium in stone formers is warranted.
There are four reasons routine
restriction of calcium has not been promoted:
1. Not all kidney stones are
made of calcium and not all sufferers have high calcium levels in their
urine;
2. Even when calcium levels
are high in urine, restricting calcium in the diet does not always lower it;
3. Studies have shown that
low levels of dietary calcium actually increase urinary levels of oxalate
and other common components of kidney stones;
4. Low serum calcium levels
stimulate loss of calcium from bone which may increase risk for
osteoporosis.
A recent study of male health
professionals found that individuals with the highest intakes of dietary
calcium actually had the lowest incidence of kidney stones. Researchers
believe calcium in the intestine combines with oxalate in the diet and
prevents it from being absorbed. Then both calcium and oxalate are excreted in
the feces instead of in the urine.
In this study, calcium
supplements did not seem to have a protective effect and even slightly
increased the risk for kidney stones. Researchers believe that supplements do
not help because they are usually taken only once a day, either between meals
or at breakfast, when few foods containing oxalate are consumed. Calcium helps
prevent kidney stones the most when it is consumed at least in small amounts
at every meal.
Reserch shows that higher
urinary oxalate levels increase risk for kidney stones more than higher levels
of urinary calcium. Only eight foods definitely raise urinary oxalate values:
spinach, tea, nuts including peanuts, chocolate, beets (the leaves and roots),
rhubarb, strawberries and wheat bran. Even prolonged restriction of these
foods is not advocated unless urinalysis shows that limiting them definitely
helps.
Some concern has also been
voiced about the effects of high intakes of ascorbic acid or vitamin C on
oxalate levels. Since ascorbic acid can be converted to oxalate, some
urologists warn against taking large doses of vitamin C. However, a recent
study published in the Journal of Urology showed that drinking a quart of
orange juice each day reduced the incidence of kidney stones in individuals
susceptible to them. The citrate contained in the juice lowered the risk for
stone formation. This is good news because the medication, potassium citrate,
often prescribed to help prevent stone recurrence, frequently causes stomach
upsets. Other juices like cranberry juice, grape juice, and other citrus
juices may also have the same positive effect.
Other dietary components may
also increase risk for kidney stones. A National Institutes of Health study
found that a high salt diet increases urinary calcium phosphate, the type of
calcium used to make stones, and decreased urinary citrate levels, the
substance that reduces risk.
A high animal protein diet also
increases risk by causing calcium to be lost from the bone and excreted into
the urine.
High fat diets are also
harmful. Fat binds with calcium in the intestines and prevents it from
combining with oxalate. More oxalate is then absorbed and excreted in the
urine increasing the risk for stone formation.
A final very important factor
is fluid intake. Drinking plenty of fluids keeps the urine dilute so stones
cannot form. Stone formers need at least two quarts of fluid a day to prevent
problems.
GENERAL RECOMMENDATIONS FOR
PREVENTION OF KIDNEY STONES
1. Drink at least 2 quarts of
fluid per day.
2. Drink a quart of orange,
grape, cranberry or grapefruit juice per day.
3. Limit high fat foods like
fried foods, butter, margarine, mayonnaise, whole fat dairy products and
high fat snack foods.
4. Eat meat, fish, and
poultry moderately.
5. Limit high oxalate foods
like spinach, tea, nuts, peanuts, chocolate, beets, beet greens, rhubards,
strawberries and wheat bran upon the advice of your doctor.
6. Only limit dairy products
and calcium-rich foods on the advice of your doctor. Usually two or three 8
ounce glasses of skim or low-fat milk or the equivalent in low fat or non-
fat cheese and yogurt are tolerated well. Eat calcium-rich foods throughout
the day, not just at one meal.
7. Avoid calcium supplements.
Sources:
1) Massey, L.D. et al. Effect of dietary oxalate and calcium on urinary oxalate
and risk of formation of calcium oxalate kidney stones, JADA, Vol 93, No. 8:
901-906, 1993.
2) Curham, G.C. et al. A
prospective study of dietary calcium and other nutrients and the risk of
symptomatic kidney stones. New Eng. J. Med. Vol 328 No. 12: 833-838, 1993.
3. Folkenberg, J. Orange juice
and a low-salt diet help prevent kidney stones. NIH Healthline, Oct./Nov., 1993.
4. Krause and Mahan, Food,
Nutrition and Diet Therapy, 1984.
Reprinted with permission from
the University of Georgia.
Crawley, C. (1995). Update on Kidney Stones. Athens, GA: University of
Georgia, Cooperative Extension Service.
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