Dietary changes that may be helpful
Increasing dietary oxalate can lead to an increase in urinary oxalate excretion. Increased
urinary oxalate increases the risk of stone formation. As a result, most doctors agree that
kidney stone formers should reduce their intake of oxalate from food as a way to reduce
urinary oxalate.2 Many foods contain oxalate; however, only a few—spinach,
rhubarb, beet greens, nuts,
chocolate, tea, bran, almonds,
peanuts, and strawberries—appear to
significantly increase urinary oxalate levels.3 4
Increased levels of urinary calcium also increases the risk of stone formation. Consumption
of animal protein from meat, dairy,
poultry, or fish increases urinary
calcium. Perhaps for this reason, consumption of animal protein has been linked to an
increased risk of forming stones5 6 7 8 and vegetarians have been reported to be at lower
risk for stone formation in some reports.9 As a result, many researchers and some
doctors believe that people with a history of kidney stone formation should restrict intake of
animal foods high in protein.
In one controlled trial, contrary to expectations, after 4.5 years of follow-up, those who
restricted their dietary protein actually had an increased risk of forming a kidney
stone, compared with the control group.10 The findings of this trial conflict with
the outcomes of most preliminary studies,11 12 and need to be confirmed
by further clinical trials.13 Other researchers have found that a low-protein diet
reduces the risk of forming stones.14 15 Although
high-protein diets should probably be avoided by people with kidney stones, the effect of
restricting dietary protein to low levels (below the RDA level of 0.8 grams per 2.2 pounds of
body weight per day) remains unclear. Until more is known, it makes sense to consume a diet
with a moderate amount of protein, perhaps partially limiting animal protein, but not limiting
protein from vegetarian sources, such as nuts
and beans.
Salt increases urinary calcium excretion in stone formers.16 17
18 In theory, this should increase the risk of forming a stone. As a result, some
researchers have suggested that reducing dietary
salt may be a useful way to decrease the chance of forming additional stones.19
20 Increasing dietary salt has also affected a variety of other risk factors in
ways that suggest an increased chance of kidney stone formation.21 Some doctors
recommend that people with a history of kidney stones reduce salt intake. To what extent such
a dietary change would reduce the risk of stone recurrence remains unclear.
Potassium reduces urinary calcium
excretion,22 and people who eat high amounts of dietary potassium appear to be at
low risk of forming kidney stones.23 Most kidney stone research involving potassium
supplementation uses the form potassium citrate. When a group of stone formers was given 5
grams of potassium citrate three times daily in addition to their regular drug treatment for
28 months, they had a significantly lower rate of stone recurrence compared to those taking
potassium for only eight months and to those taking no potassium at all.24 Although
citrate itself may lower the risk of stone recurrence (see below), in some potassium research,
a significant decrease in urinary calcium occurs even in the absence of added
citrate.25 This finding suggests that increasing potassium itself may reduce the
risk of kidney stone recurrence. The best way to increase potassium is to eat fruits and vegetables. The level of potassium in food is much
higher than the small amounts found in supplements.
Some citrate research conducted with people who have a history of kidney stones involves
supplementation with a combination of potassium
citrate and magnesium citrate. In one
double-blind trial, the recurrence rate of kidney stones dropped from 64% to 13% for those
receiving high amounts of both supplements.26 In that trial, people were instructed
to take six pills per day—enough potassium citrate to provide 1,600 mg of potassium and
enough magnesium citrate to provide 500 mg of magnesium. Both placebo and citrate groups were
also advised to restrict salt, sugar, animal
protein, and foods rich in oxalate. Other trials have also shown that potassium and magnesium
citrate supplementation reduces kidney stone recurrences.27
Citric acid (citrate) is found in many foods and may also protect against kidney stone
formation.28 29 The best food source commonly available is citrus
fruits, particularly lemons. One preliminary
trial found that drinking 2 liters (approximately 2 quarts) of lemonade per day improved the
quality of the urine in ways that are associated with kidney stone prevention.30
Lemonade was far more effective in modifying these urinary parameters than orange juice. The lemonade was made by mixing 4 oz
lemon juice with enough water to make 2
liters. The smallest amount of sweetener
possible should be added to make the taste acceptable. Further study is necessary to determine
if lemonade can prevent recurrence of kidney stones.
Drinking grapefruit juice has been linked
to an increased risk of kidney stones in two large studies.31 32 Whether
grapefruit juice actually causes kidney stone recurrence or is merely associated with
something else that increases risks remains unclear; some doctors suggest that people with a
history of stones should restrict grapefruit juice intake until more is known.
Bran, a rich source of insoluble fiber,
reduces the absorption of calcium, which in
turn causes urinary calcium to fall.33 In one trial, risk of forming kidney stones
was significantly reduced simply by adding one-half ounce of rice bran per day to the diet.34 Oat and wheat bran are also good sources of insoluble
fiber and are available in natural food stores and supermarkets. Before supplementing with
bran, however, people should check with a doctor, because some people—even a few with
kidney stones—don’t absorb enough calcium. For those people, supplementing with
bran might deprive them of much-needed calcium.
People who form kidney stones have been reported to process sugar abnormally.35 Sugar has also been
reported to increase urinary oxalate,36 and in some reports, urinary calcium as
well.37 As a result, some doctors recommend that people who form stones avoid
sugar.38 39 To what extent, if any, such a dietary change decreased the
risk of stone recurrence has not been studied and remains unclear.
Drinking water increases the volume of
urine. In the process, substances that form kidney stones are diluted, reducing the risk of
kidney stone recurrence. For this reason, people with a history of kidney stones should drink
at least two quarts per day. It is particularly important that people in hot climates increase
their fluid intake to reduce their risk.40
Drinking coffee or other caffeine-containing beverages increases urinary
calcium.41 Long-term caffeine consumers are reported to have an increased risk of
osteoporosis,42 suggesting that the
increase in urinary calcium caused by caffeine consumption may be significant. However, coffee
consists mostly of water, and increasing water consumption is known to reduce the risk of
forming a kidney stone. While many doctors are concerned about the possible negative effects
of caffeine consumption in people with a history of kidney stones, preliminary studies in both
men43 44 and women45 have found that coffee and tea consumption is actually associated with a
reduced risk of forming a kidney stone. These reports suggest that the helpful effect of
consuming more water by drinking coffee or tea may compensate for the theoretically harmful
effect that caffeine has in elevating urinary calcium. Therefore, the bulk of current research
suggests that it is not important for kidney stone formers to avoid coffee and tea.
The findings of some46 47 but not all48 studies suggest
that consumption of soft drinks may increase
the risk of forming a kidney stone. The phosphoric acid found in these beverages is thought to
affect calcium metabolism in ways that might increase kidney stone recurrence risk.
Vitamins that may be helpful
IP-6 (inositol hexaphosphate, also called
phytic acid) reduces urinary calcium levels and may reduce the risk of forming a kidney
stone.49 In one trial, 120 mg per day of IP-6 for 15 days significantly reduced the
formation of calcium oxalate crystals in the urine of people with a history of kidney stone
formation.50
In the past, doctors have sometimes recommended that people with a history of kidney stones
restrict calcium intake because a higher
calcium intake increases the amount of calcium in urine. However, calcium (from supplements or
food) binds to oxalate in the gut before either can be absorbed, thus interfering with the
absorption of oxalate. When oxalate is not absorbed, it cannot be excreted in urine. The
resulting decrease in urinary oxalate actually reduces the risk of stone
formation,51 and the reduction in urinary oxalate appears to outweigh the increase
in urinary calcium.52 In clinical studies, people who consumed more calcium in the
diet were reported to have a lower risk of forming kidney stones than people who consume less
calcium.53 54 55
However, while dietary calcium has been linked to reduction in the risk of forming
stones, calcium supplements have been associated with an increased risk in a large
study of American nurses.56 The researchers who conducted this trial speculate that
the difference in effects between dietary and supplemental calcium resulted from differences
in timing of calcium consumption. Dietary calcium is eaten with food, and so it can then block
absorption of oxalates that may be present at the same meal. In the study of American nurses,
however, most supplemental calcium was consumed apart from food.57 Calcium
taken without food will increase urinary calcium, thus increasing the risk of forming stones;
but calcium taken without food cannot reduce the absorption of oxalate from food consumed at a
different time. For this reason, these researchers speculate that calcium supplements were
linked to increased risk because they were taken between meals. Thus, calcium
supplements may be beneficial for many stone formers, as dietary calcium appears to be, but
only if taken with meals.
When doctors recommend calcium supplements to stone formers, they often suggest 800 mg per
day in the form of calcium citrate or calcium citrate malate, taken with meals. Citrate helps
reduce the risk of forming a stone (see “Dietary changes that may be helpful”
above).58 Calcium citrate has been shown to increase urinary citrate in stone
formers, which may act as protection against an increase in urinary calcium resulting from
absorption of calcium from the supplement.59
Despite the fact that calcium supplementation taken with meals may be helpful for some,
people with a history of kidney stone formation should not take calcium supplements without
the supervision of a healthcare professional. Although the increase in urinary calcium caused
by calcium supplements can be mild or even temporary,60 some stone formers show a
potentially dangerous increase in urinary calcium following calcium supplementation; this may,
in turn, increase the risk of stone formation.61 People who are
“hyperabsorbers” of calcium should not take supplemental calcium until more is
known. Using a protocol established years ago in the Journal of Urology, 24-hour
urinary calcium studies conducted both with and without calcium supplementation determine
which stone formers are calcium “hyperabsorbers.”62 Any healthcare
practitioner can order this simple test.
Increased blood levels of vitamin D are found in some kidney stone formers, according to
some,63 but not all, research.64 Until more is known, kidney stone
formers should take vitamin D supplements only
after consulting a doctor.65
Both magnesium and vitamin B6 are used by the body to convert oxalate
into other substances. Vitamin B6 deficiency leads to an increase in kidney stones as a result
of elevated urinary oxalate.66 Vitamin B6 is also known to reduce elevated urinary
oxalate in some stone formers who are not necessarily B6 deficient.67
68
Years ago, the Merck Manual recommended 100–200 mg of vitamin B6 and 200 mg
of magnesium per day for some kidney stone formers with elevated urinary oxalate.69
Most trials have shown that supplementing with magnesium70 71
72 and/or vitamin B673 74 significantly lowers the risk of forming
kidney stones. Results have varied from only a slight reduction in recurrences75 to
a greater than 90% decrease in recurrences.76
Optimal supplemental levels of vitamin B6 and magnesium for people with kidney stones
remain unknown. Some doctors advise 200–400 mg per day of magnesium. While the effective
intake of vitamin B6 appears to be as low as 10–50 mg per day, certain people with
elevated urinary oxalate may require much higher amounts, and therefore require medical
supervision. In some cases, as much as 1,000 mg of vitamin B6 per day (a potentially toxic
level) has been used successfully.77
Doctors who do advocate use of magnesium for people with a history of stone formation
generally suggest the use of magnesium citrate because citrate itself reduces kidney stone
recurrences. As with calcium supplementation, it appears important to take magnesium with
meals in order for it to reduce kidney stone risks by lowering urinary
oxalate.78
It has been suggested that people who form kidney stones should avoid vitamin C supplements, because vitamin C can convert
into oxalate and increase urinary oxalate.79 80 Initially, these
concerns were questioned because the vitamin C was converted to oxalate after urine
had left the body.81 82 However, newer trials have shown that as little
as 1 gram of vitamin C per day can increase urinary oxalate levels in some people, even those
without a history of kidney stones.83 84 In one case report, a young man
who ingested 8 grams per day of vitamin C had a dramatic increase in urinary oxalate
excretion, resulting in calcium-oxalate crystal formation and blood in the urine.85
On the other hand, in preliminary studies performed on large populations, high intake of
vitamin C was associated with no change in the risk of forming a kidney stone in
women,86 and with a reduced risk in men.87 This research suggests that
routine restriction of vitamin C to prevent stone formation is unwarranted. However, until
more is known, people with a history of kidney stones should consult a doctor before taking
large amounts (1 gram or more per day) of supplemental vitamin C.
Chondroitin sulfate may play a role in
reducing the risk of kidney stone formation. One trial found 60 mg per day of
glycosamionoglycans significantly lowered urinary oxalate levels in stone
formers.88 Chondroitin sulfate is a type of glycosaminoglycan. A decrease in
urinary oxalate levels should reduce the risk of stone formation.
In a double-blind trial, supplementation with 200 IU of synthetic vitamin E per day was found to reduce several risk
factors for kidney stone formation in people with elevated levels of urinary
oxalate.89
Are there any side effects or interactions?
Refer to the individual supplement for information about any side effects or interactions.
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