Risk Factors
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Descriptions
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Calcium
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- Calcium stone formers
generally have a high urinary calcium.
- Calcium is the most abundant
mineral in kidney stones.
- More calcium is absorbed
from the upper
intestine while more oxalate is
available in the lower intestine.
- Reducing urine calcium should
be a goal for stone formers, but not via dietary
restriction.
- Counterintuitively,
calcium reduction may promote stone formation, and doctors
should encourage patients to continue consuming
calcium-rich foods.[17]
- A 5-year randomized clinical
trial of men with a history of calcium oxalate
stones found that a normal calcium, decreased
sodium, and decreased animal protein diet was more
effective for reducing stone events than was a
restricted calcium diet (Borghi et al., 2002).
- Clearly, strong research
evidence now supports adequate calcium intake for
patients who form kidney stones. Low-fat dairy
products, green leafy vegetables, broccoli, fortified foods,
and almonds are
excellent sources.
- Patients should avoid calcium
supplements in favor of calcium-rich foods (see [12]; the new research has
associated calcium supplements with an increased
kidney stone risk).
- Low-calcium diet is bad for
two reasons:
- Lower bone
density
- If you are
on a low-calcium diet, your bone problem is
going to get worse, particularly as you grow
older.
- There is
growing evidence to indicate that stone formers
as a group have lower bone densities than the
general population.
- Higher
urinary oxalate
- Calcium and oxalate bind
in the gut and in the urine to
form a nonabsorbable compound. Low dietary
calcium permits greater free oxalate to be
absorbed in the gut and excreted in the urine,
which may be counterproductive for calcium
oxalate stone formers.
- Ordinarily,
there is an excess of calcium over oxalate in
the first part of your intestine. Since calcium and oxalate combine
easily, most of the oxalate in
your diet unites with calcium in
the upper intestine. In essence, tiny calcium oxalate stones
form inside your bowl. Since the intestine is so
wide, these tiny stones make no differences.
There is, however, very little oxalate available
as your food moves to the last part of the
intestine, the colon. And it is in the colon
that most oxalate is
absorbed by the body.
- In other
words, with ordinary bowel function, only a very
small part of the ingested oxalate is
absorbed because most of it combines with calcium in
the first part of the intestine.
- In a recent
study, men over 40 on low-calcium diets made
more stones than men on medium-calcium diets.
The increased oxalate absorption may
have been a factor.
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Oxalate
|
Oxalate is
a component, with calcium,
of the majority of kidney stones. Oxalate is
present in more than 75% of all stones. What most
people with kidney stones don't know is that oxalic
acid, like calcium, performs a vital function. 98
% of the oxalic acid in the body is produced internally
and is used for moving food through the digestive tract
by peristalsis. Oxalic acid also aids in the
absorption of calcium into the cells[7].
Based on [7], leftover oxalic acid, along with excess
calcium, is removed from the blood by the kidneys and
passes into the urine. Calcium oxalate permeates
the urine generally, but only in those people whose
urine is overloaded with acid waste does it form
stones. [7] also says that eating greens high in
oxalates in moderate amount should be OK if they are
raw.
- Calcium oxalate is
a very insoluble salt.
- Though only 10%
to 20% of urinary oxalates come from dietary sources
(Morton, Iliescu, & Wilson, 2002), dietary
reduction is commonly advised for calcium oxalate
stone formers.
- It has been
suggested that because there is much less oxalate in
the urine than calcium in the urine, urinary oxalate
concentration is much more critical to the formation
of calcium oxalate crystals than is the urinary
calcium concentration; reducing urine oxalates may
have a more powerful effect on stone formation than
can reduction of urine calcium (Morton et al.,
2002).
- Oxalate is
found only in plant foods, and the highest
concentration are in dark green, leafy vegetables
such as spinach,
and in rhubarb, chocolate, tea, okra, nuts, beans, beets, wheat bran,
and strawberries.
A thorough oxalate list can be found on the Web site
of the Oxalosis and Hyperoxaluria Foundation (http://www.ohf.org/diet.html).
For a simple list of high-oxalate foods, visit http://www.gicare.com/pated/edtgs29.htm.
- Because it
binds vital nutrients such as calcium,
long-term consumption of foods high in oxalic acid can
be problematic. Healthy individuals can safely
consume such foods in moderation, but those
with kidney
disorders, gout, rheumatoid arthritis,
or certain forms of chronic vulvar pain (vulvodynia) are
typically advised to avoid foods high in oxalic acid or oxalates.
- There are four
sources of the oxalate that appears in the urine.
- large amount
of protein
- excess
amounts of vitamin C
- Excess
amounts of vitamin C can be excreted in the
urine as oxalate.
- Vitamin C
is an acid (ascorbic acid). It tends to lower
urinary pH, which in turn lowers urinary
citrate, a natural inhibitor of kidney stones.
- Recent data
suggest that even small amounts of vitamin C
supplements-doses of 250 mg to 500 mg-can raise
urinary oxalate in some patients.
- waste
products of general metabolism
- seven foods that greatly enhance urinary oxalate excretion:
- spinach
- In various food tests, spinach seems to produce the greatest rise in urinary oxalate.
- rhubarb
- beets
- nuts
- chocolate
- tea
- One patient who started to keep a large pitcher of iced tea on his desk at work. He was consuming 4 to 6 large glasses a day. He formed his first stone 2 months later.
- For those
with excessive urinary oxalates, black and
iced tea should be eliminated because both
teas are high-oxalate beverages [5&8].
- wheat bran
- However, the
"bioavailability" of the oxalate may be different in
various foods.
- In other
words, some oxalate-containing foods are more
problematic than others.
- Each person
absorbs and excretes various dietary elements
differently.
- The problem
with oxalate recommendations
- Oxalate is
difficulty to measure in both food and body
fluids.
- The oxalate
content of food can vary greatly from one batch of
a given food to another.
- It is
believed that aging increases the amount of
oxalate in plants.
- Although oxalate has
no known function in animals, it is believed to
help plants dispose of calcium. Animals have
kidneys that excrete excess calcium. Plants do
not. The theory is that the oxalate binds to the
excess calcium, trapping it in the leaves, bark,
and skin. As older leaves are shed, the plant
disposes of unwanted calcium. As the plant gets
older, the oxalate content tends to increase.
- The
bioavailability of oxalate differs in foods.
- Strawberries and spinach are
both high-oxalate foods. However, the oxalate
in spinach is
more bioavailable than it is in strawberries.
- If oxalate
is present as oxalic acid (the form found in
younger plants), it is more bioavailable. If it
is present in food as a calcium oxalate salt
(the form found in older plants), it is less
bioavailable in the intestines, and absorption
decreases.
- Key issues in oxalate
control (note that reduction of high
oxalate foods is the goal for typical stone formers
rather than strict avoidance of all
oxalate-containing foods which would be very
difficult):
- control
portion size
- control fat
intake
- In people
prone to the overabsorption of oxalate, a
high-fat diet will cause more oxalate to be
absorbed.
- avoid vitamin
c supplements
- Dr. Rodman
advises his stone-forming patients not to take
vitamin C supplements beyond what is in a
balanced diet.
- watch for
hidden oxalates
- the peanut
oil used to cook in most Chinese food.
- drink two
large glasses of water-or more when you know you
have overindulged.
|
High Blood Uric Acid
(See Also High Animal Protein Intake)
|
- Saturation
levels of uric acid in blood may result in uric acid
stones.
- There is no
inhibitor of uric acid crystal formation (Menon
& Resnick, 2002), so dietary measures focus on
reducing uric acid and increasing urine volume.
- Some patients
with gout eventually
get uric kidney stones.
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Acid Urine
|
- If the urine is
acid most of all of the time, you are at risk for
making uric
acid stones.
- An acid urine
also decreases citrate formation (the
natural inhibitor of stones) by the kidneys, making
a calcium
oxalate stone more likely.
- Normal urinary
pH levels range between 5.0 and a little over 8.0.
This varies with the time of day, food consumption,
age, and other factors.
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Lack of Fluids (or
Dehydration)
|
- Less urinary
flow (due to less fluid intake) increases the
relative frequency of stone formation.
- Addressing
fluid intake is the only therapeutic recommendation
needed for some female stone formers. It is also one
of the most important aspects of treating urinary
tract infections in women.
- Dilution of
urine is necessary "24/7," or all day, every day.
- Patients must
accept the necessity of getting up at least twice at
night to urinate, and should consume more water each
time they rise to void [5].
- What fluids to
drink? For stone formers, you should drink more
water and avoid excess caffeine, black tea,
and grapefruit and apple juices. Lemonade is
often recommended, as it supplies dietary citrate, a
stone inhibitor and pH buffer when excreted later in
the urine. [5].
|
High Animal Protein
Intake |
- After low urinary volume,
a high
animal protein intake is the most
important factor influencing the frequency of kidney
stone disease.
- The effect
of excess
animal protein (purine=>uric acid)
is most obvious for the uric acid stone former.
- High dietary
protein is associated with increased urinary
calcium. Thus, there is a link between meat
consumption and both uric acid and calcium
stone formation.
- Too much
protein can cause dehydration, which is a cause of
kidney stones[6]
- For every 50
grams of protein digested, your body must
contribute 3 cups of water to dilute the resulting
uric acid that is excreted.
- In
comparison, metabolizing an equal amount of fat or
carbohydrates consumes less than 1/2 cup of water.
- A study of 18
hypercalciuric stone formers found that a 15-day
protein restriction had many positive effects on
urinary markers of stone risk. Namely, significant
decreases were seen in urine calcium, urine uric
acid, urine phosphate, and urine oxalate. And, for
unclear reasons, a beneficial increase in urinary
citrate was observed (Giannini et al., 1999).
- Diets high in
animal protein will increase the chance of forming
both uric
acid and calcium oxalate stones for 3
related but slightly different reasons:
- Animal
protein contains high purines, whose end product
is uric
acid. More uric acid in the urine means
that you are more likely to make a uric acid
stone. In addition, one form of uric acid, urate,
can act as a seed for calcium
oxalate stones.
- The excessive
consumption of animal protein will lower the pH of
urine, making it more acid. This lower urinary pH
makes uric acid less soluble and thereby favors
the formation of uric acid stones. It also
suppresses citrate formation
by the kidney, which reduces one the body's
natural inhibitors of calcium
oxalate formation.
- One of the
amino acids in all proteins, glycine, is
metabolized to oxalate. Another amino
acid, methionine,
drives out more calcium in the urine. A diet
higher in protein will therefore increase urinary
calcium even though the amount of calcium in the
diet remains constant.
|
Alcohol
|
By itself, alcohol does not contribute to stone
formation. However, it does have two effects that may
set the stage for stone formation.
- alcohol makes
you pass more urine and can lead to dehydration.
- alcohol
indirectly inhibits the ability of the kidneys to
excrete uric acid.
|
Sodium
|
By itself, the sodium in your
diet has only a minimal effect on the tendency of the
urine to make stones. However, an increase in dietary
sodium increases the
urinary calcium.
|
Late dinner |
- Late-night
eating is conducive to the formation of kidney
stones.
- The tendency of
more families to follow late eating habit in the
1980s and 1990s may be one reason for the rising
incidence of stone disease.
|
NSAIDs and
Acetaminophen |
Taken in large quantities over
long periods of time (no longer than 10 days, Dr.
Oz has warned), analgesics such as NSAIDs and acetaminophen can
cause damage to the papillae of the kidney. Injured
papillae can seed a stone as well as complicate the
stone-forming process. |
Diamox or Acetazolamide |
Some people with glaucoma form
kidney stones while they are on carbonic anhydrase
inhibitors (e.g., Diamox or acetazolamide)
to lower eye pressure.
- These medicines
can lower urinary citrate and increase urinary
calcium.
- Patients can
take potassium citrate supplements under doctor's
supervision while the acetazolamide is continued,
to minimize the tendency to make stones.
|
Rapid Weight Loss |
Rapid weight loss is as bad as, or
potentially worse than, overeating protein when it comes
to causing stone disease.
- Doctors
frequently give potassium bicarbonate and potassium citrate supplements
to stone formers who are beginning a weight loss
diet.
- The best way to
lose weight is to combine a nutritionally balanced
diet with exercise and behavior modifications.
|
Hot Climates
|
Kidney stones often occur in
people who move to hot, dry climates and in people whose
work or exercise causes them to perspire heavily. |
Fat Malabsorption |
Fat malabsorption can lead to an increased net oxalate
absorption and, finally, to increased urinary oxalate.
|
Problems with Voiding |
Problems with voiding make bladder stones more likely.
- The most common
cause of bladder outflow problems is enlargement of
the prostate gland in the older male.
|
Pregnancy
|
Pregnancy can
increase stone risk due to the following factors:
- Her diminished
bladder capacity
- As the fetus
grows, there is less room in the pelvis for a full
bladder.
- The pregnant
woman has to provide enough calcium in her blood to
form a whole new skeleton. Urinary calcium will rise
to its highest levels in the last trimester.
Therefore, stone formation can result from this low
urine flow and high urinary calcium.
|
Genetic Predisposition |
If one or both of your parents made stones, there is a
greater chance that you will make stones.
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Men vs. Women
|
Stone disease is much more common in men than in women
because of the following factors:
- Men generally
have a larger muscle mass than women. Hence they
have more of the daily breakdown and rebuilding of
tissue that results in metabolic waste. And an
increase in metabolic waste predispose people to
stone formation.
- Men generally
eat more meat than women do.
- The male
urinary tract is more complicated than the female
urinary tract.
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