The pain caused by a kidney stone has been described as one of the worst that humans can suffer. Certainly those who have experienced one will never forget it. Unfortunately the prevalence of kidney stones is on a dramatic rise with stone presence doubling since the 1970s (4). One in ten Canadians will have a kidney stone at some point in their life (1,2). Once you have had one kidney stone, the risk of another occurring within five to seven years is about 50% (3).
A kidney stone, a hard solid mass of mineral crystals, usually forms in the kidneys but can also develop in the bladder or in the ureter (the small tube that connects the kidney with the bladder). They vary in size from that of a grain of sand to as large as a golf ball. Large kidney stones can sit quietly in the kidney causing only a dull ache. Smaller ones, those that are 4 or 5 mm in diameter or less, can pass through the urinary tract without much problem. In-between sized stones can get stuck in the urinary tract. The amount of discomfort seems to be unrelated to stone size and the smallest of stones can be the most agonizing. Thankfully pain ends once the stone has been expelled from the body (2,3).
The urine commonly contains substances such as calcium, oxalate and phosphate. If these become highly concentrated they precipitate out into crystals that can stick together and form kidney stones. The vast majority (90%) of kidney stones are calcium based with calcium oxalate stones the most common (56 to 61%). Other types are calcium phosphate (8 to 18%), uric acid (8 to 18%), struvite (2 to 4%) and cysteine (1%) (5).
Symptoms of kidney stones (1,2)
Severe pain that starts suddenly in the small of the back under the ribs or in the lower abdomen. This pain may move to the groin and may last for a few minutes or hours, followed by periods of relief
Blood in the urine
If you also have a urinary tract infection you may experience burning during urination, the urge to urinate frequently, cloudy or foul-smelling urine, fever, chills and weakness
Factors that contribute to kidney stone formation (2)
Being male. Kidney stones are more common in men than in women.
Being of middle age.
Having a family history of kidney stones which can increase the risk two and a half fold
Being overweight or obese
Living in a hot climate
Drinking too little fluid
Consuming high doses of Vitamin C supplements
Blockage of the urinary tract
Recurrent urinary tract infections
Certain metabolic diseases such as insulin resistance, high blood pressure, gout and hyperparathyroidism (3)
Certain medication such as decongestants (ephedrine, guaifenesin), triamterene (a diuretic) and some anticonvulsants (felbamate, topiramate) (3)
Prolonged bed rest
Management of kidney stones (3)
The desired outcome from having a kidney stone is to get it out of the body, either through the stone making its own way through the urinary tract or by another means of removal from the body.
Pain can be relieved locally by warming of the abdomen and lower back. If heat is not enough, prescription medication to control pain may be required.
Nifedipine, a calcium channel blocker, can be prescribed to relax the urinary tract muscles allowing a stone to travel through more easily.
Prednisone decreases swelling through its anti-inflammatory properties.
Antibiotics can prevent or treat an infection.
Stones that won’t pass naturally can be removed by using ultrasound shock waves to break the stone into smaller pieces that will more readily travel along the urinary tract or by a surgical procedure.
What could be driving the increasing incidence of kidney stones?
Kidney stone formation can be affected by both genetic and environmental factors. Alterations in the gene pool occur extremely slowly so it is unlikely that genetics are the cause of the relatively recent rapid increase in kidney stones. On the other hand, environmental factors can induce changes over much shorter intervals. It appears that diet and climate have both had significant influences on the increasing incidence of kidney stones over the past few decades (4).
High intake of animal protein increases the formation of kidney stones (4). The common North American diet of today contains a much higher percentage of protein derived from animals than it did 50 or 60 years ago. To use the USA as an example, reports from the US Department of Agriculture Office of the Chief Economist (USDA/OCE) state that meat consumption has increased at an average rate of 1.2 pounds per person per year, from about 161 pounds of red meat, poultry and fish in 1949 to 231.8 pounds in 2008. Disturbingly, meat intake increased by 9.1 pounds per person per year in 2015 and by 3.6 pounds per person per year in 2016. It is predicted that US meat consumption will again increase during 2017 by another 2.5 pounds or more.
Climate change exerts its own effect on human biological systems and kidney stone formation. Average global temperatures are increasing (19) and there is a direct association between environmental temperature and increased kidney stone rates (4). This is most likely due to greater loss of fluid from the body through perspiration leading to dehydration (11).
A short primer on pH
At this point it would be apropos to clarify a couple of terms related to acidity in the human body. The acid/base balance in our blood is tightly controlled by body mechanisms. Even a minor deviation from normal can severely affect many of our organs. Acidity and alkalinity are expressed on the pH scale. This scale ranges from 0 (strongly acidic) to 14 (strongly basic or alkaline). The pH scale is logarithmic, meaning that for each whole number change there is a ten-fold change in the result. For example a pH of 4 is ten times more acidic than a pH of 5 and a pH of 9 is ten times more basic than a pH of 8. Right in the middle of the pH scale is a neutral pH of 7. Blood is slightly basic with a normal pH as close to 7.4 as possible although it can range between 7.35 to 7.45. A lower pH (more acid) in body terms is closer to a pH of 7.35 while a higher pH (more alkaline or basic) in body terms is closer to a pH of 7.45. The interplay between the level of acidity and micronutrients in the urine is delicate and complicated.
The diet/kidney stone connection
Examination of diet has revealed the importance of the food we eat in the production of kidney stones. In past decades patients were often advised to reduce their intake of calcium and oxalate in order to prevent the most common type of kidney stones. This has now been shown to be ineffective. However there are dietary elements that do prevent kidney stones.
The biggest risk factor for the development of kidney stones is the consumption of animal protein. Scientific trials from the late 1970s clearly illustrated this association. For instance, an interventional study performed in Britain added additional animal protein in the form of an extra can of tuna fish every day to the diet of its subjects. The resulting overall probability of forming kidney stones, calculated from a combination of risk factors, increased by 250% on the days that the study participants were eating the extra fish. Sadly, the augmented level of protein used in this study is the same level of protein eaten habitually by North Americans every day (14). The standard Western diet itself is a significant risk factor for kidney stones.
Even a modest increase in animal protein (from less than 50 gm to 77 gm per day) is associated with a 33% increased risk of kidney stones in men (7). Increases in kidney stone risk from eating animal protein is even higher in women (8). The connection of animal protein to kidney stones originates from its content of sulfur-containing amino acids. Cysteine and methionine contain the highest level of sulfur among the amino acids that are incorporated into protein. These two amino acids are found at a level 2 to 5 times higher in meat and eggs than in plant-based foods. The metabolism of these amino acids increases acidity in the body. Higher acidity in the blood affects the levels of many important micronutrients such as calcium, citrate and potassium which in turn alter the risk of producing stones in the kidney (9).
Citrate levels in the urine are reduced by a diet high in animal protein. Citrate is a common component of urine and its presence helps to prevent the formation of kidney stone (18). The combination of citrate and calcium is quite soluble in urine and even allows some calcium-based stones to dissolve away. Citrate also plays a protective role in the preservation of bone mass. Because citrate is alkalizing (lowers pH), it reduces the need for stealing calcium from the bones to neutralize an acidic environment (3,26).
Uric acid stones are the second most common type of kidney stone. Uric acid is found in much higher levels in foods of animal origin than those of plant origin. Crystals of uric acid can act as “seeds” around which calcium-based stones form or they can combine to create stones made entirely of uric acid (18). In order to minimize the effect of uric acid in the urine, the urine itself must be made to be more alkaline. One study showed that eating a vegetarian diet (removing meat completely from the picture) greatly decreases the acidity in the urine and consequently reduces the chance of uric acid crystallization by 93% (23). Additionally, uric acid stones are the most readily dissolvable of all kidney stone types. Simple dietary changes such as drinking more water and reducing animal protein and salt intake can lead to the dissolving of uric acid kidney stones completely away (24).
A 2000 study looked at low animal protein diets in people prone to developing multiple kidney stones and found that these diets result in a significant decrease of calcium in the urine and consequently fewer kidney stones. Researchers point out that only those who markedly decrease their animal protein intake show any benefit (21).
In 2014 a study from the Oxford England arm of the giant EPIC study looked directly at levels of meat in diets and showed that, compared to those with a high meat intake (>100 gm per day), moderate meat-eaters lowered their risk of kidney stones by 20%, low meat-eaters and fish-eaters reduced their risk by about 25% and vegetarians had a 30% lower risk of developing kidney stones. High intakes of fresh fruit, magnesium and fiber from whole-grains were also associated with lower risk of kidney stone formation while a high intake of zinc was associated with a higher risk (13). Meats such as beef, pork, lamb and poultry and oysters are the highest dietary sources of zinc (25).
A five year randomized trial performed in 2002 was conducted to compare two diets with the goal of preventing recurrent kidney stones (6). The first group of sixty men ate a diet containing normal amounts of daily calcium (about 30 mmol per day) but reduced amounts of animal protein (52 gm per day or less) and salt (50 mmol of sodium chloride per day or less). A second group of 60 men ate a low-calcium diet (10 mmol of calcium per day or less) with no change in amounts of animal protein or sodium. After five years, subjects in the first group exhibited a 50% lower risk of suffering another kidney stone. Calcium levels in the urine dropped significantly in both groups. On the other hand, urinary oxalate excretion increased in the men on the low-calcium diet while it decreased in those on the normal calcium/low animal protein and sodium diet. Overall results showed that the restriction of intake of animal protein and salt combined with normal calcium intake provides greater protection from kidney stones than the low-calcium diet once suggested by practitioners to prevent stone formation (6).
What about reducing the oxalate portion of the equation? Many fruits and vegetables are high in oxalate and patients suffering from recurring kidney stones in the past have been advised to cut back on oxalate-containing foods such as spinach, nuts, rhubarb, beets, tea, French fries, potato chips and chocolate. However, there is no evidence that oxalates in plant-sourced foods cause increased formation of kidney stones. In fact, whole fruits and vegetables bring with them their own benefits of reduced kidney stone risk independent of other known risk factors. In other words plant-based foods decrease the risk of kidney stones over and above the beneficial effects of simply replacing animal-based foods with plant-based foods (7,22).
What foods have the highest risk for inducing kidney stones? In 2014 researchers came up with the LAKE Score (Load of Acid to Kidney Evaluation Score) to calculate the acid load of foods at regular serving sizes. The most acid-producing food is fish such as tuna, then, in descending order, pork, poultry, cheese, beef and eggs. Grains such as bread and rice can be slightly acid-forming while pasta is not. Beans are very alkaline-forming and fruits are even better. Vegetables though are the most alkaline-forming of all (12,16). Diet modifications based on the LAKE Score can prevent both calcium and uric acid kidney stones (5,15,16).
Water dilutes the urine which prevents calcium, oxalate and uric acid from crystallizing. Studies show that a fluid intake from all sources (water, tea, coffee, juices, soups) of 2.5 liters over 24 hours results in one-third less risk of kidney stones compared to those subjects drinking only half as much fluid (7). There is no evidence from clinical trials to support any specific beverage (ex: hard vs soft water, filtered vs nonfiltered water) as superior for prevention of kidney stones (5).
A high sodium diet increases the level of calcium in the urine and decreases citrate levels. Both these changes can trigger the formation of kidney stones (5,20). Limiting sodium intake to 1500 mg per day, along with consuming adequate sources of calcium is associated with a lower risk of kidney stones (5,18,19).
Although most kidney stones contain calcium, the calcium in food is not necessarily the problem. In fact calcium can actually reduce the absorption of oxalate by binding to oxalates in foods and holding them in the digestive tract rather than allowing them to be absorbed (3,8,18). Calcium consumed as a part of food in a meal actually reduces the risk of kidney stones. However calcium in supplement form can provide too much calcium at one time leading to increased calcium in the urine on its way out of the body and raising the possibility of higher kidney stone formation.
Eating foods high in potassium can cut the risk of kidney stones in half (7). Potassium helps the kidneys conserve calcium rather than sending it away in the urine. Regular servings of fruits, vegetables and beans are an adequate source of potassium. A completely plant-based diet is an excellent way to acquire potassium (7).
Caffeine in beverages such as coffee or tea reduces the risk of kidney stones. This is because the diuretic effect of caffeine increases water loss in the urine, keeping the concentration of minerals lower and reducing the possibility of the precipitation of crystals and stone formation.
Sugar accelerates loss of calcium through the kidney leading to a higher concentration of calcium in the urine. In the Nurses’ Health Study, those who ate more sugar (60 gm or more per day) had a 50% higher risk of kidney stones than those consuming 20 gm of sugar per day (10).
Many people take Vitamin C to prevent colds but perhaps you should think twice about this if you have ever had a kidney stone or have a family history of kidney stones. A 2013 study followed 22,000 men for more than ten years and observed that those taking high doses of vitamin C doubled their risk of developing a kidney stone. This increased risk applies only to Vitamin C supplements, not Vitamin C derived from food. One 1000 mg tablet daily of Vitamin C constitutes a high dose. The mechanism for this may be that excess Vitamin C is excreted in the urine in oxalate form which we know can combine with calcium to form kidney stones (27).
High fructose corn syrup
Studies have shown that people ingesting the highest amounts of high-fructose products also have the highest risk of developing kidney stones. Other carbohydrates were not associated with an increased risk (17).
Finally, a simple list of tips for preventing kidney stones…
Avoid eating animal products. Along with their acidity increasing effects, their inherent proteins and the sodium added to products such as processed meats increase risk of kidney stones.
If you must eat meat, do not eat more in a day than an amount about the size of a deck of cards (18).
Drink plenty of liquids throughout the day. Try to ingest 10 to 12 cups of fluid daily of which half should be plain water. Drink before you are thirsty. If you wait until you are thirsty, dehydration has already started.
Include citrus fruits such as oranges and lemons regularly in your diet as they contain citrate that can block kidney stone formation.
Get your Vitamin C from fruits not supplements.
Eat lots of leafy greens and beans to be sure that you are consuming a good source of calcium.
Eat a diet rich in vegetables, fruits and beans. They provide a good sources of potassium.
Include coffee and tea in your regular diet.
Read labels and stay away from products high in high-fructose corn syrup.
Limit salt and sugar in your diet.
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