The Link Between Calcium and kidney stones
This article explores the role of dietary calcium in the formation of kidney stones.
Table of contents
- What Is Calcium?
- Where Is Calcium Found?
- What Is The Role Of Calcium In The Body?
- How Much Calcium Should People Be Consuming Each Day?
- What Are Kidney Stones?
- Dietary Management And Prevention Of Kidney Stones
- Can Diet Help Prevent Kidney Stones?
Calcium is a mineral with the chemical symbol Ca. It is a divalent cation with the atomic number of 20. Calcium is grouped within the alkaline earth metals in the periodic table and is the fifth most abundant element in our bodies. Despite this, calcium is never found free in nature since it easily forms compounds by reacting with oxygen and water. It does however occur abundantly as limestone (calcium carbonate), gypsum (calcium sulphate) and fluorite (calcium fluoride) (1). When it is found it is as a compound such as hydroxyapatite, Ca10(OH)2(PO4)6 which is the storage compound found in bone (2).
Calcium is found within nature in multiple sources and within the human body. In nature, calcium is used in alloys and in the extraction of other metals, such as thorium, from their ores. Calcium hydroxide is used to make cement through heating of limestone (calcium carbonate) (1).
Calcium is also found in many foods, with dairy foods being the best source. Milk and dairy products such as yoghurt and cheese contain a form of calcium that makes it easy for the body to absorb (3). Other good sources of calcium include nuts, canned fish with bones, leafy vegetables and dried fruit (2); however, the calcium in these foods is not as easily absorbed by the body.
Within the body, calcium is found in the skeleton and teeth. In fact the skeleton contains 99% of the body’s calcium (2) and the average human body contains about 1kg of calcium (1)! Nerve cells, body tissues, blood and other fluids also contain calcium, however, in smaller quantities (3).
Within the body, calcium plays a number of critical roles such as muscle contraction, nerve conductivity, enzyme activation, blood clotting as well as the ever important building of strong bones and teeth (2).
- How calcium plays a role in muscle contraction – calcium triggers contraction by reacting with regulatory proteins that in the absence of calcium prevent interaction of actin and myosin. In actin linked regulation, troponin and tropomyosin regulate actin by blocking sites on actin required for complex formation with myosin. In myosin-linked regulation, sites on myosin are blocked in the absence of calcium (4)
- How calcium plays a role in nerve conductivity – calcium is constantly present in extracellular compartments of organisms. For a nerve impulse to be conducted, chemicals must be able to cross the synaptic cleft, thereby transmitting the impulse from one nerve cell to the next. As the motor axon potential depolarises the nerve terminal voltage gated calcium channels open increasing the concentration of calcium on the pre-synaptic terminal. This is turn releases acetylcholine from the nerve terminal, with the end plate potential reaching the threshold level and causing the action potential to be propagated along the nerve fibre (5).
- How calcium is involved in enzyme activation – calcium is most commonly known for its regulation of calpains which are calcium dependent cysteine proteases. These enzymes , which are controlled by calcium ions, are involved in signal transduction, apoptosis and cell motility (6)
- How calcium is involved in blood clotting – calcium, alongside vitamin K and the protein fibrinogen, help the platelets form a clot.
How much calcium people require is dependent on age and sex. The following table from the National Health and Medical Research Council indicates how much calcium people should be consuming.
|Gender||Age Group (Years)||RDI|
Kidney stones are associated with chronic kidney disease (7). Kidney stone formation is a complex process and requires crystals to form, grow, aggregate and interact with a non-crystalline organic matrix, and avoid excretion while too small to cause pathology(8). These stones can block the flow of urine and cause infection, kidney damage or kidney failure.
Urine is normally supersaturated with calcium salts; yet the formation of kidney stones is a relatively rare event. Kidney stone formers are considered to have physiological abnormalities that increase their risk for kidney stones. These abnormalities include differences in renal handling of calcium and in the secretion into urine of solution stabilisers. There is evidence of a genetic component to the population distribution of these physiological behaviours (9).
The characteristic cramping and intermittent abdominal and flank pain occur as kidney stones travel within the urinary tract. The pain is often accompanied by hematuria, nausea, vomiting, fever and chills. However stones in the renal pelvis may be asymptomatic (7).
There are four major types of kidney stones:
- Stones formed from calcium not used by the bones and muscles, combined with oxalate or phosphate – these are the most common kidney stones. They are called calcium oxalate stones or calcium phosphate stones
- Stones containing magnesium and the waste product ammonia – these are called struvite stones and form after urine infections
- Uric acid stones – these are formed when there is too much acid in the urine
- Cystine stones – these are rare and hereditary (10)
Oral hydration and pain management are part of the acute treatment for all stones types (7). Most stones are treated without surgery. Ninety percent of stones pass themselves within three to six weeks. However if a stone doesn’t pass and blocks urine flow or causes bleeding and infection it may need to be removed.
Treatments for kidney stones can include:
- Extracorporeal shock-wave lithotipsy (EWSL)- ultrasound waves are used to break the kidney stone into smaller pieces, which pass out in the urine. This is used for stones less than 2cm in size
- Percutaneous nephrolithotomy – a small cut is made in the back, and an instrument removes the kidney stone
- Surgery – the stone may need to removed using traditional surgery which requires a cut in the back to access the kidney and ureter to remove the stone.
Risk factors for kidney stones include obesity, insulin resistance, gastrointestinal pathology and certain dietary patterns and medications.
Management and prevention are largely specific to the type of kidney stone. Dietary sodium, animal protein, oxalate, sucrose and ascorbic acid may promote calcium stone formation by direct or indirect effects on urine composition and urinary supersaturation as described in the following table (8):
|Sodium||High sodium intake, and a subsequent decrease in proximal sodium reabsorption reduces renal tubular calcium reabsorption||A positive association between sodium consumption and new kidney stone formation has been demonstrated in women but not men (11)(12)|
|Animal protein||Animal protein intake represents a metabolic acid load. Higher protein intake increases urinary calcium excretion (possible due to the release of calcium from bone following excess H+ buffering) and may result in hypocitaruria, a risk factor for nephrolithiasis. Dietary protein also leads to an increase in calcitriol production.||A positive association between animal protein consumption and new kidney stone formation has been shown in men but not women (12)(11)|
|Ascorbic acid||Vitamin C is metabolised to oxalate. Some studies have found increased urinary oxalate with increased vitamin c ingestion.||Measurements of urinary oxalate in the presence of ascorbate can be unreliable. Observational data collected to date fails to identify vitamin c consumption as a risk factor for stone formation (13)(14)|
However, it is important to consider that these are only proposed mechanisms and as such it is difficult to determine why stones form in individuals.
Urine calcium excretion is considered to be an important risk factor for stone formation and is elevated with increased diet acid loads, increased salt intake and inadequate vitamin D levels (7). Other things being equal, increasing the calcium concentration of the urine does increase its potential to support stone formation, and thus it might be expected that increased oral calcium intake would predispose to kidney stones. However “other things” are not actually equal. A systematic review conducted in 2008 found that increasing dietary calcium intake usually decreases urinary oxalate excretion. Oxalate concentration of urine is a powerful stone factor in its own right, and for that reason high calcium intake, within limits, typically reduces stone risk (9). This protective effect of dietary calcium in stone formation is also found in a study conducted by Taylor and Curhan (2004), however the mechanism remains unclear. Some evidence suggests that dietary calcium inhibits the absorption of oxalate in the gut and in this way serves, indirectly, as a means of reducing urinary oxalate excretion (11).
When treating kidney stones, advice has focused on reducing calcium intake, when previous kidney stones have been composed of calcium oxalate or calcium phosphate, however no prospective data supports this practice (15).
Dietary intervention has also focused on increasing fluid intake as a well accepted method for reducing kidney stone recurrence. Nevertheless, not all types of fluids may be equally beneficial; some beverages like sodas have an increased risk of stones reported, and others, like coffee and tea, have been reported to be associated with a reduced risk of stone formation (16).
A study conducted by Ferraro et al (2013) found that participants consuming one or more sugar-sweetened cola drinks per day had a 23% higher risk of developing kidney stones compared with those participants consuming less than one serving per week (p value 0.02). This study also found the same previously reported inverse relationship between coffee and tea consumption and kidney stones. Coffee and tea are sources of caffeine, which has been reported to increase dieresis moderately together with excretion of magnesium and potassium as well as calcium and sodium.(17) More recent evidence suggests that caffeine-induced natriuresis might be caused by the action on both the proximal and distal tubules (18).
Diet can play a role in kidney stones development and treatment.
Several factors are related to kidney stone formation.
- Avoid dehydration – drinking enough fluids will help to keep urine volume at an appropriate level and reduce the risk of kidney stones
- Intestinal calcium absorption is thought to play a critical role in the formation of kidney stones. New research from Seattle published in the Journal of Urology reported that a greater calcium intake is associated with lower fractional calcium absorption. This may protect against kidney stones (19). However, always discuss this with your doctor and dietitian.
- Reducing intake of soft drinks, especially drinks containing cola. Not only will this help with reducing the risk of developing kidney stones but will also help with weight maintenance.
How does calcium in the diet affect kidney stone formation?
Calcium from food does not increase the risk of calcium oxalate stones. Calcium in the digestive tract binds to oxalate from food and keeps it from entering the blood, and then the urinary tract, where it can form stones.
People who form calcium oxalate stones should include 800 mg of calcium in their diet every day, not only for kidney stone prevention but also to maintain bone density. A cup of low-fat milk contains 300 mg of calcium.
Other dairy products such as yogurt are also high in calcium. For people who have lactose intolerance and must avoid dairy products, orange juice fortified with calcium or dairy with reduced lactose content may be alternatives. Calcium supplements may increase the risk of calcium oxalate stones if they are not taken with food (20).
How does oxalate in the diet affect kidney stone formation?
Some of the oxalate in urine is made by the body. However, eating certain foods with high levels of oxalate can increase the amount of oxalate in the urine, where it combines with calcium to form calcium oxalate stones. Foods that have been shown to increase the amount of oxalate in urine include:
- wheat bran
Avoiding these foods may help reduce the amount of oxalate in the urine (20).
How a diet plan can help prevent future kidney stones
A dietitian can help a person plan meals that lower the risk of forming stones based on the type of stone the person formed in the past. A person with a history of kidney stones may want to talk to a dietitian who specializes in kidney stone prevention or nutrition for people with kidney problems.
A dietitian can also help overweight people plan meals to help them lose weight. Studies have shown that being overweight increases the risk of kidney stones, particularly uric acid stones. Diets that are low in carbohydrates have been shown to further increase the risk of uric acid stones and should be avoided.
Studies have shown the Dietary Approaches to Stop Hypertension (DASH) diet can reduce the risk of kidney stones. The DASH diet is high in fruits and vegetables, moderate in low-fat dairy products, and low in animal protein (20). More information about the DASH diet can be found on the National Heart, Lung, and Blood Institute’s website, click here.
Points to Remember
- Kidney stones can form when substances in the urine—such as calcium, oxalate, and phosphorus—become highly concentrated. Diet is one of several factors that can promote or inhibit kidney stone formation.
- The four major types of kidney stones include: calcium stones, uric acid stones, struvite stones, and cystine stones.
- Drinking enough fluid is the most important thing a person can do to prevent kidney stones.
- People who have had a kidney stone should drink enough water to make at least 2 litres of urine a day.
- Sodium, often from salt, causes the kidneys to excrete more calcium into the urine. High concentrations of calcium in the urine combine with oxalate and phosphorus to form stones. Reducing sodium intake is preferred to reducing calcium intake.
- Meats and other animal protein—such as eggs and fish—contain purines, which break down into uric acid in the urine.
- Calcium from food does not increase the risk of calcium oxalate stones. Calium in the digestive tract binds to oxalate from food and keeps it from entering the blood, and then the urinary tract, where it can form stones.
Contact us for results focused nutritional advice
This article was written by Rhiannon Welsh who is a Dietitians Association of Australia member and an Accredited Practising Dietitian.
A dietitian can help a person plan meals that lower the risk of forming stones based on the type of stone the person formed in the past.
If you have any questions about healthy calcium intake or calcium and kidney stones, contact your local doctor who will arrange for you to see a dietitian in Sydney. Contact us today!
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- Curhan GC, Willett WC, Rimm EB, Stampfer MJ. A prospective study of the intake of vitamins C and B6 and the risk of kidney stones in men. Journal of Urology. 1996; 155: p. 1847-1851.
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- US Department of Health and Human Services. National Kidney and Urologic Diseases. A service of the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK), National Institutes of Health (NIH)