Preventing kidney stones

Kidney stones develop if the calcium or uric acid salts in the urine can no longer dissolve. If the concentration of these substances is too high, the salts form crystals, which develop into stones. This risk is lower if the urine is diluted as much as possible.

One study suggests that the risk of developing a second kidney stone can be reduced by drinking more water than usual. Participants drank enough to produce about 2.5 liters of urine per day. After a period of five years, the study showed the following:

• 27 out of 100 people who drank the same amount as they did before developed another kidney stone.
• 12 out of 100 people who drank more than they did before developed another kidney stone.

The study participants found it easy to drink more fluids.

Many soft drinks like cola contain phosphoric acid (food additive E338), which may possibly increase the likelihood of kidney stones forming. There has only been one high-quality study on the question of whether not drinking soft drinks also lowers the risk of developing more kidney stones. In the study, 500 men were advised to drink fewer soft drinks. On average, they lowered their consumption to under 100 milliliters per day – that's less than half a glass. Their risk of kidney stones dropped in comparison to the group of men who didn't change their drinking habits. The study showed the following after three years:

• 41 out of 100 men who drank as many soft drinks as before developed another kidney stone.
• 34 out of 100 men who drank fewer soft drinks developed another kidney stone.

Some men dropped out of the study early – possibly because they found it difficult to change their drinking habits.

Calcium stones are often made up of calcium and oxalate ( calcium oxalate). Both of these substances are found in many different foods. Not eating foods that contain oxalate – such as rhubarb, parsley, walnuts, spinach and chocolate – is considered to be helpful. But it’s not a good idea to avoid foods that are rich in calcium, like milk, cheese or yogurt. Research shows that a low-calcium diet tends to increase the risk of kidney stones: If you get too little calcium in your diet, more oxalate can enter your urine.

It is different if calcium is used as a dietary supplement. Studies show that this increases the risk of kidney stones – at least for women in menopause.

The highest-quality study on diet and kidney stones involved men who had high levels of calcium in their blood. They were advised to do the following as part of comprehensive changes to their diet:

• Drink plenty of fluids
• Don't eat food that are rich in oxalate, like walnuts, spinach and chocolate
• Eat less animal protein, such as meat and eggs
• Stick to a low-salt diet
• Get enough – but not too much – calcium, for example in the form of milk, cheese or yogurt (about 1,000 to 1,200 milligrams per day).

The men in the comparison group were only instructed to drink more fluids and to reduce the oxalate and calcium in their diet.

The results show that a major change in diet can lower your risk of kidney stones:

• 38 out of 100 men who didn't make major changes to their diet, and in
• 20 out of 100 men who did make major changes to their diet.

Sometimes a fiber-rich diet is also recommended, but there is no scientific proof that it will prevent kidney stones.

People who develop uric acid stones often have gout or diabetes. Uric acid is a waste product generated by substances called purines. Purines are mainly produced by the body itself, but are also found in some foods. This is the reasoning behind recommending a low-purine diet with less fish, meat and seafood to prevent uric acid stones from forming.

There has not yet been any scientific research to test whether a low-purine diet can lower the risk of developing uric acid stones.

Preventive medication is an option for people who are at a greater risk of developing further kidney stones. Any of the following things may be a sign that you have a higher risk:

• if you develop kidney stones in childhood or as a teenager
• if other people in your family have kidney stones
• if you have a medical condition affecting the kidneys, urinary tract or gastrointestinal tract
• if you have an overactive parathyroid gland
• if you have had kidney stones that were caused by a urinary tract infection
• if you have uric acid stones
• if you have cystine stones (caused by a rare inherited metabolic disorder)

The type of medication that is used will depend on the type of stone:

• Citrate salts: They bind to the calcium dissolved in urine to prevent calcium crystals from developing. Citrate salts are commercially available in the form of soluble tablets, capsules, and powder. It is used to prevent calcium stones, uric acid stones and cystine stones.
• Thiazide diuretics: These medications reduce the amount of calcium entering the urine from the bloodstream. They promote the production of urine, preventing calcium stones.
• Allopurinol: This drug inhibits the breakdown of purines to uric acid, lowering the uric acid levels in the urine. Allopurinol is mostly used to prevent uric acid stones.
• Other medications: If someone has kidney stones caused by urinary tract infections, they can try to prevent them using medication containing L-methionine, which increases the acidity of the urine.
• Dietary supplements with magnesium increase the levels of magnesium in urine. The magnesium attaches to oxalate in the urine and is believed to prevent the formation of calcium oxalate stones in this way.

Doctors can use blood tests and urine tests, as well as an analysis of the kidney stones, to decide which of the medications are most suitable.

Medications to prevent kidney stones are taken once a day for life. They're usually only considered if someone has already had kidney stones at least twice.

Research shows that medicines that are commonly used to prevent kidney stones can be effective. Calcium stones are the most common type of kidney stone, and most of the studies in this area have involved people with calcium stones.

Several studies have shown that potassium citrate salts can prevent kidney stones:

• Without citrate salts, 44 out of 100 people had another kidney stone within two years.
• With citrate salts, 11 out of 100 people had another kidney stone within two years.

The possible side effects of citrate salts are stomach and bowel (gastrointestinal) problems and diarrhea. About 12 out of 100 people in the studies stopped their treatment early due to the side effects.

Research has also shown that thiazide diuretics lower the risk of kidney stones:

• Without thiazide diuretics, 47 out of 100 people had another kidney stone within three years.
• With thiazide diuretics, 24 out of 100 people had another kidney stone within three years.

Thiazide diuretics can have various side effects, including low blood pressure accompanied by dizziness and drowsiness, a dry mouth and erection problems. 8 out of 100 people in the studies stopped their treatment early due to side effects.

Allopurinol is mostly used to prevent uric acid stones. But its effectiveness has only been studied in people with calcium stones. Those studies showed that allopurinol can prevent calcium stones:

• Without allopurinol, 55 out of 100 people had another kidney stone within three and a half years.
• With allopurinol, 33 out of 100 people had another kidney stone within three and a half years.

Allopurinol may cause rashes and sometimes trigger gout attacks. But the number of people in the studies who dropped out due to side effects was no greater in the group that had the allopurinol treatment than in the group that did not.

Other types of medications used for the prevention of specific types of kidney stones, such as magnesium or L-methionine, have not yet been tested in good-quality studies, so it's not clear whether they can prevent kidney stones.