Try to avoid soft drinks

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.

In other words, drinking fewer soft drinks prevented the development of further kidney stones in 7 out of 100 men. Some men dropped out of the study early – possibly because they found it difficult to change their drinking habits.

Low-calcium diets are more likely to be harmful

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's 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.

Major change in diet could help

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.

So the dietary changes prevented further kidney stones in 18 out of 100 men.

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

The impact of a low-purine diet is not clear

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.

Citrate

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.

In other words, this medication prevented the development of another kidney stone in 33 out of 100 people.

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.

Thiazide diuretics

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.

So taking thiazide diuretics prevented the development of further kidney stones in 23 out of 100 people.

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

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.

So the medication prevented the development of a further kidney stone in 22 out of 100 people.

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 medications

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.