Why is diet important for kidney stone formers?
10% of all first-time stone formers will get another episode within 1 year and 50% within 10 years. For recurrent stone formers, the impact on their lives is enormous in terms of physical debility, social and financial implications. Prevention is paramount in these patients. In some patients, medications may be needed. In others, medications may not work – and this is where lifestyle and dietary changes are important.
Food that we eat gets broken down and absorbed in our digestive system (stomach and bowel) into our blood system. The waste products in our bloodstream are filtered by the kidneys and excreted in the urine. Therefore, the type of food that we eat contributes to the composition of the urine. Certain waste products can form stones in the urine.
Why is the type of stone that I form important?
The main types of kidney stones are calcium oxalate (most common), calcium phosphate, struvite, uric acid, and cystine. Stones can be obtained surgically or by you bringing in the ones that you passed so that they can be sent to the laboratory for analysis. Recurrent stone formers should also have a 24-hour urine collection and blood tests to screen for metabolic abnormalities.
Different dietary advice applies to different stones. For example, cutting down on oxalate intake does not help in preventing uric acid or cystine stones. Uric acid stones are more common in a diet rich in animal proteins. However, ensuring adequate fluid intake applies to all stone formers and is the most important advice of all.
How much and what types of fluid should I drink?
Drinking enough fluid keeps the urine diluted and also flushes away urine crystals or sediments before they become bigger stones.
You should drink 2.5 to 3L (8 to 10 glasses) of fluid a day. This should be adjusted according to the weather and the activity level that you do. On a hot day and when exercising, you should aim to drink more. One easy guide to assess your hydration status is to look at your urine colour; it should be pale yellow or clear.
Water is the best type of fluid to drink. Citrus drinks like orange juice, water with a touch of lemon, or lemonade contain citrate, which may act to prevent stone formation (especially calcium oxalate and uric acid stones) in the urine. However, they may increase calcium phosphate stone formation. Coffee, tea, grapefruit juice, and cola drinks contain oxalate and should be avoided by calcium oxalate stone formers. Cranberry juice may reduce urinary infections and prevent struvite stones, but it also contains oxalate.
What are the dietary recommendations?
Dietary recommendations include limiting animal protein, oxalate, and salt intake. Calcium is important for the body and restriction is usually not necessary.
Examples of foods rich in animal protein are:
- Fish (anchovies, mackerel, sardine, herring).
- Organ meat (liver, kidney).
- Red meat.
These foods are rich in purine, which is broken down into uric acid by the body, resulting in increased uric acid stone formation. The recommended daily allowance of protein is 0.8g/kg of body weight (about 160 – 210g of meat or fish).
Only about 2 – 5% of dietary oxalate is absorbed and excreted in the urine, where it can combine with calcium to form calcium oxalate stones. Therefore, a low oxalate diet is not recommended for routine stone patients unless they have high oxalate levels in the urine. Eating foods high in calcium can reduce oxalate in the urine because calcium can also combine with oxalate in the digestive tract, so less is excreted in the urine.
Foods high in oxalate include:
- Cocoa powder.
Salt is composed of sodium and chloride. When the sodium molecules are excreted into the urine by the kidneys, calcium molecules are excreted together. The calcium will then combine with oxalate or phosphate to form stones. For patients with high calcium levels in the urine, sodium restriction is an effective way of preventing stones. Most food labels have information about sodium and other nutrients. A daily sodium limit of 2400mg is recommended.
Examples of food high in sodium are:
- Saltshaker / Salt as a seasoning.
- Fast food (burgers, hotdogs).
- Processed meats (ham, bacon).
- Canned food (soups, vegetables).
- Salty snacks (potato chips).
- Chinese foods are often high in MSG (monosodium glutamate).
Restriction of calcium intake is usually not necessary because high calcium levels in the urine will not be affected by small changes in calcium intake. Calcium from food actually binds to oxalate in the digestive tract and prevents it from being excreted in the urine where it can form stones. Moreover, calcium is important for healthy bones and prevention of osteoporosis. Patients who form calcium oxalate stones should include 800 - 1000mg (but not more) of calcium in their daily diet. Those people on calcium supplements should take them with meals so the calcium can combine with the oxalate in food.
The calcium levels found in a standard serving of some high-calcium foods are:
- Yoghurt (415mg).
- Milk (400mg).
- Cheese (250mg).
- Ice cream (235mg).
- Rhubarb (265mg).
- Cheese pizza (220mg).
- Baked beans (130mg).
- Broccoli (90mg).