Q: I have calcium oxalate kidney stones and was told to avoid high-oxalate foods like spinach. Should I cut back on calcium, too?
Once you’ve had a kidney stone, making modifications to your diet is necessary to help prevent stones from recurring. And if you’ve experienced the harsh pain of passing a stone, chances are you’ll be motived to do so.
Kidney stones are becoming more common due, in part, to rising rates of obesity and Type 2 diabetes, conditions associated with developing stones.
Diet plays a role, too. Drinking too little water, for instance, increases the risk. So does consuming too much salt, meat and sugar.
What are kidney stones?
Kidney stones develop when excessive levels of certain minerals and salts in the urine (calcium, oxalate, uric acid, phosphate, cystine) form crystals that build up on the inner surfaces of the kidneys. Stones can range in size from a grain of sand to a golf ball.
Kidney stones differ depending on which chemicals they’re composed of. Calcium-containing stones are the most common type, accounting for 80 per cent of all stones and, among those, most (80 per cent) are calcium oxalate stones.
Other stones consist of uric acid, cystine or struvite. Advice about diet is slightly different for each type of stone.
A (calcium oxalate) stone-friendly diet
Modifying your diet to prevent calcium oxalate kidney stones doesn’t mean avoiding all oxalate-containing foods. Nor does it mean giving up calcium-rich foods. In fact, a low calcium diet can increase the risk of kidney stones.
The following strategies can help prevent recurrent calcium oxalate kidney stones.
Increase fluids. The most important thing to do is to drink plenty of water each day. Doing so dilutes the chemicals in your urine, making it harder for stones to form.
Drink 2 1/2 to three litres of fluids each day. Drink more during hot weather or when exercising to make up for fluids lost through sweat.
Spread fluid intake out over the course of the day. If you need a reminder to drink water, set hourly alerts on your smartphone.
Add one tablespoon of lemon juice to every cup (250 ml) of water (1/2 cup per two litres). Lemon juice is high in citrate, which helps increase the volume of urine and inhibit calcium stone formation.
Limit oxalate-rich foods. Oxalates occur naturally in plant foods; however, oxalates in many of these foods are not easily absorbed by the body.
According to Unity Health Toronto, high oxalate foods to be avoided include almonds, beets, buckwheat, cocoa powder, chocolate, okra, oranges, potatoes, spinach, dates, figs, raspberries, rhubarb, walnuts, white beans and quinoa. Other heath organizations also recommend avoiding wheat bran and black tea.
If you can’t avoid high oxalate foods, eat them in small portions and paired with a calcium-rich food (examples include dairy, calcium-fortified non-dairy milk, tofu, canned fish with bones). Calcium binds to oxalate in the digestive tract so it can’t make its way to the kidneys.
If you supplement with vitamin C, don’t take more than 1,000 milligrams. Too much can increase oxalates in the urine.
Meet calcium requirements. Getting enough calcium each day is tied to a lower risk of stone formation. Adults need 1,000 to 1,200 mg of calcium a day, depending on age.
Aim to get your calcium from foods and ideally at meals. If a supplement is needed, take it at a meal.
Reduce sodium intake. Diets high in sodium increase the amount of calcium that ends up in the urine, increasing the risk of stone formation. Keep sodium intake to less than 2,000 mg per day.
Read labels to choose lower sodium products. Limit restaurant meals and ones made from meal delivery kits, which can be very high in sodium.
Keep animal protein in check. Eating too much meat, chicken and fish may increase the risk of stones; limit to six ounces a day. More often, choose plant-based proteins such as beans, lentils, tofu, tempeh, edamame and seeds.
Eat vegetables and fruits daily. Potassium in fruits and vegetables is thought to reduce the amount of calcium that gets excreted in the urine. As well, the water in these foods can contribute to higher urine volumes.
Leslie Beck, a Toronto-based private practice dietitian, is director of food and nutrition at Medcan. Follow her on Twitter @LeslieBeckRD
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