Avoiding kidney stones

There are a lot worse things than kidney stones. But, oh my, they can cause a lot of pain as they pass through the ureters, the tubes that connect the kidneys to the bladder. And the number of Americans getting kidney stones is increasing, perhaps because of the increasing prevalence of obesity. Once you get kidney stones, the chance of getting them again is high, and much of the prevention advice is aimed at fending off a recurrence, but it may also help some with avoiding kidney stones in the first place. Here are a few pointers:

Keep your fluid intake up. Kidney stones form when certain minerals concentrate in the urine and form into crystals. Drink plenty of fluids (water is the safest bet) and you'll increase the amount of water in the urine, so those mineral concentrations don't get too high. This is old advice: recommendations to increase fluid intake to prevent kidney stone recurrence go back to the time of Hippocrates.

Eat calcium-rich foods. Calcium is a major component of about 85% of kidney stones, so it seems like you should avoid calcium in the diet, not seek it out. But most calcium stones are composed of calcium combined with a substance called oxalate. If there is plenty of calcium in your diet, the calcium binds to oxalate in the intestine before the oxalate has a chance to get into your urine. Less oxalate in the urine means fewer opportunities for calcium oxalate to form — and fewer kidney stones. Calcium-rich foods include nonfat dairy products, leafy green vegetables, and some varieties of fish (salmon is a good choice).

Reconsider calcium supplements. Results from the Harvard-based Nurses' Health Study showed that postmenopausal women who took calcium supplements were 20% more likely to develop kidney stones than women who didn't. Findings published in 2011 from the Women's Health Initiative, a large randomized trial, echoed those of the nurses' study. One explanation for calcium in food and calcium pills having different effects is that when calcium is consumed in food, it's more likely to be present in the intestine at the same time as oxalate, which can interfere with its absorption.

Moderate your sodium intake. Low-sodium diets decrease excretion of calcium and oxalate.

Moderate your protein intake. Protein can increase calcium and oxalate excretion. High-protein diets may also reduce the levels of stone-inhibiting substances in the urine.

Moderate your oxalate intake. Calcium intake and other dietary factors seem to be more important than oxalate intake in forming kidney stones, but high oxalate intake can occasionally be a factor. Oxalate-rich foods include beets, chocolate, spinach, rhubarb, and most nuts.