Ask the Doctors

Dear Doctor: Can patients with chronic kidney stones reduce their likelihood of problems by adding daily magnesium to their diet?

Dear Reader: There are many hypothetical reasons why supplemental magnesium could decrease the chance of kidney stones.

Kidney stones are quite common, with 10 percent of people in the United States developing them over their lifetime. And the rates are increasing. In fact, the incidence of kidney stones has nearly doubled over the last 30 years. The majority of stones (65 percent) are calcium oxalate; 15 percent are calcium phosphate; and the rest are uric acid or struvite. Thus, the medical community has looked for ways to decrease the amount of calcium in the urine as well as its ability to form crystals there.

Theoretically, magnesium could decrease the possibility of stone formation by binding to oxalate in the intestine, leading to less oxalate in the urine and possibly fewer calcium oxalate stones. It also could bind to oxalate once it's in the urine, enabling the oxalate to be excreted before it binds with calcium. Magnesium preparations could increase urine levels of citric acid, which could bind the calcium and lower the amount of free calcium in the urine. And magnesium could decrease the ability of calcium oxide to form crystals in the urine.

I say "theoretically" for a reason, however. Magnesium also inhibits the reabsorption of calcium by the kidneys, leading to more calcium in the urine and thus a greater chance of kidney stones. (This risk is greater when the urine is overly acidic and less so when the urine is more alkaline.) And while magnesium may increase the amount of calcium in the urine, animal studies have shown that a lower level of magnesium in the urine is actually associated with a greater risk for kidney stones.

In a 2017 study at the University of Texas, researchers analyzed the impact on women of various amounts of dietary magnesium lactate, potassium citrate (which alkalinizes the urine) and ammonium chloride. The goal was to assess how magnesium intake and the pH of urine could affect kidney stone risk. As expected, making the urine more alkaline decreased the calcium excreted in the urine, but did not decrease oxalate in the urine. The magnesium lactate actually increased urine calcium -- even with the addition of potassium citrate -- as well as the risk of calcium oxalate stone formation.

I'm not sure what supplement you've used, but not all supplements would have the same effect as magnesium lactate. Magnesium oxide, for example, provides less magnesium than magnesium lactate, which alkalinizes the urine.

One interesting note from the 2017 study is that magnesium lactate decreased the amount of phosphorus in the urine and thus may decrease the formation of calcium phosphate stones. I'm not sure if your stone was identified as containing calcium phosphate, but that may be interesting to know.

Ultimately, however, there are no good controlled trials assessing whether magnesium has any effect on people with kidney stones, so I can't make a general recommendation. Clearly, more study is needed.

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o Media Relations, UCLA Health, 924 Westwood Blvd., Suite 350, Los Angeles, CA, 90095. Owing to the volume of mail, personal replies cannot be provided.)

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