My mother is prone to kidney stones — no problems in a while, but lately she's had symptoms that made her think perhaps she was cooking up a new stone or two. I noticed she drinks a lot of Coca-Cola and wondered if this was a causative agent … but from what I see online apparently it's a cure for kidney stones. I never heard of this before, but there's stuff all over the Web and YouTube about it. What's the Straight Dope?
Illustration by Slug Signorino
You’ll never guess.
For those who haven’t heard of it, kidney flushing has indeed been enthusiastically embraced all over the Internet, on the usual holistic/herbal/spiritual sites. The reasoning goes:
1. Asparagus is good for the kidneys, and a mild diuretic to boot.
2. Coca-Cola contains phosphoric acid, and we’ve all heard it can dissolve nails.
3. Therefore, consuming huge amounts of asparagus and Coca-Cola will dissolve and flush those nasty kidney stones.
The exact instructions vary. Some specify Coke or Classic Coke; others say Diet Coke, Pepsi, or even Dr Pepper may also be efficacious. In any case, the general procedure is to cook six to eight ounces of asparagus, blend it into a smoothie, chug it, then drink either a six- or twelve-pack of the appropriate soft drink, followed by lots of water. If there are no results in one to three days, repeat until the stones dissolve or pass. Some recommend peeing through pantyhose or a coffee filter to catch the shards.
My assistant Una rifled through the scientific databases and found no studies on kidney flushing. Could it work, though? Let’s give this some thought.
Most kidney stones are made up of calcium oxalate, calcium phosphate, or both. Urinary tract infections can form struvite stones (magnesium ammonium phosphate, if that means anything to you). Uric acid stones turn up sometimes, and rarely one finds stones formed from cystine, an amino acid.
Determining what type of stones you have is critical to treating and preventing them. For example, high levels of oxalate from some foods can lead to calcium stones, struvite stones are encouraged by alkaline urine, and acidic urine contributes to uric acid stones. Cystine stones can be reduced by alkaline urine.
You see the problem: a treatment that fights one kind of stone — namely making the urine more acidic or alkaline — can be the very thing that helps another kind of stone form. If you start medicating yourself with some Internet remedy without first establishing what sort of stones you’ve got, you could make things worse.
That said, changing your diet and urine pH can in fact help prevent or sometimes reverse kidney stone formation, and with this in mind doctors have been investigating ways of modifying urine chemistry for at least 80 years. Turns out some stones can be dissolved through diet, but it generally takes weeks or months.
In the 1930s physicians were trying to dissolve kidney stones using dubious cocktails of dilute aqua regia (nitro-hydrochloric acid), ammonium chloride, ammonium nitrate, malic acid, and ash, with minimal success. In 1939 doctors at Massachusetts General Hospital reported that even tenacious calcium stones could be dissolved, but only by direct application (via catheter) of sodium citrate and citric acid, and only after nearly three weeks of treatment.
So let’s consider our remedy. Asparagus will make urine slightly more alkaline. It also contains oxalates, which will make calcium-based stones worse, and purine, which is bad for uric acid stones. Coca-Cola on the other hand will make urine more acidic. Given the relative proportions of the kidney flush mix, we’d expect the resulting mess to make your urine slightly more acidic.
Enough to make a difference? Don’t be silly. I came across a 1930s case in which a doctor treated a woman suffering from carbonate stones with the above-mentioned brew of dilute aqua regia and whatnot plus an acid ash diet, reducing her urine pH to 4.5 for several months. She passed several stones, and X-rays showed that what was left was reduced in size, but getting the last bits out required surgery. Bear in mind that nondilute aqua regia is a corrosive acid alchemists used to dissolve gold. It’s safe to say you won’t get the same results with Coke.
Except by coincidence. Kidney stones of half a centimeter or less pass spontaneously about 70 percent of the time, and stones up to one centimeter have nearly a 50 percent chance of passing without treatment.
Will you do yourself any harm? Probably not. It’s not like there’s some more effective treatment you’d have to forego to dose yourself with Coke. Then again, of the top 20 Google hits that came up for “kidney flush asparagus coke,” not one mentioned the importance of determining the type of stone before treatment, and only a couple thought it might be helpful to consult a doctor. Not to harp on the unreliability of the Internet, but on serious matters you need to speak to someone with a clue.
Send questions to Cecil via firstname.lastname@example.org.