Did drinking iced tea really cause my kidney stones?

KidneyStoners.org

Recently, you may have heard patients, newspapers, and websites all tell you, “Iced tea caused your kidney stones!” With so many sources telling you the same thing, it must be true, right?

Wouldn’t it be great if iced tea were the single underlying cause of your kidney stones? If this were really the case, you could simply remove it from your diet then viola, no more stones. But things are unfortunately usually not so simple. To help clarify this issue and give you the information to reach your own conclusions, we’ll go over the evidence for and against tea in the post below.

Who says I shouldn’t drink iced tea?

A lot of people. Here’s a collection of links to recent news articles warning you about the dangers of iced tea and kidney stones:

Loyola Medicine News Release: “Hidden hazards of iced tea: Popular summer drink may raise risk of painful kidney stones, Loyola urologist warns” (July 19, 2010)

Medical News Today: “Iced Tea’s Hidden Risk Of Painful Kidney Stones” (July 23, 2008)

The Washington Post: “Kidney Stones? Lay Off the Iced Tea” (August 15, 2008)

Livestrong.com: “Correlation Between Iced Tea & Kidney Stones” (May 23, 2011)

ABC news: “Iced tea link to kidney stones” (August 8, 2012)

If you read the articles above, you might notice that they all seem to say the similar things. That’s because Loyola re-releases their same press release each summer, which is then picked up by various news outlets to fill out their summer news.

Warnings from experts against tea drinking in kidney stone patients have actually been around for a while. Dr. Linda Massey, an expert in nutrition, wrote back in 2000 that “…it seems prudent that calcium oxalate stone formers, especially those with elevated urinary oxalate values, limit their consumption of regular black and green teas, or at least drink them with generous amounts of added milk.”

What’s in tea that could make it so bad for stone formers?

Tea contains high level of a substance called “oxalate”. Oxalate can combine with calcium to form an insoluble compound (calcium oxalate) that makes up most kidney stones. A high level of oxalate in the urine (hyperoxaluria) is therefore a risk factor for forming kidney stones.

A large amount of the oxalate that is excreted in urine is actually made by your own liver as a breakdown product of normal metabolism and is not from your diet. The amount that dietary intake contributes to urinary oxalate is currently debated, with some authors estimating only 10% while others report higher levels like 50%. (Holmes 2001 and Williams 1989)

In addition to being found in tea, other common food items that are important dietary sources of oxalate include nuts, spinach, beets, rhubarb, chocolate, breads, cereals, and potatoes. There are many other food items that also that contain high levels of oxalate which can make staying on a low oxalate diet a challenge. A list of the oxalate content of various foods is available here from Medscape.

Is diet the only reason for high urinary oxalate?

No. Less common conditions that can also lead to high levels of oxalate include enteric hyperoxaluria, caused by bowel abnormalities or bowel surgery, and primary hyperoxaluria, caused by an inherited disorder of oxalate metabolism. We won’t be discussing them here.

Okay, so tea contains a lot of oxalate. Does drinking it increase the amount of oxalate in my urine?

Maybe…early research into whether drinking tea results in high levels of urinary oxalate found that tea only increased urinary oxalate by small amounts when given to healthy volunteers. This was because while tea contains relatively high oxalate content, only a small amount of that is “bioavailable” or actually absorbed. In contrast, spinach, while also having low bioavailability, has much higher total oxalate content per serving, resulting in a much higher amount of oxalate making its way into urine. The table shown below summarizes the increase in urinary oxalate levels from a large serving of each food and the bioavailability % of oxalate for each food.

Data from Brinkley 1981 and Brinkley 1990

Another group of researchers again looked into this topic and found that in a group of healthy volunteers drinking 6 cups of two types of black tea over a 24-hour period, the total amount of oxalate excreted in urine was 28.9 and 24.0mg for each type of tea. While that sounds like a lot of oxalate, the researchers also checked the amount of oxalate excreted by the same volunteers over a 24-hour period while not drinking tea and found that their “baseline” amount was 26.8mg. When averaging both teas, the net result of drinking 6 cups of black tea was an average decrease of 0.37mg of urinary oxalate compared to not drinking tea. They concluded that, “….It would, however, be very difficult indeed to consume an excessive amount of soluble oxalate from drinking tea.” (Savage 2003)

If you’ve read this far you must be thinking that this whole tea thing must have been totally overblown – in the laboratory studies above, drinking tea led to only negligible increases or, in contrast, decreases in urinary oxalate. Well, things may be a little more complicated than that. Both studies above used normal healthy volunteers with no history of stones. But stones formers may be different than non-stone formers when it comes to oxalate absorption. More specifically, some stone formers appear to be super-absorbers of oxalate. These individuals, when tested, have higher urinary levels of oxalate than normal. They represent about 20% of all stone formers (Laminksi 1991). In a study where an oral oxalate “load” in the form of sodium oxalate was given to a group of stone formers, those who had pre-existing elevated urinary oxalate as a risk factor absorbed and excreted more oxalate than the stone formers without a history of elevated urinary oxalate. (17.2 mg versus 12.1 mg excreted in urine after six hours following the 440 mg oral load) This suggests that for some 20% of stone formers, their bodies seem to be more sensitive to dietary oxalate and therefore drinking too much tea might be a concern for them. However, note that in this study the subjects drank pure sodium oxalate and not tea so it is still unclear whether tea would be a problem even in this group of patients. (Krishnamurthy 2003)

Enough with the laboratory studies, do tea drinkers actually make more stones?

The short answer is no. In fact, tea drinkers may actually make fewer stones! Here are four studies on this topic:

Study #1. Drinking tea decreases the risk of stones among male health professionals: Curhan and colleagues looked at fluid intake from different beverages and the risk of developing kidney stones in a group of 45,289 male health professionals over a six-year period. For every 8oz daily serving of tea, the risk of developing stones was decreased by 14% in the study. (Curhan 1996)

Study #2. Drinking tea doesn’t increase or decrease the risk of stones among Finnish male smokers: Hirvonen and coworkers looked at dietary data from a study involving 27,001 Finnish male smokers, originally conducted to study lung cancer prevention. They found that drinking tea neither increased nor decreased the risk of developing kidney stones among the study subjects. (Hirvonen 1999)

Study#3. Drinking tea doesn’t increase or decrease the risk of stones among men from the Carolinas or Rockies: Shuster and colleagues performed a study involving 2,295 men from the Carolinas and the Rockies. In the study, those who were primarily tea drinkers were not at increased or decreased risk for the development of kidney stones compared to non tea drinkers. (Shuster 1985).

Study #4. Drinking tea decreases the risk of stones among female nurses.  In a study of 81,093 nurses followed over 8 years, each 8oz serving of tea that was drunk on a daily basis decreased the risk of developing stones by 8%. (Curhan 1998)

Moving off the topic of tea, it may interest you that the other beverages found to decrease the risk of stones in the above studies included (grouped by study): beer, coffee, and wine; beer; beer and coffee; coffee and wine.

Now I’m confused, if tea has a lot of oxalate, why didn’t tea drinkers make more stones?

Possible reasons for this paradox include:

1)   Not much oxalate is actually absorbed by drinking tea in most people* so the oxalate content of tea becomes a non-issue

2)   The volume of fluid in tea helps to dilute the urine, counteracting the small amount of oxalate absorbed

3)   The caffeine in tea encourages higher urine output, again diluting the small amount of oxalate

4)   If you drink tea with milk, the calcium in the milk binds the oxalate, further reducing the amount of oxalate absorbed.

*As discussed previously, this may be different in the approximate 20% of stone formers who have high urinary oxalate as a risk factor; they may be “super-absorbers”. However, we haven’t yet seen a study demonstrating significant absorption of oxalate from tea, even among this group.

Okay, what’s the bottom line? I’m a kidney stone patient – should I stop drinking tea?

Quickly summarizing the above: 1) Tea contains oxalate, which at high levels in the urine can increase your risk of stones. 2) While tea contains oxalate, not much is actually absorbed by your body in most individuals.  3) Based on the only studies available linking beverage intake to actual development of stones, drinking tea may actually decrease your risk of stones. 4) The decreased risk of stones may be due to the increased urine you make when drinking tea

Our final take home points:

• For better or worse, drinking iced tea probably did not “cause” your stone. Cutting out iced tea will probably not make your stones go away but neither do you have to swear off drinking it either.
• If you like iced tea and you are a stone former, drinking a moderate amount is unlikely to cause you to develop more stones. Based on the available research, it may actually decrease your risk of stones.
• If in doubt, ask your doctor for a 24-hour urine test to measure your urinary oxalate levels. If you have normal urinary oxalate, you don’t have to worry much about restricting the amount of oxalate you eat or drink. If however you have high urinary oxalate, work with your doctor to develop a plan to lower your oxalate intake.
• Realize that no one (including us) can really tell you for sure sure whether tea is good or bad for kidney stone patients until a well designed study is done in which a large amount of individuals are randomly assigned into tea drinking and non-tea drinking groups and then followed for many years to see which group develops more stones. This study is unfortunately unlikely to happen.
• In the end, most stone formers might do better for themselves by keeping things simple. Focus instead on the three basic dietary recommendations given below. There is less controversy about the benefits of these recommendations in reducing your risk of stones and as a bonus they will also help to improve your overall health.

1) Increase your overall fluid intake to keep your urine dilute and close to colorless

2) Watch your salt intake- try to keep it at 2300 mg a day or lower*

3) Watch your animal meat intake- try to keep it less than 6oz a day and instead eat more fruits and vegetables*

*The amount of salt and animal protein suggested above are actually the same as those recommended for all adults, stone formers or not, by the FDA and USDA. Examples of the amounts of salt and protein found in common foods are available here.

References 

Brinkley et al, “Bioavailability of oxalate in foods”, Urology 1981

Brinkley et al, “A further study of oxalate bioavailability in foods”, J Urol 1990 

Curhan et al, “Prospective study of beverage use and the risk of kidney stones”, Am J Epidemiology 1996

Curhan et al, “Beverage use and risk of kidney stones in women”, Ann Intern Med 1998

Hirvonen et al, “Nutrient intake and use of beverages and the risk of kidney stones among male smokers”, Am J Epidemiology 1999

Holmes et al, “Contribution of dietary oxalate to urinary oxalate excretion”, Kidney Intl 2001

Krishnamurthy et al, “The urinary response to an oral oxalate load in recurrent stone formers”, J Urol 2003

Laminski et al, “Hyperoxaluria in patients with recurrent calcium oxalate calculi: dietary and other risk factors”, Br J Urol 1991

Massey, “Tea Oxalate”, Nutrition Reviews 2000

Savage et al, “Bioavailability of soluble oxalate from tea and the effect of consuming milk with tea”, Eur J of Clin Nutrition 2003

Shuster et al, “Primary liquid intake and urinary stone disease”, J Chron Dis 1985 

Williams et al, “Oxalate synthesis, transport and hyperoxaluric syndromes”, J Urol 1989