I am sure that you have heard of the well-publicized study from the University of Texas that suggested that low carbohydrate diets increase the risk of kidney stones. For several days in August, this study grabbed national health news headlines with some media commentators boldly referring to this study as a “body blow” to low carbohydrate diets, particularly to the Atkins diet. However, close inspection of the study reveals a different picture that simply does not justify the hyperbole.
To fully understand this study and what it means, it is necessary to explain some basic concepts about the physiology of kidney stones and how they form. (Before your eyes glaze over, let me say that this primer will be simple and brief – I promise!) The primary job of our kidneys is to filter our blood and remove all unnecessary substances, waste products, and excess water from it. According to the Mayo Clinic, about 80 to 85 percent of kidney stones are formed when small particles of calcium are removed from the blood and enter the urine and combine with a second substance known as oxalate. Oxalate is found in many fruits, vegetables and grains and is present in our blood and urine after we digest these foods. Most kidney stones form when tiny crystals of calcium and oxalate combine and begin to grow and form hard masses. They grow much like a snowball tumbling down a hill collects more snow as it goes along. These calcium-oxalate crystals collect more and more crystals until they form hard masses that look very much like stones or rocks, hence the name.
Of the 15 to 20 percent of kidney stones that are not made up of a combination of calcium and oxalate, most are formed from a substance that results after protein metabolism known as uric acid. In other words, most kidney stones are formed from the byproducts that naturally occur as our bodies digest the food we eat.
But the substances that make up kidney stones are only half of the story. There are also substances in our urine that inhibit kidney stone formation that in effect, keep the snowball from rolling down the hill. Two of these substances are citrate and magnesium. But the most important substance that prevents kidney stones is water. When plenty of water is available for our kidneys to use to flush out waste products, calcium, oxalate, and uric acid are much less likely to collect and form stones.
In the University of Texas study entitled Effect of Low-Carbohydrate High-Protein Diets on Acid-Base Balance, Stone-Forming Propensity, and Calcium Metabolism, published in the August 2002 issue of Amercian Journal of Kidney Diseases, researchers restricted the carbohydrate intake of ten study participants to 20 to 40 grams per day, thereby inducing ketosis for six weeks. During this time, the researchers monitored concentrations of calcium, uric acid and citrate (among other substances) in the urine of the participants. The researchers found that the concentrations of calcium and uric acid increased, while concentrations of citrate fell. Since many kidney stones are made of calcium and uric acid, and citrate is believed to retard stone formation, the researchers speculated that low carbohydrate diets lead to kidney stones. Sounds simple, doesn’t it?
As many of us discover as we go through life, medical matters are rarely that simple. Such is the case with this study. It has several shortcomings that call in question this simple conclusion. First, the study was very small and very short. It studied only ten people for only six weeks. It was not long or large enough to account for the fact that our bodies have an amazing capacity to adapt to our environment, and it did not determine how our kidneys react and adapt to very low carbohydrate diets and ketosis in the medium-to-long term. In medical and research communities, the reliability of a study of such a small group for such a short duration would immediately be discounted. Small and brief studies are generally viewed as being indicators of a need for more comprehensive research at best. In fact, many of these small and brief studies are conducted simply to justify funding for larger studies, rather than to change medical practice or recommendations. In other words, studies such as these are most useful in raising and clarifying questions rather than answering them. Therefore, this study was much too small to prove that very low carbohydrate diets cause kidney stones — it simply raised a question that requires further study.
Second, the researchers did not report the levels of oxalate found in the participants-urine. Remember that I mentioned earlier that 80 to 85 percent of all kidney stones are formed from a combination of calcium and oxalate? The researchers did not report the levels of oxalate, and since it accumulates in the blood and urine after our bodies break down fruits, vegetables, and grains one could only assume that oxalate levels would be low in people who were eating between 20 to 40 grams of carbohydrate per day. Therefore, the following question forms: Since most kidney stones are made up of a combination of calcium and oxalate, would it not follow that when oxalate levels are low the likelihood that kidney stones would form would decrease? This is yet another question left unanswered by this study that will require further long-term research.
Perhaps the most serious limitation of the study is that it did not report on the critical issue of water intake. Failing to report the participants’ water intake is a major limitation because the amount of water a person drinks dramatically affects the degree of urine concentration which in turn, dramatically affects the results of urine tests. For example, calcium concentrations would test higher in the urine of people who are not well hydrated while they would test lower in the urine of people who have higher fluid intakes. Controlling, or at least tracking fluid intake before, during and after invoking ketosis would help to answer several questions that are unanswered by this study.
Lastly, the participants were on a very low carbohydrate diet of 20 to 40 grams of carbohydrate or less per day. The study findings are further limited to very low carbohydrate diets that are ketogenic, and cannot be applied to other low carbohydrate diets that call for a higher carbohydrate intake.
So, if we cannot draw definitive conclusions from this study the obvious question follows: Are there other studies that could settle the question for us? As I was preparing this article, I searched for studies that would either confirm or dispute this study. Given the enormous amount of controversy on this topic for the past several years, I was startled to find that there are very few medical studies on the subject. I was able to locate only one study published in 1983 in the journal Metabolism entitled “The Human Metabolic Response to Chronic Ketosis Without Calorie Restriction: Physical and Biochemical Adaptation,” that studied at the affects of very low carbohydrate diets and ketosis on kidney function and other important body functions. This study is remarkably similar to the University of Texas study in that it placed nine participants on a very low carbohydrate diet of twenty grams per day for four weeks. The key difference is that in this study, the researchers followed the participants for six months. They found no changes in kidney function, including no evidence of kidney stone formation, in any of the study participants.
As you may have noticed, this study has many of the same limitations as the University of Texas study. It is small and short, it studied a very low carbohydrate, ketogenic diet, and it did not report the participants’ water intake. Since both of these studies have many significant shortcomings, and the conclusions of the researchers are unproven, what are we to believe? The short answer is that the jury is still out, and neither of these studies do much to settle the question of whether or not ketogenic diets are detrimental to kidney health. They simply raise questions that have been raised many times before.
If you have concerns that a very low carbohydrate ketogenic diet may put you at increased risk for kidney stones, carefully consider your options. Kidney stones are a serious medical matter that should not be taken lightly and I know from personal experience how painful they are. When I was in college, long before I adopted my low carbohydrate lifestyle, I passed several kidney stones without the aid of medication. It is an experience I will never forget. Since I have a personal and family history of kidney stones, I carefully consider and monitor my kidney health. I am a religious water-drinker and I have successfully kept kidney stones at bay and maintained normal kidney function for nearly 20 years (even while I lost 40 pounds through my low carbohydrate lifestyle). I consult at least annually with my physician about my kidney function. My latest kidney tests (in the spring of this year) revealed normal kidney function on all parameters. If you are concerned, you too should consult with your physician about your medical status and your family history to assess your overall risk for kidney stones. Be sure to discuss this risk in the contest of all the benefits of your low carbohydrate lifestyle. For example, your kidney tests should be balanced with other test results such as lowered cholesterol, lowered blood pressure, blood sugar level control, decreased obesity, etc. If your medical consultation leaves you with ongoing concerns of your kidney stone risk, there are a couple of steps you can take to minimize your concerns without abandoning the low carbohydrate lifestyle.
First and foremost, drink plenty of water. You have heard it over and over again for years: Drinking plenty of water is essential for proper kidney function regardless of your diet. The usual medical recommendation is to drink at least 64 ounces of water per day, but many low carbohydrate experts recommend more, especially if you have a significant amount of weight to lose. A common rule of thumb is to drink half of your weight in pounds in ounces each day. For example, if you weigh 200 pounds then drinking 100 ounces of water is in order. Personally, I drink at least a gallon (128 ounces) of a combination of water and decaffeinated tea every day. How much water you should drink should be part of the consultation with your physician.
If you are very concerned, you may consider adopting a low carbohydrate plan that does not invoke ketosis, if you are not already following one. There are currently over ten low carbohydrate plans on the market and only two of them call for a restriction of carbohydrate to the point of invoking ketosis. One of them, Dr. Atkins’ New Diet Revolution, strongly recommends ketosis for weight loss and Dr. Atkins states that he has treated over 70,000 people with his diet without detrimental affects on their kidneys. In the second ketogenic diet, Protein Power, ketosis is actually optional. The authors of Protein Power state that they often recommend that dieters lower their carbohydrate intake to the point at which they are at the “cusp” of ketosis, although they also state that their diet has not had a detrimental affect on the kidneys of the thousands of patients they treated in their medical practice. Other plans such as SugarBusters and The Schwarzbein Principle offer low carbohydrate lifestyles that do not involve ketosis.
So there you have it. I know you hoped that this article would provide you with a definitive answer to the protracted, drawn-out debate about low carbohydrate diets and kidney stones, but unfortunately it remains elusive. Hopefully, the answer will come soon in the form of a well-designed, long-term study. When it does, I will be sure to report it to you!
Until then, Happy Reading!