Graph showing urine output volume by hydration level. At 100% hydration, average urine output is 50 cc/hr or 1200 cc/day. Kidney stone patients should aim for 100 cc/hr or 2400 cc/day.
Most kidney stone patients are used to being told to increase their fluid intake – for good reason, as urinating 2-2.5 liters a day has been shown to reduce the recurrence of stones by 50%. However, measuring your urine output everyday to see if it matches up to this goal isn’t very practical. So what are alternative ways to check if you are drinking enough?
Option #1: Focus on your amount of fluid intake instead:
Once a person is well hydrated, almost all of the excess fluid he or she drinks is rapidly cleared by the kidneys. So if you’ve always thought that “water goes right through me”, you’re right. The graph above, adapted from Lee’s Handbook of Physiology, 1964, shows urine volume per hour compared to hydration status.
When a person is 100% hydrated, urine volume is on average 50ml per hour. Translated into 24 hours, that’s equal to 1200 ml or 1.2 liters, which happens to be the normal urine output seen in most studies of kidney stone patients. Hydration above 100% (by drinking more than your thirst tells you to) leads to a rapid rise in urine output, as the body uses the kidneys to get rid of this “extra” fluid.
For stone formers, the recommendation is to make about 2.4 liters of urine a day. Assuming you are 100% hydrated by drinking to your thirst, you should be making 1.2 liters of urine. If you then drink another liter of fluid on top of this, you should be able to achieve a urine output close to 2.2 liters as the body will direct most of this “extra” liter of fluid into urine. (A liter of fluid is equivalent to 4.2 eight-oz cups, or about 34 oz). Some individuals truly have a lower thirst point and may be walking around in a constant state of dehydration, for them, a higher fluid intake is needed to first get to 100% hydration. Additionally, when it’s hot outside or if you are exercising, you will lose more fluid as sweat – in the heat of the desert, water losses through sweat can reach 4.9 liters a day! In these situations, a higher intake of fluid is mandatory.
Option #2: Go by the color of your urine:
Researchers have developed a urine 8-level color chart to estimate hydration status. To be well hydrated, urine color should be almost clear, very light yellow, or light yellow (1 through 3). Urine color is a good estimate of hydration for most purposes and it is generally as effective as using volume or urine specific gravity for this purpose. Keeping track of your urine color therefore can be a quick and easy way to check on your hydration status. As a stone former, you actually need to be “over” hydrated, so you should try to keep your urine almost clear (1 on the color scale). It should be noted that urine color can be altered by certain foods or vitamin intake, both of which can make your urine darker than normal.
Option #3: Measure your urine specific gravity with a dipstick:
In a study from 1991, stone formers were given urine “dipsticks”. These are small pieces of paper that show the results of chemical tests by color change. The patients were given these dipsticks and instructed to monitor their urine specific gravity. They were found to be successful in increasing their urine volume using this technique. Having a more dilute urine means having a low specific gravity. Therefore, the patients were instructed to keep their specific gravity below 1.010. While effective, the disadvantage of this approach to monitoring hydration status is the requirement to carry test strips and the need to either urinate onto the strips directly or dip the strips in a urine container.
What is normal amount of fluid intake?
You may be now wondering what is a “normal” amount of fluid intake. National survey data was used by the National Academy of Sciences to set “Adequate Intake” levels which are meant to represent enough fluid intake to avoid dehydration related medical problems. Adequate fluid intake for young men and women (19 to 30 years old) in the United States is set at 3.7 liters and 2.7 liters a day, respectively. Of this, about 81% comes from beverages with the rest coming from food moisture. Adequate daily beverage intake therefore is 3.0 liters (about 13 eight-oz cups or 101 oz) for men and 2.2 liters (about 9 eight-oz cups or 74 oz) for women. Remember that as a stone former, you may actually need to exceed these amounts.
Armstrong and colleagues, “Urinary indices during dehydration, exercise, and rehydration.” International Journal of Sports Nutrition. 1998.
Lee’s Handbook of Physiology, 1964
McCormack and colleagues, “The urine specific gravity dipstick: a useful tool to increase fluid intake in stone forming patients.” Journal of Urology. 1991.
National Academies Press, “Dietary Reference Intakes for Water, Potassium, Sodium, Chloride, and Sulfate.”, 2004. Available from http://www.nap.edu/catalog/10925.html