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Nutrition Column

Just Add Water

Most people have heard the recommendation to drink at least eight glasses of 8 ounce of liquid per day, but how was this recommendation agreed upon? To be honest, there is no single study that shows drinking eight glasses of 8 ounce of liquid per day are necessary for optimal heath. We know that humans cannot survive for more than a few days without consuming water, but researchers do not always agree on the potential benefits of water drinking.

A common claim is that increased water intake improves kidney function and clearance of toxins. However, a short term study found that increased water intake actually decreased the rate of glomerular filtration. Fortunately, there are other means by which the body can clear toxins. The same study found that the total clearance of solute increased as water intake increased, most likely due to decreased reabsorption in the kidneys. Among the solutes excreted was sodium, which plays an important role in controlling hypertension. Although clearances of certain substances are increased due to water intake, little is known about the identity of toxic substances cleared by the kidney so no concrete conclusions can be made.

Increasing water intake may also positively influence satiety. In a study by leading diet researcher Barbara Rolls, the authors reported that increasing the water content of foods decreased caloric intake. Surprisingly, adding water to a meal had no such effect. The decrease in overall caloric intake was not seen when participants were offered water in parallel with their meal. Another study published in the Journal of Clinical Endocrinology and Metabolism showed that increasing water intake increased body heat so that more calories were used via thermogenesis. Drinking two liters of cold water increased caloric expenditure approximately 95 calories per day. Despite these promising findings, there is not enough evidence to clarify the potential benefits of water intake on fighting the obesity epidemic.

Finally, a reveiw of epidemiological studies published in the European Journal of Clinical Nutrition,reported that as water intake decreases, the incidence of coronary disease, bladder cancer, and colon cancer all increase. However, it is difficult to prove a cause-and-effect relationship based upon epidemiological data alone. Do these patients become sick because they drink less water, or do they drink less water because they are sick?

There is a lack of general evidence supporting or refuting the potential health benefits of consuming water, but there are no studies that show a negative relationship between water consumption and health. As long as the water is clean and it is not consumed in excessive amounts, it is an appealing option to stay hydrated and refreshed.

Written by Ryan Serrano with Eric Sternlicht, Ph.D. Occidental College, Los Angeles, CA.