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Nutrition Basics: Fluid Requirements for Healthy Adults

Author: Stan Reents, PharmD
Original Posting: 05/06/2007 07:36 AM
Last Revision: 08/04/2020 12:38 PM

Pretty much everyone knows that if you don't drink enough fluids when you've been perspiring heavily, you can get overheated and "other bad things" might happen.

But you may not realize that maintaining an optimum state of hydration has other health benefits, too. Here's a brief list (Horswill CA, et al. 2011):


• Brain health: Inadequate hydration has been shown to impair cognitive performance in adults, in adolescents, and in children.

• Cardiovascular health: Adequate hydration lowered the risk of heart attacks in a key study (see below).

• Colon health: Adequate hydration lowers the risk of polyps and colon cancer.

• Kidney health: Adequate hydration lowers the risk of kidney stones and bladder cancer.

• Weight management: Some research, though not all, shows that drinking water before a meal can help control how much you eat.


Many people have heard the recommendation to drink 8 glasses of water per day. However, this guideline was never based on solid scientific evidence (Schnirring L. 2004). It's possible that this recommendation came from John Harvey Kellogg, MD, dating back to the late 1800's. While Dr. Kellogg is most well-known as the founder of Kellogg's, between the late 1800's and up to The Great Depression, he ran a wellness spa in Battle Creek, Michigan. Although he had many unconventional ideas about health, he was one of the first to advocate exercise, a vegetarian diet, and reducing stress (Markel H. 2011).

Until recently, about the only definitive research regarding fluid intake and health was a study of Seventh Day Adventists that was published in 2002. Researchers at Loma Linda University in California reported that people who drank at least 5 glasses of water per day had a lower risk of a heart attack during the 6-year period than those who drank 2 or fewer glasses per day (Chan J, et al. 2002).

But even this information is not very precise: how much water is "5 glasses" anyway? And what about people who live in extremely dry climates? In Arizona, for example, the humidity can get down to as low as 2%.

To establish better guidelines on fluid intake, a panel of experts from the US and Canada was convened on behalf of the Institute of Medicine. Their excellent report was made public in February 2004. Some of the recommendations are listed below:

For healthy sedentary adults living in temperate climates:

• MEN 125-oz. (3.7 liters) per day
• WOMEN 91-oz. (2.7 liters) per day

Foods supply 20% of our daily fluid intake. But stop for a minute and think about how much fluid that remaining 80% actually is. The requirement for men is roughly 3 quarts per day. How many of you men out there drink this much water every single day? For women, the requirement is over 2 quarts per day. Do you drink that much? If you are like most adults, you are in a state of mild dehydration.


Some athletes require even more fluid than what is recommended in the Institute of Medicine report. For example, researchers at West Chester University in Pennsylvania determined that their cross-country runners needed 4.6 liters per day and that football players can require as much as 12.2 liters per day. That's over 3 gallons per day! (Godek SF, et al. 2005).

At these amounts, it shouldn't be surprising that athletes don't drink enough either, even though they may know that dehydration can impair exercise performance.

The same problem exists in young athletes. In one report, 34 boys (average age 13 yrs) and 24 girls (average age 12 yrs) who participated in a 4-day soccer camp in Pennsylvania during July 2003 were studied. Most of the players were dehydrated by the second day of camp; more than half of the girls were seriously dehydrated at this point. By the last day of camp, 59% of boys and 70% of girls were significantly dehydrated. This occured despite the fact that coolers of fluids were available within 25 meters and coaches encouraged players to drink and take frequent rest breaks (Mamula PW. 2004).


Part of the problem is that the body's thirst response is not very precise. First, you can lose significant amounts of body water before you begin to feel thirsty. The thirst reflex is even more flawed in the elderly.

Even in elite athletes, the thirst reflex cannot be relied upon to maintain adequate hydration. Studies in soccer players show that athletes often do not drink enough to offset their sweat losses, regardless of whether they train in the heat (Shirreffs SM, et al. 2005) or a cooler environment (Maughan RJ, et al. 2005).

Second, drinking cold water shuts off the drive to drink before enough volume has been consumed. In addition to temperature (of the beverage), researchers at Gatorade® have shown that flavoring and sodium content can also affect how much fluid an athlete will drink before he/she feels satisfied.

Determining the correct amount of fluid an athlete needs while exercising or competing is highly variable. Specific formulas exist. See Fluids and Electrolytes During Exercise for a thorough discussion of this.


So far, we've discussed the amount of fluid that should be consumed daily. Now, what about specific types of beverages?

In March 2006, the American Journal of Clinical Nutrition published a set of guidelines that address this question. The report was produced by the "Beverage Guidance Panel", a group of scientists organized by Barry M. Popkin, PhD, a professor at the School of Public Health, University of North Carolina, Chapel Hill. The mission of this group was to develop dietary guidelines that address the persistent obesity epidemic, but their recommendations are relevant for this discussion as well.

The Panel grouped beverages into 6 categories. They ranked them from most important to least important, and provided recommended daily amounts of each:

• Level 1
(most important)
water 20 - 50 oz. per day
• Level 2 coffee & tea 0 - 40 oz. per day
(can replace
an equivalent
amount of water)
• Level 3 • low fat/skim milk
• soy beverages
0 - 16 oz. per day
• Level 4 "Non-calorically Sweetened" Beverages
(eg., Diet Coke®)
0 - 32 oz. per day
• Level 5 "Caloric Beverages
With Some Nutrients"
(eg., 100% fruit juices,
sports drinks, alcohol)
Fruit juices: 0-8 oz./day
Alcohol - women: 0-1 drinks/day
Alcohol - men: 0-2 drinks/day
• Level 6
(least important)
"Calorically-Sweetened" Beverages
(eg., non-diet soft drinks)
0 - 8 oz. per day

Regarding sports drinks (see Level 5 above), the Panel recommended that these beverages be consumed "sparingly," except by endurance athletes. Because sports drinks contain sodium, they are preferred over plain water when sweating is profuse or prolonged. This is particularly important when:

  • the sweat rate > 8 liters/hour
  • strenuous exercise lasts > 60 minutes
  • there is a deficiency of sodium or carbohydrates available for consumption

This issue is discussed in greater detail in Sports Drinks and Recovery Drinks.


I have several thoughts on these recommendations from the Beverage Guidance Panel:

Water: The water recommendation of 20-50 oz. per day seems low, particularly if we refer to the Institute of Medicine guidelines which recommend 91-125 oz. per day. 20-50 oz. is only one-half to 1.5 quarts of water per day. And allowing for a reduction if someone drinks a lot of coffee -- which most people do -- reduces it further.

Milk: The panel recommends a range of 0-16 oz. per day. This should be clarified according to age group: Children should drink milk. But, there is growing evidence that adults should not. Marlene Zuk, PhD ("Paleofantasy" 2013) states that adults do not need milk. Joel Fuhrman, MD ("The End of Heart Disease" 2016) is more emphatic: he recommends that adults not drink any milk at all. Nevertheless, milk can be an ideal post-exercise sports drink (Karp JR, et al. 2006).

In my opinion, the panel should have written the milk recommendation as "8-16 oz. per day," or even "12-16 oz. per day" for teenagers and "none" for adults. Note that some brands of chocolate milk contain high-fructose corn syrup....these brands should be avoided. (see below)

Soy beverages: A report appeared in the June 2009 issue of Men's Health magazine of a 55-yr old man who developed profound feminizing traits (development of breasts), loss of libido, and emotional changes after consuming 3 quarts of soy milk per day. When his estrogen levels were measured, they were 8 times higher than normal limits for men, and higher than normal levels for women. It turns out that soy contains at least 2 molecules that possess estrogen-like actions: daidzein and genistein. While this person was consuming an unusually high amount of soy milk every day, I caution older men to avoid consuming soy products. Most older men already have health issues due to a decline in testosterone levels....they don't need high estrogen levels complicating that picture further.

"Calorically-sweetened" beverages: Almost all non-diet soft drinks in the US are sweetened with 55% high-fructose corn syrup (HFCS) (Bray GA, et al. 2004). There is substantial research showing that HFCS contributes to obesity (Bray GA, et al. 2004) and a worse lipid profile (Krilanovich NJ. 2004). The Center for Science in the Public Interest is so concerned about HFCS that, in 2005, they submitted a petition to the FDA to have this food additive removed from our food supply. In my opinion, the recommended intake of beverages sweetened with HFCS should be zero. Period.


Q: Does the use of caffeine, alcohol, or prescription diuretics change these new fluid-intake recommendations?

ANSWER: This question requires three separate answers:

Caffeine: Most people know that caffeine and alcohol have diuretic-like effects. However, caffeine is a relatively mild diuretic and this effect appears to diminish with daily consumption. Thus, regular caffeine use by non-athletes is probably less of an issue than failure to meet these new fluid-intake recommendations. Further, the kidney can easily reverse the diuretic effect of caffeine once exercise begins (Wemple RD, et al. 1997). So, it would seem that caffeine-drinkers don't need to increase their daily fluid intake beyond the amounts recommended in the 2004 Institute of Medicine (IOM) guidelines.

Alcohol: Alcohol is a stronger diuretic than caffeine. In addition, it doesn't appear that your body develops a tolerance to the diuretic effects of alcohol like it does for caffeine. Thus, dehydration is just another in a long list of reasons to minimize the amount of alcohol you consume. On days that you might drink too much alcohol, then you definitely want to meet the 2004 IOM recommendations.

Diuretic drugs: Recommending fluid intake for someone who takes prescription diuretics is much more complicated, and can only be answered by your physician. Diuretics have many medical uses, so we cannot offer one simple answer that might apply to all these possible scenarios.

Q: Is caffeinated water (ie., bottled water with added caffeine) a bad idea?

ANSWER: Not necessarily. Though, caffeine is caffeine. Whether it comes from coffee, tea, soda pop, or caffeinated water, the effects will be the same. Caffeine is ergogenic (ie., enhances performance) in some sports. But, this is highly unlikely with caffeinated water; the amounts of caffeine in these beverages are too low: an average cup of java contains 75-150 mg caffeine, a 12-oz can of cola contains 40-50 mg, and caffeinated water contains even less than that. (Note that some energy drinks may contain very large amounts of caffeine.)


For most people, attaining these goals for fluid intake will require a substantial change in daily routine. However, keep in mind that you don't have to take it all in as water (or other beverages). Roughly 80% of your total daily fluid intake comes from drinks; the other 20% comes from fruits, vegetables, and other foods. Still, consuming this much fluid on a daily basis will require some conscious effort.

Also, these recommendations do not apply to endurance athletes, or anyone else for that matter who perspires for an extended period of time.


The Institute of Medicine report mentioned above is available as the book Dietary Reference Intakes for Water, Potassium, Sodium, Chloride and Sulfate. It can be obtained at the National Academies Press web site:

Readers may be interested in the following related stories:


Stan Reents, PharmD, is available to speak on this and many other exercise-related topics. (Here is a downloadable recording of one of his Health Talks.) He also provides a one-on-one Health Coaching Service. Contact him through the Contact Us page.


Bray GA, Nielsen SJ, Popkin BM. Consumption of high-fructose corn syrup in beverages may play a role in the epidemic of obesity. Am J Clin Nutr 2004;79:537-543. Abstract

Chan J, Knutsen SF, Blix GG, et al. Water, other fluids, and fatal coronary heart disease: The Adventist Health Study. Am J Epidemiol 2002;155:827-833. Abstract

Godek SF, Bartolozzi AR, Godek JJ. Sweat rate and fluid turnover in American football players compared with runners in a hot and humid environment. Br J Sports Med 2005;39:205-211. Abstract

Horswill CA, Janas LM. Hydration and health. Am J Lifestyle Med 2011;5:304-315. (no abstract)

Karp JR, Johnston JD, Tecklenburg S, et al. Chocolate milk as a post-exercise recovery aid. Int J Sport Nutr Exerc Metab 2006;16:78-91. Abstract

Krilanovich NJ. Fructose misuse, the obesity epidemic, the special problems of the child, and a call to action. Am J Clin Nutr 2004;80:1446-1447. Abstract

Mamula PW. Dehydration risk during summer youth sports camps. Phys Sportsmed 2004;32:15. (no abstract)

Markel H. John Harvey Kellogg and the pursuit of wellness. JAMA 2011;305:1814-1815. (no abstract)

Maughan RJ, Shirreffs SM, Merson SJ, et al. Fluid and electrolyte balance in elite male football (soccer) players training in a cool environment. J Sports Sci 2005;23:73-79. Abstract

Popkin BM, Armstrong LE, Bray GM, et al. A new proposed guidance system for beverage consumption in the United States. Am J Clin Nutr 2006;83:529-542. Abstract

Schnirring L. Dietary guidelines for water and electrolytes. What role for thirst? Phys Sportsmed 2004;32:12, 41. (no abstract)

Shirreffs SM, Aragon-Vargas LF, Chamorro M, et al. The sweating response of elite professional soccer players to training in the heat. Int J Sports Med 2005;26:90-95. Abstract

Valtin H. Drink at least eight glasses of water a day. Really? Is there scientific evidence for "8x8"? Am J Physiol Regul Integr Comp Physiol 2002;283:R993-R1004. Abstract

Vieira Potter VJ. Eat your water for health, sport performance, and weight control. Am J Lifestyle Med 2011;5:316-319. (no abstract)

Wemple RD, Lamb DR, McKeever KH. Caffeine vs caffeine-free sports drinks: Effects on urine production at rest and during prolonged exercise. Int J Sports Med 1997;18:40-46. Abstract


Stan Reents, PharmD, is a former healthcare professional. He has been a member of the American College of Lifestyle Medicine (ACLM) and a member of the American College of Sports Medicine (ACSM). In the past, he has been certified as a Health Fitness Specialist by ACSM, as a Certified Health Coach by ACE, as a Personal Trainer by ACE, and as a tennis coach by USTA. He is the author of Sport and Exercise Pharmacology (published by Human Kinetics) and has written for Runner's World magazine, Senior Softball USA, Training and Conditioning and other fitness publications.

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