Hydration for Exercise and Scuba Diving
This is the first of a series of articles about hydration for scuba fitness and diving activities. Due to the amount of varied information available on hydration, and depending on your individual knowledge, information and life experience, you may discover several myths and more importantly solutions for hydration.
Good hydration is important to a successful exercise program and especially important to scuba divers to reduce the risks associated with diving. It is commonly thought that most people are dehydrated most of the time. This general introduction reviews how much of the human body is made up of water, what water provides for the body, how fluid loss occurs, and general recommendations for daily fluid intake.
How Much Water is in the Human Body?
In an adult weighing approximately 155 pounds, the amount of water in the body is 42 liters, nearly 11 gallons or approximately 60%. The amount of water varies depending on percentage of body fat, male or female, and age. Approximately 40% (7 gallons) of the total body weight of the average person is fluid inside the cells and the remaining 20% (4 gallons) is outside the cells. The estimated blood of the average adult body (1.3 gallons) is made up of fluid both inside and outside the cells. A small portion of the fluid in the body consists of specialized fluids, such as synovial fluid which lubricates your joints. These fluids total approximately 1 to 2 liters.1
What Does Water Provide the Human Body?
Water is necessary to maintain homeostasis, the maintenance of nearly constant conditions of every physiological process of the body. "The principal medium of cells is water, which is present in most cells (except fat cells) in a concentration of 70% to 85%. Some cellular chemicals are dissolved in water and some are suspended as solid particles or membranes. Chemical reactions take place among the dissolved chemicals or at the surfaces of the suspended particles or membranes." Important ions of the cell are the electrolytes (such as potassium, magnesium, sodium and calcium) often discussed along with hydration. Ideally, fluid intake and fluid loss are balanced, creating a "steady-state" of hydration, which allows for the "continuous exchange of fluids and solutes with the external environment as well as within the different compartments of the body."1
How Does Water Loss Occur in the Human Body?
Daily water loss occurs by evaporation through the skin, through the lungs when breathing, from sweating, and by excretion.
An estimate of daily fluid loss when hydrated normally ranges from 2,300 ml (a little more than 1/2 gallon) with relative inactivity, to 6,000 ml (1 and 1/2 gallons) when engaged in prolonged heavy exercise. However, a dehydrated individual may only lose 500 ml (.13 gallons) a day while a person ingesting large quantities of water can process as much as 20 liters (5 gallons) a day through the body. "This variability is also true for most of the electrolytes of the body."1
Scuba divers lose additional fluids through increased respiratory water loss from breathing compressed air/gas and immersion diuresis. Compressed air/gas is dry when you inhale it and saturated when you exhale it. Immersion diuresis is an increased production of urine produced by the pressure of being at depth underwater and lower temperatures. Even when you are drinking adequately between dives, "your body can only absorb from 1 to 1-and-1/2 liters per hour" and fluid loss from immersion diuresis may equal the amount of fluid intake. The good news is that the timing of fluid intake before dives, even though it increases urine output, "may temporarily decrease the physiological stress of exertion."2,3
How Much Water Should You Drink?
Water intake per day for individuals between the ages of 19 and 30 years is recommended for men to be 3.7 liters (approximately 13 eight- ounce glasses) and for women to be 2.7 liters (approximately 9 eight-ounce glasses). Drinking water and other fluids makes up about 81% of this fluid intake and water contained in food makes up the other 19%.4
Additional fluid intake is required to replace fluid loss from the diuretic affects of caffeinated beverages, herbal supplements, prescription medications and alcohol. Hot and dry climates, physical activity and scuba diving also increase the need for fluid intake.
The following “Position Stand on Exercise and Fluid Replacement” from the American College of Sports Medicine (ACSM) are hydration recommendations for exercise and, which are also excellent for scuba diving. In addition to water, the ACSM suggests supplementing a sports drink, but only after an hour or more of exercise. Remember most scuba diving is moderate activity, not necessarily physical exertion equal to exercise or sports performance.
More specifically, sports drink intake is limited to 16 to 30 ounces spread over an hour, only after you have already been exercising for more than an hour. This intake is for the purpose of replenishing muscle glycogen to maintain athletic performance. While some of these drinks contain electrolytes, there are other less caloric sources for electrolytes without the risk of too much sugar or salt interfering with hydration.
ACSM Position Stand on Exercise and Fluid Replacement
"1) It is recommended that individuals consume a nutritionally balanced diet and drink adequate fluids during the 24-h period before an event, especially during the period that includes the meal prior to exercise, to promote proper hydration before exercise or competition.
2) It is recommended that individuals drink about 500 ml (about 17 ounces) of fluid about 2 h before exercise to promote adequate hydration and allow time for excretion of excess ingested water.
3) During exercise, athletes should start drinking early and at regular intervals in an attempt to consume fluids at a rate sufficient to replace all the water lost through sweating (i.e., body weight loss), or consume the maximal amount that can be tolerated.
4) It is recommended that ingested fluids be cooler than ambient temperature [between 15 and 22C (59 and 72F)] and flavored to enhance palatability and promote fluid replacement. Fluids should be readily available and served in containers that allow adequate volumes to be ingested with ease and with minimal interruption of exercise.
5) Addition of proper amounts of carbohydrates and/or electrolytes to a fluid replacement solution is recommended for exercise events of duration greater than 1 h since it does not significantly impair water delivery to the body and may enhance performance. During exercise lasting less than 1 h, there is little evidence of physiological or physical performance differences between consuming a carbohydrate-electrolyte drink and plain water.
6) During intense exercise lasting longer than 1 h, it is recommended that carbohydrates be ingested at a rate of 30-60 g h-1 to maintain oxidation of carbohydrates and delay fatigue. This rate of carbohydrate intake can be achieved without compromising fluid delivery by drinking 600-1200 ml h-1 of solutions containing 4%-8% carbohydrates (g 100 ml-1). The carbohydrates can be sugars (glucose or sucrose) or starch (e.g., maltodextrin).
7) Inclusion of sodium (0.5-0.7 g l-1 of water) in the rehydration solution ingested during exercise lasting longer than 1 h is recommended since it may be advantageous in enhancing palatability, promoting fluid retention, and possibly preventing hyponatremia in certain individuals who drink excessive quantities of fluid. There is little physiological basis for the presence of sodium in an oral rehydration solution for enhancing intestinal water absorption as long as sodium is sufficiently available from the previous meal."5, 6
Gretchen M. Ashton is registered with the National Board of Fitness Examiners. An advanced diver, personal trainer and fitness therapist, she specializes in lifestyle fitness and overcoming orthopedic challenges. Gretchen developed the FitDiver® program and is the owner of ScubaFit®. She developed and presented the first physical fitness module for diving certification for the Los Angeles County Scuba Advanced Diver Program, is a seminar speaker at the Scuba Show, and has been published in Alert Diver. Gretchen developed comprehensive peak-performance strength programs for local eco-adventure teams, fire fighters and athletes. As a public speaker and columnist, she has been featured on radio, local television news and in newspapers. As an athlete, she became the second woman in AAU Power Lifting history to receive Hall of Fame recognition setting 21 American and World Records.
1. Guyton, Arthur C., Hall, John E., Textbook of medical physiology, 11th ed., Elevier Inc., Philadelphia, Pennsylvania, 2006 p.291-306.
2.Arthur J. Bachrach and Glen H. Egstrom, Stress and Performance in Diving, Best Publishing Co., 1987 pp. 158-160.
4. Food and Nutrition Information Center (FNIC), Daily Recommended Intake of Water, United States Department of Agriculture (USDA).
5. Douglas J. Casa, PhD, ATC, FACSM, Priscilla M. Clarkson, PhD, FACSM, and William O. Roberts, MD, FACSM, American College of Sports Medicine Roundtable on Hydration and Physical Activity: Consensus Statements, Current Sports Medicine Reports 2005,4:115-127.
6. Victor A. Convertino, Ph.D., FASCM (Chair); Lawrence E. Armstrong, Ph.D., FACSM; Edward F. Coyle, Ph.D., FACSM; Gary W. Mack, Ph.D., Michael N. Sawka, Ph.D., FACSM; Leo Senay, Jr., Ph.D., FACSM; and W. Michael Sherman, Ph.D., FACSM. Exercise and Fluid Replacement, Position Stand, American College of Sports Medicine, January 1996, Vol.28, No. 1.