Water in the Body
The amount of water in the body, referred to as Total Body Water (TBW), is very tightly regulated by the body. A complex system of hormonal and kidney responses maintains the fluid levels in our bodies in response to the slightest change in hydration. It is constantly working to maintain an average TBW of about 60%. That means a person weighing 70kg (154 pounds) is made up of about 42 liters (~11 gallons) of water!All body tissue contains water. In fact, 70-80% of fat-free mass is actually water! Fat tissue on the other hand has much lower water content of about 10% water. Therefore, an athlete, who has more muscle and less fat tissue than a sedentary person, will have a higher TBW than a non-athlete. To support healthy bodily functions, it becomes especially important to increase water intake when starting a new exercise program or increasing intensity of your current one.
What Causes Dehydration?
Over the course of a day, TBW fluctuates a lot. Your body loses water due to exercise, heat, physical labor, illness, warm clothing or a combination of any of the above. In efforts to maintain a relatively constant body temperature under such conditions, the body cools itself by sweating. The salty droplets of water that form at the surface of the skin, sometimes dripping and even drenching your clothes, are the most common form of water loss. Sweating rates are different for each individual. How much you sweat will vary, depending on factors such as body composition, genetics, heat acclimation and metabolism. If you are a competitive athlete or are very active, your water requirements are probably a bit different from other athletes. Likely they differ by even more when compared to your peers that are less active or have a lower lean body mass, with your body needing greater amounts of water. Participating in regular physical exercise means you likely have a higher metabolic rate and make physical demands on your body that require your body to cool down more by sweating. Since sweating means that your body is losing water, you must hydrate and replenish.
Risks of Dehydration?
Research is beginning to recognize an increasing correlation between dehydration and chronic disease. There is evidence that chronic dehydration can promote obesity, alter metabolism and is correlated with many other chronic conditions.
Dehydration has also been shown to have a negative impact on mental performance, as well as anaerobic and aerobic exercise performance. For example, a 2% body mass loss due to water loss is recognized as a threshold where aerobic performance can decrease by 7% to as much as 60%! Though the exact physiological mechanism is not yet known, one theory is that decrease in performance is caused by a reduction in the volume of circulating blood and plasma throughout your body. With a smaller volume of blood and plasma filling the heart, a larger fraction of available oxygen in the blood is required for a given amount of work. Basically, physical activity or a typical workout routine will feel more exhausting under a dehydrated state.
How Much is Enough? It’s Not the Same for Everyone!
The Institute of Medicine (IOM) established adequate intake recommendations for water based on national surveys. The daily recommendations are 3.7 liters (~1 gallon) for men and 2.7 liters (~3/4 gallon) for women, which includes water taken in by food consumption. The recommended amount does change however, depending on factors such as the level of physical activity, age, and environment. For example, the IOM recognizes that moderately active adults in warmer climates may require a minimum of 6 liters (~1.5 gallons) of water per day. Athletes engaged in intense training programs, such as bikini and figure competitors, will need more than recommended amount of 1.5 gallons per day and each competitor will have a different need.
Tips to Keep in Mind
- It takes several hours after guzzling a bunch of water before your body is rehydrated, even if your urine turns clear soon after pounding the fluid. So, it’s best to hydrate with water consistently throughout the day.
- Thirst has been shown to be an inadequate indicator of hydration status. Once you feel thirsty you are likely already dehydrated by 1% – 2% body mass. Rehydration doesn’t occur until well after thirst has been quenched. Therefore, don’t rely solely on feeling thirsty to decide on refilling your water bottle.
- Athletes should aim to be hydrated prior to activity, hydrate extra during activity, and continue regular hydration throughout the day after exercise.
- According to an ACSM position paper, variability in sweating rates and sweat electrolyte content between individuals requires customized fluid replacement programs. Individual sweat rates can be estimated by measuring body weight before and after exercise.
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