Water Functions, Absorption and Excretion

Water Functions in the Human Body

The water in your body enables:

• The flow of the blood, lymph, the fluid around the brain and spinal cord, gastric and pancreatic juice and bile
• Urination and thus disposal of waste substances from the body
• Chemical reactions between nutrients and other substances
• Dissociation of minerals into charged particles (ions) and their flow across the cell membranes and thus conduction of electrical signals along the nerves and muscle contractions
• Sweating and thus heat regulation
• Moisturization of the skin, eyes and mucous membranes in the respiratory, gastrointestinal, urinary and genital tract
• Lubrication of the joints

How is the amount of water in your body controlled?

Two main hormones that regulate the amount of water in your body are antidiuretic hormone (ADH) and aldosterone.

When the amount of water in your body falls, your blood volume decreases and your blood sodium level increases, what stimulates the pituitary gland to secrete the antidiuretic hormone (ADH), which reduces water loss through the kidneys ^[3]. The decrease of the blood volume also stimulates the adrenal gland to secrete the hormone aldosterone, which reduces sodium and thus water excretion through the kidneys. The fall of blood pressure also stimulates aldosterone secretion and thus water retention, which helps to increase blood pressure ^[55].

Water Absorption

The flow of water from the stomach into the small intestine (gastric emptying) is limited to about 1-1.5 liters per hour and may vary a lot among individuals ^[15,16,49].

The absorption of water in the intestine is limited to about 20 liters per day, from this about 15 liters in the small intestine and 5 liters in the large intestine (colon) ^[4].

Factors that STIMULATE water absorption:

• The greater the fluid volume in the stomach the faster its emptying from the stomach ^[6].
• Carbohydrates in concentrations up to 6% added to water stimulate water absorption ^[9]. A combination of different carbohydrates, for example glucose and fructose, stimulates water absorption more than either carbohydrate alone ^[53].
• Walking or exercise up to 70% maximal intensity moderately stimulates gastric emptying of water and thus its absorption ^[5].

Factors that SLOW DOWN water absorption:

Factors that do not significantly affect water absorption of beverages: carbonation ^[21,22,23], sodium content ^[9,25] and osmolality within 200-400 mosm/kg range ^[51,54].

How do you lose water?

The Effects of Diet on the Urine Excretion (Diuresis)

A healthy adult normally excretes 800-2,000 mL of urine per day ^[34]. Excretion of less than 800 mL per day is called oliguria and more than 3 liters per day in adults or 2 liters in children is called polyuria. The urination rate may range from no urine in acute kidney failure to more than 20 liters per day due to excessive drinking, diabetes mellitus or insipidus or other health disorders ^[8].

Water. When you are healthy and well hydrated, any additional water you take with beverages or foods will be excreted with the urine within few hours. When you are dehydrated, some water you take will stay in your body but some of it will be excreted by urine in order to remove the waste products of metabolism from your body.

Salt (sodium). Water can stay in your body few hours longer if you consume it along with sodium (in sport drinks or salty foods) than when you consume it without sodium ^[8] but, eventually, you will excrete both excessive sodium and water with the urine.

Proteins. Every gram of protein you consume results in production of about 8 mL of urine, which is required to excrete the waste products of protein metabolism (urea). For, example, consumption of 50 grams of protein results in excretion of about 400 mL of the urine ^[8,33].

A low carb-diet (fasting (no food) results in increased urination ^[35,36]. After the onset of fasting or low-carb diet, glycogen in your liver and muscles will start to break down to provide glucose for the body needs. Each gram of glycogen is stored along with about 4 milliliters of water, so during the first day of a low-carb diet, when all glycogen (about 500 grams) is exhausted, about 2,000 mL of water previously bound to glycogen will be released and excreted in the urine if you continue to consume the same amount of water as before ^[37]. After glycogen exhaustion, your body fat stores will start to break down into ketones, which may be partly excreted with the urine and may drag some water with them, which may result in slightly increased total daily urination volume during the next days of fasting or low-carb diet.

Alcohol appears to stimulate the urine excretion (diuresis) in well hydrated, but not in dehydrated individuals ^[38]. In one human trial, well hydrated males after drinking one liter of beer with 4 vol % alcohol excreted about 160 milliliters more urine in average (within four hours) than after alcohol-free beer. When the same males were dehydrated, they excreted about the same amount of urine after drinking 1 liter of either alcohol-free or alcohol-containing beer ^[38]. In conclusion, alcohol in concentrations up to 4 vol% does not seem to cause dehydration ^[2,30,39].

Caffeine. According to several reviews of studies, individuals who do not regularly consume caffeine can experience transitional increase in urine volume after a single dose of caffeine greater than 250 mg (~20 fl. oz of home-brewed coffee, 60 fl oz of caffeinated cola or various amounts of tea or energy drinks) ^[41]. Individuals who regularly (every day) consume caffeine often develop tolerance to its effects, so they usually do not have increased urine output after consuming up to 250 mg caffeine per day ^[8,40,41]. In individuals with overactive bladder, caffeinated beverages can irritate the bladder what may result in increased frequency of urination but not increased volume of the urine ^[42].

Taurine, an ingredient in energy drinks, has no significant effect on the urine excretion ^[43].

Diuretics (water pills), such as furosemide, chlorothiazide and spironolactone, are prescription drugs that stimulate water excretion in the urine (diuresis). They are used to remove excessive body water in various heart, kidneys, liver, lung or hormonal disorders and in high blood pressure.

Natural diuretics. There is some evidence that certain herbs, such as birch leaves, celery seed, dandelion leaves, goldenrod, rest-harrow root and stinging nettle leaves, may mildly stimulate the excretion of the urine (diuresis) ^[44].

Immersion in water stimulates urine excretion; this is called immersion diuresis ^[45]. Exposure to cold ambient also stimulates urine excretion; this is called cold diuresis. Cold diuresis is self-limited ─ it progresses only up to a certain level of dehydration and can be prevented by a mild exercise ^[46,47].

Sweating

The loss of water through sweating depends on the activity level, ambient temperature, clothing and personal sweat rate.

Chart 3. Sweat Rates
ACTIVITY LEVEL, TEMPERATURE	AVERAGE SWEAT RATES
Sedentary, 41° F (5° C)	2 liters/day
Sedentary, temperate climate	3 liters/day
Sedentary, hot climate	4 liters/day
Active, 41° F (5° C)	3 liters/day
Active, temperate climate	5 liters/day
Physical work, hot climate	2-10 liters/8 hour shift
Light work in protective clothes, hot climate	1-2 liters/hour
Runners, temperate climate	0.7-1 liters/hour
Runners, hot climate	1-3+ liters/hour

 Chart 3 source: ^[48]