Water is the
quintessential nutrient of life, yet it is ironically often ignored as a
dietary constituent.
Individuals are
routinely at a risk of mild dehydration day to day (2). Public surveys (10,11)
and experimental trials (12,13) indicate that the general public, and most
importantly special populations such as children
and older adults, are at a risk of voluntary dehydration (14,15). Even experienced athletes can underestimate
their hydration status and may drink insufficient amounts of water
resulting in sustained dehydration (16).
Sustained dehydration
is associated with poor health (3,4)
and increases the likelihood of kidney
stones and urinary tract infection
by a significant degree (3,5). Additionally, prolonged vasoconstriction due to
chronic dehydration increase the risk of
hypertension and stroke (6).
An emergent
body of evidence also suggests water consumption (and the food we eat) affect
mental and physical performance (7). Water is essential to the maintenance of
normal physical and cognitive function (8), and there are some recommended intake guidelines of 2000 ml of
fluids for females and 2500 ml for males per day (9), or more specifically to
cover the needs of the vast majority of persons it can be recommended 3.7 L for 70 kg males and 2.7 L for 57 kg
females (17,18).
The human body
is approximately 60% to 70% water (with a range of 45-75%) (1). It can be less
with increasing body fat because fat is known
as "anhydrous" with about 10% water (17,22), however fat-free mass can be 73% water (1,17).
An average 70-kg person has approximately 42 L of total body water, with a
range of 31–51 L (1). Improper hydration will result in either dehydration or overhydration (hyponatremia). Daily water balance depends on the
net difference between water gain and water loss.
Approximately 5% to
10% of total body water is turned over daily (17,23). Respiratory water loss is influenced by temperature and humidity
and pulmonary ventilation; metabolic
water is formed by oxidation of substrates and is roughly offset by respiratory water losses; urine output generally approximates 1 to
2 L per day (17).
Comparison of water needs estimated using water
balance or water turnover methodologies for sedentary
and active
people (17).
Most of the infant’s
water needs comes from the consumption of human milk or formula, but as the
infant develops more and more water is acquired by a variety of beverages and
food (1,17). Total body water per kilogram of body mass is highest in infancy
and gradually declines (17,22) as fluid regulatory mechanisms mature.
By puberty responses such as sweating
become similar to adults (24) but thirst and hunger continue to be the primitive
biological drives that compensate well for obligatory water losses (17). For children
and adults, about 80% of total daily water intake is obtained from beverages
and about 20% from food (1).
In early infancy daily
water needs are about ≈0.6 L, and through childhood they are about ≈1.7 L
(17,25,26). For adults the daily water needs of men approach 2.5 L if sedentary
(17,27,28) and increase to about 3.2 L if performing modest physical activity
(17,29,30).
Living in a warm
environment can increase daily water needs of active adults to about 6 L
(17,31). Daily water requirements for
any given energy expenditure in temperate climates (20°C) can triple in very
hot weather (40°C) (17).
Water needs across the lifespan. Columns with
dashed horizontal lines represent requirements for girls or women in that age category. Data are from the IOM (17,1).
dashed horizontal lines represent requirements for girls or women in that age category. Data are from the IOM (17,1).
Daily water turnover
is 3.3 L and 4.5 L for sedentary and active men, respectively (17,23,32,33-39),
but for more active populations values can be as high as ≈6 L (33). Women generally
have lower daily water turnover rates ranging from 0.5—1.0 L less than males.
With aging the fluid regulatory capacity can
decline due to reduced renal
concentrating and diluting capacity (40,42,42) accompanied by a diminished thirst
drive (43); the elderly are less sensitive to the thirst mechanism due to the
deterioration of osmoreceptor sensitivity (44,45,46,47,48).
Water functions
1. Transport
functions. Water
transports nutrients from digestion to the cells. Adequate blood volume
enhances nutrient delivery.
2. Mechanical
functions, for example
lubricating the joints with synovial fluid, or the eyes with in tears to remove
any debris or dirt.
3. Regulate body temperature.
Water storage
Water is the
medium of circulatory function, biochemical reactions, metabolism, substrate
transport across cellular membranes, temperature regulation, and numerous other
physiological processes (19).
Water is stored
in 3 important spaces (20,21); body fluids are stored in
two compartments: inside the cells (intracellular ∼60%) and outside the cells (extracellular ∼40%). The extracellular compartment is further subdivided into the
intravascular and interstitial fluid compartments, which make up ∼20 and 80%, respectively (21).
1. The intravascular
space. This consists of all the blood cells and plasma inside the blood
vessels (arteries, veins, and capillaries). This is crucial for circulation,
respiration, oxygenation, and transport of nutrients and removal of waste
products.
2. The interstitial
space. This is the space
between the cells which provides the immediate microenvironment that allows for
movement of ions, proteins and nutrients across the cell barrier. When
excessive fluid accumulates in the interstitial space, edema develops.
3. The intracellular
space (60%), which is the
space inside the cells containing water and salt. Water is the most abundant
molecule in a cell. Organisms don’t tolerate much of a change of volume
in the intracellular space or its osmolality (the body's
electrolyte-water balance).
The loss or
increase in fluids and electrolytes (potassium, sodium, calcium, magnesium…)
affects cellular performance, and can cause cell death, and even death of the
entire organism.
Recommended
intake:
0-1y: 0.7-0.8L
1-3y: 1.6L
4-8y: 1.7L
9-13y: 2.1-2.4L
14-18y:
2.3-3.4L
19-70y:
2.7-3.7L
3.7 L for 70 kg
males;
2.7 L for 57 kg
females.
Up to 6L for
active adults living in a warm environment.
A better approach should be monitoring urine color. Read Hydration: Assessing hydration
status
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