How much water do you need?

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).

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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