Water as a Beverage and Constitutent of Food
Water as a Beverage and Constituent of Food
The human body has a water content that represents from 65 to 70 percent of its total mass. This means that a 70-kilogram (154-lb.) adult male has a fluid content of 45 to 50 liters of water. Only 3.6 to 4.0 liters or 8 percent of this fluid is in the bloodstream. Of the body's total water content, 60 to 70 percent is within the cells, and an additional 20 percent is in the intercellular space. These fluid compartments differ with respect to certain important constituents such as ions (electrolytes) that play an important role in cell function. The maintenance of homeostatic balance in each of these fluid compartments requires finely tuned interactions of the endocrine system and the organs involved in the absorption and excretion of minerals and other nutrients. The chemical properties of the food and water consumed can have profound effects on the function of the organs maintaining this balance. An extreme example of imbalance with fatal consequences is the dehydration that follows the consumption of salty seawater. When this occurs, the kidneys are forced to increase urine production to rid the body of the excessive salt, with the result that blood and intercellular fluid volume are reduced. In extreme cases such loss can disrupt fluid homeostasis sufficiently to result in death.
Humans differ from other primates in relying heavily on evaporative cooling achieved through the sweat response to maintain normal body temperature under increased heat loads. Sweat contains proteins and minerals in addition to water. Heavy, sweat-inducing exercise, especially under warm, humid conditions and in untrained individuals, can result in the loss of excessive salt in sweat. Water loss exceeding twenty liters per day, sometimes as much as three liters per hour, can result from vigorous exercise under desert conditions. Replacement of fluids under such circumstances often requires forcing fluids since the human thirst response lags behind actual requirements, resulting in what has been termed "voluntary dehydration."
Athletes engaged in ultra endurance competition are especially subject to the risk of dehydration, since they typically do not satisfy their fluid requirements while exercising. On the other hand, forced consumption of plain water can, under such circumstances, result in overhydration. Insufficient salt intake can also produce a serious impairment of performance. Under such circumstances, fluids containing electrolytes are necessary to sustain performance. In addition to electrolytes, glucose, or glucose-containing carbohydrates, can enhance water absorption as well as supply supplemental energy for muscle metabolism. However, a carbohydrate concentration exceeding 5 percent weight per volume may actually reduce the rate of water absorption (Rehrer, 2001). Athletic training for endurance competition routinely includes emphasis on increasing the ability to consume sufficient fluids.
Minerals in Water
In addition to their needs for energy, proteins, and fats, humans require a number of vitamins and minerals. Some of these nutrient requirements, especially those for minerals, can be partially and sometimes totally satisfied through the consumption of drinking water (Costi et al., 1999). The water people drink varies greatly from one area to another. Some communities rely on surface water drawn from rivers and lakes, often stored in reservoirs. Other communities rely on groundwater drawn from subterranean aquifers. Both surface water and groundwater can bear significant loads of salts and other minerals. Water purification systems are usually designed to keep the salinity of drinking water within prescribed limits in areas where water supplies tend to be salty. However, sodium can be detected in most water supplies, sometimes in surprisingly high concentrations. In cities such as Jiddah, Saudi Arabia, that depend entirely on desalinated seawater, the salinity of the drinking water is quite noticeable to the visitor.
Because of its unique chemical properties, water is seldom if ever found in its pure state. Even rainwater contains contaminants absorbed from the atmosphere while in the vapor phase and during its descent to Earth in its liquid phase. However, rainwater is generally considered to be "soft water," since its mineral content is lower than that of most other sources. Water "hardness" is often associated with its calcium content, since drinking water is drawn from aquifers associated with limestone deposits in many parts of the world. In these areas precipitated water percolates through topsoil rich in organic compounds, becoming increasingly acidic in the process. When this acidic water comes into contact with the calcium carbonate of the limestone bedrock, it gradually dissolves the rock and carries its soluble constituents with it into the aquifer. Depending upon the length of time over which this process occurs, the water that finally finds its way into the aquifer can carry a substantial load of calcium and carbonates. Such water may require "softening" if it is to be used to launder clothing, since it tends to precipitate soaps. However, hard water of this sort is potable and may indeed be considered quite desirable as drinking water.
Sources of Drinking Water
While the palatability of drinking water is literally a matter of taste, it is unlikely that pure, distilled water would be considered a desirable beverage. When comparisons of drinking water are made, the samples generally considered most desirable are invariably ones that bear a significant mineral content. For example, the drinking water of New York City, piped in from reservoirs in upstate New York, is considered of superior taste as a result of its mineral content. In other parts of the United States, groundwater drawn from ancient deposits is considered a precious but diminishing resource. A case in point is that of Tucson, Arizona, where Pleistocene water deposits are being withdrawn at a rate faster than normal recharge. Consequently the growing needs of this urban area are projected to be satisfied through increasing use of Colorado River water diverted to Phoenix and Tucson by the Central Arizona Project (CAP). Community resistance to the use of Colorado River water has resulted in a program focused on the "blending" of groundwater with CAP water in an effort to avoid an abrupt change in the perceived quality of the drinking water.
The features of CAP water that have most troubled the Tucson Water Authority are the substantial load of sediment it bears as well as its different mineral composition. The expense of purification to achieve a standard comparable to that of the Pleistocene groundwater long taken for granted has been prohibitive. In an initial attempt to convert to exclusive use of CAP water in a number of Tucson neighborhoods, tap water was found to be turbid, and according to many residents, it had a distinctly unpleasant flavor. It was eventually determined that part of the problem arose from the release of mineral deposits from the water pipes both within and outside residences subsequent to sustained exposure to water having a significantly different chemical profile. The release of these deposits sometimes caused leakage that resulted in property damage and claims for compensation. The problems encountered by the Tucson Water Authority were the result of an unusual set of circumstances coupled with concern for the damage that could occur if the depletion of the local groundwater supply should progress to a point where subsidence caused structural damage to buildings. Concern for the risk of subsidence was well-founded, since parts of Interstate 10 less than fifty miles from Tucson have required repeated repair due to damage directly attributable to subsidence.
The problem of subsidence is of concern in many parts of the world where increasing need for drinking water has led to the depletion of aquifers. It is a concern in the central United States, for instance, in an extensive area overlying the Ogalalla Aquifer. Although the falling water table in this region is largely the result of agricultural use of water, urban growth has had a deleterious impact on aquifer recharge with the result that drinking water supplies are increasingly drawn from surface water sources. The long-term consequence is a decline in both the quality and quantity of drinking water in a number of cities. In coastal areas, depletion of aquifers has led to saltwater incursions that seriously impact the quality of drinking water. As is sometimes the case in midcontinental regions, damage to aquifers in coastal areas can be irreversible, necessitating reliance on alternative sources of drinking water. In the event of a rise in sea level associated with global warming, such reliance will become increasingly widespread. In almost every case the mineral profile of the alternative source will differ from that of the original groundwater, often in ways that alter the taste of drinking water.
Reliance on surface water sources for drinking-water supplies presents numerous problems. Surface water is subject to forms of contamination not usually found in groundwater. During the time it takes for water to percolate from the surface into the aquifer, most of its organic contaminants are left behind. Generally minerals take their place. The array of minerals present in the soil and bedrock determine the character and taste of the groundwater. As a rule the oldest deposits are the deepest. Therefore water pumped up from deep wells is less likely to bear organic contaminants. Additionally, deep wells provide a more stable supply of water since they draw on supplies that are less dependent upon recharge to maintain a constant flow. Shallow wells, such as the tube wells in many parts of Asia, for the most part trap surface water during a rainy season for use during the dry season. Water from such wells resembles surface water more than groundwater and may therefore contain a substantial load of organic contaminants.
The organic contaminants found in the surface water in many parts of the world include agricultural fertilizers and pesticides. Agricultural runoff, which finds its way to the larger streams and rivers before eventually reaching the oceans, contaminates freshwater bodies at all points along the line. In many cases the rich mixture of organic fertilizers entering lakes and rivers stimulates the growth of microorganisms, such as algal blooms. Eutrophication, the overgrowth of algae and other plant life, depletes the water of oxygen. Oxygen depletion limits the range of aquatic species that can inhabit these bodies of freshwater. The eutrophication of freshwater lakes is a serious problem in many parts of the world. It has a serious impact on the quality and quantity of drinking water wherever it occurs.
At least in part because of general distrust in the quality of drinking water, the consumption of bottled water has increased. The commoditization of the drinking-water supply is a fairly recent phenomenon in many parts of the industrialized world. The quality of water available from this source is highly variable. Some is drawn from mountain springs, where the mineral content is consistent and organic contaminants are virtually absent. Other commercially marketed bottled water comes from more dubious sources and may contain a variety of contaminants, including pathogenic organisms in unacceptably high concentrations.
Certain mineral waters, especially those purported to have therapeutic value, have long had a market. These mineral waters were originally consumed at their sources, often at spas, where hot springs produced highly mineralized, often sulfurous water thought to have medicinal properties when consumed or used as a bath. The Romans sought thermal springs throughout the areas they occupied in Europe, and they constructed baths that served as focal points of the communities.
The Romans were well aware of the benefits of a reliable supply of pure drinking water even in the absence of thermal springs, as the many aqueducts in Europe attest. The Romans did not face the problems associated with chemical contamination that accompany modern agricultural methods. However, they were aware of the risk of disease associated with contaminated drinking water and consequently preferred to draw their water supplies from mountain sources whenever possible. To what extent this preference reflects awareness of superior taste as a beverage is not known with certainty, but Roman literature includes many references to the desirable properties of the drinking water in specific parts of the empire.
The widespread marketing of drinking water in industrialized countries reflects in part an increasing awareness of the risks of contamination of conventional sources. This awareness comes at a time of increasing concern for the future supply of potable water for a steadily growing world population. Clearly, water is a basic requirement of life. Freshwater is not evenly distributed over the face of the planet. It is most abundant in places where few people live, and the transport of large amounts of water to the arid and semiarid areas of high population density presents a serious technological challenge. Contamination, both by chemical agents and by pathogenic organisms, is a problem that requires serious attention at every governmental level. Even in the United States, where the safety of the drinking water supply is usually assumed to be assured, occasional events, like the outbreak of a waterborne disease in Milwaukee, point up the fact that surface water supplies in particular are vulnerable. However, recognition of the urgency of the problem has increased public awareness of the necessity of preserving and protecting a fundamental resource, and it appears that drinking water is receiving the level of appreciation it deserves.
See also Food Supply, Food Shortages.
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William A. Stini