Nutrient Requirements of Nonhuman Primates: Second Revised Edition

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Nutrient Requirements of Nonhuman Primates: Second Revised Edition, 2003

the composition of water can vary among municipal systems, all such systems are required, at a minimum, to meet national primary drinking-water standards (National Primary Drinking Water Regulations [NPDWR]) established by the US Environmental Protection Agency (EPA). These primary standards protect drinking-water quality by setting limits on levels of specific contaminants that can adversely affect public health and that are known to occur or can be expected to occur in public water systems. The contaminants are divided into inorganic chemicals, organic chemicals, radionuclides, and microorganisms. EPA has included two levels in the NPDWR for each contaminant. The first is the Maximum Contaminant Level Goal (MCLG), defined as the maximal level of a contaminant in drinking water at which no known or expected adverse effect on human health would occur; MCLGs are nonenforceable public-health goals. The second is the Maximum Contaminant Level (MCL), defined as the maximal per-missible level of a contaminant in water delivered to any user of a public water system; MCLs are enforceable standards. MCLGs are equal to or lower than MCLs, with margins of safety that ensure that slightly exceeding the MCL does not pose a substantial risk to public health.

EPA also has established secondary drinking-water standards (National Secondary Drinking Water Regulations) that are nonenforceable guidelines related to contaminants of drinking water that might cause cosmetic effects (such as skin or tooth discoloration) or aesthetic effects (such as taste, odor, or color). Although the EPA does not require compliance with these secondary regulations, some states comply.

It is not experimentally verified, but water-quality standards established for humans would probably be satisfactory for nonhuman primates. Detailed information on the EPA drinking-water standards for US public water systems can be obtained at http://www.epa.gov/safewater/mcl.html. Composition of the water from specific municipal water supplies can usually be obtained from public-works departments or state departments of health.

WATER REQUIREMENTS

Requirements for liquid-water intake are dictated by the need to balance total water intake and water loss when metabolic water and water from food are inadequate for that purpose. Thus, liquid-water requirements will vary with food composition, intake, and metabolism and with activity and the need to dissipate body heat. The efficiency of the latter process varies with environmental circumstances, particularly ambient temperature and relative humidity, which in turn affect food intake.

Thirst is the body’s clue that something is amiss with water balance, and it encourages the thirsty subject to seek and consume water. Little has been reported on the physiology of thirst in various species of nonhuman primates, and the issue is complicated by observations that the mechanisms involved can be different in different non-primate species (Wood et al., 1982).

Homeostatic regulation of body fluid volumes has received major attention in humans because of its importance for normal subjects and for clinical patients (Oh and Uribarri, 1999). Body fluid is an aqueous solution containing many electrolytes in intracellular and extracellular compartments. Intracellular fluid occupies not a single large compartment but myriad cell compartments, which have their characteristic environments and communicate with each other via interstitial fluid and plasma. Although cells in different tissues can vary in the solutes present and in solute concentrations, osmotic equilibrium is maintained so that the same number of water molecules surrounds each particle of solute throughout the body. Cell membranes are so permeable to water that osmolality is normally the same throughout the body fluid.

Most of the metabolic reactions of the body take place in cells. For normal operation of these reactions, optimal ionic strengths must be maintained in the cellular compartment, and the homeostatic mechanisms of the body are constantly at work to provide such an environment. Extracellular fluids (ECFs), in contrast, are not sites of major metabolic activity. Therefore, there can be substantial alterations in ionic strength of ECFs without adverse effects. The primary function of ECFs is to serve as a conduit between cells and between organs. The interstitial fluid surrounds cells and allows for slow but efficient intercellular solute exchange. Plasma is a conduit for rapid solute exchange between organs. ECFs thus regulate intracellular volume and ionic strength.

The kidneys and central nervous system are jointly responsible for maintaining the homeostasis of body fluids. When water loss exceeds water intake, increases in extracellular fluid osmolality shrink the hypothalamic osmoreceptor cells, which then signal the thirst center in the cerebral cortex and the antidiuretic hormone (ADH) releasing center in the supraoptic and paraventricular nuclei. ADH release is also regulated by nonosmotic factors, such as low effective arterial volume. ADH is released from the posterior pituitary, is carried in the plasma to the kidneys, and stimulates tubular resorption of water from the renal glomerular filtrate. At the same time or shortly thereafter, the thirst center responds by increasing the thirst drive and consequently promotes water intake (Askew, 1996; Oh and Uribarri, 1999).

Because of the complexity and interrelationships of factors affecting water requirements and the dearth of information from studies of nonhuman primates, water needs can be most safely met by providing ad libitum access. It should be noted that in group-housing situations, competi-

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