Nutrient balance technique

The nutrient balance technique, whether used for measuring N balance, calcium balance, or balance of other nutrients, is not particularly accurate. Inaccuracies are introduced with feeding. It may not always be possible to measure exactly how much food is consumed by the animal or human subject. Inaccuracies are introduced in collecting the excreta, especially as collections must be made over the course of many days. An important source of error is the fact that the value for N balance is determined by subtraction, that is, subtracting the amount excreted from that consumed. The values for balance, whether positive or negative, are often small compared to the values for the amount of nutrient consumed. 

The nutrient balance technique is sometimes used to determine requirements, as illustrated by the example of growing monkeys in the Protein chapter. This intake indicates only the amount required to maintain a constant level of the nutrient in the body. Under some conditions, the determination of the minimal intake that can support nutrient balance in an adult animal does not necessarily indicate the requirement (Mertz, 1987). Other techniques must then be used to detennine if higher, but still constant, levels are of greater value to the animal. Here, biochemical tests may be used to assess a relevant physiological function. Kopple (1987) has pointed Out a number of difficulties in using the nutrient balance technique. For example, the value for nutrient intake tends to be overestimated and that for excretion tends to be underestimated. These two factors car> lead to an artifactually high value for the nutrient balance. 

The techniques used to determine mineral requirements are the same as those used to determine energy and protein requirements. A theoretical framework is provided by using the factorial approach, whereas practical estimates of requirements can be obtained using nutrient balance or growth trials. Since the mineral contents of foods are usually expressed as the total or gross amounts present, requirements are stated in the same terms. Nutrient standards must therefore take into accoimt the differences in mineral availabUity that occiu between different species and age classes of animal (see Chapter 10). 

Commonly administered LVPs include such products as Lactated Ringers Injection USP, Sodium Chloride Injection USP (0.9%), which replenish fluids and electrolytes, and Dextrose Injection USP (5%), which provides fluid plus nutrition (calories), or various combinations of dextrose and saline. In addition, numerous other nutrient and ionic solutions are available for clinical use, the most popular of which are solutions of essential amino acids or lipid emulsions. These solutions are modified to be hypertonic, isotonic, or hypotonic to aid in maintaining both fluid, nutritional, and electrolyte balance in a particular patient according to need. Indwelling needles or catheters are required in LVP administration. Care must be taken to avoid local or systemic infections or thrombophlebitis owing to faulty injection or administration technique. 

Isotopes of oxygen and hydrogen are useful tracers of water sources because they are constituents of the water molecule itself and because they are conservative in aquifers at near-surface temperatures. Isotopic techniques take advantage of the fact that lakes and their surrounding ground-water systems are usually isotopically distinct. Applications of stable isotopes for the study of lakes were first described by Dincer (10) and were discussed in several subsequent review articles (11-14). Most applications of isotopic techniques to lake systems are designed for the determination of water balances, nutrient-uptake studies, and paleotemperature reconstructions. 

The balance study technique has been used In all of these studies. Although there are difficulties and certain problems with balance studies, nutrition researchers have not yet found a reasonable and realistic alternative to estimate nutrient needs of human subjects. Continued development and Increased sophistication of methods and analytical techniques may provide alternatives In the future, but these are not clearly available at the present time. 

Nutritional sciences employ various experimental techniques. The methods used to assess a deficiency can also be used to determine the requirement for a given nutrient. Dietary deficiency, a technique applied to animals and microoigan isms, was used in the discovery of vitamins and in proving the essential nature of certain amino acids and lipids. This book features a strong emphasis on the techniques used to assess both requirements and deficiencies- Two of the most important techniques, those involving nitrogen balance and the respiratory quotient, are covered in some detail. 

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