Diet-tissue

Nitrogen isotope ratios ( N/ " N) inerease from plants to herbivores to eami-vores and ean be used to estimate the degree of camivory in human diets. Some field studies observe a greater differenee in 5 N between trophie levels in dry, hot habitats than in wet, cool ones. Two hypotheses have been proposed to explain this variation in difference in 8 N between trophic levels. (1) Elevated excretion of -depleted urea in heat/water-stressed animals (2) recycling of nitrogen on protein-deficient diets. Both predict increased diet-tissue 8 N difference under stress. 

The increase in diet-tissue spacing has been proposed to be caused by the effects of water and heat stress on urinary nitrogen excretion. The model has been described in detail previously (Ambrose 1991) and will be briefly summarized here Nitrogen is excreted mainly as urinary urea. Its 6 N value is substantially (2-5%o) more negative than that of the diet (Steele Daniel 1978 Yoneyama et al. 1983). Under heat and water stress the concentration... 

Protein stress and recycling of nitrogen could also have the opposite effect, however. If less N-depleted N is excreted as inea, then there should be less overall enrichment in the nitrogen available for tissue synthesis. Moreover, if urea itself is recycled for protein synthesis under protein stress, which often occurs in herbivores, then the diet-tissue difference should be smaller than in unstressed individuals because urea has a substantially lower 8 N value than the diet. 

The nutrient stress hypothesis can be tested by comparing diet-tissue A N values of animals on low versus normal and high protein diets. Our controlled diet experiments, although primarily designed to trace carbon from different dietary macronutrient fractions (proteins versus carbohydrates, fats and sugars) to animal tissues under different levels of nutrient stress (Ambrose and Norr 1993) may be suitable for testing this hypothesis because they contain diets with 5, 20 and 70% protein by weight. 

Eleven controlled diet and environment experiments have been designed in a way that can be used to investigate the effects of protein nutrition and heat and/or water stress on diet-tissue A N. Laboratory rats were raised on purified, pelletized diets in which the isotopic composition of proteins, lipids and carbohydrates were well characterized and their proportions accurately and precisely measured (Ambrose and Norr 1993). Four experiments involved manipulation of temperature and/or water availability. Of these four experiments, one used a diet with high (70%) protein concentrations and heat/water stress (36°C) and three used normal (20%) protein concentrations. Seven experiments were conducted at normal temperature (21°C) with water ad libitum. Of these seven experiments, two used diets formulated with veiy low protein (5%), three with normal protein and two with high protein concentrations. 

In order to examine the effects of water and heat stress on diet-tissue... 

Diet-Tissue Differences in Collagen and Hair Nitrogen Isotopes... 

Table 12,2. Experimental conditions, mean 5 N values and diet-tissue and tissue-tissue dilTerence values (A N) of rats not subjected to water stress and/or heat stress, Diet 8 N values are those of the protein source rather than the whole diet pellets. SD = standard deviation, co-d = collagen-diet, h-d = hair-dici, f-d = flesh-diet, co-f = collagen-flesh. 

The mean difference between collagen and flesh (A Nco.f) values for the first sacrificed pairs (91 days after birth) is 0.3 1.09%o while that for third pairs (171 days after birth) is 1.4 0.45%o. This result was unanticipated but seems robust. This indicates the relationship between diet and tissue 8 N and age is complex and varies between tissues. Future studies of diet-tissue nitrogen isotope spacing will have to consider age effects. This contrasts with carbon isotopes (Ambrose and Norr 1993), where we have observed little increase in 8 C with age in the same individuals. 

Hohson, K.A. and Clark, R.G. 1992 Assessing avian diets using stable isotopes 11 factors influencing diet-tissue fractionation. The Condor 94 181-188. 

Biological factors species, strain, sex, genetic factors, disease and pathological conditions, hormonal influences, age, stress, diet, tissue and organ specificity, dose, and enzyme induction and inhibition. 

The direct determination of zinc in diet, tissue and in body fluids can be accomplished by a variety of methods. A common limitation is the chance of sample contamination prior to analysis. Some early studies using less sensitive methods may not have recognised this problem and reported erroneously high results. Older colorimetric methods required that the biological sample be efficiently digested or otherwise deproteinised, prior to formation of a coloured zinc complex. These techniques have largely been superseded by atomic absorption spectrometry which is more sensitive yet less prone to interferences. For fluids such as plasma or urine, simple dilution is all that is required prior to analysis. Tissue or diet samples only require to be dissolved in mineral acid. These simpier sample preparation procedures limit the chances of contamination. 

Proposed limits for cadmium in water, diet, tissues, air, soils, and sewage sludge for the protection of human health, plants, and animals are shown in Table 5.2. It is noteworthy that the current upper limit of 10.0 xg Cd/L in drinking water for human health protection is not sufficient to protect many species of freshwater biota against the biocidal properties of cadmium or against sublethal effects, such as reduced growth and inhibited reproduction. Ambient water quality criteria formulated for... 

Certain biochemical indices are available for determining the presence of either total EFA deficiency or -3 fatty acid deficiency. Linoleic, linolenic and oleic (18 l -9) acids compete for the same desaturation and elongation enzymes that convert these fatty acids to long chain polyunsaturated fatty acids (see Figure 2.3). The desaturases prefer co-3 over -6 over -9 fatty acids (Tinoco et al., 1979). Normally, with sufficient EFA in the diet, tissue levels of eicosatrienoic acid (20 3 -9) are very minor despite the abundance in tissues of its precursor, oleic acid. With total EFA deficiency, tissue levels of eicosatrienoic acid rise concomitant with decreased levels of AA. An increase in this triene/tetraene ratio (20 3 -9/20 4 -6) in blood and tissues is characteristic of total EFA deficiency, but not of -3 fatty acid deficiency (Holman et al., 1964). 

The necessity of polyunsaturated fatty acids as dietary components is to be ascribed to the fact that they cannot be synthesized by the animal de novo. If they are absent from the diet, tissues in which they play an integral role and enzyme systems in which they may participate will become depleted. They will then function less effectively, or they may disappear entirely. 

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