title: The quantification of dietary intake, digestion and metabolism in farm livestock and its relevance to the study of radionuclide uptake.
abstract: The transfer coefficient and biological half-life have been widely used to describe the uptake of radionuclides by farm animals. However, these parameters may be inadequate, for they take little account of the effects of the processes occurring in the animal upon the behaviour of a radionuclide. The aim of this paper is to review techniques which could be used to study the uptake of dietary radionuclides by farm animals. The main factors affecting uptake can be categorised in terms of intake, digestion and absorption, and metabolism. The estimation of the intake of a radionuclide by grazing animals is difficult. Intake estimation by determining amounts of standing herbage before and after grazing is rarely a valid method because of herbage growth. Methods based on animal measurements have wider application. Many intake methods give short-term estimates (measured over a few hours), whereas for most radionuclide studies, longer-term estimates (over days or weeks) are required. The most suitable methods use indigestible markers to determine the output of faeces and the digestibility of grazed herbage. Considerable variability in the distribution of radionuclides may exist among different plant species. To accurately estimate radionuclide intake on mixed pastures it is necessary to determine the plant species composition of the diet of animals; present methods may be inadequate and new techniques may be required. If contaminated, the drinking-water intake by animals would need to be determined. The processes of digestion (the physical disintegration and chemical breakdown by gut microbes and secreted enzymes) may affect the radionuclide uptake by an animal. Other factors, including the chemical form of the radionuclide, rate of passage of material along the gut, degree of incorporation into microbial tissue, and the absorption mechanism might also affect the degree to which the radionuclide can cross the gut wall. The apparent absorption coefficient (apparent digestibility) has been widely used in nutritional studies to quantify net uptake of a component from the gut. However, as this parameter does not adequately indicate the ability of a substance to cross the gut wall, its value in radionuclide uptake studies is rather limited. More useful, since it gives an indication of transfer across the wall of the gut, is the true absorption coefficient. The use of animals with cannulae in the digestive tract, allows the quantitative measurement of digestion and absorption in the different functional parts of the gut. The metabolism of absorbed substances by an animal involves transport (through the circulatory system), uptake into tissue cells, modification by biochemical reactions, and excretion.(ABSTRACT TRUNCATED AT 400 WORDS)
authors.affiliation: Macaulay Land Use Research Institute, Roslin, Midlothian.
authors.forename: R W
authors.initials: RW
authors.lastname: Mayes
mesh_terms: D000821:Animal Feed; D000818:Animals; D000829:Animals, Domestic; D004032:Diet; D004063:Digestion; D005507:Food Contamination, Radioactive; D007408:Intestinal Absorption; D006109:Poaceae; D011868:Radioisotopes / Q000032:analysis / Q000378:metabolism*
chemical_list: D011868:Radioisotopes