Fecal balance studies

Fecal Balance Studies (Single Isotopic Label)... 

Similarly to fecal balance studies, assessment of element absorption by plasma deconvolution is an approach adopted from non-isotopic measurements. After feeding the test meal containing the isotopic label, serum samples are taken at regular intervals over several hours. Samples are analyzed for the isotopic label to assess the appearance and disappearance of the label over time in blood. TTie area under the resulting curve for label concentration is then proportional to the fraction of label absorbed. The larger the area, the more of the label has passed the intestinal mucosa [29]. 

Most of the forementioned studies which examined the influence of various dietary fiber on the bioavailability of calcium by human subjects have depended upon the comparative measurements of calcium content of diets and calcium contents of stools and urine. As reviewed by Allen (3), calcium balance studies have distinct limitations relative to accuracy and precision. However, their ease of application and cost, laboratory equipment requirements, and real (or perceived) safety in comparison to available radioactive or stable isotope methods continue to make their use popular. In calcium balance studies, calcium absorption is assumed to be the difference between calcium excretion in the feces and calcium intake. Usually this is expressed as a percent of the calcium intake. This method assumes that all fecal calcium loss is unabsorbed dietary calcium which is, of course, untrue since appreciable amounts of calcium from the body are lost via the intestinal route through the biliary tract. Hence, calcium absorption by this method may underestimate absorption of dietary calcium but is useful for comparative purposes. It has been estimated that bile salts may contribute about 100 g calcium/day to the intestinal calcium contents. Bile salt calcium has been found to be more efficiently absorbed through the intestinal mucosa than is dietary calcium (20) but less so by other investigators (21). 

Results from some recent studies (19-36) on the effects of fiber are summarized in Table I. For the most part, the results are from multi-day balance studies. However, Turnland et al. (36) used the stable isotope fecal monitoring method to assess zinc utilization and Simpson et al. (24) measured iron absorption from a single test meal. 

To investigate the rate and the extent of the excretion of the drug/metabolites with the bile. If required, the investigation can be supplemented to study the entero-hepatic circle. The bile fistula study becomes necessary when relevant parts of the administered radioactivity are eliminated fecally suggesting a considerable biliary excretion during the mass balance study in rats. 

The data are finally drawn together for detailed discussion and evaluation with special attention to known results from other radiokinetic studies such as the mass balance study and the obtained fecal elimination or the quantitative whole body autoradiography. 

The longest balance study measuring manganese was conducted by Tipton et al. (28) who collected duplicate diets and all urinary and fecal excreta of two men for 347 days. The self-chosen diets of the two men contained an average of 3.3 and 5.5 mg Mn/day and produced positive balances of 0.8 and 2.5 mg/day, respectively. 

The results obtained In zinc balance studies during different Intake levels of calcium and phosphorus were In agreement In Zn absorption studies. Figure 1 shows data of f> Zn plasma levels and of the fecal excretions following a single oral... 

Several approaches have /been used to determine absorption of trace elements in humans. The most frequently used method has been balance studies, in which the amount of a mineral ingested is compared with the amount eliminated in the feces. However, absorption calculated from total mineral eliminated in fecal collections generally differs greatly from true absorption, since some of the mineral eliminated in the feces is of endogenous origin (1). A number of other difficulties with metabolic balance studies, such as variation in intestinal transit time and inadequate analytical precision, limit their usefulness and often result in conflicting results (2). 

Picloram is not readily metabolized and is rapidly excreted unchanged in the urine and feces of treated rats, hollowing a 10 mg kg [ C]picloram intravenous dose, the isotope was cleared biophysical-ly and excreted in the urine. Balance studies in rats indicated that 98.4% of the dose was recovered. Urinary excretion resulted in an 80-84% recovery, fecal excretion resulted in 15% recovery less than 0.5% was recovered in the bile, and virtually no radioactivity was recovered as trapped " C02 or as other volatile compounds. Studies with [ " C]piclo-ram showed that 90% of the compound fed in the diet to dogs was excreted within 48 h in the urine, with small amounts appearing in the feces. 

About half of the absorbed biotin is excreted as the metabolites bisnorbiotin, occurring from fl-oxidation of the valeric acid side chain, and biotin sulfoxide, occurring from the oxidation of the sulfur in the heterocyclic ring. The circulating plasma and urinary excretion patterns show a ratio of 3 2 1 for biotin, bisnorbiotin, and biotin sulfoxide. Minor metabolites are bisnorbiotin methyl ketone and biotin sulfone. Careful balance studies in humans, where perhaps only 1 mg is the total body content, showed that urinary excretion of biotin often exceeded dietary intake, and that in aU cases, fecal excretion was as much as three to six times greater than dietary intake because of microfloral biosynthesis. 

Bile, urine, and feces samples for metabolite profiling are collected in 0-8 and 8-24h block collection period or in one 0-24h block collection period. Development stage definitive metabolite profiling and mass balance studies generally involve urine and feces collection in 24 h block periods over 7, 10, or 15 days or until more than 85% of the radioactivity is excreted. Urine and fecal sample time points selected for metabolite profiling generally cover over 90% of the radioactivity excreted in the respective matrix. 

Mass Balance Studies. Mass balance studies may be performed using a combination of an aortic cannula, indwelling stomach tube (oral administration studies), urinary catheter, fecal catheter, and collection/extraction of flowing immersion water (branchial products). Unlike mass balance studies in mammals, such studies require this experimental format as the water serves as a common depot for all wastes and importantly serves as a confounding source for reuptake. These studies can be performed in restrained and free swimming animals, however, in either case require intensive effort and experimental diligence. 

Balance studies involving aluminium make it necessary to perform fecal analysis. A recent report for the authors laboratory describe methods of collection, shipping and analysis of these types of specimens (Brown et al., manuscript in preparation). 

Mass Balance Studies. Pharmacokinetic mass balance studies apply unlabeled, stable isotopes or radiolabeled compounds to study the extent of absorption and first-pass metabolism, distribution, and excretion of a given compound. In the microdosing approach, a C-labeled compound is administered to human volunteers at doses from as low as one microgram blood, urine, and fecal samples are collected over time and analyzed for C content by accelerator mass spectroscopy to determine half-life, plasma AUC, and maximal concentration (Cmax)- However, these methods are not very popular even when very low doses of radioactivity are involved. Highly sensitive, and more readily available, tech-niques for separation and analysis (e.g., LC-MS, LC-MS/MS) are frequently used alternatives that enable pharmacokinetic investigations and metabolite profiling of nonradiolabeled compounds. 

Extensive evaluation of existing in vitro and in vivo methods in foods indicates that the rat balance method is the most suitable practical method for predicting protein digestibility by humans. Therefore, when human balance studies cannot be used, the standardized rat fecal-balance method of [119] or [109] is recommended. 

The crudest approach to study element absorption is by taking the human body as a black box. As for chemical balance studies, element absorption can then be assessed as the difference between dietary intake of an isotopic label and its fecal recovery. This approach, however, does not deliver the fraction of element that has truly been absorbed, that is, the fraction of element in the digested food moiety that has passed the intestinal mucosa. Digestion can only occur by action of... 

True absorption, as the element fraction that is taken up by the intestinal mucosa and released into circulation, cannot be assessed by fecal balance techniques using a single oral isotopic label only. A second label needs to be administered intravenously to determine the fraction of absorbed element that re-enters the intestine by secretory pathways. Intravenous isotopic labels in metabolic studies are usually intended to mimic freshly absorbed element from the diet. This entails that labels must be injected slowly over several minutes about 1 h after test meal administration or, ideally, as an infusion over 20-30 min in physiological saline. 

The effect of low fat intakes and of crude fiber has been studied by Walker by means of balance studies on a healthy European male. When a large increase was made in the crude fiber content of equal fat diets, the amount of fecal fat excreted detectably increased. This was considered not to have been derived directly from the food. The possibility was raised by Walker that it might be of microbial origin. 

Earlier sterol balance studies showed that bile acid synthesis is frequently but not consistently increased in hypertri-glyceridemic patients particularly when measured with the isotope dilution technique. Furthermore, a negative correlation between serum cholesterol and fecal bile acids indicated that the lower the LDL level the higher the bile acid synthesis in hypertriglyceridemic patients. In view of the current concept on VLDL metabolism in type IV, the finding suggests that enhanced VLDL catabolism via a non-LDL pathway (low LDL) is associated with increased bile acid and also cholesterol synthesis. An increase of LDL i.e., development of type IIB pattern would then inhibit the production rates possibly via enhanced hepatic uptake of LDL cholesterol. 

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