Factors Influencing Absorption

In an adequate, well-balanced diet the ratio of calcium to phosphorus is of little significance from the nutritional viewpoint, but in less-balanced diets this ratio assumes some importance. At the pH of the intestine calcium phosphate is difficultly soluble, and excessive amounts of calcium will render most of the phosphate insoluble and thus not available for absorption a high ratio of phosphate to calcium has a similar effect upon the calcium. This effect is enhanced when there is a deficiency of vitamin D, the incidence of rickets being directly related both to the actual amount ingested and to the calcium-phosphorus ratio. 

In infants the ideal calcium-phosphorus ratio is 2 1, which represents the proportion of these minerals in mother s milk. With increasing age this ratio is not so important and may fall below 1. 

When fat splitting and absorption is normal the ingestion of fats has in general been found to aid calcium absorption. In conditions in which there is excessive fat excretion, however, such as sprue or idiopathic steatorrhea, calcium is lost in the feces as calcium soaps. It has also been suggested 

Addition of citrate to a rachitic diet has been observed to render it non-rachitic and to aid calcification. It is suggested that citrates lower the pH of the intestinal tract, form calcium citrate which is relatively soluble, and aid deposition subsequent to absorption by raising the pH of tissue fluids. 

Addition of protein to a basal diet has been shown to increase calcium absorption and retention in men, and addition of peptone to the diet increases calcium absorption in rats. The effect upon absorption is presumed to be due to soluble calcium compounds formed with the amino acids produced by digestion of the protein. Better retention may possibly be due to the creation of a favorable nitrogen balance which assists in the laying down of matrix necessary for deposition. 

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The following factors influence absorption from the colon ... 

Fig. 14.12 VolSurf model to correlate 49 matrix metalloproteinase inhibitors with different zinc-binding functionalities to rabbit oral bioavailability for metabolically stable compounds. (A) Semiquantitative PLS model 0.424, r 0.646, 4 PLS components) to rank novel synthesis candidates. Main factors influencing absorption, that is, lower polarity, capacity factors and increased hydrophobicity, are in agreement with global models for human intestinal absorption. (B) Distribution of polar and hydrophobic surfaces for two molecules with low (0981) and higher (2290) rabbit AUC from oral pharmacokinetic studies.

A rigorous model of the absorption column was developed for the nitric acid system. The model considers all of the key factors influencing absorption including temperature, pressure, nitrous gas concentration, and acid concentration. This model has been adapted from a similar system described in Ref. Al. Other useful models have been proposed, e.g. Ref. A2. 

This chapter provides an overview of factors affecting dermal absorption. Factors influencing absorption are among others related to the skin (e.g. anatomical site, difference between species, metabolism, etc.) and the exposure conditions (e.g. area dose, vehicle, occlusion and exposure duration). In order to provide relevant information for the risk assessment of pesticides, dermal absorption studies should take these aspects into account. With respect to the methods being used nowadays for the assessment of dermal absorption, it is important to realize that neither in vitro nor in vivo animal studies have been formally validated. Available data from various in vitro studies, however, indicate that the use of the total absorbed dose (i.e. the amount of test substance in the receptor medium plus amount in the skin) could be used in a quantitative manner in risk assessment. Tape stripping of the skin can be adequate to give a good indication of test chemical distribution, and hence its immediate bioavailability. 

VIII. Factors Influencing Absorption Phenomena in Meats/Meat Mixtures... 

VIII. FACTORS INFLUENCING ABSORPTION PHENOMENA IN MEATS/MEAT MIXTURES... 

Chamberlain (1983) estimated that about 10% of dietary lead is contributed by atmospheric fallout, but this was considered to be an underestimate by the Royal Commission on Environmental Pollution (1983). The factors influencing absorption of ingested and inhaled lead are complex and beyond the scope of this review. Current evidence suggests that some 75% of the total uptake of lead by the body is derived from dietary sources. 

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