Digestion bioavailability

Dietary protein adequacy (aside from the energy/protein relationship) is a function of several protein-dependent factors the protein content of the food, the digestibility/bioavailability of the protein/amino acids within the food, the quantity of indispensable amino acids within the protein, and the relative ratio of the amino acid content to the requirements of the person(s) or amino acid standard in question. The final protein quality score is dependent on the lowest level of bioavailable indispensable amino acid, the limiting amino acid. 

The term bioavailability has various definitions. Previously, the authors of this chapter have defined bioavailability as the proportion of a nutrient (or other food component) that is digested, absorbed and utilised in normal metabolism - with the practical measurement of bioavailability usually relying upon estimates of amounts absorbed (Southon and Faulks, 2001). Biological activity, or bioactivity , has been viewed and described as a separate stage which follows on from bioavailability in the journey of a compound from food to function. However, here we present a new definition of bioavailability that recognises the functional consequences of absorption. 

All the actual or putative functional benefits of carotenoids are dependent on their bioavailability amounts consumed, amounts released from the food structure during digestion and extent of absorption and tissue distribution. The following three sections deal with each of these issues in turn. 

The mucosa of the GIT represents an interface between the external and internal environments. The expansive surface area is necessary for the efficient hydrolysis of foodstuffs and the absorption of energy and nutrients. The mucosa also influences the systemic availability of non-nutrient compounds in the diet, both beneficial and detrimental. Digestion and absorption of glucosinolates are critical determinants of health benefits (see Chapter 4) Similarly, the bioavailability and health benefits of phytoestrogens, such as genistein (see Chapters 5 and 10) are at least partly dependent on the carrier-mediated processes of absorption associated with the GIT (Oitate et al, 2001). Moreover, the metabolic activities of the mucosa can influence the systemic concentrations and forms of dietary phytochemicals, as exemplified by research with soy isoflavones (Andlauer et al., 2000). 

SPENCER J D, ALLEE G L, SAUBER T E (2000) Phosphotus bioavailability and digestibility of normal and genetically modified low-phytate com for pigs. JAnim Sci. 78 675-81. 

FIGURE 3.2.1 In vitro digestion/Caco-2 cell model combination approach to assess carotenoid bioavailability. LCM = large chylomicrons. SCM = small chylomicrons. VLDL = very low density lipoproteins. 

The bioaccessibility of a compound can be defined as the result of complex processes occurring in the lumen of the gut to transfer the compound from a non-digested form into a potentially absorbable form. For carotenoids, these different processes include the disruption of the food matrix, the disruption of molecular linkage, the uptake in lipid droplets, and finally the formation and uptake in micelles. Thus, the bioaccessibility of carotenoids and other lipophilic pigments from foods can be characterized by the efficiency of their incorporation into the micellar fraction in the gut. The fate of a compound from its presence in food to its absorbable form is affected by many factors that must be known in order to understand and predict the efficiency of a compound s bioaccessibility and bioavailability from a certain meal. ... 

Garrett, D.A., Failla, M.L., and Samara, R.J., Development of an in vitro digestion method to assess carotenoid bioavailability from meals, J. Agric. Food Chem., 47, 4301, 1999. 

Chitchumroonchokchai, C., Schwartz, S.J., and Failla, M.L., Assessment of lutein bioavailability from meals and a supplement using simulated digestion and Caco-2 human intestinal cells, J. Nutr, 134, 2280, 2004. 

Liu, C.S., Glahn, R.P., and Liu R.H., Assessment of carotenoid bioavailability of whole foods using a Caco-2 cell culture model coupled with an in vitro digestion, J. Agric. Food Chem., 52, 4330, 2004. 

D. A. Garrett, M. L. Failla, and R. J. Sarama, Estimation of carotenoid bioavailability from fresh stir-fried vegetables using ab in vitro digestion/Caco-2 cell culture model, J. Nutr. Biochem. 11 (2000) 574—580. 

There is no solid evidence that relates human aging and reduction of carotenoid absorption. In some studies, old people have shown a lower (3-carotene absorption than that of young people (Madani and others 1989), whereas the opposite has also been reported by other studies (Sugarman and others 1991). The absorption of lipid-soluble substances, including carotenoids, is affected by any disease related to the digestion and absorption of fats (West and Castenmiller 1998). Inadequate production of lipase and bile as well as an inadequate neutralization of the chyme in the duodenum affect carotenoid bioavailability (Guyton and Hall 2001). 

Adsorbents (such as kaolin-pectin) are used for symptomatic relief (see Table 23-4). Adsorbents are nonspecific in their action they adsorb nutrients, toxins, drugs, and digestive juices. Co administration with other drugs reduces their bioavailability. 

The first consideration was the speciation and distribution of the metal in the sediment and water. Benthic organisms are exposed to surface water, pore water and sediment via the epidermis and/or the alimentary tract. Common binding sites for the metals in the sediment are iron and manganese oxides, clays, silica often with a coating of organic carbon that usually accounts for ca. 2% w/w. In a reducing environment contaminant metals will be precipitated as their sulfides. There is not necessarily a direct relationship between bioavailability and bioaccumulation, as digestion affects the availability and transport of the metals in animals, in ways that differ from those in plants. 

The in vitro estimate of potential availability was defined, somewhat arbitrarily, as calcium solubility (18,000 x g supernatant) after complete digestion. Potentially available calcium was expressed as a percentage of the total food calcium (Figure 1). With the exception of a low in vitro calcium solubility value for whole milk, our earlier data compared reasonably well with calcium bioavailability information in the literature (7.) ... 

When the estimates of bioavailability were compared ( ), in vivo absorption was higher than in vitro solubility for two of the foods We had expected absorption to be less than solubility due to physiological factors (lt9). Thus, this surprising result led to the reexamination of in vitro digestion conditions which is reported in this paper. 

Experiments were conducted to determine if varying the conditions in the in vitro digestion procedure would affect post-digestion calcium solubility and in some cases, exchangeability. This was done with two purposes to test the use of the in vitro digestion procedure for studying factors which might influence calcium bioavailability and to use the results to modify the standard procedure. 

Choice of Potential Bioavailability Criterion. It is usually assumed that calcium must be soluble and probably ionized in order to be available for absorption ( ). For the in vitro procedure, as a first approximation we chose calcium solubility after centrifugation at 18,000 x g as the measure of potential bioavailability (Figure 1). We assumed that this would probably overestimate the available calcium and later work based on fractionation might define the bioavailable calcium more precisely. The data in Table IV illustrate how the choice of criterion for "solubility" could affect the in vitro estimate of potential availability, even if in vitro conditions closely resembled in vivo conditions. Since our in vitro criterion unexpectedly underestimated calcium bioavailability for two of the three foods in the direct in vivo - in vitro comparison (8), it was necessary to determine the in vitro digestion conditions which might be limiting solubility before addressing the choice of appropriate criterion. 

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