Evaluation of foods digestibility

This chapter marks a change from qualitative to quantitative nutrition. The chapters preceding it have shown which substances are required by animals, how they are supplied from foods and the manner in which they are utilised. This chapter and those following it are concerned with the assessment of (1) the quantities in which nutrients are supplied by foods, and (2) the quantities in which they are required by different classes of farm animal. 

The potential value of a food to supply particular nutrients can be determined by chemical analysis. However, the actual value of the food to the animal can be determined only after making allowances for the inevitable losses that occur during digestion, absorption and metabolism. The first and most important loss of nutrients is represented by the proportion that is not absorbed but is excreted in the faeces. 

The digestibility of a food is most accurately defined as the proportion that is not excreted in the faeces and that is, therefore, assumed to be absorbed by the animal. It is commonly expressed in terms of dry matter and as a coefficient or a percentage. For example, if a cow ate 9 kg of hay containing 8 kg of dry matter and excreted 3 kg of dry matter in its faeces, the digestibility of the hay dry matter would be  

Digestibility coefficients can be calculated in the same way for each constituent of the dry matter. Although the proportion of the food not excreted in the faeces is commonly assumed to be equal to that which is absorbed from the digestive tract, there are objections to this assumption, which will be discussed later. 

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Demirel, S., Tuzen, M., Saracoglu, S., Soylak, M. Evaluation of various digestion procedures for trace element contents of some food materials. J. Hazard. Mater. 152, 1020-1026 (2008)... 

Sarwar, G. and McDonough, RE. Evaluation of protein digestibility-corrected amino acid score method for assessing protein quality of foods. / Assoc. Off. Anal. Chem. 73 347-356,1990. 

Current systems for the evaluation of food protein for ruminant animals involve determinations of the degradabUity of protein in the rumen, the synthesis of microbial protein, the digestion in the lower gut of both food and microbial proteins, and the efficiency of utilisation of absorbed amino adds. The methods used to determine these components of the system are described next, after which their use in the systems will be illustrated. 

Although many in vitro studies on the antioxidative property of food constituents have been reported, little is known about the biological functions of dietary antioxidants in vivo, except for several well-known antioxidants such as tocopherols, (i-carotene, and ascorbic acid. Because the bioavailability of food constituents is limited by their digestibility and metabolic fate, an oral administration trial of a dietary antioxidant is favored to evaluate its biological function. 

The impact of plant products on the metabolism of synthetic dmgs results from the inhibition or activation of cytochrome P-450 (CYP) enzymes. Evaluation of the potential activation of CYP by administration of natural plant products or dietary supplements is important for prediction of interactions between their components and dmgs. Therefore, attention is directed to research on the impact of products available on the food market known as natural non-nutritive substances on dmg absorption. Non-nutritive dietary components are mainly secondary plant metabolites, which include, among others, phenolic compounds such as phenolic acids and flavonoids. The health effects of non-nutritive substances are not yet known. So far, there is no answer on the extent to which they are absorbed and metabolized by the body, and there is no information on the permitted daily intake for these compounds. This information is particularly important because certain non-nutritive natural substances are simultaneously considered to be anti-nutritional factors, mainly because they inhibit digestion and reduce the bioavailability of nutrients or dmgs. It is also possible that they form undesirable interactions with dmgs. The positive health effects of non-nutritive natural substances are not only attributed to their antioxidant properties. These substances are involved in various metabolic... 

However, excessive consumption of non-digestible carbohydrates, such as polydextrose, can lead to gastrointestinal distress. After evaluating a series of clinical studies, the Joint FAO/WHO Expert Committee on Food Additives (JECFA) and the European Commission Scientific Committee for Eood (EC/ SCF) concluded that polydextrose was better tolerated than other digestible carbohydrates such as polyols. The committee concluded that polydextrose has a mean laxative threshold of approximately 90 g/day (1.3 g/kg body-weight) or 50 g as a single dose. See also Section 18. 

Wong, K. H. and Cheung, P. C. K. (2001). Nutritional evaluation of some subtropical red and green seaweeds. Part II. In vitro protein digestibility and amino acid profiles of protein concentrates. Food Chem. 72,11-17. 

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 Protein Digestibility Corrected Amino Acid Score (PDCAAS) is superior to other methods for evaluating the protein quality of food proteins for humans because it measures the quality of a protein based on the amino acid requirements of a 2-5 year old child (the most demanding age group), adjusted for digestibility [120]. It has replaced the Protein Evaluation Ratio (PER). 

The processes involved in the absorption, distribution, metabolism, and excretion of food additives, are dependent on their chemical structure. Some compounds such as glycerol esters, carbohydrates, proteins, or simple peptides (such as aspartame) are digested in the gastrointestinal tract, and the products are absorbed and utilized in normal intermediary metabolism. The enzymes of intermediary metabolism usually show both high specificity and high capacity. In consequence, there would be no difference between substrates which are food additives or produced from food additives, and other substrates absorbed from the diet. The possibility that structurally unrelated food additives could show either joint actions or interactions has been assessed based on their potential to share common sites and mechanisms of action or conunon pathways of elimination. The (International Life Sciences Institute) ILSI Europe Acceptable Daily Intake Task Force established an Expert Group of independent scientists to undertake an evaluation of the possibility of interactions occurring between the 350 food additives currently approved in the EU, as laid down in directives 94/35/EC, 94/36/EC, and 95/2/EC. °... 

Sessa, M. Tsao, R. Liu, R. Ferrari, G. Donsi, F., Evaluation of the stability and antioxidant aetivity of nanoencapsulated resveratrol during in vitro digestion. Journal of Agricultural and Food Chemistry (2011) 59,12352-12360. 

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