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digestibility The proportion of a foodstuff absorbed from the digestive tract into the bloodstream, normally 90–95%. It is measured as the difference between intake and faecal output, with allowance being made for that part of the faeces that is not derived from undigested food residues (shed cells of the intestinal tract, bacteria, residues of digestive juices). Digestibility measured in this way is referred to as ‘true digestibility’, as distinct from the approximate measure, ‘apparent digestibility’, which is simply the difference between intake and output.