Absorption of proteins from the gi tract

A study of the systemic absorption of peptides (3 to 51 amino acids in length) found that peptides greater than 10 amino acids in length were poorly absorbed intact from the GI tract.4 Others have reported that gastric absorption is inversely related to the size of the molecule so that small molecules are more readily absorbed than large ones.5 A number of animal feeding studies with biotechnology-derived crops have investigated the digestibility and potential systemic absorption of intact introduced proteins in various tissues and blood samples using sensitive immunological assays.6-15 These published reports confirm that proteins, including those introduced into biotechnology-derived crops, are digested and have negligible oral bioavailability.

It is recognized that for proteins stable to digestion, minute quantities can be taken up intact by Peyers patches lining the GI tract, or may pass through intestinal cells via phagocytosis or permeation between epithelial cell junctions. An example is the egg allergen ovalbumin, which is stable to digestion in simulated gastric fluid for at least 60 minutes. Most common plant proteins, in contrast, are digestible in less than 15 seconds in simulated gastric fluid (SGF).16 Egg ovalbumin was administered to rats as an oral bolus dose (50 mg/rat). Bolus dosing increases the potential for absorption due to administration of a concentrated solution straight into the stomach. As a result, higher peak blood levels are achieved compared to lower doses resulting from consumption of albumin as a component of food in the diet. Nevertheless, even after bolus dosing of the stable egg ovalbumin protein, only 0.007% to 0.008% of the administered dose was absorbed from the GI tract.17

Similar results were reported for other protein allergens that are also stable to digestion, such as the soybean allergen Gly m Bd 30 k, where only approximately 0.004% of a large bolus dose was absorbed.18 There are also human studies reporting very low blood levels (generally less than 0.0001% of ingested protein) of stable food proteins such as ovalbumin, ovomucoid, and P-lactoglobulin after consumption of foods containing these proteins.19-21 These proteins are all highly abundant allergenic proteins in foods that are comparatively stable to digestion.16 For proteins that are not stable to digestion, the potential for systemic absorption of intact protein would be expected to be orders of magnitude lower than the very low levels of absorption for stable proteins alluded to earlier. This general lack of systemic bioavailability from the GI tract for intact proteins would minimize any potential for toxicity compared with single low-molecular-weight chemical substances following oral administration.

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