Dietary proteins, absorption

Anders E, Bagnell CR Jr, Krigman M, et al. 1982. Influence of dietary protein composition on lead absorption in rats. Bull Environ Contam Toxicol 28 61-67. 

The consequence of bacterial bile acid metabolism [66, 74,77] is hardly clinically significant malabsorption [6] in otherwise healthy individuals [32,79], but in predisposed individuals this may be different. Accordingly, omeprazole interferes with the absorption of vitamin B12 [80-83] and protein assimilation [84], The mechanism for altered vitamin B12 absorption is prevention of its cleavage from dietary protein [83], for which the importance of the concurrent bacterial overgrowth has not yet been ruled out. 

Renal tubular absorption of zinc in mice was impaired by certain diuretics and was further influenced by dietary proteins (Goyer 1986). Zinc absorption in rats was depressed after consumption of high levels of inorganic iron absorption was normal with organoirons (Greger 1989). 

It has been well established that the ingestion of high dietary protein levels results in hypercalciuria in man, and that hypercalciuria is frequently accompanied by negative calcium balance (1-3). In a summary of data from nutritional surveys in the U.S., Pao (4J showed that dietary protein intake was well above the Recommended Dietary Allowances (RDA) for both men and women regardless of age (1). Although dietary calcium intakes are generally at the RDA for men, women below the age of 50 yr consume only 75% of the RDA (4J. Women above the age of 50 yr consume only two-thirds of the RDA for calcium (4). These low consumptions become critical when we consider the reduced ability for calcium absorption demonstrated in both men and women over the age of 60 yr (6). 

Protein has long been classified as a factor that causes Ca to be wastefully excreted from the body. Less is documented in relation to how it affects Ca absorption. Dawson-Hughes has reported that a dietary protein increase of 20% combined with a low Ca intake of 800 mg/day in elderly men and women lowers the amoimt of absorbable Ca by 23%. In contrast, a high protein diet (between 18.16% and 29.14% of total dietary energy from protein) in the presence of a high Ca intake... 

Huff, M.W. and Carroll, K.K. 1980. Effects of dietary protein on turnover, oxidation, and absorption of cholesterol, and on steroid excretion in rabbits. J. Lipid Res. 21, 546-558. 

We have recently reported that the action of protease inhibitors as dietary anticarcinogens may work via two mechanisms 1) An indirect effect on protein absorption and 2) A direct effect on cell transformation (Cancer Res,... 

Limited data are available about the effects of individual dietary components on absorption, and consequently the requirement, of Mn. Dietary protein and phosphorus levels (33), calcium level (34) and the effect of a partial substitution of soy protein for meat (28) have been tested in balance studies without any obvious effect of Mn absorption or retention. However, since the main route of excretion is via the bile, the conventional balance technique is probably not sensitive enough to identify dietary factors that influence Mn absorption. 

Price et al. (5) measured the retention of zinc and other minor elements in studies in which the major variables were source and level of dietary protein. Comparison of protein sources from plant and mixed sources indicated that the apparent absorption was somevJhat lower from the diets with only plant proteins than that from mixed sources. A lower level of protein (25 g/day) caused a lower retention of zinc than the moderate protein (46 g/day) diet, although conclusions were complicated because of other variables. 

Suppression of bowel flora is thought by some to be useful in hepatic encephalopathy. Here, absorption of products of bacterial breakdown of protein (ammonium, amines) in the intestine lead to cerebral symptoms and even to coma. In acute coma, neomycin 6 g/d should be given by gastric tube as prophylaxis, 1-4 g/d may be given to patients with protein intolerance who fail to respond to dietary protein restriction (see also lactulose, p. 640). 

In the cases of dietary heme and nonheme iron, the iron appears in the bloodstream bound to the transport protein transferrin. After its dissociation from dietary proteins by proteases, the heme is absorbed intact by the enterocyte. The heme i.s then degraded by heme oxidase. Heme oxidase catalyzes the Oj-depend-ent degradation of heme to biliverdm. Biliverdin is further degraded to bilirubin, which is excreted from the body in the bile. Heme absorption, as well as heme oxidase activity, is somewhat higher in the duodenum than in the jejunum and ileum, as determined in studies with rats. The heme catabolic pathway is shown in Figure 10,29, Most of the bilirubin in the body is not produced by the catabolism of dietary heme, but by the catabi lism of the heme present in old, or senescent, red blood cells, between 7S and 80% of the bilirubin formed in the body is derived from senescent red blood cells most of the remainder is derived from the normal turnover of the heme proteins in the liver. 

Food can affect both the absorption and the clearance of theophylline. One study showed that the absorption of a modified-release formulation of theophylline was very slow after an overnight fast, in contrast to absorption after a test meal (86). The effect may be dose-related. More specifically, dietary protein significantly affects theophylline clearance a low-protein diet reduces theophylline clearance and a high-protein diet increases it. The implications for clinical practice have not been elaborated, but dietary extremes are contraindicated in patients taking theophylline (51). 

---