Gastrointestinal tract nutrient absorption

Normal dietary exposure over many years has highlighted two areas of concern arising through PPT binding of nutrients in the gastrointestinal tract. Impaired absorption of trace metals and impaired protein utilization have been referred to above. 

For some nutrients the gastrointestinal tract regulates absorption based on nutrient... 

The gastrointestinal tract (GIT) is a highly specialized region of the body whose primary functions involve the processes of secretion, digestion, and absorption. Since all nutrients needed by the body, with the exception of oxygen, must first be ingested orally, processed by the GIT, and then made available for absorption into the bloodstream, the GIT represents a significant barrier and interface with the environment. The primary defense mechanisms employed by the gut... 

A high rate of ethanol consumption can lead to inhibition of gastric secretion and irritation of the gastric mucosa. Ethanol irritates the entire gastrointestinal tract, which may lead to constipation and diminished absorption of nutrients. Other pathological effects include pancreatitis and peripheral neuropathy. Severe gonadal failure is often found in both men and women, accompanied by low blood levels of sex hormones. 

Most food absorption takes place in the small intestine. The gastrointestinal tract possesses specialized carrier systems for certain nutrients such as carbohydrates, amino acids, calcium, and sodium. Some xenobiotics use these routes of passage through the cells, while others enter through passive diffusion. 

In ruminants, avoparcin has a dual action. It acts in the rumen by enhancing fermentation, and in the intestine by improving the absorption of nutrients. Following feeding to animals, avoparcin is virtually unabsorbed from the gastrointestinal tract and is rapidly eliminated in the form of the parent compound. As a result, no withdrawal period is required. 

The various lipoid barriers of the gastrointestinal tract, the kidney tubules, and the central nervous system (CNS) allow the absorption of essential nutrients, guard against the uncontrollable disposal of electrolytes and other substances, and prevent the entrance of potentially toxic materials. 

Calcium has long been implicated as a dietary factor which inhibits the absorption of manganese. Since manganese is better absorbed in the oxidized (+2) than in the reduced state (+3 or +4), any factor which increases gastrointestinal tract pH (increases alkalinity) would be expected to inhibit manganese utilization. Calcium carbonate, a commonly used antacid by humans as well as a calcium nutrient supplement, has been found to have a greater adverse effect on apparent manganese absorption in humans than does milk. 

Drugs for obesity act either on the gastrointestinal tract to lower nutrient absorption or centrally to reduce food intake by decreasing appetite or increasing satiety (appetite suppressants). 

Many of the safety issues in parenteral nutrition relate to the fact that the process is inherently unphysiological (5). Instead of periodic ingestion of nutrients via the gastrointestinal tract resulting in gradual entry of nutrients into the blood, nutrients are infused directly at a constant rate. The gastrointestinal tract as a mediator of nutrient absorption, the periodicity of nutrient administration, and the natural biorhjdhms of hormoue secretion are all lost. 

Facilitated diffusion is very similar to passive diffusion with the difference that transfer across membranes is assisted by the participation of carrier proteins embedded in the membrane bilayer. Again, the direction of passage will be from the side of the membrane with high concentration of a chemical to the side with low concentration this also occurs without energy expenditure by the cell. Such a process is somewhat specific in the sense that it applies to molecules that are able to bind to a carrier protein. Absorption of nutrients such as glucose and amino acids across the epithelial membrane of the gastrointestinal tract occurs by facilitated diffusion. Since a finite number of carriers are available for transport, the process is saturable at high concentrations of the transported molecules and competition for transport may occur between molecules of similar structure. 

Chemical interactions in the gastrointestinal tract between nutrients and drugs may considerably reduce the absorption of some drugs calcium ions from dairy products form insoluble and therefore nonabsorbable complexes with the antibiotic tetracycline. On the other hand, certain drugs are irritants to the gastrointestinal tract (nonsteroidal antiinflammatory drugs and potassium chloride tablets) and must be ingested with food. 

In children and adults with celiac disease, with impaired absorption of nutrients from the gastrointestinal tract, the availability of tryptophan to the brain, as indicated by the plasma ratio of tryptophan to other large neutral amino acids (LNAA), is low, especially in those with depression.43-44... 

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