Assimilation from Intestine

D-Fructose in the human diet derives mainly from sucrose, fruits, and honey. Sucrose is /I-D-fructofuranosyl a-D-glucopyranoside, and, after hydrolysis by invertase (EC 3.2.1.26), to D-glucose and D-fructose, can be absorbed from the small intestine. In the human intestine, invertase, as well as a-D-glucosidases, is developed very early in fetal life, and even appears much earlier than lactase (EC 3.2.1.23). There is no significant, intestinal transport of unhydrolyzed sucrose, and, in animal experiments, sucrose administered by injection is quantitatively excreted in the urine.1 Intestinal invertase is produced by mucosal cells localized in the brush-border membrane of the mucosal epithelia. Invertase is not secreted,1-4 and little or no invertase (sucrase) has been found in the intestinal lumen.1 The specific localization of sucrase at the mucosal, luminal interface is thought to be of functional importance in coupling sucrose digestion to transport.1 

Hydrolysis of sucrose occurs rapidly within the outer portion of the brush-border, plasma membrane, whereupon most of the monosaccharides released are transported across a permeability barrier located in the inner portion of the same membrane. Some hydrolytic products diffuse backward from the mucosal site of formation, and accumulate in the lumen.5-8 The mucosal site of hydrolysis is identified by the use of fluorescent antibodies against sucrase8 and by differential centrifugation of intestinal preparations.2 

The more rapid absorption of D-fructose derived from sucrose over that of administered free D-fructose can be observed clinically. Thus, 100 g of D-fructose administered by mouth usually results in an osmotic diarrhea, whereas 200 g or more of sucrose similarly administered rarely produces diarrhea.9 

Existence of an active-transport system for D-glucose in mammalian intestine has been recognized for some time, but the mechanism of D-fructose transport is still controversial. Part of the controversy can be attributed to species differences in transport systems. Although the rate of uptake of D-fructose is lower than that observed for actively transported sugars, such as D-glucose and D-galactose (see Table I), 

Absorptive Rate11 of Monosaccharides by the Human Jejunum, Compared to tbe Rate of Absorption of D-Glucose 

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Since in mammalian species metals first need to be assimilated from dietary sources in the intestinal tract and subsequently transported to the cells of the different organs of the body through the bloodstream, we will restrict ourselves in this section to the transport of metal ions across the enterocytes of the upper part of the small intestine (essentially the duodenum), where essentially all of the uptake of dietary constituents, whether they be metal ions, carbohydrates, fats, amino acids, vitamins, etc., takes place. We will then briefly review the mechanisms by which metal ions are transported across the plasma membrane of mammalian cells and enter the cytoplasm, as we did for bacteria, fungi and plants. The specific molecules involved in extracellular metal ion transport in the circulation will be dealt with in Chapter 8. 

Assimilation from the Gastrointestinal Tract. There are several sources of variation in the assimilation of actinides from the mammalian intestine. Among the most important are chemical form and age of the animal. The comparative metabolism of different actinides administered to rats as nitrates has been studied by Sullivan and Crosby (25,26). The consistency of experimental technique in their administration of actinides to adult and newborn rats permits comparisons between the assimilation of different actinides. The fractional assimilation, expressed as the amount in liver and carcass together, 7 days post-dose, was approximately IO for isotopes of U, Pu, Am and Cm in adult rats. In young rats, the assimilation was about two orders of magnitude greater. 

Lipoproteins are assembled in two organs, the small intestine and the liver. The lipoproteins assembled in the intestine contain the lipids assimilated from the diet. These lipoproteins, called chylomicrons, leave the enterocyte and enter the bloodstream via the Lymphatic system. The lipoproteins assembled in the liver contain lipids originating from the bloodstream and from de novo synthesis in the liver. The term de novo simply means "newly made from simple components" as opposed to "acquired from the diet" or "recycled from preexisting complex components." These lipoproteins, called very-low-dcnslty lipoproteins (VLDLs), are secreted from the liver into the bloodstream. The liver also synthesizes and secretes other Lipoproteins called high-density Lipoproteins (HDLs), which interact with the chylomicrons and VLDLs in the bloodstream and promote their maturation and function. The data in Table 6-4 show that chylomicrons contain a small proportion of protein, whereas HDLs have a relatively high protein content. Of greater interest is the identity and function of the proteins that constitute these particles. These proteins confer specific properties to lipoprotein particles, as detailed later in this chapter. 

By the end of the small intestine, deposition is almost complete and there is no need for intestinal secretions to aid assimilation. The principal role of the colon is to resorb water and reclaim sodium however, complex carbohydrate components of vegetable origin have nutritional value but are relatively resistant to attack from intestinal secretion. In the caecum, a complex bacterial environment digests the soluble, fermentable carbohydrates to yield short-chain fatty acids, which are assimilated into the systemic circulation by the colon, together with vitamin K released from the plant material. 

Bile has long been attributed an important role in medicine [1]. The effect of an impaired bile flow to the intestine has been known to result in steatorrhea — fat malabsorption — and defective absorption of fat-soluble vitamins, notably vitamin K [2], Thus, it is obvious that bile is important for fat assimilation from the intestine. However, it is equally apparent that when fat absorption after bile obstruction or diversion could be studied by quantitative methods, the malabsorption was found to be only partial [3]. In fact, it has seemed surprising that some 60-70% of a normal fat load is absorbed in man and the experimental animal in the absence of bile in the intestine. The absorption of nonpolar lipids, however, is much less efficient, and cholesterol absorption has been reported to have an absolute requirement for the presence of bile salts [4]. Of the bile components important for fat absorption bile salts have been ascribed the main role although experimental results are accumulating regarding the role of bile phospholipids in the specific uptake of sterols by the intestine [5]. 

In humans, iron is assimilated from food in the intestine. A protein called tranrferrin hinds iron and transports it across the intestinal vrall to distribute it to other tissues in the body. The normal adult body contains about 4 g of iron. At any one time, about 3 g of this iron is in the hlood, mostly in the form of hemoglobin. Most of the remainder is carried hy transferrin. 

Toxicity of cadmium increases in cases of zinc deficiency, due to the zinc substitution in biological systems, which leads to functional disorders. Cadmium reduces assimilation of vitamins C and D. However, a large amount of these vitamins in the diet will decrease the toxicity of cadmium through the reduction of its absorption from the intestinal tract (Friberg et ah, 1986 Hill, 1996 McLaughlin et ah, 1999). 

Animal organisms generally require effective assistance of intestinal flora, as in ruminants, to assimilate inorganic nitrogen into body protein. This accounts for the human needs of a daily requirement of 70-80 grams of protein, However, over half of the protein-constituent amino adds can be derived from other amino acids by their own enzymic reactions. Thus, amino acids are classified as essential or nonessential. Amino acid requirements vary with the physiological state of the animal, age. and possibly with the nature of the intestinal flora. 

Gardner, M.L. 1984. Intestinal assimilation of intact peptides and proteins from the diet—a neglected field Biol. Rev. 59, 289-331. 

The rate at which cholesterol and triglycerides enter the circulation from the liver and small intestine depends on the supply of the lipid and proteins necessary to form the lipoprotein complexes. Although the protein component must he synthesized, the lipids can be obtained either from de novo biosynthesis in the ti.ssucs or from the diet. Reduction of plasma lipids by diet can delay the development of atherosclerosis. Furthermore, the u.sc of drugs that decrease assimilation of lipids into the body plus diet decreases mortality from cardiovascular disca.se. ... 

About 70 percent of the phosphorus in foods is assimilated into our bodies, unlike calcium, of which only 20 to 30 percent is absorbed from food in the small intestine. Excess phosphorus and magnesium in the blood hinder absorption of calcium from food. (Calcium, in turn, hinders absorption of iron.) If the intake of calcium, phosphorus, or vitamin D is too low, bones don t grow properly. 

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