Intestinal Absorption and Secretion of Vitamin

Some 80% to 95% of dieteuy ascorbate is absorbed at inteikes up to about 100 mg per day the absorption of larger tunounts of the viteunin is lower, falling from 50% of a 1.5-g dose to 25% of a 6-g dose, and 16% of a 12 g-dose (Rivers, 1987). Unabsorbed ascorbate from high doses is a substrate for intestinal bacteritil metabolism. 

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Although it is being found that vitamin D metaboUtes play a role ia many different biological functions, metaboHsm primarily occurs to maintain the calcium homeostasis of the body. When calcium semm levels fall below the normal range, 1 a,25-dihydroxy-vitainin is made when calcium levels are at or above this level, 24,25-dihydroxycholecalciferol is made, and 1 a-hydroxylase activity is discontiaued. The calcium homeostasis mechanism iavolves a hypocalcemic stimulus, which iaduces the secretion of parathyroid hormone. This causes phosphate diuresis ia the kidney, which stimulates the 1 a-hydroxylase activity and causes the hydroxylation of 25-hydroxy-vitamin D to 1 a,25-dihydroxycholecalciferol. Parathyroid hormone and 1,25-dihydroxycholecalciferol act at the bone site cooperatively to stimulate calcium mobilization from the bone (see Hormones). Calcium blood levels are also iafluenced by the effects of the metaboUte on intestinal absorption and renal resorption. 

Calcium is present in three forms e.g., as free calcium ion, bound to plasma protein albumin and in diffusable complexes. The endocrine system, through parathyroid hormone and calcitonin, helps in keeping the concentration of ionized plasma calcium in normal level. Decrease in plasma levels of ionized calcium leads to increased parathyroid hormone secretion. Parathyroid hormone tends to increase plasma calcium level by increasing bone resorption, increasing intestinal absorption and increasing reabsorption of calcium in kidney. Vitamin D acts by stimulating... 

Deficiencies of vitamin K and indeed all the other fat soluble vitamins are more likely to occur as a result of impairment in fat absorption than from dietary insufficiency. This could occur when the secretion of bile salts is restricted (as in biliary obstruction), when sections of the gut have been removed or damaged by surgery or in diseases, such as tropical sprue and cystic fibrosis, that are associated with poor intestinal absorption. Even when normal absorptive mechanisms are functioning well, some fat is necessary in the diet to improve the absorption and utilization of fat soluble vitamins. There is little evidence, however, that, within the normal range of fat intakes, the amount of dietary fat significantly affects the utilization of fat soluble vitamins. 

Bde salts, cholesterol, phosphoHpids, and other minor components are secreted by the Hver. Bile salts serve three significant physiological functions. The hydrophilic carboxylate group, which is attached via an alkyl chain to the hydrophobic steroid skeleton, allows the bile salts to form water-soluble micelles with cholesterol and phosphoHpids in the bile. These micelles assist in the solvation of cholesterol. By solvating cholesterol, bile salts contribute to the homeostatic regulation of the amount of cholesterol in the whole body. Bile salts are also necessary for the intestinal absorption of dietary fats and fat-soluble vitamins (24—26). 

The steroid hormone 1,25-dihydroxy vitamin D3 (calcitriol) slowly increases both intestinal calcium absorption and bone resorption, and is also stimulated through low calcium levels. In contrast, calcitonin rapidly inhibits osteoclast activity and thus decreases serum calcium levels. Calcitonin is secreted by the clear cells of the thyroid and inhibits osteoclast activity by increasing the intracellular cyclic AMP content via binding to a specific cell surface receptor, thus causing a contraction of the resorbing cell membrane. The biological relevance of calcitonin in human calcium homeostasis is not well established. 

In general, ethanol in low to moderate amounts, is relatively benign to most body systems. A moderate amount of ethanol causes peripheral vasodilation, especially of cutaneous vessels, and stimulates the secretion of salivary and gastric fluids the latter action may aid digestion. On the other hand, ethanol consumption in high concentrations, as found in undiluted spirits, can induce hemorrhagic lesions in the duodenum, inhibit intestinal brush border enzymes, inhibit the uptake of amino acids, and limit the absorption of vitamins and minerals. In addition, ethanol can reduce blood testosterone levels, resulting in sexual dysfunction. 

MecftanismofAction Afat-soluble vitamin that is essentiai for absorption, utiiization of caicium phosphate, and normai caicification of bone. Therapeutic Effect Stimu-iates caicium and phosphate absorption from smaii intestine, promotes secretion of caicium from bone to biood, promotes renai tubuie phosphate resorption, acts on bone ceiis to stimuiate skeietai growth and on parathyroid giand to suppress hormone synthesis and secretion. 

Mechanism of Action A fat-soluble vitamin that stimulates calcium and phosphate absorption from small intestine, promotes secretion of calcium from bone to blood, and promotes resorption of phosphate in renal tubules also acts on bone cells to stimulate skeletal growth and on parathyroid gland to suppress hormone synthesis... 

An inadequate snpply of bile contributes to vitamin A deficiency because of disturbances of the intestinal track which prevent the effective absorption of the vitamin. In an average adult, from one-half to one liter of bile is secreted every 24 hours, the quantity depending upon the amount and kind of food eaten. The absence or lack of secretion of bile is known as acholia. 

Vitamin E, like neutral lipids, requires apoB lipoproteins at every stage of its transport (Fig. 27-2). Dietary vitamin E becomes emulsified in micelles produced during the digestive phase of lipid absorption and permeates the intestinal epithelium, similar to fatty acids and cholesterol. Uptake of vitamin E by enterocytes appears to be concentration dependent. Within intestinal cells, vitamin E is packaged into chylomicrons and secreted into lymph. During blood circulation of chylomicrons, some vitamin E may be released to the tissues as a consequence of partial lipolysis of these particles by endothelial cell-anchored lipoprotein lipase. The rest remains associated with chylomicron remnants. Remnant particles are mainly endocy-tosed by the liver and degraded, resulting in the release of fat-soluble vitamins. 

The principal physiological role of vitamin D is in the maintenance of the plasma concentration of calcium. Calcitriol acts to increase intestinal absorption of calcium, to reduce its excretion by increasing reabsorption in the distal renal tubule, and to mobilize the mineral from bone - of the 25 mol of calcium in the adult body, 99% is in bone. The daily intake of calcium is around 25 mmol, and intestinal secretions add an additional 7 mmol to the intestinal contents 10 to 14 mmol of this is normally absorbed, with 18 to 22 mmol excreted in feces. Bone turnover accounts for exchange of 10 mmol of calcium between bone and plasma daily. The kidneys filter some 240 mmol of calcium daily, almost all of which is reabsorbed urinary excretion of calcium is about 3 to 7 mmol per day. 

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