Absorption calcium, dietary requirements

Most of the forementioned studies which examined the influence of various dietary fiber on the bioavailability of calcium by human subjects have depended upon the comparative measurements of calcium content of diets and calcium contents of stools and urine. As reviewed by Allen (3), calcium balance studies have distinct limitations relative to accuracy and precision. However, their ease of application and cost, laboratory equipment requirements, and real (or perceived) safety in comparison to available radioactive or stable isotope methods continue to make their use popular. In calcium balance studies, calcium absorption is assumed to be the difference between calcium excretion in the feces and calcium intake. Usually this is expressed as a percent of the calcium intake. This method assumes that all fecal calcium loss is unabsorbed dietary calcium which is, of course, untrue since appreciable amounts of calcium from the body are lost via the intestinal route through the biliary tract. Hence, calcium absorption by this method may underestimate absorption of dietary calcium but is useful for comparative purposes. It has been estimated that bile salts may contribute about 100 g calcium/day to the intestinal calcium contents. Bile salt calcium has been found to be more efficiently absorbed through the intestinal mucosa than is dietary calcium (20) but less so by other investigators (21). 

Administration. Most children and adults do not ingest sufficient dietary calcium and require supplements (see Tables 88-5 and 88-6). Individuals with certain characteristics or conditions—such as lactose intolerance nondairy vegetarian diet malnutrition low-fat diets and glucocorticoid, antiresorptive, or parathyroid therapy—also require evaluation for calcium supplementation. To ensure adequate calcium absorption, 25(OH) vitamin D concentrations should be maintained in the normal range." ... 

Calcium carbonate is the salt of choice because it contains the highest amount of elemental calcium and is the least expensive (see Table 88-5). The fraction of calcium absorbed is dose-limited, so maximum single doses of 600 mg or less of elemental calcium are recommended. Calcium carbonate tablets should be taken with meals to enhance absorption. Calcium citrate absorption is acid-independent and need not be administered with meals. Although tricalcium phosphate contains 39% calcium, nonabsorbable calcium-phosphorus complexes may limit overall calcium absorption compared to other products. This product may be required for up to 10% of seniors with hypophosphatemia that cannot be resolved with increased dietary intake. Disintegration and dissolution rates vary significantly between products and lots. Products with good disintegration and dissolution rates and lead contents of less than 1 mcg/day should be recommended. 

The amount of each element required in daily dietary intake varies with the individual bioavailabihty of the mineral nutrient. BioavailabiUty depends both on body need as deterrnined by absorption and excretion patterns of the element and by general solubiUty, and on the absence of substances that may cause formation of iasoluble products, eg, calcium phosphate, Ca2(P0 2- some cases, additional requirements exist either for transport of substances or for uptake or binding. For example, calcium-binding proteias are iavolved ia calcium transport an intrinsic factor is needed for vitamin cobalt,... 

One method of treatment is to inject calcitonin, which decreases blood Ca " concentration and increases bone calcification (33). Another is to increase the release of calcitonin into the blood by increasing the blood level of Ca " ( 4). This latter treatment is accompHshed by increasing Ca " absorption from the intestine requiring dietary calcium supplements and avoidance of high phosphate diets. The latter decrease Ca " absorption by precipitation of the insoluble calcium phosphate. 

Vitamin D is not strictly a vitamin since it can be synthesized in the skin, and under most conditions that is its major source. Only when sunlight is inadequate is a dietary source required. The main function of vitamin D is in the regulation of calcium absorption and homeostasis most of its actions are mediated by way of nuclear receptors that regulate gene expression. Deficiency—leading to rickets in children and osteomalacia in adults—continues to be a problem in northern latitudes, where sunlight exposure is poor. 

The concept of bioavailability was developed to explain the difference between the total amount of mineral in a food and the amount which was used by the individual consuming the food. Over the past sixty years or more, there have been numerous studies related to dietary calcium requirements and bioavailability (1,2). As a result, much is known about non-calcium food components which influence the absorption and utilization of dietary calcium under experimental conditions. What now is lacking is a detailed knowledge of how these factors interact with calcium under normal conditions of ingestion in meals. 

With normal exposure to sunlight enough 7-dehydrocholesterol is converted to cholecalciferol in the skin that it was concluded that no dietary vitamin D is required by most adults except during pregnancy. However, recently it has been recognized that old and sick adults probably need 400-600 I.U. per day to maintain calcium absorption arid to prevent osteoporesis and fractures/ " 1 It is usual-... 

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. 

Copper is recognized as an essential metalloelement like sodium, potassium, magnesium, calcium, iron, zinc, chromium, vanadium and manganese [1]. Like essential amino acids, essential fatty acids and essential cofactors (vitamins), essential metalloelements are required for normal metabolic processes but cannot be synthesized de novo and daily dietary intake and absorption are required. The adult body contains between 1.4 mg (22 pmol) and... 

The direct measurement in the diet of substances known to adversely affect zinc absorption. This most commonly would require determination of the phytic acid content. Substances such as dietary fibre, calcium, iron, tin, oxalate and others, are also important in certain instances. 

Absorption of mineral elements is either by simple diffusion or by carrier-mediated transport. The exact mechanisms for aU minerals have not been estabhshed, but the absorption of calcimn, for example, is regulated by 1,25-dehydroxycholecalciferol (see p. 80). Low alimentary pH favoiu caldinn absorption, but absorption is inhibited by a number of dietary factors such as the presence of oxalates and phytates. An excess of either calcium or phosphorus interferes with the absorption of the other. The absorption of calcium is also influenced by the requirements of the animal. For example, the absorption of calcium from the digestive tract of laying hens is much greater when shell formation is in progress than when the shell gland is inactive. 

For these reasons, many Americans turn to dietary supplements to meet their daily calcium requirements. There are two major chemical forms of dietary calcium supplements calcium carbonate and calcium citrate. Calcium carbonate is the least expensive and contains 40% calcium by weight. It requires stomach acid for proper absorption, so it must be taken with meals. It may not be as well absorbed by individuals taking common acid-blocking medications. Calcium... 

The very high requirement of birds for manganese, compared with that of mammals, even under the most favorable dietary conditions, calls for some comment. To some extent this is undoubtedly due to lower absorption from the gut. Injection of manganese in quantities equivalent to 6 to 10 p.p.m. of the diet is completely effective in preventing perosis. Four to five times these quantities are needed in diets containing ordinary levels of calcium, phosphorus, and iron. It appears, therefore, that only... 

By increasing the absorptive capacity of the gut and by regulating the renal (kidney) excretion, the body can either adapt itself to (1) reduced dietary intakes of calcium, or (2) increased requirements for calcium. 

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