Calcium factors influencing absorption

Calcification proceeds in the adult as well as during growth, and an adequate supply of calcium and phosphorus is thus essential throughout life to a healthy skeletal structure, including teeth in which mineral turnover proceeds after growth at a reduced rate. Certain factors, therefore, which influence absorption or retention of these elements can bring about demineralization of bone in some conditions. Their effects are discussed in the following sections. 

Absorption of calcium is influenced by many factors. The maintenance of normal gastric acidity facilitates absorption, and vitamin D enhances it. Absorption is increased by protein, lactose, and citrates in the diet. Absorption is decreased by the presence of phytates (in cereals), oxalates, and fatty acids, since in each instance insoluble calcium salts are formed. The ratio of calcium to phosphorus in the diet influences absorption. Calcium enters the gastrointestinal tract not only in food but as a component of digestive secretions as much as 0.3 to 0.8 g. daily may be secreted into the gastrointestinal tract. Reabsorption of this calcium is influenced by the factors just enumerated. 

A number of dietary factors in addition to need and the amount of calcium ingested influence calcium absorption some enhancing it, others interfering with it... 

Contraction of muscle follows an increase of Ca " in the muscle cell as a result of nerve stimulation. This initiates processes which cause the proteins myosin and actin to be drawn together making the cell shorter and thicker. The return of the Ca " to its storage site, the sarcoplasmic reticulum, by an active pump mechanism allows the contracted muscle to relax (27). Calcium ion, also a factor in the release of acetylcholine on stimulation of nerve cells, influences the permeabiUty of cell membranes activates enzymes, such as adenosine triphosphatase (ATPase), Hpase, and some proteolytic enzymes and facihtates intestinal absorption of vitamin B 2 [68-19-9] (28). 

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. 

As shown in Table III, mean fecal calcium losses tended to be higher when the higher fat diet was fed in comparison to results when the lower fat diet was fed. Therefore, apparent calcium absorption was higher when the low fat diet was fed. These differences were significant at only the P< 0.075 level hence, only a trend was illustrated. In this study no attempt was made to equalize fatty acid proportionality patterns or cholesterol intake. These or other dietary or non-dietary factors may have influenced the observed apparent trends. Other studies with human adults have not demonstrated any apparent influence on level of dietary fat on calcium absorption. 

Much else is known about GI absorption. Individuals vary in the extent to which they can absorb the same chemical, and absorption can be influenced by individual factors such as age, sex, health status, and even dietary habits. People who consume large amounts of fiber may absorb less calcium and iron than those who eat less. The GI tract is not fully developed until about 24 months after birth, and infants absorb metals such as lead and certain organic chemicals more readily than do adults. 

WA water quality labs by atomic absorption and autoanalyzer techniques. Charge balance calculations Indicated that all dissolved species of significance were analyzed. Comparison of filtered and unflltered aliquots suggested that un-lonlzed species were not present In appreciable quantities. Sampling and analysis uncertainties were determined by the operation of two co-located samplers for 16 weeks. The calcium and sulfate data were corrected for the Influence of sea salt to aid In the separation of the factors. This correction was calculated from bulk sea water composition and the chloride concentration In rainwater (11). Non seasalt sulfate and calcium are termed "excess" and flagged by a ... 

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. 

Intestinal calcium absorption is influenced by dietary factors. Lactose and other sugars increase water absorption, thereby enhancing passive calcium uptake. The effect of lactose is especially valuable because of its presence in milk, a major source of calcium. Lactose also increases absorption of other metal ions. This effect may contribute to the incidence of lead poisoning (plumbism) among young inner-city children exposed to high dietary levels of both lead and lactose. 

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. 

Also it is important to remember that, in addition to age and to pregnancy and lactation of women, calcium requirements are affected by the several factors that influence calcium absorption and excretion. Calcium is a good example, therefore, of a nutrient whose requirement cannot be decided on its own the factors that influence calcium absorption and excretion must also be considered. For this reason, calcium supplementation should be determined by a knowledgeble nutritionist or M.D. 

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