Dietary calcium, absorption

PTH is secreted from the parathyroid glands in response to a low plasma concentration of ionized (free) calcium. PTH immediately causes the transfer of labile calcium stores from bone into the bloodstream. PTH increases rates of dietary calcium absorption by the intestine indirectly via the vitamin D3 system activation of enterocyte activity. Within the kidney, PTH directly stimulates calcium reabsorption and a phosphate diuresis. 

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. 

Dietary calcium intake appears to affect lead absorption. An inverse relationship has been observed between dietary calcium intake and blood lead concentration in children, suggesting that children who are calcium deficient may absorb more lead than calcium replete children (Mahaffey et al. 1986 Ziegler et al. 1978). An effect of calcium on lead absorption is also evident in adults. In experimental studies of adults, absorption of a single dose of lead (100-300 ig lead chloride) was lower when the lead was... 

Calcium Rat Lead in tissues and severity of effect at low levels of dietary calcium Low dietary calcium (0.1 %) increase lead absorption and severity of effects Six and Goyer 1970 Mahaffey et al. 1973... 

Calcium Pig Lead in tissues at low levels of dietary calcium Increased absorption of lead with low dietary calcium Hsu et al. 1975... 

Fullmer CS, Rosen JF. 1990. Effect of dietary calcium and lead status on intestinal calcium absorption. Environ Res 51 91-99. 

Calcium oxalate (723) occurs as the monohydrate (whewellite, the thermodynamically stable form under ambient conditions (724)), the dihydrate (weddellite) in plant calcium stores and in sap, or the trihydrate (725). Calcium oxalate also plays a structural role in plants. Oxalate, for example from excessive amounts of rhubarb or spinach, inhibits absorption of Ca2+ from the GIT precipitation of calcium oxalate is the reason for the toxicity of oxalates. Calcium oxalate may also occur in man, where it can appear as minute star-shaped crystals in the urine. It is the main constituent of the majority of urinary calculi in man (726,727). The relationships between dietary calcium... 

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. 

Calcium retention is dependent on two factors, absorption and excretion. Normal subjects have been observed to have apparent calcium absorptions of 23 (sd = 12) to 27 (sd = 17) percent of the calcium from normal diets (21,22). For 20 women aged 55 to 65 consuming 629 (se = 92) milligrams dietary calcium daily, the apparent absorption was 32.1 (se = 1.9) percent (23). An apparent calcium absorption of 29.5 percent (n = 130) may be calculated from data published by Heaney et al. (15). Apparent absorption values from 29 to 42 percent may be calculated from data published by Linkswiler (24,25). However, much lower apparent absorption values of 6 to 15 percent may also be calculated from data published from the same laboratory (26). Although there is considerable variability in the apparent absorption values determined from many studies, a conservative value of 25 percent seems realistic for normal people consuming typical diets. 

Dietary phosphorus also affects calcium metabolism. Polyphosphate decreases calcium absorption in young men while orthophosphate supplement does not (26J. However, in the rat all forms of phosphate decrease calcium absorption about equally (31). However, widely divergent dietary calcium phosphorus ratios do not seem to affect calcium utilization by people as long as there is adequate phosphorus intake (32). In general phosphorus stimulates calcium retention in man (3277... 

Table I. Comparison of Various Dietary and Physiological Factors on Apparent Calcium Absorption by Rats and Humans...

Apparent absorption (intake minus fecal excretion) of calcium decreased when the diet contained muffins with added sodium phytate to increase the molar ratio of phytate/calcium from 0.04 to 0.14 and 0.24. One-half of the men excreted more calcium in feces than was consumed when the high phytate diet was consumed. People consuming diets with molar ratios of phytate/calcium exceeding 0.2 may be at risk of calcium deficiency because of low bioavailability of dietary calcium unless physiological adjustments can be accomplished that maintain homeostasis. 

Our studies do not resolve the question of phytate vs fiber for the effect of wheat bran on dietary calcium bioavailability. Phytate level clearly affected apparent absorption of calcium in HS-II in the presence of an amount of the water insoluble fraction of dephytinized bran equivalent to 12 g of untreated bran and the phytate supplied as sodium phytate. An additional trial using untreated bran and the same amount of fiber as the water insoluble fraction with sodium phytate could resolve the question of fiber vs phytate. In HS-I, the balances were positive when a relatively large amount of bran, 36 g/day, was consumed. Calcium intakes were possibly higher than most men consume, but under the dietary conditions imposed for 15 days, the phytate and fiber of 36 g of bran did not express an adverse effect on calcium balance. 

Age, calcium intake, hormonal status, exercise and vitamin status have all been implicated in the development of osteoporosis. Estrogen levels represent an important factor in skeletal calcium retention and homeostasis. In therapeutic trials in which post-menopausal women were given daily doses of estrogens, such therapy has been demonstrated to be partially effective in reducing the rate of bone resorption. However, this therapy has the concomitant hazard of endometrial cancer (10). Vitamin D and its hormones have been given considerable attention in the more recent studies. Without adequate dietary and tissue levels of such vitamins, calcium absorption and bone status will be impaired. 

It has been well established that the ingestion of high dietary protein levels results in hypercalciuria in man, and that hypercalciuria is frequently accompanied by negative calcium balance (1-3). In a summary of data from nutritional surveys in the U.S., Pao (4J showed that dietary protein intake was well above the Recommended Dietary Allowances (RDA) for both men and women regardless of age (1). Although dietary calcium intakes are generally at the RDA for men, women below the age of 50 yr consume only 75% of the RDA (4J. Women above the age of 50 yr consume only two-thirds of the RDA for calcium (4). These low consumptions become critical when we consider the reduced ability for calcium absorption demonstrated in both men and women over the age of 60 yr (6). 

More than 40 years ago, calcium absorption from brown (whole wheat) bread which was fed to human subjects was found to be poorer than was that when white (extracted wheat flour) was fed 04,5). Since then, many studies have sought to define the extent of inhibition of calcium intestinal bioavailability by various forms of dietary fiber with mixed results and conclusions (6-18). 

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). 

Wheat bran has been the fiber source most commonly used to study effects of dietary fiber on calcium absorption in controlled laboratory studies. However, wheat bran and other forms of fiber as they occur in food products present several disadvantages in terms of definition and by concurrently altering intakes of other substances or materials known or suspected of having an adverse effect on the bioavailability of calcium such as phytates and oxalates (5,13,17,22-28). Several studies have been conducted which have sought to separate or compare the effects of phytate and fiber... 

Bile acids and salts have been found to enhance the absorption of both calcium and vitamin D hence, to increase calcium absorption both directly and indirectly (3,37). However, the ability of some dietary fibers such as lignin and pectin to absorb conjugated and deconjugated bile salts onto their surfaces to be excreted in the feces (a mechanism credited to the hypocholesterolemic effect of some dietary fibers) may result in an overall decrease in calcium absorption from the gastrointestinal tract (7,33,38-40). 

Several other properties of selected dietary fibers may influence the bioavailability of calcium directly or indirectly. Those fibers which have cation exchange capabilities such as acid polysaccharides due to free carboxyl groups on the sugar residues may bind minerals such as calcium (3,17,33,36). Loss of calcium binding protein as a result of mucosa injury caused by the feeding of some kinds of dietary fiber was credited as being the partial cause of decreased calcium absorption in everted, rat gut sac studies by Oku et al. (16). 

Interest in the possible connection between intake of fat and absorption of calcium was generated by the concurrent massive losses of calcium in patients with steatorrhea, fatty diarrhea (46, 47). Ordinarily, however, fat is very efficiently absorbed from the gastrointestinal tract. Results of several studies in human adults and children indicate little or no effect of level of dietary fat on absorption of calcium (48-54). However, influence of level of dietary fat on calcium absorption in rat studies has produced conflicting results (55-57). 

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