Calcium excretion dietary fiber

JAHNEN A, HEYNCK H, GERTZ B, CLABEN A, HESSE A (1992) Dietary fiber the effectiveness of a high bran intake in reducing renal calcium excretion. Urol Res, 20 3-6. 

Many other dietary factors have been reported to affect calcium bioavailability. Phytate, fiber, cellulose, uronic acids, sodium alginate, oxalate, fat (only in the presence of steatorrhea), and alcohol have been reported to decrease calcium bioavailability (15). Lactose and medium chain triglyceride increase it (15). FTuoride also affects calcium retention primarily by stimulating bone formation thereby decreasing calcium excretion (33-38). The effects of fluoride on calcium utilization have been variable (34,38,39). 

In the studies on humans there appeared to be decreased calcium balances when 200 g or more of spinach per day was included in the diet. In two of the studies in which women were fed spinach, calcium intakes were below the Recommended Dietary Allowance of 800 mg/day (37). Some studies were conducted for short period of a week or less, which may not be sufficient time to adjust to a change in diet. From measurement of calcium excretion in urine after a test meal, it was shown that the calcium in oxalate-containing vegetables was less well-absorbed than that of milk or of vegetables not containing oxalic acid. However, this would not necessarily affect calcium balance, since the total amount of calcium in the diet would have to be considered. The effect of a combination of oxalic acid and fiber on calcium bioavailability should be further investigated. 

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). 

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). 

Cellulose. Fiber components differ in their ability to bind to bile (41), the primary route of excretion of manganese (9). Cellulose, the most abundant natural fiber, has been studied for its potential effects on bile and lipid metabolism (42). It is naturally present in high fiber diets and is also commonly added to many commercially-produced food products. Previous studies have shown cellulose to decrease intestinal uptake of a variety of minerals, including zinc, phosphorus, calcium, magnesium, and iron (32,43,44). In many cases, however, the effect of cellulose has not been considered to be anti-nutritional (32). It has been generally believed that cellulose has a lower binding affinity for minerals than other constituents of dietary fiber (33,45). 

Figure 1. Calcium intake and excretion of older rats (63 days) and younger rats (33 days) on a low fiber diet (1), raw spinach (2), cooked spinach (3), filter paper (4), and high filter paper (5). Dietary intake is indicated by the top of the bar. Balance is indicated by the bottom of the bar. Adapted from Ref. 12.

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