Urine, calcium, balance measurements

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). 

Procedures which can be used to give an indication of the status of certain substances within the body, e.g. fat, calcium, and nitrogen. Balance studies involve measuring the intake of the substance and comparing it with the amount lost in faeces and urine. Positive balances occur when intake exceeds loss, for example nitrogen and calcium in pregnancy. Negative balances are found when loss exceeds intake, for example in malabsorption. 

Our simple approach to determining the optimal Ca/P ratio for intravenous feeding solutions was to simply alter the ratio of calcium to phosphate in these solutions and measure the only external loss of calcium and phosphate which was in the urine. We initially assumed that the difference between the intake and urinary loss of calcium and phosphate would measure the retention of these elements. The results of seven balance studies at varying Ca/P ratios are shown in Figure 5. 

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