Balance meal size

As noted above, disagreement has often been observed among different studies on the effects of fiber, phytic acid and protein source on mineral utilization. Some possible reasons include (a) estimates of absorption from single meals (with or without previous consumption of the same foods used in the test meal which may also affect results) may not always be equivalent to results from multi-day balance studies, (b) in balance studies, the failure to allow sufficient time (e.g., 1-2 weeks or more) for adaptation may alter the findings, (c) variations in the compositions of meals or diets, including mineral levels, between studies may influence the results obtained, and (d) the persons used as subjects vary and this may have an affect. In addition, in the fiber studies, the levels, types, and particle size of fiber fed have varied widely and levels of other possibly confounding components (e.g., caffeine, tanins, oxalates) may have differed. 

The enterohepatic circulation of bile acids is characterized by a large pool of bile acids (2,15,26) which cycles many times (probably six to ten) each day. The size of the pool is determined by the efficiency of intestinal absorption and by the rate of hepatic synthesis of bile acids from cholesterol (94). The efficiency of absorption is high—in health probably greater than 98 %— and the amount of bile acids not absorbed is balanced by hepatic synthesis (Fig. 17). Bile acids are excreted only in feces, and their nucleus is considered invulnerable to bacterial attack therefore, the measurement of fecal bile acids either by chemical estimation or isotope dilution techniques indicates hepatic synthesis (94). The pool size may be estimated directly by isotope dilution, and an indirect estimate of pool size can be obtained by measuring jejunal bile acid concentration during digestion of a test meal. 

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