No abstract

Antacids used to decrease phosphorus absorption in patients with renal failure may be toxic. To find more efficient or less toxic binders, a three-part study was conducted. First, theoretical calculations showed that phosphorus binding occurs in the following order of avidity: Al3+ > H+ > Ca2+ > Mg2+. In the presence of acid (as in the stomach), aluminum can therefore bind phosphorus better than calcium or magnesium. Second, in vitro studies showed that the time required to reach equilibrium varied from 10 min to 3 wk among different compounds, depending upon solubility in acid and neutral solutions. Third, the relative order of effectiveness of binders in vivo was accurately predicted from theoretical and in vitro results; specifically, calcium acetate and aluminum carbonate gel were superior to calcium carbonate or calcium citrate in inhibiting dietary phosphorus absorption in normal subjects. We concluded that: (a) inhibition of phosphorus absorption by binders involves a complex interplay between chemical reactions and ion transport processes in the stomach and small intestine; (b) theoretical and in vitro studies can identify potentially better in vivo phosphorus binders; and (c) calcium acetate, not previously used for medical purposes, is approximately as efficient as aluminum carbonate gel and more efficient as a phosphorus binder than other currently used calcium salts.

The purpose of this study was to measure magnesium absorption over the wide range of intakes to which the intestine may be exposed from food and/or magnesium-containing medications. Net magnesium absorption was measured in normal subjects after they ingested a standard meal supplemented with 0, 10, 20, 40, and 80 mEq of magnesium acetate. Although absorption increased with each increment in intake, fractional magnesium absorption fell progressively (from 65% at the lowest to 11% at the highest intake) so that absorption as a function of intake was curvilinear. This absorption-intake relationship was almost perfectly represented by an equation containing a hyperbolic function plus a linear function. Our results are statistically compatible with a magnesium absorption process that simultaneously uses a mechanism that reaches an absorptive maximum, plus a mechanism that endlessly absorbs a defined fraction (7%) of ingested magnesium. Compared to previous studies of calcium absorption, much less magnesium than calcium was absorbed at intakes above 8 mEq/meal, apparently due to greater restriction of intestinal permeability to magnesium. We also found that magnesium from a high magnesiumcontaining food source, almonds, was just as bioavailable as from soluble magnesium acetate. In contrast, magnesium absorption from commercially available enteric-coated magnesium chloride was much less than from magnesium acetate, suggesting that enteric coating can impair magnesium bioavailability. (J. Clin. Invest. 1991.88:396-402.)

The purpose of these studies was to gain insight into the pathophysiology of pure osmotic diarrhea and the osmotic diarrhea caused by carbohydrate malabsorption.

No abstract

Background: Roux-en-Y gastric bypass (RYGB) restricts food intake, and when the Roux limb is elongated to 150 cm, the procedure is believed to induce malabsorption. Objective: Our objective was to measure total reduction in intestinal absorption of combustible energy after RYGB and the extent to which this was due to restriction of food intake or malabsorption of ingested macronutrients. Design: Long-limb RYGB was performed in 9 severely obese patients. Dietary intake and intestinal absorption of fat, protein, carbohydrate, and combustible energy were measured before and at 2 intervals after bypass. By using coefficients of absorption to measure absorptive function, equations were developed to calculate the daily gram and kilocalorie quantities of ingested macronutrients that were not absorbed because of malabsorption or restricted food intake. Results: Coefficients of fat absorption were 92 6 1.3% before bypass, 72 6 5.5% 5 mo after bypass, and 68 6 8.7% 14 mo after bypass. There were no statistically significant effects of RYGB on protein or carbohydrate absorption coefficients, although protein coefficients decreased substantially in some patients. Five months after bypass, malabsorption reduced absorption of combustible energy by 124 6 57 kcal/d, whereas restriction of food intake reduced energy absorption by 2062 6 271 kcal/d. Fourteen months after bypass, malabsorption reduced energy absorption by 172 6 60 kcal/d compared with 1418 6 171 kcal/d caused by restricted food intake. Conclusion: On average, malabsorption accounted for '6% and 11% of the total reduction in combustible energy absorption at 5 and 14 mo, respectively, after this gastric bypass procedure. Am J Clin Nutr 2010;92:704-13.

Whether ingested calcium is absorbed more efficiently from freely water-soluble calcium salts than from poorly soluble salts is unclear. It is also unknown whether calcium is absorbed better from dairy products than from calcium salts. Using a method by which the net absorption of calcium can be accurately measured after a single dose, we studied eight healthy fasting subjects after they took a 500-mg dose of calcium from each of five calcium salts with various degrees of water solubility and from milk. The order of administration of the agents given was randomly determined. The mean (+/- SEM) net calcium absorption, in decreasing order of the solubility of the salts, was 32 +/- 4 percent from calcium acetate, 32 +/- 4 percent from calcium lactate, 27 +/- 3 percent from calcium gluconate, 30 +/- 3 percent from calcium citrate, and 39 +/- 3 percent from calcium carbonate. The differences in absorption were not statistically significant according to analysis of variance. On the basis of in vitro solubility experiments in acid mediums, we hypothesize that acid dissolution in the gastrointestinal tract may be responsible for the similar absorption of calcium from salts with widely different water solubilities. Calcium absorption from whole milk (31 +/- 3 percent) was similar to absorption from calcium salts. We conclude that calcium absorption from carbonate, acetate, lactate, gluconate, and citrate salts of calcium, and from whole milk, is similar in fasting healthy young subjects. Further study will be required to determine whether the results would be different in older subjects, with a higher dose of calcium, or if the calcium was ingested with food.

Calcium salts are increasingly used as phosphorus binders in patients with chronic renal failure. Calcium carbonate is the principal salt presently utilized, however, other calcium salts may be more effective and safer phosphorus binders. Theoretical calculations, in vitro experiments, and in vivo studies in normal subjects have shown calcium acetate to be a more effective phosphorus binder than other calcium salts. This salt has not previously been studied in patients with chronic renal failure. We used a one-meal gastrointestinal balance technique to measure phosphorus absorption, calcium absorption and phosphorus binding in six patients with chronic renal failure. Calcium acetate was compared with calcium carbonate and placebo. Equivalent doses (50 mEq Ca++) of calcium acetate bound more than twice as much phosphorus (106 +/- 23 mg) as calcium carbonate (43 +/- 39 mg) P less than 0.05. When phosphorus binding was factored for calcium absorption, calcium acetate bound 0.44 mEq HPO4 =/mEq absorbed Ca++ compared with 0.16 mEq HPO4 = bound/mEq Ca++ absorbed with calcium carbonate. More efficient phosphorus binding permits serum phosphorus concentration to be controlled with lower doses of calcium salts. The higher phosphorus binding/calcium absorption ratio coupled with a lower dose indicates that less calcium will be absorbed when calcium acetate is used for phosphorus control. Markedly positive calcium balance, hypercalcemia and ectopic calcification should be less likely to occur with this drug than other calcium salts.