Influence of dietary composition on the phosphate-binding ability of lanthanum during in vitro digestion

Abstract

As lanthanum (La) can bind phosphate in a wide pH range, it is used as a phosphate binder for patients with chronic renal failure. To clarify the influence of dietary composition on the phosphate-binding ability of La, the bioaccessibility of phosphorus (P) in different meals and feeds with or without supplemented La was measured. Lyophilized powder of a hospital breakfast (mainly composed of bread and dairy products), a hospital lunch (mainly composed of rice and grilled fish) and a hospital supper (mainly composed of rice and deep fried chicken), as well as powder of animal feeds composed of casein, soybean isolate protein or wheat gluten were used. Lanthanum carbonate was added at 5 mg La/g for the hospital meals and at 5 to 20 mg La/g for the animal feeds. One g of each sample with and without La was incubated with 10 mL of 0.1 M HCl containing 0.5% pepsin at 37°C for 2 h. After the pepsin digestion, pancreatin (final concentration, 0.5%) was added and the digest was further incubated at pH 7.4 and 37°C for 8 h. After the pepsin digestion, the bioaccessibility of P in the lunch and dinner samples was reduced by La, and was further decreased after the pancreatin digestion. In the breakfast sample, the effects of La were first noted after the pancreatin digestion. The degree of reduction was in the order of dinner > lunch > breakfast samples. In all meals, the amount of soluble La in water was less than 1 %, but it increased to 71-94% with 0.1 M HCl. After the pancreatin digestion, the amount of soluble La in the lunch and dinner samples returned to approximately 1 %; however, that in the breakfast sample was 32%. In the case of animal feeds, the degree of reduction of soluble P by La was in the order of gluten > soybean > casein, and the amount of soluble La after the pancreatin digestion was significantly higher in the casein feed than in the other feeds. These results suggest that binding of La to phosphate occurs either in the stomach or duodenum, but not in the stomach, and binding is considered to be low in the presence of dairy products. In addition, La may be solubilized by gastric juice and then be mostly re-precipitated in the duodenum. On the other hand, when dairy products are present, some La remains soluble even in the duodenum.