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Effects of dairy constituents on calcium bioavailability : impact on utilization as indexed by bone mineral composition and biomechanics

University of British Columbia

Calcium bioavailability was investigated using isotopic intestinal absorption, and balance study techniques with bone mineralization and biomechanics as endpoint determinants of calcium utilization. In experiment 1, lactose enhancement of paracellular calcium absorption was confirmed, and it was suggested that a critical luminal concentration of lactose was necessary for its action. Despite the enhanced intestinal absorption of calcium in animals fed the 50% lactose containing diet, bone mineralization was not different from controls; and further, a decrease in bone strength of these animals was found to be secondary to nutrient malabsorption. In this study, there was no evidence to indicate a difference in the bioavailability of calcium from milk (colloidal) or yogurt (ionized) sources. In experiment 2, the absorption of calcium from the ileum was significantly enhanced in[formula omitted] normal Wistar rats fed milk protein diets containing casein as compared to whey and soy protein diets. The increase in absorbed calcium was shown to have little physiological significance in bone mineralization and biomechanics when animals were fed a diet adequate in dietary calcium. In experiment 3, paracellular calcium absorption was similar between genetically spontaneously hypertensive (SHR) and normotensive control Wistar-Kyoto (WKY) rats, suggesting that differences in calcium metabolism between these two strains was not due to differences in ileal calcium transport. The effect of casein and soy protein diets containing high (2.0%), adequate (0.5%) and low (0.05%) levels of calcium, respectively on calcium bioavailability and subsequent utilization was determined in SHR and WKY animals. Ileal calcium absorption was greater in casein fed animals than those fed soy at the adequate and low levels of calcium. Femur calcification was enhanced by casein diets at the high and medium levels of dietary calcium only. Femur biomechanics were not influenced by dietary protein source, but were however, adversely affected by the low dietary calcium level. In experiment 4, the effect of dietary fortification with casein phosphopeptides (CPP) was investigated in casein and soy fed SHR animals. CPP added to casein and soy protein diets appeared to result in a greater ileal absorption of calcium. This increase in calcium bioavailability from the casein diet had little effect however, on bone mineralization and biomechanical strength, due to the excretion of excess absorbed calcium. In experiment 5, severe thermal processing of dietary proteins was shown to reduce in vitro digestibility. Animals fed heat denatured casein and soy diets exhibited reduced intestinal calcium absorption, calcium balance as well as bone mineralization and biomechanics. These effects were influenced by the nutrient malabsorption experienced by these animals. In experiment 6, a low (6%) protein, low phosphorus diet resulted in decreased food intake and animal growth. However, ileal calcium absorption (% dose) was similar between 6% and 20% protein fed animals. A low level of dietary protein influenced calcium balance and utilization for bone mineralization and biomechanical strength. These results indicate that paracellular calcium absorption may not necessarily equate with those obtained from a calcium balance study. Notwithstanding, calcium bioavailability from the ileum was shown to be enhanced in animals fed casein, which was likely due to the production of bioactive peptides (CPP) that are involved in sequestering calcium and retaining it in a soluble form. By reducing protein digestibility with heat denaturation, this effect was lost. Reducing the protein content in the diet however, did not reduce the enhancement of calcium bioavailability observed in casein fed animals. Finally, bone mineralization and biomechanical parameters were shown to be sensitive indicators of calcium utilization from diets that varied in calcium content or bioavailability of this mineral.

Text

2010-10-21

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http://hdl.handle.net/2429/29408

Food Science

University of British Columbia

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