Effect of dietary calcium and phosphorus on intestinal calcium absorption and vitamin D metabolism.

To understand better dietary regulation of intestinal calcium absorption, a quantitative assessment of the metabolites in plasma and duodenum of rats given daily doses of radioactive vitamin D₃ and diets differing in calcium and phosphorus content was made. All known vitamin D metabolites were ultimately identified by high-pressure liquid chromatography. In addition to the known metabolites (25-hydroxyvitamin D₃, 24,25-dihydroxyvitamin D₃, 1,25-dihydroxyvitamin D₃, 25,26-dihydroxyvitamin D₃, and 1,24,25-trihydroxyvitamin D₃), several new and unidentified metabolites were found. In addition to 1,25-dihydroxyvitamin D₃ and 1,24,25-trihydroxyvitamin D₃, the levels of some of the unknown metabolites could be correlated with intestinal calcium transport. However, whether or not any of these metabolites plays a role in the stimulation of intestinal calcium absorption by low dietary calcium or low dietary phosphorus remains unknown.     

FGF23/Klotho axis: phosphorus, mineral metabolism and beyond

In this work we summarizes the steps that allowed the identification of the fibroblast growth factor (FGF) 23/Klotho axis as the principal regulator of phosphate homeostasis, exerting actions on intestine, bone, parathyroid glands, and kidney. We review the not fully understood mechanisms of action of this axis on the regulation of mineral homeostasis and, in addition, we discuss its potential role in the pathophysiology of chronic kidney disease and the associated complications. We also reflect the actual tendency to consider the components of this system as better predictors of the pathological conditions frequently associated to mineral disorders, and review some recent studies linking these components to cardiovascular disease even in population without mineral disorders. Finally, we consider the numerous processes in which Klotho is involved, including anti-ageing and mineral control processes, independently of its functions as obligated-coreceptor for FGF23.     

Bone calcium/phosphorus ratio determination using dual energy X-ray method

Non-invasive dual energy methods have been used extensively on osteoporosis diagnosis estimating parameters, such as, Bone Mineral Density (BMD) and Bone Mineral Content (BMC). In this study, an X-ray dual energy method (XRDE) was developed for the estimation of the bone Calcium-to-Phosphorous (Ca/P) mass ratio, as a bone quality index. The optimized irradiation parameters were assessed by performing analytical model simulations. X-ray tube output, filter material and thickness were used as input parameters. A single exposure technique, combined with K-edge filtering, was applied. The optimal X-ray spectra were selected according to the resulted precision and accuracy values. Experimental evaluation was performed on an XRDE system incorporating a Cadmium Telluride (CdTe) photon counting detector and three bone phantoms with different nominal mass Ca/P ratios. Additionally, the phantoms’ mass Ca/P ratios were validated with energy-dispersive X-ray spectroscopy (EDX). Simulation results showed that the optimum filter atomic number (Z) ranges between 57 and 70. The optimum spectrum was obtained at 100 kVp, filtered with Cerium (Ce), with a surface density of 0.88 g/cm². All Ca/P ratio measurements were found to be accurate to within 1.6% of the nominal values, while the precision ranged between 0.91 and 1.37%. The accuracy and precision values of the proposed non-invasive method contributes to the assessment of the bone quality state through the mass Ca/P ratio determination.     

Endocrine regulation of phosphorus and calcium metabolism during pregnancy in domestic ruminants

In pregnant domestic ruminants (cows, ewes, goats) foetal plasma calcium and inorganic phosphorus concentrations are higher than those measured in the dam. The foetus regulates its own calcaemia and phosphataemia. Changes in maternal plasma calcium levels have no significant effect on foetal calcaemia. Calcium and phosphorus are transported from the dam to the foetus according to a one-way process, the transport from the foetus to the dam being negligible. An important part of the calcium transferred to the foetus comes from the maternal skeleton. The true molecular mechanisms involved in placental transport of calcium are still unknown. This is an active transport, stimulated by vitamin D metabolites (of maternal, foetal or placental origin) and maternal prolactin. Maternal calcitonin protects the skeleton of the pregnant (and lactating) female ruminant against excessive demineralization, partly by modulating placental transport of calcium during periods of intense mineralization of foetal skeleton.     

Effect of dietary calcium and phosphorus depletion on vitamin D metabolism and calcium binding protein in the growing pig

Twenty-four young pigs were divided into three groups and each fed a replete, low calcium (Ca) or low phosphorus (P) diet. It was found that the deficient diets induced rises in renal 25 hydroxy-vitamin D 1,hydroxylase (1-hydroxylase) activity, circulating 1,25 dihydroxy-vitamin D3 (1,25 (OH)2-D3) and Ca binding protein (CaBP) and intestinal 1,25(OH)2D3 and CaBP. All these rises were statistically significant in the low Ca group but only the rises in the 1-hydroxylase activity and intestinal 1,25(OH)2D3 were significant in the low P group. A high degree of correlation existed between the parameters. There was no enhancement of intestinal 1,25(OH)2D3 or CaBP concentration relative to the 1-hydroxylase activity in the low P pigs as occurs in the chick. The low-P-induced rise in 1-hydroxylase activity was independent of parathyroid hormone.     

Effects of mineral and rapeseed phosphorus supplementation on phytate degradation in dairy cows

The objective of this study was to evaluate the effects of diet composition on phytate (InsP₆) degradation in dairy cows. In Experiment 1, four diets that differed in the amount and source of phosphorus (P) were fed to 24 lactating cows in a 4 × 4 Latin Square design. The control diet (Diet C) contained 4.18 g P/kg dry matter (DM). Diet MP contained additional mineral P (5.11 g P/kg DM), Diet RS contained rapeseed and rapeseed meal as organic P sources (5.26 g P/kg DM) and Diet RSM contained rapeseed meal and rapeseed oil (5.04 g P/kg DM). Total P (tP) and InsP₆ excretion in faeces were measured. In Experiment 2, we used a rumen simulation technique (Rusitec) to estimate ruminal disappearance of tP and InsP₆ from Diets C, MP and RSM. In Experiment 1, tP concentration in faeces increased with tP intake and was highest for Diets RS and RSM. The source of supplemented P had no influence on tP digestibility, but tP digestibility was reduced for Diets MP, RS and RSM in comparison to that for Diet C. InsP₆ disappearance decreased in Diet MP (85.0%) and increased in Diets RS (92.7%) and RSM (94.0%) compared to that in Diet C (90.0%). In Experiment 2, P source influenced ruminal tP disappearance (Diet MP, 78.6%; Diet RSM, 75.3%). InsP₆ disappearance for Diet C (98.1%) was higher than that for Diets MP (95.6%) and RSM (94.9%). The results confirmed the high potential of ruminants to degrade InsP₆, but differences in diet composition influenced InsP₆ disappearance. Further studies of the site of InsP₆ degradation are required to understand the relevance of InsP₆ degradation for the absorption of P.     

Effects of physical training on bone mineral density and bone metabolism

The purpose of this study was to examine the influences of long-term walking training and walking and jumping training on bone mineral density (BMD) and bone metabolism. Data from 28 healthy premenopausal women was assessed. The subjects were divided into the walking group (WG; 17 women mean+/-SE age 35+/-2 years), and the walking and jumping group (WJG; 11 women mean+/-SE age 39+/-1 years). BMD was measured in the lumbar spine and proximal femur using dual energy X-ray absorptiometry (DXA). As markers of bone metabolism, this study was to measure bone formation markers, bone-alkaline phosphatase (B-ALP: measured by enzyme immunoassay/EIA) and osteocalcin (BGP: by radioimmunoassay/RI) as well as bone resorption markers, parathyroid hormone (PTH: measured by/RI) and type I collagen cross-linked N-telopeptides (NTx: by EIA). Despite the significant decrease in body weight (p<0.05), no corresponding decrease in BMD was observed. Moreover, no significant difference in bone markers BGP, PTH, and NTx was observed. B-ALP was significantly increased (p<0.05) after one year, and the rate of this increase was greater in the WJG than in the WG. It is thus concluded that walking training for one year is beneficial for the promotion of bone formation, and that jumping stimulus maintain BMD effectively.