Effect of dietary phosphorus on the development of bone mineral density of pigs assessed using dual energy X-ray absorptiometry

Two experiments were performed in weaner, grower and finisher pigs in order to monitor the changes in skeletal area bone mineral density (aBMD, g/cm2) and dietary phosphorus (P) concentrations using dual energy X-ray absorptiometry (DXA) scan data. In experiment 1, 24 female pigs were selected at 8 kg (SD ± 1.04 kg) and individually fed either high (3.7 g/kg) or low (3.0 g/kg) digestible P diets for 8 weeks (with reductions of 0.5 g/kg digestible P at 4 weeks) until they reached ca. 30 kg body weight. The pigs were anaesthetised and scanned at the start of the study (day 0) and after 28 and 56 days. In experiment 2, 30 female pigs were selected at 34 kg (SD ± 2.3 kg) and individually fed high (2.8 g/kg), medium (2.2 g/kg) or low (1.6 g/kg) digestible P until slaughter at 100 kg (SD ± 4.3 kg). These pigs were scanned at the start of the experiment (day 0) and after 35 and 70 days. One foot from each pig in experiment 2 was collected at slaughter for analysis of metacarpal cross section area, moment of inertia, dry matter, apparent density, aBMD, and ash percentage. In both experiments, there were significant diet × time interactions (P < 0.01-experiment 1; P < 0.001-experiment 2) on whole body aBMD. Values of whole body aBMD (g/cm2) measured in pigs fed high and low P diets (experiment 1) were: 0.44, 0.50 and 0.62 vs. 0.45, 0.48 and 0.52 (SEM = 0.020) for days 0, 28 and 56; respectively. In experiment 2, whole body aBMD values (g/cm2) for days 0, 35 and 70 were: 0.55, 0.75 and 0.97 vs. 0.56, 0.70 and 0.88 vs. 0.56, 0.64 and 0.73 (SEM = 0.013) in pigs fed high, medium and low P diets; respectively. In experiment 2, metacarpal apparent density from pigs fed the high P diets was higher (P < 0.001) than pigs fed medium and low P diets. The metacarpal aBMD was higher (P < 0.001) in pigs fed the high and medium P diets than the low P diet. In conclusion, young pigs are highly sensitive to dietary P as their average weekly increases in aBMD for high and low digestible P diets were 0.023 and 0.009 g/cm2; respectively. Finisher pigs accumulate aBMD at much higher rates (0.041, 0.032 and 0.017 g/cm2/week for high, medium and low digestible P diets; respectively) than weaner pigs.