Markers of bone turnover and biomechanical properties of the third metacarpal bone of growing pigs subjected to the different dietary phosphorus and calcium content

Eighteen weaned piglets were subjected to deficiency, adequate or excess of dietary phosphorus and calcium (group C₋₅₀, C and C₊₅₀, respectively) during growth from 33 to 110 day of age. However, the ratio of calcium/digestible phosphorus in all feeds was the same and consistent with the nutritional recommendation. Performance, bone growth, concentration of osteocalcin (OC) and carboxyterminal telopeptide of type I collagen (CTX-I), densitometry, geometry and mechanical properties of the 3rd metacarpal bone were measured. Growth rate and final body weight did not differ between C and C₊₅₀ animals, however, they grew approximately 5.9% faster and were heavier by about 4.0% than C₋₅₀ pigs. In turn, mass and length of bones were similar in C₋₅₀ and C₊₅₀ groups, however, they were approximately 11% lighter and 4% shorter compared to C pigs. Serum content of OC took the following order: C₋₅₀ < C < C₊₅₀ group (P<0.01). Content of CTX-I did not differ between C and C₊₅₀ animals, however they were lower by 36% (P<0.01) than in the C₋₅₀ pigs. All measured geometry parameters (cortical wall thickness, cross section area and cortical index) did not differ between C and C₊₅₀ pigs and were higher than in C₋₅₀ pigs (P<0.01). Bones of C₋₅₀ pigs contained less minerals than bones of the C and C₊₅₀ pigs (5.24 g vs mean 6.14 g, P<0.01). The 3rd metacarpal bone mineral density (g/cm²) was the lowest in pigs in group C₋₅₀, higher in pigs in group C, and the highest in animals in group C₊₅₀ (0.398, 0.431 and 0.475, respectively, P<0.01). Maximum strength of bones was similar in the C and C₊₅₀ pigs, however it was 13% greater (P<0.01) compared to C₋₅₀ animals. In turn, elastic strength and elastic deflection did not differ between the C and C₋₅₀ animals and were over 20% lower (P<0.01) than in the C₊₅₀ pigs. Whereas, there was no significant difference between groups in bone stiffness (mean 27.13 mm). The results of the present study proved that even a strong excess of phosphorus and calcium content in the diet for growing pigs is inadvisable and does not improve performance and bones parameters. While excessively lowering the dietary phosphorus and calcium concentration (e.g. to reduce environmental pollution) may adversely affect growth rate and bone characteristics.