Influence of high and low protein intakes on age-related bone loss in rats submitted to adequate or restricted energy conditions

Low energy and protein intake has been suggested to contribute to the increased incidence of osteoporosis in the elderly. The impact of dietary protein on bone health is still a matter of debate. Therefore, we examined the effect of the modulation of protein intake under adequate or deficient energy conditions on bone status in 16-month-old male rats. The animals were randomly allocated to six groups (n = 10/group). Control animals were fed a diet providing either a normal-protein content (13%, C-NP) or a high-protein content (26%) (C-HP). The other groups received a 40% protein/energy-restricted diet (PER-NP and PER-HP) or a normal protein/energy-restricted diet (ER-NP and ER-HP). After 5 months of the experiment, protein intake (13% or 26%) did not modulate calcium retention or bone status in those rats, although a low-grade metabolic acidosis was induced with the HP diet. Both restrictions (PER and ER) decreased femoral bone mineral density and fracture load. Plasma osteocalcin and urinary deoxypyridinoline levels were lowered, suggesting a decrease in bone turnover in the PER and ER groups. Circulating insulin-like growth factor-I levels were also lowered by dietary restrictions, together with calcium retention. Adequate protein intake in the ER condition did not elicit any bone-sparing effect compared to PER rats. In conclusion, both energy and protein deficiencies may contribute to age-related bone loss. This study highlights the importance of sustaining adequate energy and protein provision to preserve skeletal integrity in the elderly.