The isotopic N turnover rate as a proxy to evaluate in the long-term the protein turnover in growing ruminants

Protein turnover is an energy-consuming process essential for ensuring the maintenance of living organisms. Gold standard methods for protein turnover measurement are based on intravenous infusions of stable isotopes. Although accurate they have inherent drawbacks precluding their generalization for large farm animals and during long time periods. We proposed here a non-invasive proxy of the whole-body fractional protein degradation (WBFPDR; protein turnover for a growing animal) in the long term and in a large number of beef cattle. The proxy is based on the rate at which urine-N and plasma proteins are progressively depleted in 15N after a slight decrease in the isotopic N composition of diet (i.e. diet-switch). We aimed to test the ability of this proxy to adequately discriminate the WBFPDR of 36 growing-fattening young bulls assigned to different dietary treatments known to impact the protein turnover rate: the protein content and amino acid profile. To achieve this objective, the experimental diets were enriched with 15N labeled-urea during 35 days while the animals were adapted to diets. After stopping the 15N labeled-urea administration the animals were thereafter sampled for spot urines (n = 13) and blood (n = 10) over 5 months and analyzed for their 15N enrichments in total N and plasma proteins, respectively. Adequately fitting the 15N kinetics in plasma proteins and urines required mono- and bi-exponential models, respectively, and the model parameters were compared across dietary conditions using a non-linear mixed effect model. The single 15N depletion rate found in plasma proteins represented their fractional synthesis rate, whereas the slowest depletion rate found in urines was interpreted as a proxy of the WBFPDR. The proxy here tested in urines suggested different WBFPDR values between Normal vs High protein diets but not between balanced vs unbalanced methionine diets. In contrast, the proxy tested in plasma indicated that both dietary conditions affected the fractional synthesis rate of plasma proteins. We consider that the rate at which urines are progressively 15N-depleted following an isotopic diet-switch could be proposed as a non-invasive proxy of the long-term whole-body fractional protein degradation rate for large farm animals.