Glycine Equivalents in Low Protein Diets

A second experiment was completed to determine if supplementing glycine precursors have the same effect on performance as glycine in reduced protein diets. Ross 308 cockerels were fed one of two protein levels; an industry-standard protein diet with meat and bone meal or a plant-based diet approximately 3% lower in crude protein. In the low protein diet, glycine, serine, and threonine were assigned a glycine equivalence and supplemented to match the glycine in the standard protein diet at 1.558 and 1.390% in grower and finisher diets respectively. Reducing the dietary protein by 3% reduced feed efficiency, however, the supplementation of glycine or serine did not affect performance. Threonine supplementation further reduced feed efficiency and decreased body weight gain in reduced protein diets. Therefore, glycine did not appear to be limiting in the reduced protein diets investigated, however, using threonine to supplement glycine requirements exacerbated the performance reducing effects of reduced protein diets.
In addition to the treatments described above another glycine equivalence level of serine and threonine at 1.800% was also investigated in the second experiment. Using these treatments the effects of reduced protein diets on glycine and uric acid metabolism could be investigated. The in vivo conversion of serine and threonine to glycine was evident in the blood plasma, however, reducing dietary protein reduced blood serum uric acid. The hepatic expression of genes for enzymes associated with threonine degradation to glycine, glycine degradation, and uric acid-synthesis were down-regulated in reduced protein diets. The supplementation of excess essential amino acids and non-specific nitrogen may not fulfil the non-essential amino acid requirements for efficient growth as the synthesis of non-essential amino acids is altered in reduced protein diets.