Reducing protein levels lowered lean growth rates in growing pigs. Researchers at the University of Nebraska conducted a nitrogen balance study to determine why lean growth rates of pigs fed low-protein diets supplemented with crystalline amino acids (lysine, threonine, tryptophan & methionine) are lower than those of pigs fed a standard corn-soybean meal diet. The six diets were 1) 18 percent crude protein (CP); 2) 16 percent CP; 3) 14 percent CP; 4) 14 percent CP plus AA; 5) 12 percent plus AA; and 6) 12 percent CP plus AA. The AA provided the same level of amino acids as in the intact proteins that were 4 percent higher in crude protein. The results of the metabolism study indicated that nitrogen retention decreased in both the standard diets and the equivalent amino acid supplement diets as crude protein decreased. Nitrogen retention was consistently lower in the equivalent amino acid supplemented diets compared to the higher protein standard diets. Amino acid supplementation failed to achieve the same nitrogen retention as seen in the corn-soybean meal diets. The authors suggested that dietary factors other than the amino acid concentration accounts for the reduction in lean muscle growth observed in pigs fed low-protein amino acid supplemented diets.

Figueroa, J.L., and co-workers. 1999. Nitrogen metabolism of growing gilts fed standard corn-soybean meal diets or low-protein diets supplemented with crystalline amino acids. J. Animal Sci. 77(Supplement 1): 196, abstract 328.