The Journal of Nutrition Vol. 128 No. 12 December 1998, pp. 2528-2528

LETTER TO THE EDITOR:

Response to Drs. Leterme and Sève

	LETTER

Dear Dr. Suttie,

This is a response to the "Letter to the Editor" written by Drs. Leterme and Sève regarding an article by Caine et al. (1998) published in the J. Nutr. 128: 598-605. We appreciate the interest they have indicated in our research study. Concerning guanidination of lysine in the test proteins that were used in this study, these authors have, for the most part, summarized part of our discussion about the limitations of the guanidination procedure. In particular, these authors mention differences in the ileal endogenous recoveries and true digestibilities of unguanidinated and total lysine. We also had similar concerns about the uniformity of lysine guanidination in the test proteins. Indeed, for this reason we included the footnotes in Tables 3 and 4, so that individuals reading the article could evaluate the limitation/efficiency of the guanidination. As was discussed in our article, although there are a number of published articles using guanidinated protein sources to study digestibilities in animals, in so far as we are aware, ours is the first article that has reported amino acid profiles of test proteins before and after guanidination.

The reason for using a ratio of ratios to determine endogenous recoveries of amino acids was not clear in the letter by Drs. Leterme and Sève. Moreover, it is not clear how they resolved Equation 3' to get Equation 3. Their approach was to calculate endogenous recoveries from the difference between the true digestibility of homoarginine and apparent digestibilities of the amino acids. Of course, when endogenous amino acid recoveries, determined using this approach, are subsequently used to calculate true digestibilities from Equation 6 in our study, these values will give the true digestibility of homoarginine. This is essentially a forward-and-back calculation, and then one must assume that the true digestibilities of homoarginine and dietary amino acids are always the same. In our study, endogenous amino acid recoveries were determined by difference using the ratio of homoarginine to amino acids in the diet and ileal digesta as suggested by Siriwan et al. (1994). This gives a value which, when multiplied by total amino acid flow, is a qualitative estimate of endogenous ileal recovery. For a hypothetical example, assume the concentrations of homoarginine and an amino acid in the diet and ileal digesta were 15 and 20 g/kg dry matter (DM) and 4 and 10 g/kg DM, respectively. If the total ileal flow of this amino acid is 3 g/kg DM, then its endogenous recovery will be 1 g/kg DM of ileal digesta. Using our Equation 6 to calculate true digestibility gives a value of 90% for this amino acid. Implicit in this approach is the underlying assumption that the absorption of homoarginine is a one-way process (homoarginine is not recycled back into the intestinal lumen), while amino acids of endogenous origin are being recycled into the intestinal lumen. On this basis, changes in the rate of absorption of dietary and endogenous amino acids will occur due to antinutritional or other factors (Souffrant 1991). Although, the potential absorption of all amino acids was assumed to be, more or less, similar to homoarginine. It should also be mentioned that there were large differences in endogenous recoveries of amino acids between the test proteins, presumably, because of the difference in their soybean trypsin inhibitor content. We concluded, therefore, that the homoarginine ratio method was an effective approach to determine qualitative differences between similar test protein sources. In this respect, the values reported were supposed to be interpreted as relative and not absolute values. Unfortunately, there is not yet a definitive technique to determine quantitative estimates of ileal recoveries of endogenous amino acids in pigs (Tamminga et al. 1995).

William Caine
Department of Animal Nutrition
Wageningen Institute of Animal Sciences
Wageningen Agricultural University
Wageningen 6709 PG
The Netherlands

	FOOTNOTES

Manuscript received 24 July 1998. Initial reviews completed . Revision accepted 17 August 1998.

	LITERATURE CITED

• Siriwan P., Bryden W. L., Annison E. F. Use of guanidinated dietary protein to measure losses of endogenous amino acids in poultry. Br. J. Nutr. 1994; 71:515-529[Medline]
• Souffrant, W. C. (1991) Endogenous nitrogen losses during digestion in pigs. In: Proceedings of the 5th International Symposium on Digestive Physiology in Pigs. EAAP Publication No. 54 (Verstegen, M. W. A., Huisman, J. and den Hartog, L. A., eds.), pp. 147-166. PUDOC, Wageningen (Doorwerth), The Netherlands.
• Tamminga S., Schulze H., van Bruchem J., Huisman J. The nutritional significance of endogenous N-losses along the gastro-intestinal tract of farm animals. Arch. Anim. Nutr. 1995; 48:9-22

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