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LETTER TO THE EDITOR |
Physiology of Nutrition and Feeding Control Unit
INRA-AgroParisTech
Paris Cedex 05
E-mail: gaudicho{at}inapg.fr
Dear Sir:
In our recent article entitled "Compared with casein or total milk protein, digestion of milk soluble proteins is too rapid to sustain the anabolic postprandial amino acid requirement" (1) we showed that casein ingested as a single protein meal in resting healthy human volunteers was better retained than was whey protein. This effect was mainly related to the fast absorption and subsequent enhanced postprandial oxidation of whey protein–derived amino acids. This result was obtained by directly measuring the postprandial metabolic fate of dietary protein based on tracer kinetic data. This was also in line with previous observations from our group and from others, which showed that the gastric emptying and amino acid absorption rates are major factors modulating postprandial amino acid metabolism (2–5). This issue is extensively discussed in the article.
In a letter to the editor, Manninen claimed that we misinterpreted our data. He wrote that our results suggest that slow proteins are better retained than are fast proteins and argued that these results are erroneous because the anabolic benefit of whey protein has been reported after intense exercise training. First, our conclusion was not a suggestion but a direct demonstration based on tracer kinetic data, and Manninen did not provide any data to suggest any misinterpretation of our data. Second, our study was conducted in healthy sedentary humans, and we did not extrapolate our findings to exercising subjects, especially bodybuilders. We mentioned in our discussion that whey protein had a positive effect on protein balance in the elderly (6, 7). The study mentioned by Manninen (8) was conducted under very specific conditions, ie, in bodybuilders supplemented with 
1.4 g casein · kg–1 · d–1 of either casein or whey protein, corresponding to 112 g protein as supplement per day.
Manninen is right to postulate that humans rarely consume a unique source of protein. We fully agree and admit that the final interest of our study would be achieved by integrating general dietary habits, as was partly the case because the protein was given acutely. During the week preceding the test, volunteers consumed a habitual mixed diet, although it was standardized for protein as energy. We also draw attention to our final postulate that fast and slow proteins may exert a synergistic effect because the postprandial nitrogen utilization of total milk protein was not intermediary between casein and whey protein, but was the highest that we reported.
ACKNOWLEDGMENTS
The authors had no conflict of interest to declare.
REFERENCES
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