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American Journal of Clinical Nutrition, Vol. 85, No. 6, 1664, June 2007
© 2007 American Society for Nutrition

LETTER TO THE EDITOR

Postprandial nitrogen utilization and misinterpretation of data

Anssi H Manninen

Advanced Research Press, Inc
690 Route 25A
Setauket, NY 11733
E-mail: sportsnutrition{at}luukku.com

Dear Sir:

A recent study by Lacroix et al (1) compared the postprandial utilization of nitrogen from 3 different proteins, namely micellar casein ("slow protein"), milk soluble protein isolate ("fast protein"), and total milk protein (another "slow protein"). Their data suggest that slow protein induces better postprandial nitrogen utilization than does fast protein. According to Lacroix et al, "This result, together with the hypoaminoacidemia observed 4 h after the ingestion of [milk soluble protein isolate], strongly suggests that a too-rapid dietary [amino acid] delivery cannot support the anabolic requirement throughout the postprandial period."

In my view, this is a misleading statement, because adult humans rarely, if ever, consume their dietary protein from one source only. Both fast and slow proteins have their advantages. For example, fast-acting protein hydrolysates may offer some advantages immediately after exercise (2). In addition, frequent ingestion of fast proteins may optimize muscle protein anabolism. Cribb et al (3) examined the effects of supplementation with hydrolyzed whey protein and casein on muscle strength and body composition during a 10-wk, supervised, resistance-training program. The results indicate that the whey hydrolysate group achieved a significantly greater gain in lean body mass than did the casein group (5.0 compared with 0.8 kg). The whey hydrolysate group also achieved significantly greater improvements in muscle strength than did the casein group in each assessment of strength. When the strength changes were expressed relative to body weight, the whey group still achieved significantly greater improvements in strength than did the casein group.

The superiority of whey protein hydrolysate may have something to do with its strong insulinotropic effects and its rapid absorption and uptake (2). A surge of amino acids is rapidly transported to muscle tissue, where they may help trigger muscle protein synthesis at an accelerated rate. In contrast, slow-acting proteins may be better at minimizing muscle protein catabolism during prolonged periods between eating.

ACKNOWLEDGMENTS

The author is a consultant to BioQuest Pharmaceuticals Inc.

REFERENCES

  1. Lacroix M, Bos C, Léonil J, et al. Compared with casein or total milk protein, digestion of milk soluble proteins is too rapid to sustain the anabolic postprandial amino acid requirement. Am J Clin Nutr 2006;84:1070–9.[Abstract/Free Full Text]
  2. Manninen AH. Hyperinsulinaemia, hyperaminoacidaemia and post-exercise muscle anabolism: the search for the optimal recovery drink. Br J Sports Med 2006;40:900–5.[Abstract/Free Full Text]
  3. Cribb PJ, Williams AD, Carey MF, Hayes A. The effect of whey isolate and resistance training on strength, body composition and plasma glutamine. Int J Sports Nutr Exerc Metab 2006;16:494–509.[Medline]




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