Postprandial nitrogen utilization and misinterpretation of data.

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 toorapid 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.