Coingestion of whey protein and casein in a mixed meal: demonstration of a more sustained anabolic effect of casein

Am J Physiol Endocrinol Metab. 2012 Jul 1;303(1):E152-62. doi: 10.1152/ajpendo.00106.2012. Epub 2012 May 8.


When consumed separately, whey protein (WP) is more rapidly absorbed into circulation than casein (Cas), which prompted the concept of rapid and slow dietary protein. It is unclear whether these proteins have similar metabolic fates when coingested as in milk. We determined the rate of appearance across the splanchnic bed and the rate of disappearance across the leg of phenylalanine (Phe) from coingested, intrinsically labeled WP and Cas. Either [¹⁵N]Phe or [¹³C-ring C₆]Phe was infused in lactating cows, and the labeled WP and Cas from their milk were collected. To determine the fate of Phe derived from different protein sources, 18 healthy participants were studied after ingestion of one of the following: 1) [¹⁵N]WP, [¹³C]Cas, and lactose; 2) [¹³C]WP, [¹⁵N]Cas, and lactose; 3) lactose alone. At 80-120 min, the rates of appearance (R(a)) across the splanchnic bed of Phe from WP and Cas were similar [0.068 ± 0.010 vs. 0.070 ± 0.009%/min; not significant (ns)]. At time 220-260 min, Phe appearance from WP had slowed (0.039 ± 0.008%/min, P < 0.05) whereas Phe appearance from Cas was sustained (0.068 ± 0.013%/min). Similarly, accretion rates across the leg of Phe absorbed from WP and Cas were not different at 80-120 min (0.011 ± 0.002 vs. 0.012 ± 0.003%/min; ns), but they were significantly lower for WP (0.007 ± 0.002%/min) at 220-260 min than for Cas (0.013 ± 0.002%/min) at 220-260 min. Early after meal ingestion, amino acid absorption and retention across the leg were similar for WP and Cas, but as rates for WP waned, absorption and assimilation into skeletal muscle were better retained for Cas.

Publication types

  • Comparative Study
  • Randomized Controlled Trial
  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Adult
  • Amino Acids / blood*
  • Amino Acids / metabolism
  • Anabolic Agents / metabolism*
  • Carbon Isotopes
  • Caseins / metabolism*
  • Catheters, Indwelling
  • Female
  • Femoral Artery
  • Femoral Vein
  • Hepatic Veins
  • Humans
  • Intestinal Absorption
  • Kinetics
  • Male
  • Milk Proteins / metabolism*
  • Nitrogen Isotopes
  • Phenylalanine / blood
  • Phenylalanine / metabolism
  • Protein Biosynthesis*
  • Quadriceps Muscle / metabolism*
  • Whey Proteins
  • Young Adult


  • Amino Acids
  • Anabolic Agents
  • Carbon Isotopes
  • Caseins
  • Milk Proteins
  • Nitrogen Isotopes
  • Whey Proteins
  • Phenylalanine