Background
Dietary protein digestion and amino acid absorption are strongly modulated by numerous factors that define the food matrix. It has been speculated that protein ingested in liquid form is more rapidly digested and absorbed when compared to the ingestion in a solid form. Here we assessed the post-prandial plasma amino acid availability following the ingestion of a single bolus of protein provided in either liquid or solid form.

Methods
12 healthy, young females were included in this randomized cross-over study. On two separate test days, participants ingested 20 g milk protein concentrate in solid form (protein bar) or in liquid form (protein drink). Products were matched for ingredients, macro- and micronutrient composition. On each test day, arterialized blood samples were collected frequently for up to 4 h following protein ingestion to assess the post-prandial rise in plasma amino acid concentrations. 

Results
Protein ingestion strongly elevated circulating plasma amino acid concentrations (P<0.001), with no significant difference between treatments (P=0.088). The incremental area under the curve of the post-prandial rise in total plasma amino acid concentrations did not differ following bar versus beverage consumption (160±73 vs 160±71 mmol·L^-1·4 h^-1, respectively; P=0.992). Furthermore, no difference in peak amino acid concentration (3872±481 vs 3833±867 µmol·L^-1, respectively; P=0.891) or time to peak (56±20 vs 53±20 min, respectively; P=0.674) was observed between treatments.

Conclusion
Ingestion of protein in liquid or solid form does not modulate post-prandial amino acid availability in healthy, female adults. Differences in protein digestion and amino acid absorption due to differences in food matrix are not attributed to merely the food form (solid vs liquid) of the ingested protein source.