Human Data on Circulating Glycine, Proline, and Hydroxyproline
Yes, there is robust human data demonstrating increases in circulating glycine, proline, and hydroxyproline following collagen hydrolysate ingestion.
Direct Human Evidence
Multiple human studies have directly measured and confirmed elevations in these amino acids in plasma after collagen consumption:
Hydroxyprolyl-glycine (Hyp-Gly) and prolyl-hydroxyproline (Pro-Hyp) were quantified in healthy human volunteers (n=5) after collagen hydrolysate ingestion using liquid chromatography-tandem mass spectrometry, providing the first direct quantification of food-derived Hyp-Gly in human plasma 1.
A recent 2025 study in 45 young adults (21 male, 24 female) demonstrated that ingestion of 30g hydrolyzed collagen substantially increased circulating plasma amino acid concentrations, particularly glycine 2. This was a well-controlled, randomized, double-blind study using stable isotope methodology with L-[ring-¹³C₆]-phenylalanine infusions.
Hemodialysis patients showed significantly elevated plasma concentrations of glycine (p<0.01), hydroxyproline (p<0.05), and proline (p<0.10) compared to healthy controls, with these levels normalizing during erythropoietin therapy 3. While this population differs from healthy individuals, it demonstrates the measurability and physiological relevance of these amino acids in human circulation.
Absorption and Bioavailability
The evidence confirms that orally administered collagen hydrolysate is efficiently absorbed in humans, with intact prolyl-hydroxyproline and hydroxyprolyl-glycine reaching systemic circulation through conserved PEPT1/PEPT2 transporters 4. The bioavailability varies with source, dose, and measurement method, but the fundamental absorption and circulation of these peptides is well-established across species including humans.
Important Caveats
While the increases in circulating amino acids are well-documented, the 2025 study found that these elevations did not translate to increased myofibrillar or muscle connective protein synthesis rates in healthy, recreationally active young adults at rest or during recovery from exercise 2. This highlights that circulating levels alone do not guarantee functional tissue incorporation, though other mechanisms (Keap1-Nrf2 axis, HDAC/HAT modulation, glycine-dependent glutathione synthesis) may mediate systemic effects 4.
The human data is strongest for acute pharmacokinetic measurements and less robust for long-term tissue-level outcomes, which remain an area requiring further investigation.