Does Whey Protein Increase Circulating Glycine, Proline, and Hydroxyproline?
No, whey protein ingestion actually decreases circulating glycine levels, and does not meaningfully increase proline or hydroxyproline concentrations. In fact, whey protein consumption leads to a postprandial decline in plasma glycine availability that requires collagen co-ingestion to prevent 1.
The Glycine Paradox with Whey Protein
The most recent and highest-quality evidence demonstrates that whey protein ingestion causes plasma glycine concentrations to decline following consumption 1, 2. This is a critical finding because:
- When 30g of whey protein alone was ingested post-exercise, plasma glycine availability averaged -8.9 mmol·360 min/L (negative incremental area under curve), indicating net depletion rather than increase 1
- Plasma glycine and nonessential amino acid concentrations specifically declined following whey protein ingestion in controlled studies 1
- This decline occurs despite whey protein strongly increasing total plasma amino acid concentrations and essential amino acids like leucine 2
Proline and Hydroxyproline Response
Whey protein does not substantially increase proline or hydroxyproline levels:
- Studies comparing whey versus collagen protein show that plasma proline concentrations increased more with collagen ingestion compared to whey 2
- Hydroxyproline is essentially absent from whey protein, as it is specific to collagen-containing proteins
- The glycine-proline-hydroxyproline tripeptide sequence is characteristic of collagen, not whey 3
The Solution: Collagen Co-Ingestion
Adding just 5g of collagen to 25g of whey protein is sufficient to prevent the glycine decline and optimize amino acid availability 1:
- 0g collagen (30g whey alone): -8.9 mmol·360 min/L glycine availability
- 5g collagen (25g whey + 5g collagen): +9.2 mmol·360 min/L glycine availability
- 10g collagen: +23.1 mmol·360 min/L
- 15g collagen: +39.8 mmol·360 min/L
This whey-collagen blend (5:1 ratio) has been validated to increase both myofibrillar and muscle connective protein synthesis rates 4, addressing the limitation that whey alone only stimulates myofibrillar but not connective tissue protein synthesis 5.
Clinical Implications
While whey protein is recommended in guidelines for increasing lean body mass due to its high leucine content and rapid digestion 6, 5, it paradoxically depletes glycine stores. This matters because:
- Glycine availability is associated with satiety and appetite regulation 7
- Glycine is essential for connective tissue protein synthesis 2, 8
- The glycine-proline-hydroxyproline sequence has anti-glycation and cytoprotective effects 3
Bottom line: Whey protein alone decreases circulating glycine and does not increase proline or hydroxyproline. If these amino acids are desired, collagen must be added to or substituted for whey protein.