Effects of collagen peptides on skin health	Type of study	Results/conclusion
Increase of hyaluronic acid production in dermal fibroblasts	In vitro. Cell culture using human dermal fibroblasts [47] .	The collagen peptides (type I) enhanced cell proliferation (1.5-fold) and hyaluronic acid synthesis (3.8-fold), which was concomitant with a 2.3-fold elevation of hyaluronan synthase 2 (HAS2) mRNA levels. This indicates that the collagen peptides stimulate hyaluronic acid synthesis mediated by activation of HAS2 transcription.
	In vivo. Oral intake of collagen peptides in hairless mice [48] .	The collagen peptides suppressed the negative effects of the UVB radiation exposure, improving skin elasticity and dermal hyaluronic acid content (after 4 weeks of supplementation).
Improvement of skin barrier function by increasing the water content of the stratum corneum	In vivo. Oral intake of collagen peptides in hairless mice [48] .	The collagen peptides prevented the increase in transepidermal water loss (TEWL) and the decrease in stratum corneum water content (after 4 weeks of supplementation).
	In vivo. Oral intake of collagen peptides in hairless mice [49] .	Daily administration of collagen peptides improved skin barrier dysfunction (decreased TEWL, and significantly increased water content of stratum corneum).
	In vivo. Oral intake of collagen peptides in hairless mice [50] .	The collagen peptides suppressed the UV-B-induced decrease in skin hydration and hyperplasia of the epidermis.
Induction of the synthesis of collagen on the mRNA and protein level	In vivo. Oral intake of collagen peptides in Sprague-Dawley rats [36] .	The collagen peptides increased the expressions of pro-collagen type I and III mRNA via the activation of the Smad signaling pathway with up-regulated TGF-βRII (TβRII) expression level. The collagen peptides also inhibited collagen degradation through attenuating MMP-1 expression and increasing tissue inhibitors of metalloproteinases-1 expression.
	In vivo. Oral intake of collagen peptides in Wistar rats [51] .	The relative amount of type I and IV collagens significantly increased after four weeks of collagen peptides administration, in relation to the control group. The supplement also decreased proenzyme and active forms of MMP2, suggesting that the collagen peptides may act on aging through stimulation of anabolic processes in skin tissue.
Induction of the production of stronger collagen fibrils	In vivo. Oral intake of collagen peptides in pigs [52] .	Fibroblast density and diameter and density of collagen fibrils were significantly larger in the collagen peptide group, after 62 days. The ratio of dermatan sulfate (derived from fibril-bound decorin) was also the largest in the collagen peptide group. This suggests that ingestion of collagen peptide induces increased fibroblast density and enhances the formation of collagen fibrils in the dermis.
Promotion of skin fibroblasts growth	In vitro. Cell culture, using mouse skin fibroblasts [53] .	Prolyl-hydroxyproline (found in human peripheral blood after ingestion of collagen peptide) enhanced the growth of fibroblasts in a dose-dependent manner.
Induction of fibroblast migration (wound healing)	In vitro. Cell culture, using human dermal fibroblasts [54] .	Cleavage collagen exhibited chemotactic activity.
	In vitro. Cell culture, using human dermal fibroblasts [55] .	Peptides from digested collagen (types I, II, and III) exhibited chemotactic activity, suggesting that they can attract fibroblasts to affect the repair of damaged tissue.
Skin hydration, collagen density, and fragmentation of dermal collagen network	In vivo. Two placebo-controlled clinical trials in human subjects (oral supplementation with 10 g of collagen peptides) [34] .	The collagen peptides increased skin hydration (after 8 weeks) and collagen density in the dermis (after 4 weeks). The fragmentation of the dermal collagen network significantly decreased (after 4 weeks). Effects seem to be mediated by induction of collagen and glycosaminoglycan production.