Collagen and Hair Density: Breaking Down the Reilly et al. Study

What the Reilly et al. study actually measured

The Reilly et al. research examined the effect of a hydrolysed collagen supplement on skin and hair parameters in women.¹ Over a defined study period, researchers took scalp measurements at baseline and at follow-up, counting actual hair strands per square centimetre in the same spots on participants' heads.

This matters because it's objective. No self-reporting, no subjective "my hair feels fuller." Just microscopic counts of hair density in a precise area. The study found that women taking the collagen peptide experienced a measurable increase in strand count.

A reported ~27% increase in hair count was observed in the collagen group compared to placebo, alongside other skin and scalp improvements.¹

The 27% increase in hair strand density explained

Here's where it's easy to overstate the finding. A 27% increase sounds remarkable. And in laboratory terms, it is. But what does that actually mean for how your hair looks?

Human hair density varies enormously by ethnicity, age, and genetics. Caucasian women typically have around 100 to 150 hairs per square centimetre. People of African descent often have lower strand counts, around 80 to 120, but thicker individual strands. People of East Asian descent often have even higher baseline densities.

The 27% improvement means you're moving from noticeable thinning toward something closer to your genetic baseline. It's not the same as regrowing a full head of hair. It's restoring density you've already begun to lose.

The study participants were women with existing hair thinning. They weren't bald. They weren't experiencing significant hair loss from chemotherapy or alopecia areata. They were noticing density loss, probably over months or years, and the collagen peptide helped restore some of that lost thickness.

That distinction matters enormously. A 27% gain when you've lost 30% is different from a 27% gain in someone with hair that's never thinned.

Why collagen matters for hair structure

Hair grows from follicles embedded deep in the scalp. The follicle itself is wrapped in a structure called the dermal papilla, which is rich in collagen and other structural proteins. Type I collagen, the most abundant form in your body, is part of the architecture that supports hair growth.²

When collagen breaks down, which happens with age, sun damage, inflammation, and poor nutrition, the dermal papilla weakens. The follicle doesn't receive the same structural support. Hair cycles into the shedding phase faster. New hairs grow in thinner. The net result is lower density.

Collagen peptides are hydrolysed collagen, broken down into small chains your digestive system can actually absorb. Once absorbed, your body can use those amino acids, particularly glycine and proline, to rebuild connective tissue, including the collagen matrix around hair follicles.

Hair follicle biology and collagen

Understanding why collagen matters requires understanding the hair growth cycle. Hair follicles have three phases: anagen (growth), catagen (transition), and telogen (resting/shedding).

A healthy follicle spends 2 to 7 years in anagen, growing hair. During this time, the dermal papilla is actively nourishing the follicle cells. When collagen integrity is compromised, the follicle can't sustain the anagen phase as long. It shifts into catagen and telogen earlier. Hair falls out sooner. New hair grows in thinner.

Collagen supplementation, combined with other nutrients like iron, zinc, B vitamins, and vitamin C, supports the dermal papilla's ability to maintain healthy anagen phases. This means more hairs stay in the growth phase longer, and new hairs grow in thicker.

This is the mechanism. But the real question is whether it works consistently, and for whom.

What realistic expectations look like

The Reilly et al. study is solid evidence. But it's one study. The women who benefited most tended to have moderate thinning, not severe alopecia. They took the supplement consistently over several months. They were likely in their 30s to 50s, when collagen loss is noticeable but hair follicles haven't completely shut down.

If your hair thinning is recent and mild to moderate, collagen peptides have decent evidence behind them. You're probably looking at improvements that take 3 to 6 months to become visible. That aligns with the hair growth cycle. New hairs take time to grow.

If your hair loss is severe, or if you have pattern baldness, collagen alone likely won't reverse it. Hair follicle miniaturisation is a different process, driven largely by genetics and hormones. Collagen supports the environment but doesn't override genetics.

Results depend on baseline density, how much you've lost, how consistently you take the supplement, and whether your thinning is driven by nutritional factors or by hormonal or genetic factors that collagen can't address.

The study also didn't isolate collagen in a vacuum. Study participants were presumably eating otherwise normally, getting other nutrients, managing their stress. Collagen peptides work best as part of a foundation of good nutrition, not as a standalone fix.

Diet and supplementation strategy

If you're using collagen for hair health, support it with the nutrients that make it work.

Iron is critical. Low iron has been associated with hair loss.³ Get tested first. If you're deficient, focus on heme iron sources, red meat and organ meats, which are absorbed far more efficiently than plant-based iron.

Zinc is equally important. It's involved in hair growth and the immune regulation around the follicle. Beef, shellfish, and pumpkin seeds are excellent sources.

Vitamin C supports collagen synthesis. Don't rely on supplements. Eat citrus fruits, berries, and leafy greens. The food matrix delivers vitamin C with cofactors your body actually uses.

Copper is often overlooked but critical for melanin production, which gives hair its colour and resilience. Shellfish, organ meats, and nuts provide it.

B vitamins, particularly B12 and folate, support hair follicle health. If you're plant-based, you almost certainly need supplementation here. Animal products, particularly organ meats and eggs, deliver these naturally.

The bottom line

The Reilly et al. research shows that collagen peptides can meaningfully increase hair strand density in women experiencing age-related or nutritional thinning. A 27% improvement is real and measurable. But it's not a cure for genetic hair loss, and it takes time.

If you're noticing your hair getting thinner and it's not driven by pattern baldness genetics, collagen peptides are worth trying. Aim for at least 3 to 6 months. Look for hydrolysed collagen peptides from reputable sources. And build them into a diet rich in other nutrients that support hair health: iron, zinc, B vitamins, and omega-3 fatty acids.

Hair density changes slowly. But with the right support, it can improve just as gradually. That's the promise of the research, and what the Reilly et al. study bears out.

References

1. 1. Reilly DM et al. A clinical trial shows improvement in skin collagen, hydration, elasticity, wrinkles, scalp, and hair condition following 12-week oral intake of a supplement containing hydrolysed collagen. Dermatol Res Pract. 2024;2024:8752787. PMID: 39021368.
2. 2. Gelse K, Pöschl E, Aigner T. Collagens — structure, function, and biosynthesis. Adv Drug Deliv Rev. 2003;55(12):1531-46. PMID: 14623400.
3. 3. Trost LB, Bergfeld WF, Calogeras E. The diagnosis and treatment of iron deficiency and its potential relationship to hair loss. J Am Acad Dermatol. 2006;54(5):824-44. PMID: 16635664.