My dog Dexter has not always had the best gut. Over the years, various digestive complaints have led us — as they lead a lot of clients — through stool testing, probiotics, dietary adjustments, and yes, collagen. A Bull Shih with a compromised gut barrier and a cautious owner who happens to run a functional medicine practice. He has been a willing, if occasionally reluctant, participant in the clinical process.
I mention Dexter not because canine gut health is the primary subject of this post, but because the collagen question in his case was handled exactly as it should be in any case: testing first, identification of the specific problem, targeted intervention with the most appropriate form, clear rationale for why it was indicated. Not because collagen is trending. Not because an Instagram account with 400,000 followers said to put it in your morning coffee. Because the GI-MAP showed a specific pattern that collagen peptides address.
That is the entire argument of this post in miniature. Collagen can be a genuinely useful clinical intervention. It is not a universal supplement that everyone needs, and the form, dose, source, and timing matter enormously — none of which the market selling it at scale has any interest in discussing.
What Collagen Actually Is
Collagen is the most abundant structural protein in the human body — the primary component of connective tissue, comprising tendons, ligaments, cartilage, bone matrix, skin dermis, gut wall, blood vessels, and the extracellular matrix of almost every organ. It is a triple helix of amino acid chains, predominantly glycine, proline, and hydroxyproline, providing tensile strength and structural integrity to tissues that need to hold shape under load.
There are at least 28 types of collagen. Type I is the most abundant and the most commercially relevant — found in skin, bone, tendons, and ligaments. Type II is cartilage-specific. Type III is found alongside Type I in skin and blood vessels. Type IV forms the basement membrane of epithelial and endothelial cells — relevant to gut barrier integrity. Most collagen supplements do not specify which type they contain, or contain a blend that may or may not correspond to the tissue you are trying to support.
The body synthesises collagen in fibroblasts (connective tissue), chondroblasts (cartilage), osteoblasts (bone), and various other specialised cells. The synthesis process requires specific cofactors without which collagen cannot be produced regardless of how much precursor material is available. This is the clinical fact that the supplement industry consistently omits.
The Cofactor Problem — Why Topical Collagen Is Mostly Marketing
Collagen synthesis requires:
This list matters for one specific reason: applying collagen topically to the skin addresses none of these mechanisms. Collagen molecules are too large to penetrate the skin barrier — they sit on the surface and are washed off. The skin's ability to retain water temporarily improves (because of the humectant properties of peptides on the surface), which produces a temporary plumping effect, which the beauty industry calls "visibly reducing fine lines." It is not collagen synthesis. It is surface hydration. These are not the same thing.
The appropriate response to declining skin collagen — which is a real phenomenon, occurring at roughly 1% per year from the third decade — is not topical collagen application. It is ensuring adequate vitamin C, zinc, copper, and iron status through testing and targeted repletion. It is reducing the drivers of collagen degradation: UV exposure, high blood sugar (glycation crosslinks collagen abnormally), smoking, chronic inflammation. None of these fit into a 30-second Instagram reel.
"Applying collagen to your skin is like pouring protein powder on your muscles and expecting them to grow. The substrate is irrelevant without the synthesis machinery — and the synthesis machinery requires cofactors that most people with collagen-related concerns are deficient in."
When Oral Collagen Supplementation Is Actually Warranted
The picture for oral collagen supplementation is more nuanced. Hydrolysed collagen peptides — collagen broken down into short-chain peptides — are absorbed in the small intestine and can be detected in blood and target tissues within hours. There is genuine evidence that specific collagen peptides (particularly those from Peptan and Verisol — the two most studied commercial collagen peptide sources) accumulate in skin dermis and cartilage and stimulate local fibroblast collagen synthesis. This is a real effect, not wishful thinking.
The clinical indications where the evidence supports collagen supplementation:
- Gut barrier repair — glycine and collagen peptides support tight junction integrity and mucosal healing. In clients with confirmed intestinal permeability on GI-MAP (elevated zonulin, low secretory IgA, mucosal damage patterns), targeted collagen peptide supplementation is a legitimate intervention in the gut repair protocol — typically in combination with L-glutamine and zinc carnosine.
- Joint and cartilage support — Type II collagen in undenatured form (UC-II) has specific evidence for osteoarthritis and joint pain at doses of 40mg daily — not grams, which is the standard serving size on most commercial products. UC-II works through oral tolerisation of the immune response, a different mechanism from simply providing collagen building blocks.
- Skin ageing with confirmed cofactor adequacy — if vitamin C, zinc, and copper status are confirmed adequate through testing, specific hydrolysed collagen peptides at 5–10g daily have evidence for improved skin elasticity and hydration in controlled trials. Without cofactor adequacy, you are supplementing the substrate without the tools to use it.
- Post-surgical or injury tissue repair — connective tissue repair after injury has an elevated collagen turnover demand. Targeted collagen supplementation with adequate vitamin C in the peri-surgical period has evidence for improved tissue healing rates.
- High training loads — athletes placing high demands on tendons and ligaments show benefit from collagen supplementation combined with vitamin C taken 30–60 minutes before exercise, when the target tissue is most receptive to amino acid uptake.
The Bone Broth Hierarchy
I have not made bone broth regularly for years. I used to — and it is genuinely the most complete source of collagen precursors, glycine, gelatin, and the full matrix of minerals that support connective tissue — but modern life and the demands of running a clinical practice have made it an occasional pleasure rather than a daily practice. I note this to avoid the hypocrisy of recommending something I do not currently do with any consistency.
The hierarchy of collagen sources, from most to least complete:
- Homemade bone broth — slow-cooked (8–24 hours) bones with cartilage, marrow, and connective tissue. Highest glycine content, full mineral matrix, natural gelatin, no processing artefacts. Time-intensive, which is the practical barrier for most people.
- Quality ready-made bone broth — Best Bone Broth, Freja, Osso Good are the brands I rate. Look for long cook times (the label will usually specify), bones from grass-fed animals, and a broth that gels when refrigerated — that is the collagen content made visible.
- Clinical-grade hydrolysed collagen peptides — Designs for Health Whole Body Collagen uses Verisol (skin-specific) and Peptan (gut and joint) collagen peptides. It is the practitioner-grade option in this category. Jarrow FormulasCollagen is a solid second. These are not equivalent to bone broth but are the most evidence-based supplement form.
- Consumer-grade collagen sachets and powders — variable quality, frequently underdosed relative to the studies they reference, often combined with sugars and flavourings that counteract the benefit. The subscription model that delivers a monthly quantity regardless of clinical need is the commercial standard. It is not the clinical standard.
The collagen market illustrates a pattern that repeats across supplement categories: a genuinely useful compound with specific clinical indications is identified, the research is extracted and simplified beyond recognition, a mass-market product is built around the simplified story, doses are set by what can be manufactured and priced competitively rather than what was used in the relevant studies, the product is positioned as appropriate for everyone, and the subscription model ensures revenue regardless of whether the individual is responding.
The same pattern applies to vitamin D (everyone takes it, few check their blood level, fewer check parathyroid hormone and calcium alongside it, almost nobody adjusts dose based on response). Creatine (well-evidenced, widely supplemented, and largely appropriate — one of the few supplements where the mass-market recommendation roughly matches the clinical one, though the timing and co-supplementation details are still ignored). CBD (the evidence base is genuinely interesting for specific indications; the evidence for general wellness use is not). Vitamin C (the clinical dose varies by individual oxidative stress status; the 1000mg supplement in a plastic bottle from the supermarket ignores all of this).
Functional medicine loses its clinical credibility the moment it adopts the same "everyone needs this" logic it criticises in conventional medicine. The antidote is the same thing it always is: test first, identify the specific need, match the intervention to the finding, monitor the response, and adjust. Collagen supplementation without testing the cofactors required for collagen synthesis is guesswork with a label.
A Note on Creatine, CBD, and the Trend Supplement Pattern
Creatine deserves brief mention as the most vindicated supplement in this category — 30 years of research, consistent evidence for muscle power output, cognitive function (particularly in older adults and vegetarians), and more recently creatine's role in cellular energy beyond muscle. The mass-market recommendation — 5g daily — is actually close to the clinical dose for most people. Creatine monohydrate from a reputable manufacturer is one of the few supplements where the consumer product broadly matches the clinical recommendation. It is the exception that proves the rule.
CBD is the opposite. The evidence for CBD in specific clinical contexts — treatment-resistant epilepsy, certain anxiety disorders, neuropathic pain — is genuinely interesting and growing. The evidence for CBD oil dropped under your tongue twice a day as a general wellness supplement is thin. The products sold at high-street pharmacies and Instagram-targeted brands contain doses so far below those used in clinical research that the biological plausibility of effect is minimal. The mechanism is real. The product bearing the mechanism's name is frequently not delivering it.
The unifying theme: a compound with genuine clinical applications is extracted from its clinical context, simplified, dosed commercially rather than clinically, and sold to everyone. The people for whom it is genuinely indicated benefit less than they could because the product is not optimised for them. The people for whom it is not indicated spend money on something that the evidence never supported for their situation.
The Skin Is a Window, Not a Wall
The beauty industry's fundamental error is treating skin quality as a surface problem to be solved with surface interventions. Skin condition — its texture, elasticity, hydration, tone, propensity to inflammation — is a visible output of systemic biology. Gut health, nutrient status, inflammation load, hormonal balance, sleep quality, UV exposure, blood sugar regulation: these are the inputs that determine skin quality. The topical product addresses none of them.
In clinical practice, the clients with the most dramatic skin changes are not the ones who changed their skincare routine. They are the ones who resolved their gut dysbiosis, repleted their zinc and vitamin C, normalised their blood sugar, and addressed their chronic inflammation. The skin changed because the underlying biology changed. It did not change because they applied something expensive to the surface.
This is the same point made about every supplement in this post: the substrate matters less than the system it operates in. Understanding the system — through testing, pattern recognition, and targeted intervention — is the clinical work. Everything else is the supplement industry selling shortcuts to people who have not yet been told that the shortcut does not go where they think it does.
Collagen product recommendations — for those who have addressed the cofactor question and have a specific clinical indication:
Know what you actually need before you buy anything.
The TDG blood chemistry panel tests the cofactors that collagen synthesis requires — vitamin C status markers, zinc, copper, iron. The GI-MAP tells you whether gut barrier support is the priority. Test first. Supplement to the finding.
TDG Five-Test Programme → Blood Chemistry Health Audit — £295