If you have spent any time in tendon-rehab corners of the internet, you have met the claim: take 15 grams of gelatin an hour before loading your tendons, and you will make more collagen. It is repeated with the confidence of a settled protocol. The study usually cited for it — Shaw and colleagues, published in the American Journal of Clinical Nutrition in 2017 — is real, and the result it reports is genuinely interesting. But the study did not test tendon loading, did not measure tendon outcomes, and did not study anyone with an injury. The frustration is that a careful mechanistic paper has been flattened into a prescription. The useful response is not to dismiss the paper but to read it at the resolution it deserves, then decide what, if anything, belongs in your own rehab.
What the study actually did
Shaw et al. ran a randomized, double-blinded, crossover trial in eight healthy, recreationally active young men (mean age 27 ± 6 years). Each participant completed all three conditions — placebo, 5 g gelatin, and 15 g gelatin — with a four-day washout between trials. Every drink contained 48 mg of vitamin C, so the comparison was gelatin dose, not gelatin versus vitamin C. The exercise stimulus was 6 minutes of rope skipping, performed three times per day for three days, with the supplement taken one hour before each bout (Shaw et al., 2017).
That is the whole intervention. No resistance band. No targeted eccentric heel raise. No isometric hold against a strap. Rope skipping is intermittent whole-body activity that loads the Achilles and patellar tendons among many other structures, but the study did not quantify tendon load, did not control it beyond the skipping duration, and did not measure it.
Three outcome layers, and why they matter separately
The paper reports three distinct kinds of result. Collapsing them into one headline is where most of the confusion starts.
1. Circulating amino acids
Supplementation with increasing amounts of gelatin raised circulating glycine, proline, hydroxyproline, and hydroxylysine, with concentrations peaking about one hour after ingestion. This is a pharmacokinetic finding: gelatin is digested, its constituent amino acids appear in blood, and the peak roughly coincides with the pre-exercise window the authors chose. It tells you the supplement was absorbed. It does not tell you where those amino acids went or what they built.
2. PINP — a collagen synthesis biomarker
The primary blood outcome was PINP, the amino-terminal propeptide of type I collagen. When cells synthesize type I collagen, the propeptide is cleaved and released into circulation, so a rise in serum PINP is used as a marker of increased type I collagen synthesis somewhere in the body. Participants who took 15 g gelatin one hour before exercise showed roughly double the PINP response compared with the other conditions.
Here is the critical distinction. PINP is a systemic marker. It does not tell you which tissue produced the collagen. A rise in serum PINP after rope skipping and gelatin could reflect bone, skin, tendon, ligament, fascia, or some combination — the assay cannot localize it. It is also a marker of synthesis, not of tissue quality. More collagen being made is not the same as stronger, stiffer, or better-organized tendon. And a biomarker response is not a clinical outcome: it says nothing about pain, function, load tolerance, or healing.
3. Engineered ligament constructs
The third layer is in vitro. The researchers grew ligament-like constructs from human anterior cruciate ligament cells and treated them for six days with serum drawn from participants before supplementation or one hour after consuming placebo, 5 g, or 15 g gelatin. Constructs treated with post-gelatin serum showed increased collagen content and improved mechanical properties.
This is a mechanistically suggestive result, and it is worth taking seriously as a hypothesis. It is not a tendon outcome in a living person. It is a tissue-engineered model treated with serum in a dish. The gap between that and your Achilles tendon after a calf strain is wide, and the study makes no claim to have crossed it.
What the study did not measure
It is worth stating this list plainly, because each item is something the popular claim implies:
- Tendon collagen synthesis. PINP is systemic; the study did not biopsy tendon or use any tendon-specific measure.
- Tendon mechanics. No stiffness, no cross-sectional area, no ultrasound, no elastography, no tensile testing in humans.
- Clinical outcomes. No pain scores, no function scores, no return-to-activity measures, no injury rates.
- An injured population. Participants were healthy young men. No tendinopathy, no post-surgical stiffness, no persistent pain.
- Adults 30–55. The mean age was 27. Extrapolating to a 48-year-old with a grumpy patellar tendon is an inference, not a finding.
- Targeted tendon loading. Rope skipping is not a controlled tendon-loading protocol, and the study did not match or measure load.
- Dose-response beyond two doses. The comparison was 0, 5, and 15 g. There is no evidence here about 2.5 g, 30 g, or repeated daily dosing over weeks.
- Timing beyond one hour. The one-hour window was the design choice, not a tested optimum. No other timing was compared.
The authors themselves were measured about this. Their conclusion was that the data “suggest” a benefit and “could play a beneficial role” in injury prevention and tissue repair — language of hypothesis, not of protocol.
Why the “before tendon loading” framing is an import, not a finding
The study did place supplementation one hour before exercise, and the amino acid peak at one hour is a real observation. But the exercise was intermittent whole-body skipping, not the kind of controlled, progressive tendon loading a rehab plan uses. When the claim becomes “15 g of gelatin one hour before tendon loading,” two things have happened: a specific loading context has been substituted for rope skipping, and a biomarker result has been promoted to a treatment rule. Neither move is supported by the paper. The one-hour timing is a reasonable starting point for a hypothesis about nutrient availability at the time of loading — it is not an established dosing schedule.
How to think about this if you are mid-rehab
The honest position is that gelatin is a cheap, generally safe food-derived supplement with an interesting mechanistic signal and no demonstrated tendon-specific clinical benefit. That is not a reason to take it, and it is not a reason to avoid it. It is a reason to keep it in perspective.
Here is a decision framework, clearly labeled as reasoning rather than evidence:
- Ask what problem you are trying to solve. If it is “my tendon does not tolerate load,” the intervention with the strongest evidence is progressive loading, not a supplement. Gelatin does not substitute for that.
- Treat gelatin as optional and low-stakes. If you want to try 15 g of gelatin with a source of vitamin C about an hour before your loading session, the retrieved evidence does not establish benefit, but it also does not flag harm in healthy adults at that dose. Over-the-counter gelatin is not a regulated drug, so quality varies; that is a practical consideration, not a study finding.
- Do not let it displace the loading. The variable most likely to change your tendon’s capacity is the loading program — its progression, its dosage, and your consistency with it. A supplement taken before a session you skip is worth nothing.
- Track something that matters. If you are going to spend money and attention on this, decide in advance what would tell you it is or is not working: a specific loading milestone you can now tolerate, a pain-free range you could not reach, a measured change in your home test. A supplement with no defined success criterion is just a habit.
One more caution about the biomarker. It is tempting to reason that if 15 g doubled PINP, more gelatin or more frequent dosing would do more. The study does not support that inference. There is no dose-response data above 15 g, no data on repeated daily dosing over weeks, and no evidence that a larger PINP rise translates into better tendon tissue. Biomarker chasing is a well-worn trap.
What would actually change the recommendation
If you want a clear standard for when “15 g before loading” graduates from internet folklore to a defensible protocol, here is what the evidence would need to show:
- Tendon-specific outcomes in humans. Trials measuring tendon collagen content, cross-sectional area, stiffness, or mechanical properties after gelatin or collagen supplementation with controlled loading — not serum markers, not engineered constructs.
- Injured and rehab populations. People with tendinopathy, post-surgical stiffness, or persistent pain, and adults in the 30–55 range, not only healthy men in their twenties.
- Dose-response and timing data. Comparisons beyond 5 g versus 15 g, and timing comparisons beyond a single one-hour pre-exercise window.
- Clinical endpoints. Pain, function, and return to activity — the outcomes you actually care about — not just surrogate markers.
Until those exist, the accurate summary is this: Shaw et al. showed that 15 g of vitamin C–enriched gelatin before intermittent exercise raised a systemic collagen synthesis marker in eight healthy young men, and that serum from those men improved engineered ligament constructs in a dish. That is a genuinely interesting mechanistic signal. It is not a tendon-loading protocol, and it does not tell you whether your tendon will feel better. Your loading program, progressed by criteria and measured with a band, a scale, a tape, and a stopwatch, remains the intervention with the evidence behind it.
FAQ
Does the study prove gelatin strengthens tendons?
No. It measured a systemic collagen synthesis marker (PINP) and an in vitro engineered ligament response. It did not measure tendon strength, stiffness, or structure in any participant.
Was the gelatin taken before tendon loading?
It was taken one hour before 6 minutes of rope skipping, repeated three times daily for three days. Rope skipping loads tendons, but the study did not control or measure tendon load, and it is not equivalent to a targeted rehab loading session.
Does the 15 g dose apply to me if I am 45 and recovering from a tendon injury?
The study enrolled eight healthy men with a mean age of 27. There were no injured participants and no adults in the 30–55 range as a defined group. Applying the dose to an injured 45-year-old is an extrapolation, not a finding.
Should I take more than 15 g, or take it more often?
The study compared 0, 5, and 15 g and did not test higher doses or long-term repeated dosing. There is no evidence here that more is better, and no evidence that a larger PINP rise means better tendon tissue.
Is gelatin safe?
The retrieved study does not report a safety analysis; it was a short mechanistic trial in eight healthy men. Gelatin is a common food derivative, but supplement quality and any personal medical considerations are outside what this paper can tell you. If you have a medical condition or take medication, that is a conversation for your clinician, not a blog post.
What should I do instead?
Keep the loading program as the centerpiece. Progressive, criteria-based tendon loading has the stronger evidence base for changing load tolerance. If you want to try gelatin alongside it, treat it as an optional, low-stakes experiment with a defined success criterion — not as the intervention.