Collagen, fasting and peptides for loose skin: what the studies actually tested
· 4 min read · Dan d'Auvergne-Massie
The short answer
None of the three has good evidence for tightening loose skin after weight loss. Collagen trials studied skin ageing, and the higher-quality ones showed no significant effect; no clinical trial shows fasting reduces loose skin; and peptide injections sold online are not approved in Australia, with reported harms including liver damage and severe allergic reactions.
- Collagen evidence
- 2025 analysis, 23 trials, 1,474 participants
- Independent collagen trials
- No effect on hydration, elasticity or wrinkles
- Fasting and autophagy
- No clinical trial showing it reduces loose skin
- Online peptide injections
- Not approved in Australia; TGA warnings
General information, not medical advice. Read the disclaimer.
If you have spent any time in weight loss forums or on social media, you have met the claims. Collagen powder will firm your skin back up. Extended fasting triggers autophagy, which "eats" loose skin. Peptide injections will rebuild it. They are persuasive because each contains a grain of real biology. What matters is what was actually tested, in whom, and what it found.
Collagen supplements
Collagen is the main structural protein in skin, so the idea that eating it helps skin is intuitive. The best evidence we have is a 2025 analysis that pooled 23 randomised trials with 1,474 participants. [1]
Taken all together, the trials suggested collagen improved skin hydration, elasticity and wrinkles. But when the researchers split the trials by who paid for them, the trials not funded by pharmaceutical companies showed no effect on any of those outcomes, while the company-funded ones did. Splitting by quality told a similar story: the higher-quality trials showed no significant effect in any category. Their conclusion was that there is currently no clinical evidence to support collagen supplements for preventing or treating skin ageing. [1]
Two more things are worth knowing. Those trials were about skin ageing, mostly facial wrinkles and hydration, not loose skin after major weight loss, which is a different problem. And even if collagen did improve skin quality, a better-hydrated fold of surplus skin is still a fold of surplus skin. Nothing you eat removes tissue.
If you like your collagen drink, it is unlikely to hurt you. It is just unlikely to do what the label implies. The money is better spent on ordinary protein, which does have evidence for protecting muscle while you lose weight. [5]
Fasting and autophagy
Autophagy is a real process: cells break down and recycle their own worn-out components, and fasting does increase it. The leap is from that to the claim that fasting will make the body "eat" loose skin. We found no clinical trial showing that fasting, extended or intermittent, reduces loose skin after weight loss, and the claim does not fit what loose skin is. Surplus skin is living, healthy tissue, not waste, and autophagy works inside cells rather than removing whole folds of skin.
That is not a verdict on fasting as a way of eating, which is a separate conversation to have with your GP or dietitian, particularly if you are on a medication that affects appetite. It is only to say that loose skin is not a good reason to do it.
Peptide injections
This is the one to take seriously, because the risk is real.
Some skincare contains peptides, such as copper peptides, applied to the skin as a cosmetic. That is not what this section is about. It is about unapproved peptide products sold online, often as powders or injections in vials, under names the TGA has listed such as BPC-157, TB-500, GHK-Cu, CJC-1295 and retatrutide. [2] Online they are promoted for everything from anti-ageing and skin to healing and weight loss.
In 2026 the Therapeutic Goods Administration issued a series of warnings about these products. Its key points: [2] [3] [4]
- They are not approved in Australia and have not been assessed for safety, quality or effectiveness.
- Many arrive as powders or injections in poorly labelled or unmarked vials, so there is no way to know what is in them, how much, or whether they are sterile.
- Reported harms include liver damage, severe allergic reactions requiring hospital treatment, and inflammation and other complications needing medical attention.
- The TGA has made them a compliance priority, targeting unlawful importation, supply and advertising.
Australia's Chief Medical Officer, Professor Michael Kidd, put it plainly: Australians should not use these products. [3] There is no evidence they tighten loose skin, and good evidence they can hurt you. This site will not discuss sources or doses for them.
A quick way to test a claim
When you see a product promising to fix loose skin, three questions sort most claims quickly:
- Was it tested in people with loose skin after weight loss, or in something else, like facial wrinkles or cells in a dish?
- Who paid for the research, and does the effect survive in the independent studies?
- Does the claim fit what loose skin is? Anything that promises to shrink a fold of surplus skin from the inside is promising something nothing non-surgical has been shown to do.
The nutrition guide covers what food can genuinely do for skin, and the non-surgical limits post covers where the real ceiling sits for treatments that do work.
Frequently asked questions
There is no good evidence it does. A 2025 analysis of 23 trials found no benefit in the independently funded or higher-quality studies, and those trials were about skin ageing, not loose skin after weight loss. No supplement removes surplus skin.
We found no clinical trial showing this. Autophagy is real, but it recycles components inside cells; it does not remove folds of healthy skin. Loose skin is not a good reason to fast.
Unapproved peptide products sold online, such as BPC-157, TB-500 and injectable GHK-Cu, have not been assessed by the TGA for safety, quality or effectiveness. The TGA has reported liver damage and severe allergic reactions requiring hospitalisation, and Australia's Chief Medical Officer has said Australians should not use them.
Keep reading
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Sources cited
- Myung SK, Park Y. Effects of collagen supplements on skin aging: a systematic review and meta-analysis of randomized controlled trials. American Journal of Medicine, 2025.
- Therapeutic Goods Administration. Understanding your responsibilities when importing, compounding and supplying unapproved peptide products, April 2026.
- Therapeutic Goods Administration. Concerns regarding the public health risks associated with unapproved peptide products, June 2026.
- Therapeutic Goods Administration. TGA warning on the risks of importing unapproved peptide products, May 2026.
- Kokura Y, Ueshima J, Saino Y, Maeda K. Enhanced protein intake on maintaining muscle mass, strength, and physical function in adults with overweight/obesity: a systematic review and meta-analysis. Clinical Nutrition ESPEN, 2024.
Written by
Dan d'Auvergne-Massie· Editor, Skin After Weight Loss
Lost 36kg while on a prescription GLP-1 medicine and spent the next two years researching what actually tightens skin after rapid weight loss. Runs a small findability agency and previously worked in health technology. Not a doctor: every clinical claim on this site is sourced.
Last updated by the editor:
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