Peptides are short chains of amino acids. Insulin is one, collagen is one too, and the most-searched category of 2026 is one as well: GLP-1, BPC-157, and growth-hormone peptides. Here is what peptides are, what they do, and which blood values can matter if you use them.
Honestly: not all peptides are the same, and most articles on "peptides" mix harmless collagen supplements with research compounds in a way that leaves nobody clearer. We will take them category by category.
What are peptides?
A peptide is a short chain of amino acids, shorter than a protein. The cut-off sits around 50 amino acids: shorter chains are called peptides, longer ones are called proteins. Your body makes them itself, like insulin and oxytocin, and you also get them from food like collagen or whey.
The word was first used in 1902 by the German chemist Emil Fischer. For a long time it stayed a specialist term. That has changed in recent years: peptides show up on supplement shelves, in weight-loss injections, and in protocols circulating online among athletes and biohackers.
Not all peptides are the same.
The sections below split them into four groups, plus the blood values that may matter if you are considering or already using a protocol.
What types of peptides exist?
Peptides fall roughly into four groups: endogenous peptides like insulin, food peptides like collagen, registered medications like semaglutide (GLP-1), and research peptides like BPC-157 and sermorelin. The medical and legal picture is different for each group.
Endogenous peptides
These are the peptides your body produces. Insulin regulates blood sugar. Oxytocin plays a role in bonding and childbirth. Glucagon does roughly the opposite of insulin. This group is well studied: insulin has been used clinically since 1921.
Food peptides
Collagen peptides are the best-selling. The idea: hydrolysed collagen is absorbed as smaller peptides, and some of those fragments may influence skin and joints. The research base is mixed: some small studies show effects on skin elasticity, others do not. Whey derivatives like beta-lactoglobulin peptides also belong in this group.
Registered peptide medications
The best known are GLP-1 agonists like semaglutide (Wegovy, Ozempic) and tirzepatide (Mounjaro). These are prescribed for type 2 diabetes or weight loss, assessed by the EMA, and available only through a doctor. They have a known side-effect profile. For the specific combination of GLP-1 and muscle retention we wrote a separate guide: GLP-1 weight loss without muscle loss.
Research peptides
BPC-157, TB-500, sermorelin, ipamorelin, CJC-1295. These peptides are not approved for human use in the Netherlands. They are sold online under the "for research purposes only" label. The human research base is limited: many animal studies, few randomised clinical trials.
Which peptides do people use for muscle and recovery?
For muscle building and recovery, the biohacker community looks mainly at growth-hormone secretagogues (sermorelin, ipamorelin, CJC-1295) and recovery peptides (BPC-157, TB-500). The claims are large. The human research base is much smaller. Animal studies show interesting effects, but randomised trials in people are scarce.
Growth-hormone secretagogues
Sermorelin, ipamorelin and CJC-1295 stimulate your pituitary to release growth hormone (GH). More GH typically means more IGF-1, and IGF-1 is the marker you see in blood work (Junnila et al., 2013). What sellers claim: more muscle growth, better recovery, better sleep. What the literature shows in healthy adults: most data comes from small, uncontrolled studies or animal work.
BPC-157 and TB-500
BPC-157 is positioned as a "body protective compound", originally isolated from stomach lining. TB-500 (a thymosin beta-4 fragment) is sold for tissue repair. Animal studies show accelerated wound healing and tissue regeneration. Human data is limited to small uncontrolled studies (Berlanga-Acosta et al., 2022).
That is what we know. That is what we do not.
Are peptides safe?
Peptide safety depends on which peptide and how you use it. Endogenous and food peptides are well studied. Registered GLP-1 medications have been assessed by the EMA with a known side-effect profile. Research peptides have not: source, dose, and purity are often unclear.
For GLP-1 medications, the most common side effects are nausea, gastrointestinal complaints, and in rare cases pancreatitis (EMA product information, 2024). For research peptides it is different: the compounds often have not passed through a regulated production chain. What is in the vial is not easily verified. That is a real risk, and this is exactly where blood values come in.
Not to "screen", but to monitor change.
Are peptides legal in the Netherlands?
In the Netherlands, registered peptide medications like semaglutide are only available by prescription through a doctor. Research peptides like BPC-157 sit in a grey area. They are not approved for human use by the CBG or EMA. Selling them for human consumption is, in principle, not permitted.
The practice: they are sold via international web shops under the "for research purposes only" label. Possession for personal use is rarely prosecuted. The legal responsibility sits with the seller, and with you if you use them.
No judgement, just the facts.
Which blood values matter when you use peptides
Which blood values matter depends on the peptide. GH secretagogues mainly affect IGF-1, glucose, and lipids. GLP-1 affects glucose, HbA1c, and the liver. For anyone considering starting: a baseline of hormone, sugar and lipid values gives the starting point to measure change against.
GH secretagogues (sermorelin, ipamorelin, CJC-1295)
- IGF-1: the direct marker of GH activity. If the peptide does what people claim, this moves.
- Fasting glucose and HbA1c: GH can raise insulin resistance. You would see it here.
- Lipid panel: for the wider metabolic picture.
- Prolactin: can rise with some GH secretagogues.
GLP-1 (semaglutide, tirzepatide)
- HbA1c and fasting glucose: primary metabolic markers, also what the medication targets.
- Lipid panel: weight loss typically changes this.
- Liver (ALT, AST): standard safety markers.
- Kidney (creatinine, eGFR): dehydration is a risk with rapid weight loss, especially in the early weeks.
Research peptides (BPC-157, TB-500)
- Complete blood count (CBC): basic safety marker.
- CRP: inflammation marker, relevant for recovery claims.
- Liver and kidney: to gauge whether source quality is hurting you somewhere.
- Before-after measurement: without a baseline, change is invisible.
General baseline (for anyone considering)
- Testosterone (total and free): starting point for men considering optimisation.
- IGF-1: starting point for the GH axis.
- Thyroid (TSH, free T4): affects recovery and metabolism.
- CBC and CMP: basic blood picture and organ function.
- Lipid panel: metabolic starting point.
This is not testing advice, it is a reference frame. Whether blood work makes sense for you, and when, is best discussed with your GP.
When does blood work make sense with peptide use?
Blood work with peptide use comes down to one thing: locking in a starting point. Without a baseline, you cannot measure change. Most people who consider this choose a measurement before they start and a repeat afterwards. Whether that makes sense for you is for your GP to weigh in on.
Many of the markers that matter overlap with what we measure for TRT monitoring: testosterone, IGF-1, lipids, glucose, HbA1c, liver, kidney. Anyone already on TRT who is considering peptides already has most of those markers in their history.
If you are thinking about a peptide protocol, or you are already in one, a baseline blood test with testosterone, IGF-1, fasting glucose, HbA1c and lipids gives you the starting point to measure change later. The comprehensive hormone and metabolic panel from Enhanced covers these markers and the safety markers around them.
Discuss your results with your GP.
Every blood test result includes a professional assessment from a BIG-registered doctor. For treatment decisions, discuss your results with your GP.
References
- Berlanga-Acosta J et al. Body-protective compound (BPC)-157: known properties and possible applications. World Journal of Gastroenterology. 2022.
- Junnila RK et al. The GH/IGF-1 axis in ageing and longevity. Nature Reviews Endocrinology. 2013;9(6):366-376.
- European Medicines Agency (EMA). Wegovy (semaglutide), product information. 2024 update.
- RIVM. Consumer information on supplements. 2024.
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