Peptides explained: types, use and blood values in 2026

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, without hype and without judgement.

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.

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, and that distinction is exactly what decides the medical and legal picture.

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 picture is different for each group.

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 stimulate your pituitary to release growth hormone (GH), which typically raises your IGF-1, the marker you see in blood work. BPC-157 and TB-500 are sold for tissue repair, but human data is limited to small, uncontrolled studies. That is what we know. The rest is still open.

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 European Medicines Agency (EMA), with a known side-effect profile: usually nausea and gastrointestinal complaints, rarely pancreatitis. Research peptides have not been assessed that way: source, dose, and purity are often unclear.

The Dutch public health institute (RIVM) warns that products from unregulated channels can contain something other than what the label states. 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: not approved for human use by the CBG or EMA, and selling them for human consumption is in principle not permitted. In practice they are sold under the "for research purposes only" label. The legal responsibility sits with the seller, and with you if you use them.

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. 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.
• Prolactin: can rise with some GH secretagogues.

GLP-1 (semaglutide, tirzepatide)

• HbA1c and fasting glucose: primary metabolic markers, also what the medication targets.
• ALT and AST: standard safety markers.
• Creatinine and eGFR: dehydration is a risk with rapid weight loss.

Research peptides (BPC-157, TB-500)

• 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.

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. Many of the markers that matter overlap with what we measure for TRT monitoring: testosterone, IGF-1, lipids, glucose, HbA1c, liver and kidney.

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 panel from Enhanced covers these markers and the safety markers around them. See also the guide the 360 health panel.

My advice: you do not start a peptide protocol blind. Lock in a baseline first and discuss your plan and your results with your GP.

References

1. European Medicines Agency (EMA). Wegovy (semaglutide), product information. 2024.
2. RIVM (Dutch National Institute for Public Health). Information on supplements and products from unregulated channels. Accessed 2026.
3. Gezondheidsraad (Dutch Health Council). Position on self-medication and unregistered substances. Accessed 2026.

Disclaimer

Every blood test result includes a professional assessment by a BIG-registered doctor. This article gives general information and is not a substitute for medical advice. A blood test is a tool to walk into the conversation with your GP better informed, not a diagnosis in itself. For treatment decisions, discuss your results with your GP.