Does Collagen Break a Fast? Calories and Protein Explained
Does collagen break a fast? On a strict zero-calorie fast, yes, because collagen is protein and protein has calories. USDA lists dry unsweetened gelatin, the closest entry it has and our stand-in here, at 335 kcal and 85.6 g of protein per 100 g. On other kinds of fast the answer depends on your goal, and for insulin, appetite and autophagy the research on collagen is thin.
There is no official definition of "breaking a fast". This page sets out what the sources say for each fasting goal. It is not medical or diet advice.
Short answer
- Does collagen have calories?
- Yes. US label rules let protein be counted at 4 calories per gram, one of several methods they allow, and USDA lists dry gelatin at 335 kcal per 100 g.
- Does it break a zero-calorie fast?
- Yes. A zero-calorie fast, as one review describes it, permits only non-caloric drinks.
- Does it affect insulin or autophagy?
- Collagen raised insulin in two trials. No study has measured autophagy after collagen, so that part is unknown.
Sources: 21 CFR 101.9 nutrition labeling (eCFR); USDA FoodData Central: dry unsweetened gelatin; Health effects of fasting review, 2020 (Europe PMC); Collagen peptides and appetite after exercise trial, 2025 (PubMed); Porcine collagen hydrolysate and glucose trial, 2025 (PubMed)
Does it break a fast? It depends on which fast
Fasting plans set different rules, so the same scoop of collagen can break one fast and fit inside another. This table matches each common goal with what the sources show.
| Fasting goal or rule | What the sources say | Collagen breaks it? |
|---|---|---|
| Zero-calorie fast | Only non-caloric drinks are permitted | Yes: it has calories |
| Modified alternate-day fast | Some plans allow about 25% of usual calories on fasting days | Not necessarily, if it fits inside the allowance |
| Keeping insulin low | Collagen raised insulin after exercise; 4 weeks raised fasting insulin slightly | It raised insulin in these trials |
| Blood sugar | Daily glucose did not change over 4 weeks; one 5 g product lowered the blood sugar rise after a meal | Mixed: daily glucose unchanged; the after-meal response rose slightly in one trial and fell in another |
| Appetite | One trial: less food eaten later; another: no change in hunger | Mixed results |
| Autophagy | Switched on by amino acid starvation; never measured after collagen | Unknown |
If your plan is about calories, the answer is arithmetic. If it is about a hormone or a cell process, the answer is mostly "not tested", as the sections below explain.
Sources: Health effects of fasting review, 2020 (Europe PMC); Caloric restriction and fasting review, 2021 (Europe PMC); Collagen peptides and appetite after exercise trial, 2025 (PubMed); Porcine collagen hydrolysate and glucose trial, 2025 (PubMed); Collagen hydrolysate glucose tolerance study, 2024 (PubMed); Whey or collagen and appetite pilot trial, 2022 (PubMed); Amino acids and autophagy review, 2015 (PubMed)
How many calories are in collagen?
We found no USDA entry named collagen peptides, so the closest official figure is for dry unsweetened gelatin: 335 kcal and 85.6 g of protein per 100 g.
Gelatin is used here only because it is the closest USDA entry. No source we used states how gelatin relates to collagen peptides, or how closely their nutrition matches. Treat the gelatin figures as a stand-in, and read the panel on your own product for its real numbers.
US labeling rules give a second way to estimate. Calories may be worked out with general factors of 4 calories per gram of protein, 4 per gram of carbohydrate and 9 per gram of fat. Collagen is a protein, so the protein factor of 4 per gram is the one that applies to it.
Sources: USDA FoodData Central: dry unsweetened gelatin; 21 CFR 101.9 nutrition labeling (eCFR)
Calories by amount, using the gelatin figures
Even a small amount is above zero. We scaled the USDA gelatin figures to three amounts: 1 g, and the 5 g and 15 g amounts used in the trials below.
| Amount of dry powder | Calories (approx.) | Protein (approx.) |
|---|---|---|
| 1 g | About 3 kcal | About 0.9 g |
| 5 g | About 17 kcal | About 4.3 g |
| 15 g | About 50 kcal | About 12.8 g |
These amounts are for the arithmetic only, not a suggestion of how much to take. For what collagen is and what it was tested for, see our collagen peptides guide, and for how it compares with another protein powder, see collagen vs whey protein.
Can a "0 calories" label still break a fast?
Yes, on a strict fast. US labeling rules say amounts less than 5 calories may be shown as zero. So a product that reads 0 calories can still contain a few. By the gelatin figures above, 1 g of dry powder has about 3 kcal, which is under that line.
Sources: 21 CFR 101.9 nutrition labeling (eCFR); USDA FoodData Central: dry unsweetened gelatin
Does collagen raise insulin or blood sugar?
In the trials we found, collagen raised insulin but did not raise daily blood sugar. If your fast is meant to keep insulin low, that matters.
A 2025 randomized trial in physically active women gave 15 g a day of collagen peptides or a taste-matched control with no energy, for 7 days. Taken after exercise, collagen raised GLP-1 and insulin levels and cut the amount of food eaten at a later meal.
A 4-week randomized trial of porcine collagen hydrolysate in men and women with overweight or obesity found no change in day-to-day glucose. It did find rises in glucose swings, the blood sugar response after meals, fasting insulin and two insulin-related scores (HOMA-IR and HOMA-β). The authors said these changes were small, with limited clinical relevance.
A third, early study points the other way on blood sugar. A specific collagen hydrolysate called H80 reduced the blood sugar response after a meal at a 5 g dose in healthy, normal and prediabetic people, in what the authors call a first proof of concept study. None of these trials gave collagen during a fast.
Sources: Collagen peptides and appetite after exercise trial, 2025 (PubMed); Porcine collagen hydrolysate and glucose trial, 2025 (PubMed); Collagen hydrolysate glucose tolerance study, 2024 (PubMed)
Does collagen help with hunger during a fast?
The two trials we found disagree. One saw less food eaten later; the other saw no change in hunger at all.
The 2025 trial above found collagen after exercise reduced how much women chose to eat at the next meal. A 2022 crossover pilot trial in healthy women compared whey protein and collagen. It found no difference between groups in hunger, desire to eat, fullness or food eaten. Neither trial was run during a fasting window.
Sources: Collagen peptides and appetite after exercise trial, 2025 (PubMed); Whey or collagen and appetite pilot trial, 2022 (PubMed)
Does collagen stop autophagy?
No one knows: no study has measured autophagy after collagen. The reasoning that it might comes from cell biology, and the human data are limited.
Autophagy is the cell's clean-up process. A review of cell signaling explains that autophagy is switched on by amino acid starvation through two key pathways. Collagen is protein, and protein is built from amino acids, which is why people expect it to interfere.
A 2018 review found that fasting and calorie restriction both turn up autophagy, with evidence of it in many tissues in response to food deprivation. But human data are thin. A 2022 study protocol says autophagy has been hard to measure in people. A 2025 analysis notes that nutrient restriction stimulates autophagy in animal studies, but that this had never been shown in humans. Its own exploratory result suggests autophagy may be increased by intermittent nutrient restriction in humans.
One 2026 randomized crossover trial in healthy adults cuts against the simple story. Reducing protein intake did not change autophagic flux in blood. So whether a small amount of collagen during a fast changes autophagy in people is an open question.
Sources: Amino acids and autophagy review, 2015 (PubMed); Fasting and autophagy review, 2018 (PubMed); Break-Fast study protocol, 2022 (PubMed); Time-restricted eating and autophagy analysis, 2025 (PubMed); Reduced protein and autophagy trial, 2026 (PubMed)
When should I take collagen when intermittent fasting?
No study we found tested the timing of collagen around a fasting window, so there is no evidence-based best time.
What follows from the sources is simple. Collagen has calories, so inside a zero-calorie window it is outside the rules, while inside the eating window it counts as part of that day's food. For a plan that allows some calories on fasting days, such as about 25% of usual intake, it counts toward that allowance. Whether timing changes any benefit of collagen itself has not been studied. If you are wondering about skin side effects too, see can collagen cause breakouts.
Sources: Health effects of fasting review, 2020 (Europe PMC); Caloric restriction and fasting review, 2021 (Europe PMC)
Common questions
When should I take collagen when intermittent fasting?+
No study we found tested collagen timing around a fasting window. What the sources do show: collagen is protein with calories, so on a zero-calorie fast it falls outside the rules. Taken inside the eating window, it is part of the food for that day.
How many calories are in collagen powder?+
We found no USDA entry for collagen peptides. The closest entry is dry unsweetened gelatin, which USDA lists at 335 kcal and 85.6 g of protein per 100 g. Check your own product label, since products vary.
Does collagen spike insulin?+
It can raise insulin. In a 2025 trial, collagen peptides after exercise raised GLP-1 and insulin. In a 4-week trial, collagen hydrolysate raised fasting insulin a little but did not change daily glucose, and the authors called the changes small.
Does collagen break autophagy?+
No study has measured autophagy after collagen. Autophagy is switched on in cells by amino acid starvation, and collagen is protein. But a 2026 trial found that eating less protein did not change autophagy in blood cells, so the effect of a small amount is unknown.
Does collagen with 0 calories on the label break a fast?+
US label rules let amounts under 5 calories be shown as zero. So a zero on a label can mean a small number of calories, not none at all.
Sources: USDA FoodData Central: dry unsweetened gelatin; 21 CFR 101.9 nutrition labeling (eCFR); Collagen peptides and appetite after exercise trial, 2025 (PubMed); Porcine collagen hydrolysate and glucose trial, 2025 (PubMed); Amino acids and autophagy review, 2015 (PubMed); Reduced protein and autophagy trial, 2026 (PubMed)
Related reading
This guide is for education only and is not medical or diet advice. It describes what labeling rules, USDA data and published studies say about collagen and fasting. Talk to a licensed clinician before changing how you eat or fast.