If you do intermittent fasting, you have probably wondered whether your morning creatine, collagen scoop, electrolyte drink or multivitamin "breaks" the fast. The internet's answers are confident and contradictory, and many come from brands that sell those products. The honest answer: you can take many supplements while fasting, but whether one "breaks" the fast depends on what you want the fast to do, and for several popular claims we found no human evidence either way.
This guide goes through the common supplements one by one, sorted by the goal of your fast: weight loss, blood sugar and insulin, or autophagy and "longevity." It also covers the very different rules for fasting before a blood test or surgery, and who should not fast at all.
The short version
- For weight loss, calories decide. In most head-to-head trials, time-restricted eating (eating only within a set window each day) did about as well as ordinary calorie cutting; in a year-long US trial, both groups ended up eating about 400 calories a day less. A supplement with a few calories will not undo that.
- For insulin, what you take matters more than the calories. Plain creatine did not raise insulin in a trial, while 10 grams of collagen taken alone did. Protein and amino acids raise insulin most when they come with carbohydrate.
- Usually fine in a fasting window: water, black coffee, plain tea, plain electrolytes, plain creatine and diet sweeteners (which acted like water in short trials).
- Better with your first meal: fish oil and vitamin D (absorbed better with fat), multivitamins, and gummies, which contain sugar.
- Autophagy claims are unproven in people. We found no human study showing that any supplement or calorie amount switches autophagy off.
- Diabetes medicines need a plan first. If you take insulin or sulfonylureas (a type of diabetes pill), fasting can cause dangerously low blood sugar, so talk to your clinician before fasting or changing doses; see who should not fast.
- Medical fasts are different. Before a fasting blood test, plain water only, no coffee or gum, and tell the lab about supplements, especially biotin. Before surgery, follow your surgical team's instructions.
It depends on why you are fasting
"Breaking a fast" is not one thing. A supplement can be irrelevant for one goal and matter for another:
- Weight loss. What counts is how much you eat over the day. In a year-long trial in China, adding an 8-hour eating window to calorie restriction did not lead to significantly more weight loss than calorie restriction alone. In a year-long US trial, people on time-restricted eating ate about 425 fewer calories a day and a calorie-counting group about 405 fewer, and both lost a similar amount of weight. A 2026 Cochrane review (a systematic summary of the trials) found that intermittent fasting may make little to no difference in weight loss compared with regular diet advice (low-certainty evidence). One exception: in a 14-week trial, eating only between 7 a.m. and 3 p.m. on top of calorie cutting gave about 2.3 kg (about 5 pounds) more weight loss than eating over 12 or more hours. A 5 to 40 calorie supplement is small against a deficit of several hundred calories (our arithmetic).
- Blood sugar and insulin. Insulin is the hormone that moves sugar from your blood into your cells; it rises after you eat and falls while you fast. Anything with carbohydrate or protein can raise it, while things like creatine, plain electrolytes and plain pills mostly do not. One small controlled-feeding study in men with prediabetes found that eating early in the day improved insulin sensitivity (how strongly your body responds to insulin) without weight loss, so timing is not irrelevant. A larger 14-week trial of early eating, however, found no difference in glucose or insulin levels.
- Autophagy and "longevity." Autophagy is the cell's recycling process. The evidence that fasting boosts it comes mostly from animals, and human studies mostly measure it in blood cells, which may not reflect other tissues (see below).
- A medical fast before a blood test or surgery has strict rules of its own (see below).
- A religious fast follows its own rules, which differ between traditions. Follow your tradition and its authorities; this guide does not give religious rulings.
Supplements at a glance
| Supplement | Weight-loss fast | Strict or insulin fast |
|---|---|---|
| Water, black coffee, plain tea | Fine | No calories; caffeine may briefly lower insulin sensitivity |
| Plain electrolytes (no sugar) | Fine | Fine |
| Creatine monohydrate in water | Fine | Fine; did not raise insulin |
| Diet sweeteners (sucralose, stevia) | Fine | Acted like water in short trials; one 2-week sucralose trial lowered insulin sensitivity |
| Collagen peptides (about 10 g) | Small (about 40 calories) | Breaks it; raised insulin |
| Bone broth | Adds calories | Breaks it |
| Protein shakes, BCAAs | Adds calories | Breaks it |
| Fish oil capsules | Small (about 9 calories per 1 g capsule) | Small; absorbed better with a meal |
| MCT oil, ketone drinks | Adds calories | Adds calories; raises ketones |
| Multivitamin tablets | Fine | Fine; often better with food |
| Gummy vitamins | Adds sugar | Breaks it |
| Apple cider vinegar | A few calories | Little evidence either way |
"Fine" means it adds few or no calories and has no known insulin effect for that goal, not that it has been tested inside a fasting window. "Strict" means a clean fast: nothing that adds meaningful calories or raises insulin. Flavored products can contain sugar or maltodextrin, so check the label.
Does creatine break a fast?
Plain creatine monohydrate has no calories, and in a trial of 24 men, creatine dissolved in water "had no effect on serum insulin concentration," while creatine taken with a large dose of sugar raised insulin sharply. For a weight-loss or insulin-focused fast, a plain scoop of creatine in water is fine.
Taking creatine with a lot of carbohydrate can increase how much the muscles store: in the same trial of 24 men, adding about 93 grams of sugar to each dose made the rise in muscle creatine 60 percent greater over a 5-day loading phase. A second small study found about 25 percent more creatine kept in the body over one day when each dose came with about 50 grams each of protein and carbohydrate, although the link between insulin and creatine uptake weakened across that day's doses. We found no study comparing creatine taken in a fasting window with creatine taken with meals over weeks of regular use. Flavored creatines and pre-workouts may contain sugar, so read the label. See creatine benefits, dosing and myths.
Does collagen or bone broth break a fast?
Collagen peptides are protein, about 40 calories in a 10 gram scoop by our arithmetic (4 calories per gram of protein). In the one study we found that measured collagen taken alone after an overnight fast, 10 grams raised insulin by about 7 microunits per milliliter, compared with almost no change on placebo. That study tested one patented collagen product, chosen because it stimulates gut hormones. It was funded by the collagen maker, and half of its authors work for the company, although an outside research firm ran it.
So collagen breaks a fast if your goal is to keep insulin low, and it is a small amount of calories if your goal is weight loss. If you want to keep your window "clean," move it to your first meal. Bone broth is food too, with protein and calories, and its amino acid content varies widely between recipes. It breaks a strict fast; for a weight-loss fast, count it toward your calories.
Do BCAAs and protein break a fast?
Protein raises insulin, and whey does so strongly: in small studies a whey drink or meal produced much larger insulin responses than matched controls. Branched-chain amino acids (BCAAs), especially leucine, help drive that response. But the big insulin rises in those studies came when amino acids were taken with glucose. In 13 healthy adults after a 12-hour fast, leucine taken alone raised insulin only "modestly" and did not change blood sugar.
BCAAs and protein still supply calories (about 4 per gram) and amino acids, which signal the body that food has arrived. For any strict fast, they break it. For a weight-loss window, a protein shake is a small meal, and it is usually better to count it as your first one. See our ranking of BCAA and EAA supplements.
Do electrolytes break a fast?
Plain sodium, potassium and magnesium contain no calories and have no known effect on insulin, so plain electrolytes do not break a fast for weight or insulin goals. Many electrolyte drink mixes contain sugar or maltodextrin, which do.
Do you need them? For a typical 16 to 24 hour fast, we found no trial testing electrolyte supplements for fasting headaches or cramps. In medically supervised multi-day fasts, blood sodium drifted down slightly. In a review of 768 supervised water-only fasts, one person was hospitalized for severe low sodium and one for dehydration. That is why fasts longer than a day or two belong under medical supervision. People on salt-restricted diets, with kidney disease, or taking medicines that raise potassium should not add sodium or potassium supplements without advice. See do you need electrolytes?
Do vitamins break a fast?
A multivitamin tablet has trivial calories, so it does not break a fast for weight loss. But taking it with your first meal is usually the better choice:
- Fat-soluble vitamins absorb better with fat. In 50 older adults, a single dose of vitamin D reached a 32 percent higher peak when taken with a meal containing fat than with a fat-free meal.
- Upset stomach. Many people find vitamins and minerals harder on the stomach without food, although we found no trial measuring this for multivitamins. High doses of zinc can cause nausea.
- Iron is often advised away from meals, and especially away from coffee and tea, which cut iron absorption from a bread meal by roughly 50 to more than 90 percent in one study. Iron pills commonly cause stomach upset, so if it bothers you, taking it with a little food is a common compromise.
- Gummy vitamins contain sugar, so they break a fast for every goal.
Fasting regimens can lower intake of some nutrients, such as calcium, magnesium, potassium and some B vitamins, according to a 2025 review, so a multivitamin can make sense; just take it with food. Probiotic survival was best with or just before a meal in a lab model, though we found no human trial on probiotic timing.
Does fish oil or MCT oil break a fast?
Fish oil. A 1 gram capsule has about 9 calories, which is small. But fish oil, especially the ethyl ester form used in many products, is absorbed much better with fat in the meal: in one study, ethyl ester absorption roughly tripled with a high-fat meal. So there is little reason to take fish oil in your fasting window.
MCT oil. MCT oil is pure fat, roughly 160 calories in a 20 ml serving by our arithmetic. Ketones are fuels the liver makes from fat when carbohydrate is scarce, as during a fast, and ketosis simply means your blood ketones are raised. MCT oil raises ketones, about twice as much without a meal in one study, so it does not stop ketosis, but it clearly breaks a calorie fast.
Ketone drinks. Ketone esters and ketone salts raise blood ketones and lowered blood sugar in small studies, including ones by researchers who hold patents on ketone drinks and direct a company developing them. They are a fuel, so they supply energy rather than keeping you fasted.
Does black coffee break a fast? Plus tea and sweeteners
Black coffee and plain tea have almost no calories. In a small trial of 10 healthy adults after a 10-hour fast, black coffee did not change blood sugar or blood fats compared with water (insulin was not measured). Caffeine can reduce insulin sensitivity for a while: by about 15 percent when given into a vein in one lab study. That may slightly affect your next meal's blood sugar, but for weight loss black coffee is fine. Cream, sugar, butter or MCT oil in coffee add calories. If coffee on an empty stomach gives you jitters or heartburn, see our guide to coffee sensitivity.
Diet sweeteners. A 2023 analysis of 36 short trials found that drinks sweetened with sucralose, stevia or other non-calorie sweeteners, taken alone, had no effect on blood sugar or insulin, similar to water. Several of its authors had ties to the sweetener industry. A smaller study in 17 people with obesity found that sucralose taken just before a sugar drink increased the insulin response to that sugar by about 20 percent. A two-week trial found a modest drop in insulin sensitivity, so the long-term picture is not settled. Erythritol carries a separate question: observational studies linked higher blood levels to heart attack and stroke risk, and it increased platelet clotting activity in small experiments, which shows a link, not proof of harm.
Does apple cider vinegar break a fast? Plus fiber and berberine
Apple cider vinegar has a few calories per tablespoon. A tablespoon diluted in water barely touches a weight-loss fast, but many ACV gummies add sugar, which does break a fast. Vinegar's blood sugar effects come from taking it with a carbohydrate meal, and we found no evidence that it does anything for the fast itself. It also slows stomach emptying, which may be a problem for people with diabetes-related gastroparesis. A widely reported 2024 weight-loss trial of apple cider vinegar was retracted in 2025. See apple cider vinegar gummies vs liquid.
Psyllium and other fiber supply little usable energy and are designed to be taken with water and meals; we found no fasting-window studies. Berberine lowered blood sugar in trials in people with type 2 diabetes and caused stomach upset in about a third of them, so it makes more sense around meals, and it may add to the risk of low blood sugar if you take diabetes medicines while fasting.
Do supplements break autophagy?
Autophagy is the process cells use to break down and recycle their own parts. Fasting reliably boosts it in mice, and that research is where most online claims come from. In people, the evidence is thin. A 2026 review by autophagy researchers concluded that human data are "much more limited" and that the few studies measuring the actual rate of autophagy have often been negative or inconclusive. Even "how long a human must fast for in order to activate autophagy" remains "unresolved," the authors wrote. Mice start to die after about 3 days without food, while humans can survive weeks, so mouse timelines do not translate.
On a cellular level, amino acids such as leucine switch on mTOR, a growth signal that tells cells to build rather than recycle. That signal suppresses autophagy, which is why people worry about collagen, BCAAs and protein. But in a 2026 trial in healthy adults, cutting protein from 20 to 10 percent of calories for 4 weeks, without cutting total calories, did not change autophagy measured in blood cells. Single-meal studies are mixed. The 2026 review describes one human study in which an autophagy marker in some blood cells fell after a high-protein meal, and another that measured the actual rate of autophagy and found no change after a high-protein drink. In PubMed searches through November 2026, we found no human study showing that a specific number of calories, a specific number of grams of protein, or a supplement such as collagen, BCAAs, creatine, fish oil or coffee switches autophagy off.
If autophagy is your goal, the most cautious approach is to keep protein and amino acid supplements for your eating window, while knowing that the benefit itself has not been shown in people.
How many calories break a fast? The 50-calorie rule
We found no scientifically tested cutoff. The idea that anything under 50 calories keeps you fasted is an internet rule of thumb, and we found no research source for it. Research "fasts" often allow food: many alternate-day fasting trials permit up to 500 calories on fast days, a clinic's supervised fasting program provides 200 to 250 calories a day and still produces ketosis, and the commercial "fasting mimicking diet" provides roughly 700 to 1,100 calories a day. What matters depends on the goal: total calories for weight, and the type of nutrient for insulin.
The same logic settles "dirty" versus "clean" fasting. Clean fasting means only water, black coffee or plain tea during the window; dirty fasting is an informal term for allowing a small amount of calories, such as a splash of cream in coffee. For weight loss, those calories simply count toward your daily total. For an insulin-focused fast, clean is the more cautious choice.
Fasting before blood tests and surgery
A medical fast is stricter than any lifestyle fast, and the rules exist to keep your results accurate and your procedure safe.
- Fasting blood tests. According to MedlinePlus, fasting means nothing but plain water, usually for 8 to 12 hours: no coffee, juice or flavored water, and no gum. Ask your provider whether to take your usual medicines, do not stop them unless told to, and tell your provider about vitamins and supplements.
- Biotin. High-dose biotin, common in hair, skin and nail products, can cause falsely high or low results on some lab tests, including a falsely low result for troponin, a heart attack test. The FDA says there is not enough information to know whether stopping biotin for any number of hours before testing prevents this, so tell your doctor and the lab if you take it.
- Before surgery. For healthy adults having elective procedures, the American Society of Anesthesiologists allows clear liquids, including black coffee and clear tea, up to 2 hours before anesthesia, and a light meal up to 6 hours before. Milky protein shakes are not clear liquids, and the society's 2023 update found too little evidence to recommend clear protein-containing drinks over other clear liquids, so ask your team before having one. Its guidelines do not cover supplement pills, so follow your own surgical team's instructions, which are often stricter.
- Supplements and surgery. The society's patient guidance warns that some supplements can increase bleeding or interact with anesthesia, and says that in some cases you may be asked to stop supplements at least two weeks before a procedure. Tell the team about everything you take.
Who should not fast
- Diabetes medicines. If you take insulin or sulfonylureas (diabetes pills such as glipizide or glyburide), fasting can cause dangerously low blood sugar. In a 12-week trial of people with type 2 diabetes eating about 500 to 600 calories on two days a week, low blood sugar was about twice as common despite lower medication doses. Do not fast or change doses without your clinician. Meglitinides such as repaglinide can also cause low blood sugar. SGLT2 inhibitors make you pass more water, so on fasting days, when you also get less fluid from food, ask whether the dose should change to avoid dehydration.
- Others who should avoid intermittent fasting, according to expert guidance, include people who are pregnant or breastfeeding, young children, adults of advanced age and frail older adults, people with eating disorders, people with dementia, and people with weakened immune systems, including transplant recipients on immune-suppressing medicines.
- Longer fasts of several days carry risks such as dehydration and low sodium, and restarting food after a long fast needs care. Do them only with medical supervision.
- Some medicines must be taken with food. Do not skip or move medicine doses to fit a fasting window without asking your pharmacist or doctor.
Frequently asked questions
Do supplements break a fast?
You can take many supplements while fasting, but whether one breaks the fast depends on your goal. For weight loss, what matters is total calories, so low-calorie supplements make little difference. For keeping insulin low, plain creatine, plain electrolytes, black coffee and diet sweeteners look fine, while collagen, protein, BCAAs and gummies do not. For autophagy, human evidence is limited and mixed, and we found no study showing that a supplement switches it off.
Does creatine break a fast?
Plain creatine monohydrate has no calories, and in a trial it did not raise insulin when taken in water. It does not break a fast for weight or insulin goals. Flavored creatines and pre-workouts may contain sugar, so check the label.
Does collagen break a fast?
For an insulin-focused or strict fast, yes. A 10 gram scoop is about 40 calories of protein, and in one study funded by a collagen maker, 10 grams of its patented collagen taken alone after an overnight fast raised insulin. For a weight-loss fast it is a small amount of calories.
Does bone broth break a fast?
Yes, for a strict or insulin-focused fast. Bone broth is food: it contains protein and calories, and its amino acid content varies widely between recipes. For a weight-loss fast, count it toward your calories or have it with your first meal.
Do electrolytes break a fast?
Plain sodium, potassium and magnesium have no calories and no known effect on insulin, so they do not break a fast for weight loss or insulin goals. Many electrolyte mixes contain sugar or maltodextrin, which do. People with kidney disease or on medicines that raise potassium should ask before adding electrolytes.
Can you take vitamins while fasting?
Multivitamin tablets have trivial calories, but they are usually better with your first meal: fat-soluble vitamins such as vitamin D absorb better with fat, and many people find vitamins harder on an empty stomach. Gummy vitamins contain sugar and break a fast.
Does fish oil break a fast?
A 1 gram capsule has about 9 calories, so it barely affects a weight-loss fast. But fish oil is absorbed much better with a meal containing fat, so there is little reason to take it during the fast.
Do BCAAs break a fast?
Yes, for a strict or insulin-focused fast. BCAAs supply calories and amino acids that signal food to the body. Leucine alone raised insulin only modestly in a small study, with much larger rises when amino acids came with carbohydrate.
Does apple cider vinegar break a fast?
A tablespoon diluted in water has only a few calories, so it makes little difference to a weight-loss fast. Its blood sugar effects were seen when it was taken with a carbohydrate meal, and we found no evidence that it helps the fast itself. Many ACV gummies add sugar, which does break a fast. In a small study, vinegar slowed stomach emptying in people with diabetes-related gastroparesis, so they should ask their doctor first.
Does black coffee break a fast?
Black coffee has almost no calories and did not change fasting blood sugar in a small trial, so it is fine for a weight-loss fast. Cream, sugar, butter or MCT oil add calories. Before a fasting blood test, skip coffee and drink only water unless told otherwise.
How many calories break a fast?
We found no scientifically tested cutoff. The 50-calorie rule is an internet rule of thumb. Many research fasting programs allow some food on fast days, from up to 500 calories in alternate-day fasting to roughly 700 to 1,100 in the fasting mimicking diet. What matters depends on your goal: total calories for weight, and the type of nutrient for insulin.
What is the difference between dirty and clean fasting?
Clean fasting means only water, black coffee or plain tea during the fasting window. Dirty fasting is an informal term for allowing a small amount of calories, such as a splash of cream in coffee. For weight loss, those calories simply count toward your daily total; for an insulin-focused fast, clean fasting is the more cautious choice.
Do supplements break autophagy?
No one knows. Fasting boosts autophagy in mice, but in people the evidence is limited, measurements in people mostly rely on blood cells, and we found no human study showing that any calorie amount or supplement switches it off.
Can you take supplements before a fasting blood test?
Follow your provider's instructions. MedlinePlus advises plain water only for a fasting blood test and telling your provider about vitamins and supplements. High-dose biotin can distort some lab results, including a heart attack test, so mention it before testing.
Is intermittent fasting safe if you take diabetes medicine?
Not without a plan from your clinician. If you take insulin or sulfonylureas (diabetes pills such as glipizide or glyburide), fasting can cause dangerously low blood sugar. In a 12-week trial of people with type 2 diabetes eating about 500 to 600 calories on two days a week, low blood sugar was about twice as common despite lower medication doses. Talk to your clinician before fasting or changing doses, and remember that glucose-lowering supplements such as berberine may add to the risk.
The bottom line
Whether a supplement breaks your fast depends on why you fast. For weight loss, a few calories from collagen, fish oil or apple cider vinegar make little difference, because results come from total calories. For keeping insulin low, plain creatine, electrolytes, black coffee and diet sweeteners look fine, while collagen, protein and BCAAs do not. Autophagy claims, including any calorie cutoff, have not been shown in people.
Practical rules: keep plain creatine, plain electrolytes and black coffee in your window if you like, move protein, collagen, fish oil, fat-soluble vitamins and gummies to your first meal, follow medical fasting instructions exactly, and talk to your doctor before fasting if you take diabetes medicines or fall into a higher-risk group.