Whether something breaks your fast depends on why you’re fasting. A splash of cream in your coffee will spike your insulin differently than it affects autophagy, and a zero-calorie sweetener might trigger hormonal responses even without actual calories. The short answer: anything with calories technically breaks a fast, but the practical answer is more nuanced than that.
Most people fasting for weight loss, blood sugar control, or cellular repair care about two things: whether something raises insulin and whether it stops fat burning. Here’s what the evidence says about the most common things people consume during a fasting window.
Drinks That Keep Your Fast Intact
Plain water, black coffee, and unsweetened tea are the safest choices. They contain zero calories and don’t trigger a meaningful insulin response. Sparkling water and mineral water are equally fine.
Black coffee actually supports some fasting goals. It promotes fat oxidation and can make hunger more manageable. Just keep it black. Adding sugar, milk, cream, or flavored syrups introduces calories and protein that will shift your body out of a fasted state.
Artificial Sweeteners: A Gray Area
Diet sodas, zero-calorie drink mixes, and sweetener packets are where things get complicated. They contain no calories, but your body doesn’t always respond to them that way.
Your brain can trigger a small burst of insulin just from tasting something sweet, a reflex called the cephalic phase insulin response. Some studies have found that sucralose triggers this response in certain people, particularly those with overweight or obesity, while others show no effect at all. In one study, aspartame alone didn’t raise insulin levels, but when participants could also see and smell a palatable meal, the combination did produce a response. The takeaway: artificial sweeteners don’t reliably break a fast in everyone, but they can in some people, especially in the context of food cues.
If your primary goal is weight loss and you need a diet soda to get through your fasting window, the tiny potential insulin blip is unlikely to cancel out the benefits of not eating for 16 hours. If you’re fasting specifically for insulin sensitivity or metabolic reset, plain water is the cleaner choice.
What About Stevia?
Stevia is calorie-free and plant-derived, but it still activates sweet taste receptors. The research on whether it triggers an insulin response mirrors the broader sweetener data: inconsistent results across studies, with some people responding and others not. It’s a better option than sugar, but not guaranteed to be metabolically invisible.
Lemon Water and Apple Cider Vinegar
A squeeze of lemon in water adds roughly 1 to 3 calories. That’s not enough to meaningfully raise insulin or halt fat burning. Lemon water is generally considered safe during a fast.
Apple cider vinegar is similarly low-calorie (about 3 calories per tablespoon) and may actually support fasting goals. Acetic acid, the active compound in vinegar, has been shown to blunt blood sugar spikes when consumed around meals and can promote feelings of fullness. Some people drink it diluted during a fast for this reason. It won’t break your fast, but be cautious if you take diabetes medications, since vinegar can influence blood sugar and potassium levels in ways that interact with those drugs.
Fat, Butter, and MCT Oil
Bulletproof coffee (black coffee blended with butter or MCT oil) is popular in fasting communities because pure fat produces a minimal insulin response compared to protein or carbohydrates. A tablespoon of butter has about 100 calories, all from fat.
Here’s the tradeoff: pure fat won’t spike your insulin much and can keep you in ketosis, but it absolutely provides calories, which means your body is processing energy rather than relying entirely on stored fuel. Fat also activates a cellular nutrient-sensing pathway that inhibits autophagy, the cellular cleanup process many people fast to promote. If you’re fasting for weight loss and bulletproof coffee helps you stick to a shorter eating window, it’s a reasonable compromise. If you’re fasting for autophagy or deep metabolic benefits, it counts as breaking your fast.
Protein and Amino Acids
Protein is the clearest fast-breaker after carbohydrates. Even small amounts of protein stimulate insulin release and activate a growth-signaling pathway in your cells called mTOR that directly shuts down autophagy. This is why bone broth, collagen powder, and protein shakes all break a fast, even if they’re low in calories.
BCAAs (branched-chain amino acids) deserve a specific mention because many people take them as a workout supplement. BCAAs contain calories and are, by definition, amino acids. They stimulate muscle protein synthesis, which is the opposite of the catabolic state your body enters during a fast. They will interrupt a fasted state. Some people argue that the 20 or so calories in a BCAA supplement still preserve most fasting benefits, but if you’re being strict about your fast, take them in your eating window instead.
Sugar-Free Gum and Mints
A piece of sugar-free gum typically contains 1 to 5 calories, usually from sugar alcohols like xylitol or sorbitol. Xylitol scores a 7 on the glycemic index compared to regular sugar’s 60, meaning its effect on blood sugar is negligible. One or two pieces of gum during a fast won’t produce a meaningful metabolic disruption.
That said, chewing gum does stimulate digestive enzymes and stomach acid production. If you’re fasting partly to give your digestive system a rest, that’s worth considering. For most people fasting for weight loss or blood sugar control, a piece of sugar-free gum is fine.
Supplements and Medications
Most medications won’t break your fast from a metabolic standpoint, and you should never skip prescribed medications to preserve a fasting window.
For supplements, the key question is whether they contain calories or need fat for absorption:
- Water-soluble vitamins (vitamin C, B vitamins) are calorie-free and fine to take during a fast, though some people find they cause nausea on an empty stomach.
- Fat-soluble vitamins (vitamins A, D, E, K) and fish oil capsules need dietary fat to be absorbed properly. Taking them during a fast means you’ll absorb less of the nutrient. Save these for your eating window.
- Creatine is calorie-free and doesn’t trigger an insulin response. It won’t break your fast.
- Electrolytes (sodium, potassium, magnesium) without added sugar are safe during a fast and can actually help prevent the headaches and lightheadedness that sometimes come with longer fasting periods.
The Practical Threshold
If you want a simple rule: anything under roughly 10 calories with no protein and no carbohydrates is unlikely to produce a hormonal response significant enough to cancel out the benefits of your fast. This covers black coffee, plain tea, a squeeze of lemon, apple cider vinegar, and sugar-free gum.
Once you introduce protein, carbohydrates, or enough calories to require digestion, your body shifts from a fasted state into a fed state. The speed of that shift depends on what you consume. A handful of nuts will raise insulin less dramatically than a glass of orange juice, but both end the fast. Pure fat sits in the middle, preserving some fasting benefits (low insulin, continued ketosis) while canceling others (autophagy, complete digestive rest).
Your fasting goal determines where you draw the line. For weight loss, keeping insulin low is what matters most, and small amounts of fat or a zero-calorie sweetener are unlikely to derail your progress. For autophagy and cellular repair, even amino acids and calories from fat can interrupt the process. Match your strictness to your objective.