Any food or drink containing calories can technically break your fast, but the real answer depends on what you’re fasting for. Someone fasting for fat loss has different thresholds than someone fasting to promote cellular cleanup (autophagy). A splash of cream in your coffee and a bowl of oatmeal both “break” a fast, but they don’t break it in the same way or to the same degree. Here’s what actually matters.
Why Your Fasting Goal Changes the Answer
Fasting triggers several distinct metabolic shifts. After hours without food, your body exhausts its sugar stores and begins burning fat for fuel. Separately, fasting suppresses the growth-signaling pathway in your cells, which allows autophagy (your body’s internal recycling system) to ramp up. These processes have different triggers and different breaking points.
If your goal is fat loss, the main concern is anything that raises insulin significantly, because insulin tells your body to store energy rather than burn it. Small amounts of calories, especially from fat, may not meaningfully spike insulin. If your goal is autophagy, even small amounts of protein or amino acids can reactivate growth signaling and slow that cellular cleanup. And if you’re fasting for blood sugar control, anything that raises glucose matters, even if the calorie count is low.
The “50-Calorie Rule” and Why It’s Fuzzy
You’ve probably heard that anything under 50 calories won’t break your fast. There is no peer-reviewed study that validates this specific number. It likely originated from the observation that very small caloric intakes don’t produce a measurable insulin spike in most people, but individual responses vary. The safest assumption: any calories can interrupt fasting to some degree. Zero calories is the only guarantee.
Drinks That Are Safe During a Fast
Water, black coffee, and plain tea are the gold standard. They contain zero calories and don’t trigger an insulin response. Sparkling water is fine too, as long as it has no added sweeteners or flavors.
Apple cider vinegar is a common question. One tablespoon contains about 3 calories and less than 1 gram of carbs, which is unlikely to affect your fast. Some evidence actually suggests vinegar may help lower blood sugar levels and support the goals people are fasting for in the first place. Dilute it in water if you use it, since straight vinegar is harsh on your teeth and stomach lining.
A squeeze of lemon in your water adds roughly 1 to 3 calories. That’s negligible for fat-loss fasting, though purists avoiding all caloric intake will want to skip it.
Drinks That Will Break Your Fast
Anything with sugar, milk, cream, or significant calories ends your fasted state. That includes:
- Juice and smoothies: Even “healthy” green juices contain enough sugar to spike insulin immediately.
- Milk and cream in coffee: A tablespoon of heavy cream has about 50 calories and will blunt fat burning. A splash of milk (whole, oat, almond with sweetener) has a similar effect.
- Bone broth: Often recommended during fasts, but a cup typically contains 30 to 50 calories plus protein. The protein content is enough to activate growth signaling and interrupt autophagy. It’s a reasonable choice if you’re easing into fasting and need something to get through the day, but it does break a strict fast.
- Coconut water and sports drinks: Both contain sugar and calories.
Artificial Sweeteners: A Gray Area
Diet sodas, zero-calorie drink mixes, and sweeteners like stevia, monk fruit, and sucralose are technically calorie-free, which makes them tempting during a fast. The concern has been that sweet taste alone might trigger a “cephalic phase insulin response,” where your body releases insulin just from tasting something sweet, before any sugar hits your bloodstream.
Research on this has been mixed. A study in The American Journal of Clinical Nutrition found no cephalic phase insulin response when participants tasted sweet tablets (including those with the artificial sweetener aspartame) for five minutes. Plasma insulin did not rise. That said, some sweeteners in powder or liquid form contain fillers like maltodextrin or dextrose that do contain calories and raise blood sugar. Check the ingredients rather than trusting the “zero calorie” label on the front.
For fat-loss fasting, plain artificial sweeteners are unlikely to cause problems. For autophagy-focused fasting, the safest bet is to avoid them entirely, since the research isn’t conclusive enough to give a clear pass.
Supplements and Medications
Most plain capsule or tablet supplements are fine during a fast. A standard multivitamin pill, fish oil capsule, or magnesium tablet contains minimal calories from binders and fillers, not enough to meaningfully raise insulin in most people.
Gummy vitamins are a different story. They typically contain 2 to 5 grams of sugar per serving and 10 to 20 calories, often from corn syrup or sugar alcohols. That’s enough carbohydrate to spike insulin and halt fat burning. If you take gummy vitamins, save them for your eating window.
Watch for supplements that list maltodextrin, dextrose, or sugar alcohols in their ingredients. These are common binders and flavor enhancers that raise blood glucose. Protein powders, BCAAs (branched-chain amino acids), and collagen supplements all contain enough protein and calories to break a fast outright. BCAAs are particularly worth noting because they directly activate the growth-signaling pathway that suppresses autophagy.
Medications generally take priority over fasting. If your doctor prescribed something that needs to be taken with food or on a specific schedule, follow those instructions rather than adjusting around a fasting window.
Common Foods People Think Are Safe
A few items trip people up because they seem minor or “healthy”:
- Bulletproof coffee (coffee with butter or MCT oil): This can contain 200 or more calories. It will absolutely break your fast, even though some proponents claim fat doesn’t “count.” Fat doesn’t spike insulin as sharply as carbs, but it still provides energy your body will burn instead of stored fat.
- A handful of nuts: Even a small portion has 80 to 100 calories with protein and fat. That’s a snack, not a fast.
- Sugar-free gum: Most contain 1 to 5 calories per piece from sugar alcohols. One piece is unlikely to matter for fat loss. Chewing several pieces over hours could add up.
- Cinnamon: A small pinch in coffee adds less than 2 calories. It’s unlikely to affect your fast and may support blood sugar regulation.
A Simple Way to Think About It
If you’re fasting for weight loss, your main enemies are sugar, protein, and significant calories from any source. Small amounts of fat (under 10 to 15 calories) are the least likely to interfere, but zero is still better. If you’re fasting for autophagy, protein and amino acids matter most, since even small amounts reactivate the growth pathways that autophagy depends on being suppressed. Stick to water, black coffee, and plain tea for the cleanest fast regardless of your goal.
The practical takeaway: black coffee, plain tea, water, a pinch of salt, and a splash of apple cider vinegar are safe for nearly everyone. Everything else falls on a spectrum, and where you draw the line depends on why you’re fasting in the first place.