What you can have while fasting depends entirely on why you’re fasting. If your goal is weight loss, you have more flexibility than someone fasting for cellular cleanup (autophagy) or gut rest. The simplest rule: water, black coffee, and plain tea are safe across every fasting goal. Everything else falls on a spectrum.
Why Your Fasting Goal Changes the Rules
The term “dirty fasting” describes consuming up to about 100 calories during your fasting window, things like a splash of milk in coffee or a cup of bone broth. If weight loss is your only goal, dirty fasting can still work because you’re consuming far fewer calories than a normal eating day. But there’s no evidence that dirty fasting delivers the same benefits as zero-calorie fasting for reducing insulin resistance, stabilizing blood sugar, or triggering autophagy. The only way to guarantee you’re in a fully fasted state is to consume zero calories during your window.
That said, most people doing intermittent fasting (16:8 or similar) are primarily after fat loss and metabolic improvement. If that’s you, a few calories from cream or broth won’t derail your results. If you’re fasting specifically for autophagy or doing extended fasts beyond 24 hours, stricter rules apply.
Drinks That Won’t Break Your Fast
Water is the obvious baseline, still or sparkling. Black coffee is not only safe but potentially beneficial: caffeine is a potent autophagy stimulant, and in mouse studies, coffee rapidly triggered autophagy within hours at a human-equivalent dose. Interestingly, decaf coffee worked just as well. The autophagy-promoting effect appears to come from polyphenols (particularly chlorogenic acid, the most abundant antioxidant in coffee beans) rather than the caffeine itself. Coffee is also associated with lower risk of type 2 diabetes over the long term.
Plain tea, whether green, black, or herbal, is similarly fine. Like coffee, tea contains polyphenols that may support fasting benefits. Just skip the honey, sugar, or milk.
Apple cider vinegar diluted in water (one to two tablespoons in eight ounces) is another common fasting drink. One small study found it lowered both blood glucose and insulin in participants with diabetes, which aligns with fasting goals rather than working against them. A squeeze of lemon juice in water adds minimal calories and is generally considered compatible with fasting, though there’s less formal research on its metabolic effects.
Sweeteners: Stevia, Erythritol, and Sucralose
Stevia appears to be the safest sweetener option during a fast. A meta-analysis found that stevia consumption has no significant effect on insulin concentration. When a stevia-sweetened beverage replaced a sugar-sweetened one with a meal, both blood sugar and insulin responses decreased in healthy individuals.
Erythritol, a sugar alcohol with essentially zero calories, is another option that most fasting practitioners consider acceptable. Sucralose is more controversial. While it’s calorie-free, some research suggests artificial sweeteners may interact with sweet taste receptors in the gut, potentially affecting hormones like GLP-1 and insulin. If you want to play it safe, stevia or plain beverages are your best bet.
Sugar-Free Gum and Mints
Sugar-free gum is unlikely to break your fast. A 2015 study of 12 fasting people found that chewing sugar-free gum for 30 minutes did not affect insulin levels. Another study involving 59 people with gestational diabetes showed no effect on blood sugar after chewing gum. The calorie content is negligible, typically two to five calories per piece. If gum helps you get through a fasting window without eating, it’s a reasonable tradeoff.
What About Fats, Bone Broth, and Protein
Pure fats like MCT oil, butter, or coconut oil are a gray area. Your body can convert MCTs into ketones, which provide fat-based energy and help maintain ketosis. This is why “bulletproof coffee” (coffee blended with butter or MCT oil) is popular among keto fasters. Fat doesn’t spike insulin the way protein or carbohydrates do, so it preserves some fasting benefits. But it does contain calories, and it will interrupt autophagy. If you’re doing a fat-focused fast for ketone production, small amounts of pure fat may fit your protocol. For a strict fast, skip them.
Bone broth is a popular choice during extended fasts, but a single cup contains around 10 grams of protein. Protein triggers an insulin response and activates growth-signaling pathways that directly oppose autophagy. Bone broth is better thought of as a gentle re-feed or a tool for making longer fasts more sustainable, not something that keeps you in a fasted state. If you’re doing a 24-hour or multi-day fast and bone broth is the difference between continuing and quitting, it’s a practical compromise.
Electrolytes Are Essential, Not Optional
Your body flushes electrolytes faster when you’re not eating, especially during longer fasts or fasts combined with a low-carb diet. The headaches, fatigue, and muscle cramps people call “keto flu” are often just electrolyte depletion. Electrolyte supplements or electrolyte water won’t break your fast as long as they contain zero calories, sugar, or protein.
Recommended daily ranges during fasting are higher than you might expect: roughly 4,000 to 7,000 milligrams of sodium, 1,000 to 4,700 milligrams of potassium, and 400 to 600 milligrams of magnesium. You can get sodium from adding salt to water, potassium from a lite salt blend (which mixes sodium and potassium chloride), and magnesium from a supplement. Look for electrolyte powders free of sugar and artificial sweeteners if you want a simpler option.
Supplements and Vitamins
Some supplements are fine during a fast, others aren’t. The dividing line is whether they contain calories from fat, protein, or carbohydrates.
- Generally safe while fasting: Basic multivitamins, individual minerals (magnesium, zinc, iron), herbal supplements, and water-soluble vitamins like B and C, as long as they don’t contain sweeteners, oils, or fillers with calories.
- Best saved for your eating window: Fat-soluble vitamins like vitamin D (which needs dietary fat for proper absorption), fish oil, CoQ10, and any supplement containing protein, amino acids, or collagen.
If a supplement’s label lists calories, fat, or protein, take it with food. You’ll absorb more of it anyway.
Quick Reference by Fasting Goal
For weight loss, you can afford the most flexibility. Black coffee with a splash of cream, bone broth, sugar-free gum, stevia-sweetened drinks, and even small amounts of MCT oil are unlikely to meaningfully slow fat loss, especially within a 16:8 or 18:6 eating schedule.
For blood sugar and insulin control, stick to zero-calorie options: water, plain coffee, plain tea, apple cider vinegar in water, stevia if needed, and electrolytes. Avoid anything with protein, fat, or caloric sweeteners.
For autophagy, keep it as clean as possible. Water, black coffee (which may actually enhance autophagy), plain tea, salt, and calorie-free electrolytes. Even small amounts of protein or fat can interrupt the cellular cleanup process you’re trying to activate.