What Is Allowed During Intermittent Fasting?

During a fasting window, you are allowed to consume water, plain black coffee, plain tea, and sparkling water — all with zero calories. These drinks keep your fast fully intact. Anything containing calories, protein, carbohydrates, or fat will break your fast and should be saved for your eating window.

Why This Matters

One of the most common reasons people accidentally break their fast is not knowing what they can and cannot consume. A splash of milk in your morning coffee, a stick of gum, or a flavored sparkling water can all trigger an insulin response and end your fasting state before you realize it. Getting this right is the difference between a productive fast and wasted effort — especially if your goal is weight loss, metabolic reset, or improved insulin sensitivity.

The good news is that the permitted list is simple once you know the rules. You have more options than you might think, and none of them require giving up your morning routine.

What Science Says About Fasting and Calories

The core principle of intermittent fasting is maintaining a state where insulin levels remain low and the body draws on stored fat for fuel. Research published in journals including Cell Metabolism and Obesity Reviews consistently shows that this metabolic shift — from glucose to fat burning — begins roughly 12 to 16 hours after your last meal, depending on individual metabolism.

What disrupts this state? Any meaningful caloric intake. Even small amounts of certain macronutrients — particularly carbohydrates and protein — can stimulate insulin secretion, which signals the body to stop burning stored fat. This is why the rules around fasting beverages matter so much.

Here is what research and clinical practice agree on:

Fully permitted (will not break your fast):

• Water — still or sparkling, plain
• Black coffee — no milk, cream, sugar, or flavored syrups
• Plain green tea, black tea, herbal tea — unsweetened, no additives
• Electrolyte supplements — only if they contain zero calories and zero sweeteners (check labels carefully)

Will break your fast:

• Milk, cream, half-and-half, or any dairy added to coffee or tea
• Fruit juice, smoothies, protein shakes
• Bulletproof coffee (butter and MCT oil contain calories and fat)
• Sweetened beverages — including those with artificial sweeteners, which can still spike insulin in some people
• Bone broth (contains protein and some calories)
• Chewing gum that contains sugar or sugar alcohols
• Any food, even small amounts

Gray area — approach with caution:

• Artificial sweeteners: Some studies suggest sweeteners like sucralose and aspartame trigger an insulin response even without calories. The evidence is mixed, but if you are not seeing results, eliminating sweeteners entirely is worth testing.
• Sparkling water with "natural flavors": Most are fine, but some flavored waters contain trace sugars. Read the label — it must say zero calories and zero grams of sugar.
• Apple cider vinegar: A tablespoon in water is commonly used during fasting and is unlikely to break a fast due to its negligible calorie content, but the evidence is not conclusive.

Practical Tips for Your Fasting Window

Start with water first. Begin your fasting day with a large glass of water. Most morning hunger is actually mild dehydration. Hydrating immediately reduces cravings and supports the detoxification processes your body runs during the fasted state.

Black coffee is your ally. Caffeine modestly suppresses appetite and has been shown to enhance fat oxidation during fasting. If you are new to black coffee, try reducing the milk gradually over a week rather than stopping abruptly. Many people adjust and come to prefer it.

Brew tea in advance. Herbal teas — chamomile, peppermint, ginger — are all permitted and can make long fasting windows more comfortable. Keeping a thermos nearby gives you something warm to sip when hunger peaks, typically around hour 13 to 15 of a fast.

Read every label. Products marketed as "zero calorie" sometimes contain small amounts of carbohydrates, sweeteners, or flavoring agents. A label that says "0 calories" but lists sugar alcohols or natural flavors warrants a closer look.

Plan your eating window break intentionally. What you eat when you break your fast matters almost as much as the fast itself. Breaking a 16-hour fast with a high-sugar, refined-carbohydrate meal will spike insulin sharply. Prioritize protein and fiber-rich foods for your first meal to extend the metabolic benefits of your fast.

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Frequently Asked Questions

Does black coffee break a fast?

No. Plain black coffee — with nothing added — does not break a fast. It contains virtually zero calories and does not trigger a meaningful insulin response. In fact, caffeine may enhance fat burning during fasting. The moment you add milk, cream, sugar, or any flavoring, however, the fast is broken.

Can I drink sparkling water during intermittent fasting?

Yes, plain sparkling water is permitted during a fast. The carbonation does not affect insulin or fat burning. However, flavored sparkling waters must be checked carefully — they must show zero calories and zero grams of sugar on the label to be considered safe during a fasting window.

Do artificial sweeteners break a fast?

This is debated among researchers. Some studies suggest certain artificial sweeteners — particularly sucralose — can provoke an insulin response even without calories. If you are fasting for metabolic benefits or weight loss and are not seeing results, eliminating all sweeteners during your fasting window is a reasonable experiment.

Can I have bone broth while fasting?

Technically, bone broth contains calories and protein, which means it breaks a strict fast. However, some practitioners use it during extended fasting periods (24 hours or longer) as a way to maintain electrolytes while keeping caloric intake minimal. For standard 16:8 or 18:6 fasting, save bone broth for your eating window.