Can You Drink Coffee When Fasting 16/8?

December 19, 2025 •

3 min read

According to a 2021 study, black instant coffee does not significantly affect fasting glucose levels. This confirms that for most people, you can drink coffee when fasting 16/8, as long as it's prepared without caloric additives.

Quick Summary

Black coffee is generally permitted during a 16/8 fast, providing energy and appetite suppression without disrupting the fasted state. It is crucial to consume it plain, avoiding sugar, milk, or cream. For certain health goals like autophagy, strict adherence to black-only is recommended.

Key Points

Black Coffee is Safe: Plain black coffee contains virtually no calories and will not break a 16/8 fast.
Avoid All Additives: Any additions like sugar, milk, or cream will introduce calories and trigger an insulin response, ending your fasted state.
Enhances Fasting Benefits: The caffeine in black coffee can help suppress appetite and boost fat burning during your fasting window.
Consider Your Goals: For a strict fast focused on autophagy, it is best to avoid even zero-calorie sweeteners, though they generally don't affect weight loss.
Stay Hydrated: Coffee is a diuretic, so remember to drink plenty of water alongside it to prevent dehydration and manage hunger.
Listen to Your Body: If you experience stomach upset or jitters, reduce your intake, switch to decaf, or consider less acidic options like cold brew.

The Core Principle: Black is Best

For those practicing 16/8 intermittent fasting, the central rule regarding beverages is to avoid any calorie intake during the 16-hour fasting window. A standard cup of plain, black coffee contains a negligible 2–5 calories and virtually no macronutrients, meaning it will not trigger a significant insulin response. This is why black coffee is widely considered a safe and beneficial addition to your fasting routine. It keeps your body in a fasted, fat-burning state while offering a welcome boost of energy and appetite suppression.

What Happens When You Drink Coffee While Fasting?

Drinking coffee during a fast can offer several benefits beyond simply getting your caffeine fix. Research suggests that caffeine can enhance the metabolic effects of fasting by increasing the rate of fat oxidation and boosting metabolism. It can also make the fasting period more comfortable by suppressing hunger and improving mental clarity. Additionally, some studies indicate that coffee's polyphenols may help enhance autophagy, the cellular 'cleanup' process that fasting promotes.

Why Additives Break Your Fast

The reason black coffee is the only acceptable option is due to how other ingredients affect your body. Any source of calories, whether from sugar, milk, or certain sweeteners, will elicit an insulin response. This response tells your body that the fast is over, shifting it out of its fat-burning state.

Sugar: A single teaspoon contains about 16 kcal and is pure carbohydrates, causing an immediate insulin spike that ends your fast. This includes honey, syrup, and other caloric sweeteners.
Milk and Cream: Dairy products contain lactose (sugar) and protein. Even a small amount adds enough calories to trigger an insulin response and disrupt a strict fast, especially if your goal is autophagy. This applies to both cow's milk and most plant-based milks like oat and almond milk.
Bulletproof Coffee: While popular in some keto circles, adding butter or MCT oil introduces a significant number of calories (100–200 kcal) and definitively breaks a classic fast.
Collagen or Protein Powder: These contain calories and protein that trigger a digestive and insulin response, negating the benefits of fasting.

Fasting Goals: How 'Clean' Should Your Coffee Be?

The strictness of your fasting can determine how you approach your morning brew. The goal of your intermittent fast, such as weight loss or cellular repair, dictates how liberal you can be with what you add to your coffee.

Comparison Table: Coffee Additives and Fasting Goals


Additive	Affects Weight Loss?	Affects Autophagy?	Recommended for a Strict Fast?
Black Coffee	No, can enhance it	No, may enhance it	Yes
Zero-Calorie Sweeteners	No, but individual response varies	May slightly inhibit (debated)	No (best to avoid)
Small Splash of Milk	Minimal effect for some ('dirty fast')	Likely inhibits	No
Bulletproof Coffee	Technically breaks fast due to calories	Stops fast	No

Best Practices for Coffee Consumption During 16/8

For the best results, timing and moderation are key. Drink your coffee black and avoid having it too late in the day to protect your sleep schedule. If black coffee is too harsh on an empty stomach, consider options like cold brew, which is less acidic and often smoother. You can also experiment with zero-calorie additions, but be mindful of your specific fasting goals. It's always a good practice to stay hydrated by drinking water alongside your coffee.

Conclusion

Yes, you can confidently drink coffee while following a 16/8 intermittent fasting schedule, provided you stick to black coffee. This calorie-free beverage can serve as a beneficial tool for managing hunger and boosting energy during your fasting window, and may even enhance the metabolic benefits of your fast. However, adding any milk, sugar, or other caloric ingredients will technically break your fast by causing an insulin spike. For the most effective and 'cleanest' fast, enjoy your coffee pure and in moderation.

Resources

Healthline: 16/8 Intermittent Fasting: Meal Plan, Benefits, and More

Frequently Asked Questions

Yes, adding any amount of milk, including a 'small splash,' introduces calories and can trigger an insulin response, which breaks the fast. For the cleanest fast, stick to black coffee.

While zero-calorie sweeteners generally don't break a fast calorically, they are a debated topic. Some argue the sweet taste can trigger a cephalic-phase insulin response or increase cravings, potentially affecting your fast. For a purist's approach, it's best to avoid them.

Yes, bulletproof coffee, made with butter or MCT oil, contains significant calories (100–200 kcal), which definitively breaks a traditional fast. While it provides fatty acids, it prevents the cellular repair benefits of true fasting.

Yes, plain instant coffee is acceptable during your fast, as long as it has no added sugars or other caloric additives. Like regular black coffee, it contains very few calories.

Many people find it beneficial to drink coffee in the morning or early in their fasting window. This helps suppress appetite and boost energy. Drinking it too late can interfere with sleep, so time it wisely.

Evidence suggests that black coffee may actually enhance autophagy, not disrupt it, due to its polyphenol content. However, adding any calories will stop this process, so a 'clean' fast with black coffee is necessary.

Yes, plain decaf coffee is an excellent fasting-friendly alternative, especially if you are sensitive to caffeine. It retains many of coffee's beneficial compounds and won't break your fast.