During a fast, you can take in more than just plain water without undoing the metabolic benefits. Black coffee, plain tea, sparkling water, and certain supplements are all generally safe choices that won’t trigger a meaningful insulin response or spike blood glucose. The key is avoiding anything with significant calories, protein, or carbohydrates, since those nutrients activate the cellular pathways that fasting is designed to suppress.
What counts as “breaking” a fast depends on your goal. If you’re fasting for weight loss, a tiny number of calories is unlikely to matter. If you’re fasting to maximize autophagy (your body’s cellular cleanup process), the bar is higher, because even small amounts of amino acids can flip the switch back on. This guide covers both scenarios.
Beverages That Won’t Break Your Fast
Water is the obvious baseline, but you have more options than you might think. Black coffee and plain tea (green, black, herbal) contain essentially zero calories and do not raise blood glucose or insulin in any meaningful way. Coffee can actually support some fasting goals by mildly stimulating autophagy.
Sparkling water and mineral water are also fine, and the added minerals can help with electrolyte balance. A squeeze of lemon or lime in your water adds a negligible number of calories (roughly 1 to 3) and won’t produce an insulin response worth worrying about.
What to avoid in your beverages: milk, cream, sugar, honey, juice, and any caloric add-ins. Even a splash of cream adds fat and protein that can activate nutrient-sensing pathways in your cells. If your coffee isn’t palatable black, consider switching to a smoother roast or cold brew, which tends to be less bitter.
Zero-Calorie Sweeteners: Which Ones Are Safe
Not all artificial and natural sweeteners behave the same way during a fast. Research on aspartame and saccharin found that neither sweetener affected blood glucose levels in fasting subjects, including people with diabetes. Insulin levels showed a statistically significant but physiologically tiny bump with aspartame in healthy subjects, small enough that researchers concluded it didn’t disrupt blood glucose regulation.
Stevia and monk fruit are plant-based sweeteners with zero calories. They may cause a very small insulin nudge, but even if they do, the result is a slight dip in blood sugar, which if anything deepens the fasted state rather than disrupting it.
Erythritol is another option that doesn’t raise blood glucose or insulin. It actually slows gastric emptying and triggers gut hormones related to satiety, which can help with hunger during your fasting window. In people with diabetes, regular erythritol use has been shown to improve long-term blood sugar markers.
The sweeteners most likely to cause problems are those with hidden calories. Maltodextrin and dextrose, which show up as bulking agents in some sweetener packets, are rapidly absorbed carbohydrates. Check the ingredient list on any sweetener you plan to use.
Electrolytes and Minerals
Sodium, potassium, and magnesium are the three electrolytes most commonly depleted during fasting, especially fasts lasting longer than 16 hours. You lose sodium through urine at a higher rate when insulin is low, and that sodium loss pulls potassium and water along with it. This is often the cause of headaches, dizziness, and muscle cramps that people blame on hunger.
Adding a pinch of salt (roughly a quarter teaspoon) to your water a few times a day can prevent most of these symptoms. Electrolyte powders or tablets are another option, but read the label carefully. Many contain sugar, maltodextrin, or enough flavoring to add meaningful calories. Look for products that list zero calories and no carbohydrate-based fillers.
Magnesium supplements in pill form are fine during a fast, as they contain no calories. Magnesium citrate or glycinate are common forms that are well absorbed without food.
Which Supplements You Can and Can’t Take
The general rule: if a supplement contains fats, oils, protein, amino acids, or carbohydrates, save it for your eating window. This includes fish oil, collagen powder, branched-chain amino acids (BCAAs), protein powder, and gummy vitamins (which are loaded with sugar).
Vitamin D is a fat-soluble vitamin that absorbs poorly without dietary fat. Taking it during a fast means most of it passes through without being used. The same applies to vitamins A, E, and K. Save all fat-soluble vitamins for a meal.
Supplements that are generally safe during a fast include:
- Water-soluble vitamins like B-complex and vitamin C, which don’t need fat for absorption
- Magnesium in capsule or tablet form
- Iron, which is actually better absorbed on an empty stomach (though it can cause nausea in some people)
- Iodine and zinc in small doses
Be aware that some supplements, particularly iron and zinc, can cause stomach discomfort without food. If you experience nausea, move them to your eating window instead.
Medications During a Fast
Most medications can be taken during a fasting window with a glass of water. “Take on an empty stomach” typically means 30 minutes or more before eating, which aligns naturally with a fasting schedule. The water you use to wash down a pill does not break a fast.
Avoid taking medications with milk or fruit juice, as both can interfere with drug absorption. Stick to plain water. If your medication specifically requires food (the label or your pharmacist will tell you this), take it at the start of your eating window rather than skipping it to preserve your fast. Medication adherence always takes priority over fasting.
What Actually Breaks a Fast
Your body exits the fasted state when nutrient-sensing pathways detect incoming fuel. The most powerful trigger is amino acids (from protein), which directly activate a cellular complex called mTOR. When mTOR switches on, autophagy stops. Glucose and insulin spikes from carbohydrates do the same thing. Fat has the smallest impact on these pathways, which is why some fasting protocols allow a small amount of fat (like butter in coffee), though purists avoid it.
There’s no universally agreed-upon calorie threshold that “officially” breaks a fast, because the answer depends on the type of calorie. A few calories from fat behave differently than a few calories from protein. As a practical guideline, anything under roughly 5 to 10 calories from non-protein sources is unlikely to produce a measurable metabolic shift. But consuming even a small amount of protein or carbohydrate can blunt autophagy, since amino acids and glucose are the specific signals your cells monitor.
For weight loss purposes, the threshold is more forgiving. A splash of cream in your coffee won’t derail your caloric deficit. For autophagy and cellular repair, cleaner fasting (water, black coffee, plain tea, electrolytes) gives you the most benefit.
Oral Hygiene Products
Toothpaste and mouthwash do not break a fast as long as you’re not swallowing them. The sweeteners in toothpaste never reach your gut, so they can’t trigger an insulin response. Continue brushing and flossing normally. Fasting is not a reason to skip oral care.
Practical Fasting-Window Routine
A solid approach for a typical 16:8 or 18:6 fast looks like this: start your morning with water and a pinch of salt. If you drink coffee or tea, keep it black or add only a zero-calorie sweetener like stevia or erythritol. Take any water-soluble vitamins or calorie-free supplements in the morning if that’s your preference. Sip water with electrolytes throughout the fasting window, especially if you’re active or in warm weather.
Save your fat-soluble vitamins (D, A, E, K), fish oil, collagen, and any supplements that contain calories for your first meal. If you take medications that require food, align them with the start of your eating window. Keep calorie-free beverages varied if hunger is an issue. Sparkling water, herbal tea, and iced black coffee all help break the monotony without breaking the fast.