Water, black coffee, and plain tea are the safest choices during a fast, but several other drinks and additions can fit depending on your fasting goals. The key factor is whether something triggers an insulin response or delivers enough calories to shift your body out of its fasted metabolic state. Most people fasting for weight loss or blood sugar control have more flexibility than someone fasting strictly for cellular repair.
Water, Coffee, and Tea
Plain water is the obvious baseline. Sparkling water and mineral water are equally fine, as long as they contain no added sugars or flavors. Staying hydrated actually matters more during a fast because you’re not getting the water that normally comes from food.
Black coffee not only avoids breaking a fast but may actively support it. Caffeine and other natural compounds in coffee activate the same cellular cleanup processes that fasting promotes. These self-repair pathways, where your cells break down and recycle damaged components, are one of the main biological benefits of fasting. Coffee enhances rather than interrupts this process. The antioxidants in coffee can also improve insulin sensitivity, which complements what fasting does on its own. The catch: it has to be black. No cream, no sugar, no flavored syrups.
Plain tea, whether green, black, white, or herbal, follows the same logic. It contains zero calories and won’t trigger an insulin response. Green tea in particular contains compounds that support fat oxidation, making it a popular pairing with fasting protocols.
What You Can Add to Your Drinks
A squeeze of lemon or lime juice in water adds a negligible number of calories and won’t meaningfully affect your fast. It can make plain water easier to drink across a long fasting window.
Apple cider vinegar is another common addition. One tablespoon contains roughly 3 calories and less than 1 gram of carbs, so it won’t disrupt a fast. There’s an added benefit: vinegar intake is associated with improved blood sugar levels and reduced weight. It may also help you feel fuller during your fasting window, making the hours easier to get through. Dilute it in water rather than drinking it straight, since the acidity can irritate your throat and tooth enamel.
A pinch of salt or electrolytes (without added sugar) is not only acceptable but sometimes necessary, especially during longer fasts. Sodium, potassium, and magnesium help prevent the headaches, dizziness, and fatigue that often hit during extended fasting periods.
Artificial Sweeteners: It Depends on Which One
This is where things get complicated. Not all zero-calorie sweeteners behave the same way in your body.
Sucralose (the sweetener in Splenda) appears to trigger a real insulin response. In one study, people given sucralose had 20% higher blood insulin levels than those given plain water. Their bodies also cleared the insulin more slowly. Scientists believe this happens because sucralose activates sweet taste receptors in the mouth, which tells the brain food is coming. Your pancreas releases insulin in anticipation. This is called a cephalic phase insulin response, and it works against what fasting is trying to accomplish.
Aspartame (found in Diet Coke and Equal) does not appear to raise insulin levels in short-term studies. Stevia also generally performs well in this regard. However, there’s a longer-term concern with artificial sweeteners broadly: they can alter gut bacteria in ways that worsen blood sugar regulation over time. A 2022 study found that saccharin promoted glucose intolerance in more than half of healthy subjects tested. So while a single packet of stevia in your morning tea probably won’t derail a fast, relying heavily on artificial sweeteners during your fasting window may undermine the metabolic benefits you’re after.
Sugar-Free Gum and Mints
Most people assume sugar-free gum is harmless during a fast, but the evidence suggests otherwise. Chewing sugar-free gum triggers measurable increases in insulin, cortisol, and other hormones, even though the gum itself contains minimal calories. The act of chewing alone sets off a cascade of physiological changes. Your body interprets the chewing motion and sweet taste as signals that food is arriving, and it responds accordingly. If you’re fasting for metabolic benefits, skip the gum.
MCT Oil and the “Dirty Fast”
Some fasting approaches allow small amounts of pure fat, particularly MCT oil or a pat of butter in coffee (sometimes called “bulletproof coffee”). This is often called a “dirty fast” because it technically delivers calories while trying to preserve some fasting benefits.
The logic: pure fat produces very little insulin response compared to protein or carbohydrates. Your body converts MCTs into ketones, which serve as an alternative fuel source for your brain and muscles. This keeps you in a fat-burning state similar to what happens during a true fast. One study of people with diabetes found that daily MCT oil consumption reduced body weight, waist circumference, and insulin resistance compared to other fats.
The tradeoff is real, though. Adding fat means adding calories, which interrupts the calorie-free state that drives autophagy, your body’s deep cellular cleanup process. If you’re fasting primarily for weight management or to stay in ketosis, a small amount of MCT oil or coconut oil in your coffee is a reasonable compromise. If you’re fasting for cellular repair, it’s better to stick with zero-calorie options.
Bone Broth: A Gray Area
Bone broth contains roughly 8 to 10 grams of protein per cup, which places it firmly outside a strict fast. Protein, especially in that quantity, stimulates insulin and activates growth-signaling pathways that oppose the cellular repair processes fasting triggers.
That said, bone broth is commonly used during longer fasts (24 hours or more) as a way to get electrolytes and amino acids without eating solid food. The amino acid glycine, which is abundant in bone broth, may actually support gut lining integrity during extended fasts. For someone doing a multi-day fast who needs to maintain lean muscle and prevent excessive fatigue, a cup of bone broth is a practical tool. For a standard 16:8 intermittent fasting window, it’s not necessary and will blunt some of the metabolic benefits.
Quick Reference by Fasting Goal
- Fasting for weight loss: Water, black coffee, plain tea, lemon water, apple cider vinegar, electrolytes, and small amounts of MCT oil are all reasonable. Avoid anything with protein or sugar.
- Fasting for blood sugar control: Stick with water, black coffee, plain tea, and apple cider vinegar. Avoid sucralose and sugar-free gum, both of which trigger insulin responses.
- Fasting for autophagy and cellular repair: Keep it as clean as possible. Water, black coffee, plain tea, and electrolytes only. Even small amounts of fat or protein can activate the growth pathways that oppose cellular cleanup.
The strictness that matters depends entirely on why you’re fasting. Someone using a 16:8 eating window to manage their weight doesn’t need to follow the same rules as someone doing a 48-hour fast for cellular repair. Knowing your goal lets you decide where on the spectrum, from pure water to bone broth, your fasting window should land.