What you can take while fasting depends on why you’re fasting. If your goal is weight loss, anything with zero or near-zero calories keeps you on track. If you’re fasting for cellular cleanup (autophagy), even small amounts of protein or amino acids can interrupt the process. And if you’re on medication, some drugs simply aren’t safe on an empty stomach. Here’s a practical breakdown of what works, what doesn’t, and what falls in the gray zone.
Drinks That Won’t Break Your Fast
Water is the obvious baseline, but plain black coffee and unsweetened tea are also safe for most fasting goals. Coffee has negligible calories and actually increases fatty acid mobilization, meaning it helps your body tap into stored fat more efficiently. Green tea and black tea offer similar benefits. The key word is “plain”: no cream, no sugar, no flavored syrups.
Sparkling water and mineral water are fine too, and mineral water has the added benefit of contributing small amounts of electrolytes. Just check labels for added sugars or flavors that sneak in calories.
Electrolytes Are the Priority Supplement
The single most important thing to supplement during a fast is electrolytes, especially if you’re fasting longer than 16 hours. When you stop eating, your body sheds water and flushes sodium, potassium, and magnesium along with it. That’s why fasting headaches, muscle cramps, dizziness, and fatigue are so common.
The targets for a low-carb or fasting state are roughly 3,000 to 5,000 mg of sodium, 3,000 to 4,000 mg of potassium, and 300 to 500 mg of magnesium per day. You won’t hit those numbers from water alone. Options include:
- Sodium: A pinch of sea salt or pink salt in your water, or sugar-free electrolyte packets.
- Potassium: Look for electrolyte powders that include potassium, or use a lite salt (a blend of sodium chloride and potassium chloride) in water.
- Magnesium: A supplement in glycinate or citrate form is easiest on the stomach. Avoid magnesium oxide, which is poorly absorbed and can cause diarrhea.
Electrolyte supplements with zero calories and no sugar won’t break your fast by any definition. They’re not optional for extended fasts. They’re essential.
Apple Cider Vinegar
A tablespoon or two of apple cider vinegar diluted in water contains roughly 3 calories, which is negligible. It won’t spike insulin on its own during fasting. Where it shows real benefit is around meals: in one study, about 2 tablespoons of vinegar taken before eating increased muscle glucose uptake by roughly 30% compared to placebo, while lowering post-meal blood sugar, insulin, and triglycerides. So if you’re doing intermittent fasting, taking ACV right before you break your fast may help your body handle that first meal better.
What About Bone Broth?
Bone broth occupies a gray zone. A single cup contains anywhere from 31 to 86 calories and 5 to 9 grams of protein, depending on how it’s made. That protein will trigger an insulin response. For a strict fast focused on autophagy or maximum fat burning, bone broth technically breaks the fast.
That said, many people doing multi-day fasts use bone broth strategically to maintain electrolytes and make the fast more sustainable. If your main goal is calorie restriction or you’re easing into longer fasts, a cup of bone broth can be a practical compromise. If your goal is staying in a deep fasted state, skip it.
Sweeteners: More Complicated Than They Seem
Zero-calorie sweeteners seem like they should be safe during a fast, but the picture is murkier than the label suggests. Your body responds to sweet taste itself, not just to calories. Sucralose, for example, has been shown to raise blood insulin levels even without any actual sugar entering the bloodstream. The sweet taste triggers receptors in the gut that stimulate hormones involved in insulin release, potentially undermining the metabolic benefits of fasting.
Stevia and erythritol appear to have smaller effects on insulin, but research is still mixed. If you’re fasting for metabolic benefits, the safest approach is to skip all sweeteners, including in flavored electrolyte mixes. If you’re fasting purely for calorie control, a small amount of stevia in your coffee is unlikely to matter.
Supplements That Are Safe While Fasting
Most basic vitamins and minerals won’t break a fast. Electrolytes (as mentioned above), vitamin D, B vitamins, and fish oil in small doses are all generally fine. A few things to keep in mind:
- Fat-soluble vitamins (A, D, E, K) absorb much better with food. Taking them during your eating window will give you more benefit for the same dose.
- Iron and zinc can cause nausea on an empty stomach. Save these for meals.
- Creatine and collagen are calorie-containing supplements. Creatine has minimal calories and likely won’t affect your fast, but collagen is a protein and will.
What to Avoid: Protein and Amino Acids
If autophagy is part of your fasting goal, protein and amino acid supplements are the biggest things to avoid. Branched-chain amino acids (BCAAs), in particular, activate a growth-signaling pathway called mTOR that directly shuts down autophagy. In cell studies, BCAAs impaired the formation of the cellular structures responsible for cleaning up damaged proteins and organelles. Even small amounts of protein, whether from a supplement, bone broth, or a splash of milk in your coffee, send a signal that nutrients are available, which switches your body from cleanup mode to building mode.
Animal studies suggest autophagy ramps up significantly between 24 and 48 hours of fasting, though the exact timeline in humans isn’t well established. What is clear is that consuming protein or amino acids during that window will blunt the process.
Medications and Fasting
Some medications should not be taken on an empty stomach. NSAIDs like ibuprofen and aspirin are the most common example. Guidelines across multiple countries recommend taking these with food to reduce the risk of stomach irritation and, in rarer cases, gastrointestinal bleeding. If you take NSAIDs regularly and want to fast, schedule them during your eating window.
Other medications that commonly require food include certain antibiotics, blood pressure drugs, and diabetes medications. If you take prescription medication of any kind and plan to fast for more than 16 hours, check with your pharmacist about timing. Some drugs depend on food for proper absorption, while others can cause dangerous blood sugar drops when taken without eating. This is one area where individual circumstances matter more than general rules.
A Quick Reference by Fasting Goal
Your fasting goal determines where to draw the line:
- Weight loss / calorie restriction: Water, black coffee, plain tea, electrolytes, zero-calorie sweeteners, ACV, and most supplements are fine. Bone broth in moderation is acceptable.
- Metabolic benefits (insulin sensitivity, fat burning): Stick to water, black coffee, plain tea, electrolytes, and ACV. Avoid sweeteners, bone broth, and anything with protein or calories.
- Autophagy: Water, plain salt, black coffee, and plain tea only. Avoid all protein, amino acids, sweeteners, and caloric supplements. Even small protein inputs will interrupt cellular cleanup.