If you're doing 16:8 — fasting for 16 hours and eating within an 8-hour window — you've probably already run into the question that trips up almost everyone new to this protocol: what am I actually allowed to drink during those 16 hours?
The short answer is that several drinks are completely fine during your fasting window, and a few popular ones can quietly break your fast without you realizing it. Let's go through exactly what works, what doesn't, and why.
Does Drinking Anything Break a 16:8 Fast?
Technically, yes — anything with calories breaks a fast in the strictest sense. But most people doing 16:8 aren't chasing a zero-calorie technicality. They're trying to avoid spiking insulin, triggering digestion, or interrupting the metabolic benefits they're fasting for in the first place.
That means the real question isn't "does this have zero calories" — it's "does this keep me in a fasted metabolic state." A few drinks clear that bar easily. Others don't, even though they seem harmless.
What You Can Drink During Your 16-Hour Window
Water is the obvious one, and it should be your baseline. But pure water alone often isn't enough — especially past hour 10 or 12, when your body has used up a lot of its stored electrolytes and you start feeling the classic fasting headache or fatigue.
Black coffee is fine and, for many people, genuinely helpful during a fast — it can blunt hunger and give you an energy lift. Just watch the cream, sugar, and flavored syrups, which all break your fast.
Plain tea — green, black, herbal — is also fasting-safe. Green tea in particular has compounds that support metabolism, which is part of why we use it in FAST:RX.
Electrolytes are the one people most often get wrong. Many assume any electrolyte drink will break a fast because it has flavor or because it's not "just water." But pure electrolytes — sodium, potassium, magnesium — have effectively zero calories and don't trigger an insulin response. They're not just fasting-safe, they're often the missing piece that makes a 16-hour fast actually feel manageable. We go deep on this exact question in Can You Drink Electrolytes While Fasting? if you want the full breakdown.
Apple cider vinegar, diluted in water, is generally considered fasting-safe in small amounts and is a popular addition for people focused on blood sugar support.
What to Avoid During Your Fast
Bone broth has calories and protein — it's a great fast-breaker, but it's not a fasting drink.
Diet soda is technically zero-calorie, but artificial sweeteners can trigger an insulin response in some people and may affect gut bacteria in ways that work against your fasting goals.
Protein shakes, BCAAs, and anything with sweeteners or flavoring beyond trace amounts will break a fast — these are designed to be metabolically active, which is the opposite of what you want during your fasting window.
Cream, milk, or sugar in your coffee or tea — even a small splash adds enough calories and triggers enough of a digestive response to take you out of a fasted state.
Why Electrolytes Matter More in 16:8 Than Other Protocols
Sixteen hours is long enough that your body starts shedding sodium and water — a natural response when insulin levels drop during a fast. This is why so many people on 16:8 hit a wall around hour 12 to 14: headache, brain fog, lightheadedness. It's almost never actually hunger. It's electrolyte depletion.
This is the exact gap FAST:RX was built to fill. It's a ready-to-drink can with the electrolytes, vitamins, and ginseng your body needs during a fast — zero sugar, zero calories, and formulated specifically so it won't break your fast or kick you out of ketosis. No mixing a powder, no guessing at ratios. You crack it open and you're covered for the rest of your window.
When to Drink What During Your 16-Hour Fast
A simple way to structure it:
Morning (hours 1–6): Black coffee or tea if you want the energy lift. Plain water alongside.
Midday (hours 6–12): This is when electrolytes start to matter. A can of FAST:RX here covers you through the toughest stretch of the fast.
Late afternoon (hours 12–16): Stay on water and electrolytes. This is when headaches and fatigue are most likely to show up if you haven't been replacing electrolytes — and the easiest time to give up on your fast early if you feel bad. Staying ahead of it here is what makes the difference between finishing strong and breaking early.
The Bottom Line
16:8 is one of the most sustainable fasting protocols precisely because it leaves room for black coffee, tea, and proper hydration — you're not white-knuckling sixteen hours on water alone. The drinks that keep you in a fasted state are the same ones that make the fast easier to stick with. Get the electrolytes right and the rest of the protocol takes care of itself.
Try FAST:RX during your next 16:8 window — available on Amazon here.
