This is the question everyone asks first, and the internet answers it with far more confidence than the research supports.
What can you drink?
- Water. Lots of it. More than feels necessary.
- Black coffee and plain tea. Nobody serious argues with these.
- Zero-calorie drinks. Technically fine. Some women find they make hunger louder, which is worth testing on yourself.
- Anything with milk, sugar, or sweetener in it. That’s food.
Does coffee affect the fast?
Less than you’d think. In a twenty-four-week randomized trial in adults at risk of type 2 diabetes, daily coffee didn’t improve insulin sensitivity. That’s the most direct human evidence there is, and it points at coffee being fairly neutral.2
There’s a wrinkle. A review of short trials found caffeine raised blood sugar in people with type 2 diabetes. That was a diabetes population over brief trials, so it doesn’t transfer cleanly to a healthy woman with a black coffee at nine in the morning. It’s why you’ll see the claim repeated.3
Does it help with hunger?
A little, maybe, and less than you’d hope. In a study comparing coffee, caffeine alone, decaf, and a placebo between breakfast and lunch, there were no real differences in appetite or in how much people ate afterward.1
Twelve people, so a small study. The honest read is that coffee’s effect on appetite is modest.
That said, a warm drink in your hand at the hour you’d normally eat is genuinely useful. It doesn’t need a mechanism to work.
What about coffee and autophagy?
You’ll see it claimed that coffee boosts autophagy during a fast. That comes from a study in mice, where coffee raised autophagy markers in liver, muscle and heart.4
It’s a real finding. It hasn’t been shown in people, and the authors themselves call the health link speculation.
If someone tells you coffee does something specific to your cells during a fast, they’re quoting a mouse.
What if you’re cutting back?
Do it gradually. In a survey of over eleven thousand people, about one in nine daily caffeine drinkers reported symptoms on stopping, and women reported symptoms bad enough to disrupt their day more often than men did.5
In the small experiment that went with it, people who tapered had far less trouble than people who quit cold.