Quick answers about fasting

Short answers to the questions people ask most once they start fasting: what you can have without breaking a fast, what your body may be doing as the hours go by, what to eat and what helps when fasting feels hard. Each page starts with the short answer, then explains it with the studies behind it.

Does it break a fast?

Drink or supplement	Short answer
Lemon water	No.
Tea	No, if it's plain.
Milk in coffee	A splash: barely. More: yes.
Diet soda	Not really.
Gum	Sugar-free, occasionally: no.
Bone broth	Yes.
A protein shake	Yes.
Vitamins	Many do.
Creatine	No, if it's plain.
Apple cider vinegar	No.
Sweeteners	Zero-calorie ones: not really.
Electrolytes	Sugar-free ones: no.
Sparkling water	No, if it's plain.
Bulletproof coffee	Yes, in calories.
Almond or oat milk	A splash: barely. A latte: yes.
Collagen	Yes.
Alcohol	Yes.
Medication	Usually, but ask.
Toothpaste	No.

What happens, hour by hour

After	What happens
12 hours	The Fuel Switch begins.
14 hours	You're in the Fuel Switch.
16 hours	Fat becomes a major fuel.
18 hours	You're in Fat Burning.
20 hours	Deep in Fat Burning, near Ketosis.
24 hours	Ketosis begins.
36 hours	Deeper ketosis, and possibly more autophagy.
48 hours	A deeply fasted state.
72 hours	Extended fasting territory.

When fasting feels hard

Question	Short answer
Headache while fasting: why it happens and what helps	Usually water or caffeine.
Feeling dizzy while fasting: what to do	Often salt and water.
Trouble sleeping while fasting?	Usually not the fast itself.
Constipation while fasting: what helps	Common, and usually easy to fix.
Bad breath while fasting: why, and what helps	Common, and usually harmless.
Feeling cold while fasting: is it normal?	Often, on longer fasts.
Does intermittent fasting cause hair loss?	Not on its own, but fast weight loss can.
Intermittent fasting and your period	Timing, no. Too little food, yes.

What to eat

Question	Short answer
What to eat during intermittent fasting	Protein and plants, first.
A 16:8 meal plan	Two meals, maybe a snack.
Intermittent fasting shopping list	Mostly ordinary food.

Fasting and your health

Question	Short answer
Does intermittent fasting lower cholesterol?	Often, a little.
How much weight can you lose with intermittent fasting?	A few percent, over months.
Intermittent fasting vs calorie counting	About the same.
Intermittent fasting vs keto	Different tools.

Fasting through life

Question	Short answer
How to start intermittent fasting	Start small.
Intermittent fasting over the holidays	Keep it loose.
Ramadan and intermittent fasting	They're different.
Intermittent fasting for women over 40	It can work well.
Intermittent fasting for shift workers	Yes, with care.

To see where you are in a fast right now, try the fasting stage calculator.

Sources

1. Grajower MM, Horne BD. Clinical Management of Intermittent Fasting in Patients with Diabetes Mellitus. Nutrients. 2019;11(4):873. doi.org/10.3390/nu11040873