LEARN · 23 guides
Fasting guides
Short guides with the science behind fasting, from your first fast to expert level. Each one is a shorter version of a guide in Fasting Tracker and lists the studies it draws on.
Your first guide · FreeIntermittent fasting 101What it is, what happens in your body, what the research really shows and how to start well
Start here
The essentials for your first weeks
Your first week, day by dayWhat most people feel as their body adapts, and simple ways to make each day easier
Hunger: why it comes and how to ride it outHunger comes in waves that rise and fall on their own. Here's the science, and what actually helps
Fasting myths, bustedBreakfast, starvation mode, muscle loss and more: what the trials actually found
The science
What happens inside you, explained simply
- Fasting stages, hour by hourWhat your body may be doing from your last meal to 72 hours of fasting, and roughly when each stage begins
The metabolic switchHow your body moves from burning sugar to burning fat, when it happens and what speeds it up
Autophagy: your cells' recycling systemWhat autophagy is, what fasting does to it, and why nobody knows the exact hour it starts
Insulin and fastingThe hormone that decides whether you store or burn, how insulin resistance develops and what fasting can do about it
Fasting and your heartBlood pressure, cholesterol and what the trials really show, including the questions that remain open
Your gut while you fastThe cleaning wave that only runs on an empty stomach, what fasting does to your gut bacteria, and how to feed them well
Fasting and your brainFocus, mood and that "hangry" feeling: what happens in your head when you skip a meal, and what's still hype
Fasting and longevityWill fasting help you live longer? What animals, the CALERIE trial and "biological age" studies can and can't tell us
Eat and drink
Drinks, first meals and nutrients
What breaks a fast (and what doesn't)Coffee, milk, sweeteners and supplements, judged by what they do to your body
Coffee and fastingWhy black coffee is a faster's friend, what it does to fat burning and blood sugar, and how much is too much
How to break your fast wellYour first meal shapes the rest of your day. Here's what the research says about getting it right
Water, salt and electrolytesWhy fasting changes how your body handles water and sodium, and when it's worth doing something about it
Train, sleep, live
Fit fasting into a real life
Training while fastingStrength, cardio and when to eat, so fasting supports your workouts instead of costing you muscle
Sleep and fastingHow sleep shapes your hunger, whether fasting affects your nights, and how to time your last meal
Your body clock: when you eat mattersWhy the same meal affects you differently in the morning and at night, and how to choose your eating window
Fasting for womenHormones, cycles, PCOS and menopause: what the studies in women actually show
Advanced
For experienced fasters
Protein and muscle: fasting without losing strengthHow much you need, how to fit it into fewer meals, and why it matters most when you're losing weight
Extended fasts (24–72 h): doing them safelyWhat happens in your body after the first day, who should avoid them, and how to start and finish well
Plateaus: what to do when progress stallsWhy weight loss slows down, how to tell a real plateau from normal ups and downs, and what actually helps
GLP-1 medicines and fastingHow semaglutide and tirzepatide work, why protecting muscle matters on them, and how to use a fasting window safely alongside
These guides explain general research and aren't medical advice. Talk to your doctor before changing how you eat, especially if you have a medical condition.
A note on health
Fasting Tracker is designed for general wellness and is not a medical device. Fasting is not right for everyone. If you are pregnant, diabetic, have a history of eating disorders or take medication, talk to your doctor first, and before any extended fast.