The windows, and what changes between them.
The numbers describe one thing only: the hours of the day within which you eat. What follows from that is where the real differences are.
Reading the notation
The pair of numbers is hours fasting to hours eating, across a 24-hour day. 16:8 is sixteen hours without food and an eight-hour window in which you eat. 18:6 is an eighteen-hour fast and a six-hour window. 20:4 is twenty and four. OMAD, one meal a day, compresses it further still.
Note what the notation does not say: it says nothing about what or how much you eat. A fasting window is a schedule, not a diet.
What actually differs
The number of eating occasions. An eight-hour window comfortably holds two or three meals. A four-hour window holds one large meal or two small ones. OMAD holds one. This is the difference people feel most.
How much has to fit per meal. If your intake is roughly the same, a shorter window means larger individual meals. For some people that is satisfying; for others it is uncomfortable, and it is worth knowing which you are before committing.
Nutritional practicality. Short windows make composition harder rather than impossible. A secondary analysis of a randomized trial reported in Nutrients (2025) found that women with overweight or obesity eating within ten hours or less took in less potassium, magnesium and phosphorus than a control group, and concluded such a window might raise the risk of micronutrient inadequacy. Mamerow and colleagues found in the Journal of Nutrition (2014) that spreading protein evenly across three meals stimulated 24-hour muscle protein synthesis 25% more than concentrating most of it in the evening meal. The shorter the window, the more deliberate the composition has to be.
Social and practical fit. A window that excludes family dinner or the times you are actually hungry is a window you will abandon. This is the factor most people underweight and it is usually the deciding one.
What the fasting hours are doing
During the hours after a meal the body is primarily using and storing the energy just consumed. As those hours extend, liver glycogen draws down and fatty acids are mobilised in its place. Anton and colleagues, reviewing the mechanism in Obesity (2018), put that crossover somewhere between roughly 12 and 36 hours after the last meal, depending on how much liver glycogen you started with and how active you are during the fast. There is no single hour at which it happens.
Research into what longer fasts do at a cellular level is active and genuinely interesting, but it is also frequently overstated in popular summaries. The National Institute on Aging says in Calorie Restriction and Fasting Diets: What Do We Know? that most work so far has focused on weight loss in people with obesity, that only a few small clinical trials have been run, and that there is not yet enough evidence to recommend any such eating pattern to the public. What is well supported is the mechanism above.
The part nobody advertises
For a great many people, the reason a fasting window changes anything is straightforward: eating within fewer hours makes it easier to eat less without counting, because the late-evening hours where unplanned eating tends to happen are closed.
That is a real and useful effect. It is also why fasting stops doing anything for people who compensate fully within the window. The schedule was never the active ingredient on its own.
Who should be careful
Intermittent fasting is not appropriate for everyone. Varady and colleagues, setting out indications and contraindications in Nature Reviews Endocrinology (2022), advise against it in pregnancy and while breastfeeding, in anyone with a history of an eating disorder, in children under twelve, and at a BMI below 18.5. Diabetes is not a flat no, but it does need supervision: Albosta and Bakke note in Clinical Diabetes and Endocrinology (2021) that anyone taking insulin or a sulfonylurea should be monitored closely so that fasting does not cause hypoglycemia. And if you take a medication that has to be taken with food, the schedule has to be built around it. Any of those is a decision to make with a clinician rather than an app.
That is not boilerplate. Meal timing interacts directly with several common medications: Bushra and colleagues, reviewing food-drug interactions in the Oman Medical Journal (2011), note that lovastatin should be taken with food while rosuvastatin should not, that tetracycline needs an empty stomach an hour before or two hours after a meal, and that azithromycin loses 43% of its bioavailability when taken with food.
How Loopa handles this
Loopa supports 16:8, 18:6, 20:4, OMAD or a window you set yourself, with a live timer that runs as a Live Activity on the iPhone lock screen and a plain explanation of what is happening at each stage. If you are unsure which fits, the coach reads your own history, and will tell you to skip fasting entirely if that is what your logs point to. Fasting is a Premium feature; food, weight and water logging stay free.
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Questions people ask
What does 16:8 mean?
Sixteen hours without eating and an eight-hour window in which you eat, across a 24-hour day. The notation describes the schedule only. It says nothing about what or how much you eat inside the window.
Which fasting window is best?
There is no single best one, and which suits you depends on your schedule, your health and what you will actually sustain. The practical differences are how many meals fit in the window and how large each has to be. Nutritional adequacy is the other one: a secondary analysis of a randomized trial in Nutrients (2025) found that women with overweight or obesity eating within ten hours or less took in less potassium, magnesium and phosphorus, and concluded the window might raise the risk of micronutrient inadequacy. A window that clashes with your life is one you will drop, whatever its theoretical merits. Varady and colleagues advise against fasting altogether in pregnancy, while breastfeeding and with a history of an eating disorder (Nature Reviews Endocrinology, 2022), and anyone on insulin or a sulfonylurea should decide this with a clinician.
Is a longer fast better than a shorter one?
Not automatically. Longer fasting windows compress eating occasions further, which makes nutritional adequacy harder and, for many people, sustainability worse. The shift toward drawing on stored energy is not a threshold that unlocks at a set hour: Anton and colleagues put the crossover anywhere between roughly 12 and 36 hours depending on your starting glycogen and your activity (Obesity, 2018). The National Institute on Aging says there is not yet enough evidence to recommend any particular fasting pattern to the public (Calorie Restriction and Fasting Diets: What Do We Know?).
Does fasting work if I eat the same amount?
The schedule itself is not the active ingredient. Much of the practical effect for most people comes from a shorter window making it easier to eat less without deliberately counting, particularly by closing the late-evening hours. If intake is fully compensated within the window, the change is largely to meal timing rather than to energy balance.
Can I drink anything while fasting?
Practices vary and definitions differ between approaches, which is worth knowing before comparing two sets of advice that appear to contradict each other. If you take medication, some must be taken with food and some must not. Bushra and colleagues give lovastatin, rosuvastatin, tetracycline and azithromycin as examples in the Oman Medical Journal (2011). That is a question for your pharmacist or doctor, and it matters more than any protocol detail.
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