How long should you fast for weight loss

Common fasting windows

Short answer: for most people, a daily fast of 12 to 16 hours is long enough. Longer fasts don't burn fat faster on their own. What matters is whether your fasting window helps you eat less overall.

See sample schedules on the fasting schedules page or build your own with the intermittent fasting calculator.

What the research says

In a 12-month trial published in the New England Journal of Medicine in 2022, people who ate in an 8-hour window and cut calories lost about the same weight as people who only cut calories. Fasting is a tool for eating less, not a fat-burning switch.

How to pick your window

• Start at 12 hours, mostly overnight.
• Add an hour every few days if it feels easy.
• Stop at the longest window you can keep without bingeing later.
• Keep protein high in the window you eat. See the protein calculator.

A sample 16:8 day

Common fasting mistakes

• Overeating in the window. Fasting only helps if total calories drop.
• Skimping on protein. Fewer meals make it harder to hit your target.
• Jumping straight to long fasts. Build up gradually.
• Ignoring fluids. Drink water through the fast.

What happens during a fast

For the first several hours after a meal, your body runs mostly on the food you just ate. As the hours pass, it draws on stored carbs in the liver and gradually burns more fat.

Researchers call this the metabolic switch, and it often starts somewhere after about 12 hours without food. Most people get there overnight anyway. Over a week, fat loss still depends on total calories, not on how long each fast lasts.

More on the research

• NEJM 2022: 139 adults with obesity cut calories for 12 months, with or without an 8-hour eating window. Both groups lost about 6 to 8 kg on average, and the difference wasn't significant.
• JAMA Internal Medicine 2020: In a 12-week trial, 16:8 eating without calorie targets led to very small weight loss that wasn't different from a regular 3-meal schedule.
• Overall: Fasting seems to work about as well as standard calorie cutting, no better. It's a good fit if skipping late-night snacks or breakfast comes easily to you.

Longer fasts and OMAD

Fasts of 20 hours or more, or one meal a day, make it hard to eat enough protein, fiber and nutrients in a short window. Many people get headaches, dizziness or a big rebound appetite. Fasts longer than 24 hours, and especially multi-day fasts, should only happen with medical supervision.

Compare the trade-offs on the OMAD and 16:8 pages.

Getting through the hungry hours

• Drink water, sparkling water, black coffee or plain tea
• Make your last meal rich in protein and fiber so it lasts
• Keep busy during your usual snack time; hunger often comes in waves that pass
• Protect your sleep, since tired days make fasting harder

If 16 hours feels rough, try 14:10. Small wins count.

Make the eating window count

A shorter window means fewer meals, so each one needs to pull its weight. Build meals around protein and vegetables, add a fiber-rich carb, and keep sweets and fried foods for occasional treats.

A 16:8 window with 2 meals and a snack makes a protein goal much easier to reach than OMAD. Plan your meals with the macro calculator.

Is fasting right for you

Fasting tends to suit people who aren't hungry in the morning, who snack late at night, or who like simple rules. It tends not to suit people who get shaky or irritable without food, train early, or find that restriction leads to bingeing.

If it doesn't fit, regular meals with a modest deficit work just as well.

Who should be careful

Check with a clinician first if you take insulin or a sulfonylurea, are pregnant or breastfeeding, have a history of eating disorders, or take a GLP-1 medicine and already eat very little. See fasting and GLP-1s.