At one point, the health and wellness industry advised eating five to six small meals per day for optimal metabolism. The claim was that smaller meals lead to weight loss, while a fasted state puts the body into “starvation mode,” causing it to hold on to calories.
New research now reveals the exact opposite — that fasting (the practice of going extended periods without food) may boost metabolism, improve insulin sensitivity and help transition your body into a fat-burning state.
Intermittent fasting works similar to a ketogenic diet, depriving your body of glucose to transition to a fat-burning state. Fasting has many health benefits, including weight loss, fat loss and balanced blood sugar. It might prevent Alzheimer’s and chronic diseases and lead to a longer lifespan[*]. Many proponents of fasting say that mentally, fasting is easier to handle than counting calories. Intermittent fasting is extremely popular among those following low carb diets like keto because it helps induce a state of ketosis[*].
There are multiple approaches to fasting. You’ll learn about each intermittent fasting schedule below, and how they benefit individuals based on their goals, lifestyle and activity level.
Types of Intermittent Fasting
Intermittent fasting is a cycle between eating within a certain window of time, then abstaining from food for a defined time period. The length and frequency of your fasting window will depend on the approach you take.
Daily Window Fasting
A daily window fasting protocol involves eating within a certain time period, usually lasting eight to 10 hours, every day of the week. One such method, commonly called the 16/8 fasting, involves eating all meals within eight hours, then not eating the remaining 16 hours of the day. For example, an individual could eat breakfast at 10 a.m. and dinner at 6 p.m., then fast until breakfast the next day.
If you’re new to fasting, you might consider lengthening your eating window. Start with 10 hours (for example, eating breakfast at 9 a.m. and dinner at 7 p.m.) and progress from there. Those experienced with fasting might conduct 20-hour fasts, eating within just a four-hour time frame.
With daily window fasting, consistency matters more than the length of your eating window. Veteran fasters might eat one large meal per day (23:1), while an individual just starting out may eat within 10 hours. The key is to repeat the same cycle of fasting and eating periods each day of the week.
Daily Window Fasting Schedules
Try these various schedules, selecting the eating plan that works best for your lifestyle and experience with fasting. Remember to repeat the same cycle every day.
- 14:10, eat between 9 a.m. – 7 p.m.; fast between 7 p.m. – 9 a.m. the following day
- 16:8, eat between 10 a.m. – 6 p.m.; fast between 6 p.m. – 9 a.m. the following day
- 20:4, eat between 12 p.m. – 4 p.m., fast between 4 p.m. – 12 p.m. the following day
- 23:1, eat one big meal per day, at whatever hour you wish
If you’re new to intermittent fasting and the practice intimidates you, a good gateway is to skip meals. If you eat a large dinner, try skipping breakfast. If you’re busy midday, skip lunch and eat an early dinner.
While conventional wisdom once told us never to skip meals and labeled breakfast as the “most important meal of the day,” new research is questioning these beliefs. Skipping meals can boost your metabolism, not hinder it. It seems that skipping breakfast could be just as beneficial as skipping dinner (so take your pick)[*].
When you skip one meal, be extra careful to not overeat at the next. The goal of skipping meals is to become an intuitive eater — eating only when you are hungry, not simply because it’s “that time” to enjoy a meal. It’s an not an excuse to binge at your next sit-down meal, or practice other forms of disordered eating.
Skipping Meals Schedule
If you choose to skip meals rather than adhere to a regular fasting schedule, your eating schedule might look different each day of the week. Start by skipping one meal when you’re not hungry, then aim to skip several meals scattered throughout the week.
- Skip breakfast: Eat only lunch and dinner
- Skip lunch: Eat breakfast, fast throughout the workday, then eat dinner
- Skip dinner: Enjoy breakfast and lunch then do an overnight fast
Fasting on Alternate Days
Alternate-day fasting consists of fasting one day, then eating regularly the next. On this schedule, your caloric intake on fasting days will be roughly 25% of your usual calories. For example, if you typically eat a 2,000 calorie diet, you will limit your calorie intake to 500 calories during your fasting time.
In a study done by the National Institute of Health, there were many success stories associated with alternate-day fasts. In fact, alternate-day fasting was shown as more effective than calorie restriction, or following a consistent low-calorie diet. It was also shown to increase life span, maintain muscle mass and decrease inflammation [*].
Alternate-Day Fasting Schedule
To stick to an alternate-day fasting schedule, follow the cycle outlined below. You can start any day of the week as long as the cycle remains the same. While only four days are listed, keep the cycle going throughout the entire week.
- Day 1: Eat 25% of usual calories (roughly 500 calories throughout the day)
- Day 2: Eat 1,600 – 2,000 calories
- Day 3: Eat 25% of usual calories (roughly 500 calories throughout the day)
- Day 4: Eat 1,600 – 2,000 calories
For a 24-hour fast (also called eat stop eat), you will select one day per week and not eat on that particular day week to week. For example, you could eat breakfast at 7 a.m. on Mondays, then not eat until 7 a.m. on Tuesdays. If this seems to extreme to you, you could aim to eat in the middle of the day, around 2 p.m., so it doesn’t feel as though you’re going an entire day without eating.
The 24-hour fasting approach has been shown to be extremely effective at losing body fat. Most individuals only do one 24-hour fast per week. Those who are experienced with fasting may choose up to two separate days per week to conduct a 24-hour fast. However, there are conflicting claims as to whether going 48 hours without food in a single week might is helpful or harmful[*].
24-Hour Fasting Schedule
To complete a 24-hour fast, choose the day that you are least physically active. Perhaps choose a rest day from the gym, or a day where you will do light yoga rather than rigorous exercise. A fast schedule might include:
- Sunday: Eat 1,600 – 2,000 calories and conduct regular physical activity.
- Monday: Eat lunch at noon, fast the remainder of the day. Take a rest day from the gym.
- Tuesday: Fast through breakfast, eat lunch and conclude your fast. Resume physical activity and eat dinner as usual.
Frequency of Intermittent Fasting
Now that you understand how to implement an intermittent fasting schedule, it raises a question: How often should you fast?
The answer varies from individual to individual. Most individuals implement the above schedules every week or every other week. If you’re new to fasting, start with a moderate schedule, trying it every other week or every three weeks. If your body adapts well, aim for a regular, weekly schedule.
There’s no wrong answer here. Pay close attention to how your body responds to your fasting schedule, and adjust as needed. Keep in mind that life changes can happen. You may need to tweak your schedule to allow for social gatherings, vacations and physical activity or competition.
A Final Note on Calories and Nutrition
When implementing an intermittent fasting schedule, make sure you get enough calories during your eating windows. Be mindful of what you’re eating in between fasting periods, focusing on real, whole foods so you get the most nutrients per calorie.
Fasting is no excuse to skimp on optimal nutrition. In fact, since you are eating less, you should aim to get the most nutrients you possibly can extract from those meals. Don’t allow fasting to be the reason to eat donuts, soda and candy.
No one way of fasting is perfect for everyone, but just about any healthy person can benefit from intermittent fasting. Those who are intimidated by an intermittent fasting schedule might try skipping meals, then moving into a 16:8 fast. If you find these beneficial, move on to an alternate-day fast where you still consume 500 calories on fasting days. Finally, try a full 24-hour fast one day per week.
Intermittent fasting is generally safe to do as often as you’d like. Just make sure you’re eating enough food in between and eating whole foods. And lastly, don’t be afraid to experiment and find what works best for you.
Dr. Anthony Gustin is a board-certified sports chiropractor, functional medicine practitioner, entrepreneur, podcast host, and founder of Perfect Keto.
After growing his sports rehab and functional medicine clinics to six locations in San Francisco, he shifted his mission to help as many people as possible achieve optimal health and well-being.