Intermittent fasting (IF) has gained popularity in the last several years as an effective and sustainable way to lose weight. Some people come to IF because they're on a keto diet, and both IF and eating keto can lower insulin levels to help with weight loss. Other people have found IF because scientific research shows intermittent fasting can help with weight loss, as well offer other health benefits such as increased energy, improved digestion and reduced bloating, and improved memory, sleep, and skin. IF can also reduce inflammation and the risk of certain conditions like cardiovascular disease, cancer, and asthma, and increase longevity. What a long list, right?!
What's great about intermittent fasting is that it focuses on when you eat, not what you eat. It doesn't matter if you're plant-based, vegetarian, gluten-free, paleo, or eating low-carb - just eat the way you want in your eating window. Since you're only eating a certain number of hours a day, restrictive diets aren't necessary, and also not recommended. Many people find it easy to fit IF into their lives since seven to nine hours of the fast are spent sleeping.
Intermittent Fasting Methods and Their Health Benefits
Amy Shah, MD, a double board-certified medical doctor and wellness expert specializing in food allergies, hormones, and gut health, said that intermittent fasting is similar to exercise in that there are so many different forms and intensities. Here are the most common methods of intermittent fasting and their health benefits:
Intermittent Fasting Method Description Health Benefits 12:12 Fast 12 hours and eat in a 12-hour window. Example: eat 7 a.m. until 7 p.m. every day. This means you can eat three regularly timed meals with snacks in between. Improved digestion, increased energy, improved sleep 14:10 Fast 14 hours and eat in a 10-hour window. Example: eat 9 a.m. until 7 p.m. every day. This means you can eat three regularly timed meals with snacks in between. Improved digestion, increased energy, improved sleep, weight loss 16:8 or Leangains Probably the most well-known form of IF, fast 16 hours and eat in an eight-hour window. Example: eat from noon until 8 p.m. every day, basically just skipping breakfast. Some prefer to eat earlier in the day, such as from 7 a.m to 3 p.m., so they skip dinner. Improved digestion, increased energy, improved sleep, improved mental clarity, weight loss 18:6 Fast 18 hours and eat in a six-hour window. Example: eat from noon until 6 p.m. every day. Improved digestion, increased energy, improved sleep, improved mental clarity, weight loss 20:4 Fast 20 hours and eat in a four-hour window. Example: eat from 1 p.m. until 5 p.m. every day. Improved digestion, increased energy, improved sleep, improved mental clarity, weight loss 5:2 or The Fast Diet Eat normally five days a week. On two nonconsecutive days, fast or eat 500 calories a day (600 calories for men). Improved digestion, increased energy, improved sleep, improved mental clarity, weight loss, autophagy (if fasting over 24 hours) Alternate-Day Fasting or 4:3 Fast one day (or, in a modified version, eat up to 500 calories that day), eat normally the next day, and repeat, fasting every other day. Some people choose to do 4:3 instead, so they choose three days to fast such as Monday, Wednesday, and Friday, so every week is the same. Improved digestion, increased energy, improved sleep, improved mental clarity, weight loss, autophagy (if fasting over 24 hours) Eat Stop Eat Fast 24 hours once or twice a week. Improved digestion, increased energy, improved sleep, improved mental clarity, weight loss, autophagy (if fasting over 24 hours)
Is Intermittent Fasting Healthy Long-Term?
As you can see from the chart, different IF methods will offer different health benefits. Dr. Shah said when you do fasting overnight, which she calls circadian fasting, you will get benefits in your insulin levels, blood sugar, inflammation, and brain-derived neurotrophic factor, plus lower blood pressure and lower risk of cancer recurrence. It may also help with weight loss. "[T]his is the area where there is a lot of debate. Some studies show no significant difference between IF and traditional eating-healthy plans when it comes to weight loss," Dr. Shah said.
If your goal is to improve energy, digestion, and sleep, then just 12 to 16 hours of fasting is a great method. If you're looking for autophagy, which is a full cleansing of cells, you need at least 24 hours of fasting. Autophagy can help get rid of weak or damaged cells, which could help prevent conditions such as Parkinson's, dementia, and cancer.
If your goal is weight loss, endocrinologist Braden Barnett, MD, a medical advisor to the diabetes-management app Klinio, said that any form of daily fasting can offer results. Some people may lose weight with fasting as little as 12 to 16 hours a day. But others may find they need to fast 18 hours a day or for 24-hour stretches.
Dr. Barnett said that weight loss through fasting has offered many health benefits to his patients, including improved insulin sensitivity (better blood-sugar levels), lower blood pressure and cholesterol levels, as well as reduced risk or severity of cardiovascular issues such as heart attacks and strokes. Intermittent fasters may also have reduced sleep apnea and depression, and improved quality of life, overall physical function, and mobility. They may even experience antiaging properties such as reduced oxidative damage to DNA, which can lead to increased longevity.
Who Should Not Do Intermittent Fasting?
Anyone pregnant, breastfeeding, underweight, or who has a history with disordered eating should not fast, warned Dr. Barnett. People on insulin or blood-sugar-lowering meds who might need to eat more frequently should also avoid IF, said Dr. Shah, who added that any person with a medical diagnosis or medication should check with their doctor about any special concerns.
What Is the Healthiest Form of Intermittent Fasting?
If you're just starting IF, try the least intense method with the shortest fasting time, either 12:12 or 14:10. You can do this every day or a few times a week. Then, if you choose, you can gradually build up to 16:8, 18:6, or 24-hour fasts one to three times a week to experience weight loss, autophagy, and disease prevention. "I personally think that prolonged fasting (over 24 hours) probably offers the most antiaging benefits, but is also the hardest to execute," said Dr. Barnett.
The healthiest form of intermittent fasting is the one that feels easy, and the one you can sustain while still feeling well both physically and mentally. Don't think of IF as a diet - it's a lifestyle you can maintain long-term because it's flexible. You can tweak your fasting schedule to fit with your life and your goals. Maybe you do 18:6 Monday through Friday, and don't fast at all on the weekends. Maybe you do a "meal-less Monday" and eat normally the rest of the week. Or you might love fasting every day for 14 hours. Mix it up, play around, and find the IF pattern that works for you. This month you might love skipping breakfast with 16:8, but next month you might find you'd rather skip dinner. IF should make your life easier and healthier!
Aside from the health benefits mentioned above, many people will also experience freedom from restrictive diets and food rules, improved confidence, and an improved relationship with food and their bodies. But IF isn't for everyone, so consult with your doctor and do your own personal research before trying any methods of intermittent fasting.