What Is Intermittent Fasting? What Are the Different Types?

Intermittent fasting involves eating for a certain period of time (eating window) and then not eating for a certain period of time (fasting window). Here are the different types:

• 12:12 [3]: You eat for 12 hours of the day and fast for the other 12. For example you'd eat from 7:00 a.m. until 7:00 p.m. every day. This is perfect for IF newbies as it's consistent every day, you can eat three regular-timed meals, and it can help those who tend to overindulge with bedtime snacks.
• 16:8 [4] or Leangains: Probably the most popular form of IF, you eat eight hours of the day and fast for 16. An example would be eating from noon until 8 p.m., basically just skipping breakfast. This is something you can do every day, and still be able to enjoy dinner with the family or out with friends. If you're a morning eater, you can eat from 8 a.m. until 4 p.m. — you can make the eight-hour eating window work for you.
• 5:2 [5] or Fast Diet: Twice a week (nonconsecutive days) you restrict calories to around 500 calories a day (600 calories for men), and on the other five days, you eat as if you are not on a diet. On the fasting days, you can eat three small meals or two slightly larger meals (lunch and dinner).
• Eat, Stop, Eat [6]: Once or twice a week, you fast for 24 hours — you never go a day without eating. Eat dinner Monday night, finishing by 7:00 p.m., fast all day Tuesday, then eat dinner at 7:00 p.m. on Tuesday night. Choose the one or two days that work with your schedule. This might be a good option if you've tried 16:8 or 5:2 and have hit a plateau.
• Alternate Day Fasting [7]: You fast one day (or in a modified version, eat up to 500 calories that day), then eat normally the next day, and repeat. You essentially fast every other day. Alternate-day fasting might be a good plan if it fits your lifestyle, if you have a lot of weight to lose and want to see quicker results, or if you've tried 16:8 or 5:2 and have hit a weight-loss plateau.
• 20:4 or Warrior Diet [8]: You fast for 20 hours out of the day, then have a four-hour eating window. An example would be eating from 2:00 p.m. until 6:00 p.m. This is an advanced method of IF, but some people with busy schedules or issues with eating all day prefer this method.

How Do I Choose the Right Method?

Research all the methods and choose the one that best suits your lifestyle. Take into consideration your work schedule, your workout routine, your family life, and your goals. There is no one plan that's best [9] or the most effective — it's whichever plan is best for you, one that you can easily sustain.

Many people begin with the 12:12 plan, then move toward 16:8. Some may prefer doing 5:2, because they only have to think about fasting twice a week. Try a method for a few weeks and see how you feel, and then modify if you need to.

Won't I Be Starving and Tired?!

When you're used to eating every three to four hours, yes, you will be hungry. You may get a headache, you might feel tired, cranky, irritable, and maybe even nauseous. Know that these feelings won't last.

Fasting will be challenging for the first four to seven days, and maybe even for longer, but Dr. Jason Fung, MD, author of The Complete Guide to Fasting [10] reassures that your body quickly adjusts. After the first week or two, your mind will feel clear, you'll likely have more energy, and you'll actually feel less hungry.

How Can I Alleviate Symptoms?

To prevent headaches, hunger pangs, fatigue, and other side effects [11], drink plenty of water [12]. Pound a big glass right when you wake up and drink throughout your fast.

Caffeine can also help [13], so sip on black coffee or green tea (unsweetened and without milk or creamer). Going for a walk or light stretching can also make you feel better. Rest when you need to during this adjustment period.

Will Intermittent Fasting Help Me Lose Weight?

Many people start doing intermittent fasting to lose weight [14]. Since you're not eating for long periods of time, there are fewer opportunities to consume calories, which is why IF helps people eat fewer calories effortlessly. Dr. Luiza Petre [15], MD, a weight-loss specialist and board-certified cardiologist, told POPSUGAR that IF also raises human growth hormone and lowers insulin levels, both of which help burn more fat.

Are There Any Other Health Benefits?

Intermittent fasting has many health benefits [16], and keeping these in mind can help you stay motivated on days when you don't feel like fasting.

• Improved digestion and decreased belly bloat
• More energy
• Improved focus and mental clarity
• Disease prevention [17] and autophagy [18] (when bad or weak cells are destroyed and new stronger ones are created)
• Help overcoming sugar cravings [19] and overeating
• Improved sleep [20]

What Can I Drink During My Fasting Window?

This is a short list, so it won't be hard to remember. You can have water of any kind, including sparkling and mineral, just avoid any that contain flavors or sweeteners, even stevia. Some experts say you can have a slice of lemon in your water or a splash of apple cider vinegar [21]. If this is necessary for your compliance, go for it, but if not, drink it plain.

Black coffee and herbal tea is also allowed, just avoid cream or sweeteners — this includes Bulletproof Coffee [22]. Avoid BCAAs [23], smoothies, protein shakes, Gatorade, and broth. The point here is that if you take in calories, then you're not technically fasting.

What Should I Eat and Drink During My Eating Window?

One of the amazing things about intermittent fasting [24] is there are no restrictions on what you can eat during your eating window. You can eat whatever foods you like, including the ones you may have cut out when trying to lose weight in the past [25]. Since you're saving calories by fasting during certain times of the day, you won't feel bad about eating pizza or ice cream or having a glass of wine.

That being said, you'll have the best results if you focus on mostly healthy, whole foods. Avoid refined carbs, sugar, and fast food, and load up on the lean proteins, healthy fats, whole grains, and fresh fruits and veggies. If you have medications or vitamins, take them during this time.

You can structure your meals and snacks however you like [26] during your eating window.
Just make sure you eat enough during your eating windows, meaning eating until satiation (but not overeating). This will help you feel more satisfied when you're fasting.

Can I Exercise?

Yes, you can exercise and do intermittent fasting [27]! In fact some athletes prefer doing workouts fasted to optimize their energy levels, build more muscle mass [28], and to burn more fat [29]. This may take some getting used to though, especially if you usually eat before a workout and if your workouts are vigorous.

Focus on getting your IF schedule under control, which might mean putting your workouts on hold, doing easier forms of exercise, or changing the times you work out to suit your IF schedule. Gradually work up to your regular workout routine; this shouldn't take more than a few weeks.

What Are Intermittent Fasting No-Nos?

Here are some mistakes to avoid when intermittent fasting [30]:

• Choosing the wrong plan: Be realistic about what plan will work with your lifestyle and your personality. Alternate day fasting [31] may sound good on paper because it gets results quickly, but if you have a stressful job where you work long hours, this will be hard to maintain.
• Doing too much, too soon: Maybe you want to work up to 20:4, but you'll find the most success with sticking with IF if you start off with 12:12 to allow your body time to adjust.
• Giving up too soon: Intermittent fasting takes a certain amount of discipline and a period of time where you may not feel good or see results. Unless you're experiencing medical issues, stick with it for at least a month before throwing in the towel.
• Eating too much during your eating window: Don't make up for lost time by eating way more [32] than you would if you weren't fasting. Intermittent fasting doesn't give you a green light to overeat because you will gain weight.
• Eating too little: If you don't eat enough, you'll feel especially hungry the next day, which can cause you to overeat.
• Eating the wrong foods: Skip the junk and get your fill of protein, healthy fats, fiber, and complex carbs. This will satiate you longer, continue to help you build muscle, maintain a healthy brain, and just make you feel more energetic and happier overall.

How Do I Know If It's Working?

Intermittent fasting allows you to tap into the fat stores you have built up. One way to know this is happening is if you weigh yourself throughout your IF journey. Be aware that working out can cause muscle gain, which will actually show numbers increasing on the scale, so also be sure to take measurements and progress pics [33]. The numbers won't mean a thing if your before-and-after photos prove IF is working.

Since weight loss isn't the only benefit of IF [34], also take note of how you feel. Do you have more energy, a clearer mind, better digestion [35], and are you sleeping better? Do you feel more in control of your eating habits [36] and your cravings? Do you have a lightness about you, do you feel happier [37]? Write all this down in a journal so you can look back and determine if intermittent fasting is working for you [38].