Everything You Need to Know Before Trying Intermittent Fasting

Oren Aks/Thrillist

Choosing not to eat for extended periods of time -- called intermittent fasting -- sounds like torture, or some kind of crash diet that's doomed to fail. Aren't you supposed to eat small meals throughout the day to keep your metabolism going? Won't you get so hungry that you'll inhale an entire box of pizza rolls and maybe an actual pizza at your next meal?

But intermittent fasting may actually be an effective strategy for weight loss, among other benefits -- research suggests it helps extend life, decreases fasting insulin levels, and increases metabolic rate. As a bonus, some people have been able to lose weight on intermittent fasting plans while still eating their favorite foods.

It sounds like a late-night infomercial, but most major religions recommend some form of fasting, so it's not like this is a new fad diet. If you're considering taking the plunge, here's what you need to know.

There are three basic ways to do it

Leangains, UpDayDownDay, and the Warrior diet are the three main intermittent fasting strategies, with infinite variations possible. Leangains is the most popular form of intermittent fasting, requiring you to eat during an eight-hour window and fast the other 16 each day. People typically eat their last meal before 8 or 9pm, and won't eat again until 12 or 1pm the next afternoon.

This is the easiest plan to stick to because it's as simple as cutting out breakfast. Dr. Adam Splaver, clinical cardiologist and co-founder of NanoHealth Associates, recommends the Leangains plan to his patients, especially those starting out on intermittent fasting for the first time, because it's the most tolerable and fits into people's schedules the best.

The UpDayDownDay plan is based more on a weekly schedule. People may eat normally for five or six days, then fast the other one or two. On fasting days, people usually cut their calories to 500 or less, and still reap the benefits of intermittent fasting. Dr. Splaver warns this may be difficult for people to stick to since your body is used to getting food at regular intervals, so on fasting days you may experience mood swings and hunger pangs.

The Warrior diet is the most extreme, as suggested by its name. WARRIORS don't need to eat. On this plan, you eat during a 4-hour window, and fast the other 20 hours. It's the most difficult to stick to, since you have to do it every day, and still designed for you to to eat your daily calories within that small window.

It's important on any IF plan to make sure that the calories you eat during the feeding times -- sounds like the zoo! -- don't exceed your daily recommended amount.

In general, common sense prevails; maintaining healthy meals during feeding times is still important, and daily calories can add up. So don't reward yourself for making it through a fast by gorging at an all-you-can-eat Chinese buffet.

You'll be hangry at first

There's no way to sugarcoat this; you'll be hungry and cranky for the first few days. Making any kind of major changes to your eating schedule will do this as your body adjusts to what time it expects food. Warn all of your loved ones that you may turn into a monster for a little bit.

"In that first week or so, you may feel tired, you may feel a little bit sluggish, some people may get a little bit angry, but it’s a metabolic adjustment that’s going on with your brain," Dr. Splaver says.

It's the worst in the beginning, but should taper off after your body gets used to this new way of eating, especially with the leangains plan, which is a more regular eating and fasting schedule.

It’s not ideal for sedentary people

IF has been called the lazy person's diet, but that's not entirely accurate. Getting some exercise is important if you want to reap the benefits of intermittent fasting. "If you are sedentary, it’s going to be difficult for you to lose weight because you’ve programmed your body to store whatever you’re eating," Dr. Splaver says. So make sure you're scheduling moderate exercise (ideally during your feeding window, so you have the energy to workout).

It’s also not great for super-active people

"[For] someone who is consistently very active, a diet like this may be detrimental because you will break down muscle," Dr. Splaver explains. People who are incredibly active -- think serious athletes, extreme Crossfitters, etc. -- need extra protein, carbs, and fat to actively build muscle, which they may not get with intermittent fasting. The long stretches of time without eating could also combat the gains you're getting at the gym.

But for the average person who works out a few days a week and does a mix of cardio and moderate weight training, Dr. Splaver says intermittent fasting is a good option.

It’s a great way to jumpstart major weight loss

People have found major success with intermittent fasting, especially when it seems like nothing else has been working. "Intermittent fasting is great if you want to jumpstart the process, if you want to lose 10-20 lbs quickly," Dr. Splaver says. "In order to maintain and then continue that loss long term, switch to something that’s more manageable from a lifestyle perspective." But he adds that it's not ideal for someone who has 50 or more pounds to lose, because it's much harder to keep off that major weight loss in the long run.

But it's not for people who want to lose a few pounds

Dr. Splaver notes that IF isn't as effective for people who want to lose only a couple pounds. It may take longer, and the body just doesn't reap the same benefits as someone who has 20 or more to lose.

IF is best followed for a couple months

Intermittent fasting may be a great way to shed pounds, but isn't really designed to be sustainable over long periods of time. Dr. Splaver suggests it's an ideal to try it for one to three months, then wean yourself off of it and onto a low-glycemic diet. He suggests eating plans like the South Beach and Zone, which are lower in carbohydrates and plentiful in vegetables, healthy fats, and lean protein.

Check with your doctor first

Ah, the caveat that follows any diet change! It's especially true here -- before embarking on an intermittent fasting plan, it's best to check with your doctor first to see if this type of eating will work with your body type and lifestyle. IF isn't ideal for people with diabetes, a history of disordered eating, or other health complications, so make sure you get the go-ahead from your doc before taking the plunge.

Sign up here for our daily Thrillist email, and get your fix of the best in food/drink/fun.

Christina Stiehl is a Health and Fitness Staff Writer for Thrillist. She's not above a good trip to the Pizza Hut lunch buffet. Follow her: @ChristinaStiehl.