Walk into a mechanic’s workshop and you won’t see just one device hanging on the wall. You’ll spot a peg board covered in tools of all different shapes and sizes. Each has a purpose and a value, but you wouldn’t necessarily use all of them at the same time.
Diet strategies are not much different: There are a lot of different tools you can use to keep your body working and look the way you want. Understanding what those tools are, and when and how you can best use them, will help you keep your eating on track.
One such dietary tool that’s received a lot of attention is intermittent fasting (IF). Read enough stories about intermittent fasting, and it starts to sound like magic, with benefits going well beyond weight loss. Proponents say—and some preliminary research agrees—that IF can help improve important health biomarkers (like fasting blood glucose and triglyceride levels), turn back the clock on time (anti-aging), and even help fight neurodegenerative diseases (health defense).
Intermittent fasting success depends a lot on your personal preferences, schedule, and how you feel when you fast.
But, as you’ll see, those benefits vary from person-to-person—and also depend on the style of intermittent fasting you prefer to follow.
Should you try IF? Here’s a guide that will help you answer those questions and help you understand the dietary tools that might be best for your body.
What exactly is intermittent fasting, anyway?
At the most basic level, intermittent fasting is something everyone does every day — it’s a break between meals. The most common of which occurs when you fast between your final meal of the day (usually dinner) and breakfast the following morning. (Hence the name, “break the fast.”)
Within the health and fitness realm, however, people use the term “intermittent fasting” to describe times when you intentionally extend that overnight fast for periods of time lasting anywhere from 12 to 24 hours.
The Lean Gains approach, for example, advocates a 16-hour fast. So if you started eating at 8 am, for instance, you would finish eating for the day at 4 pm, then fast for the rest of the day, and start eating again at 8 am the next day. The hours that you fast don’t matter, just that you go 16 consecutive hours without eating followed by 8 hours where you do. That cycle repeats every day.
There’s also the Warrior Diet, which is a 20-hour fast coupled with a 4-hour eating window. And the Eat Stop Eat protocol, which incorporates one full 24-hour break from eating at least one day per week. But then the rest of the week you eat on any schedule that you desire.
We’ll weigh the pros and cons of each of these approaches in a bit. For now, let’s address the even larger question at hand…
Does intermittent fasting work?
For all the health benefits fasting recently linked to intermittent fasting, there’s one overarching reason why most people try it: To lose fat.
And if that’s your goal, then the answer is yes, intermittent fasting might help — but not for the reasons you think.
There isn’t some magic at work that’s responsible for the numerous intermittent fasting success stories.
The reason people following fasting protocols are able to lose weight is quite straightforward: They eat fewer calories than they burn throughout the day because the “eating windows” or weekly fast makes it harder to overeat.
Limiting the hours when you can eat helps you eat fewer calories overall. Think about it: let’s say your plan for weight loss required you eating 2,000 calories per day. It’s going to be easier to stick to that goal if you can only eat for an 8-hour window in a day as opposed to 14 hours. When you consider fat loss alone (more to come on the other health, longevity, and disease-fighting benefits), intermittent fasting provides an easy-to-follow structure that naturally creates habits that make it harder to overeat.
Can you still over-eat within a limited time window and gain weight? Of course. But that’s the case with any eating approach. But instead of thinking about how many meals to eat, you just set a start time and stop time to your meals, and then eat in a way that feels best for you — assuming you stay within the amounts that you should be eating.
The benefit is that this provides lots of flexibility and allows you to select the eating window (or style of IF — we’ll cover all of these to help you find the best option for you) that fits your lifestyle. Perhaps you skip breakfast, have your first meal around noon, and then end in the early evening. Or you could push back later, and then cater to your late-night eating preferences. Or maybe you do the opposite — start eating early and end in the early evening to avoid the late-night snack habit. Any of these approaches can work — it’s all about your preference.
All these intermittent fasting schedules can create an energy deficit that leads to weight and fat loss. (Again, we’ll cover a detailed breakdown of how to make this happen a little later in this post.) And while the details of nutrition still matter — proteins, carbs, and fats — it’s the simplified approach to eating less overall that makes intermittent fasting popular.
“Yes, the macronutrient splits matter a little bit. Yes, timing matters maybe a bit more. But to the largest extent, all the data suggests the real contributor to fat loss and weight loss is total calories,” says Anthony D’Orazio, director of nutrition and physique at Complete Human Performance.
And when you consider that fewer hours during the day to eat means fewer calories (or having one day — like in Eat Stop Eat method — where you don’t eat at all), you can see how week over week, it’s easy to limit your calories. After all, that’s what really works with fat loss. Thinking less about any given meal or one day, and instead of seeing the big picture and trying to limit total calories on a weekly or monthly basis. When the deficit adds up over time, so does your weight loss.
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OK, but will intermittent fasting work for you?
While the answer to that will be different for every reader, it’s worth noting that certain people tend to do better with fasting than others.
For example, research suggests that fasting works well for men. For example, this eight-week study of young males who had experience with resistance-training showed that those who ate only during an eight-hour window lost 16.4 percent of their fat mass, compared to just under 3 percent for a group who ate the same calories over a longer period.
Krista Scott-Dixon, director of curriculum at Precision Nutrition, cautions that those results may not translate to everyone, since the study’s subjects “are naturally lean, and already forget to eat half the time.”
On the other hand, there’s a mixed bag of results for women. While fasting can offer the same daily structure that helps restrict calories by having fewer hours to eat, the problems is that women tend to experience more unwanted side effects from fasting, especially with a prolonged fasting period (think 16 hours or longer) because of their hormonal environment.
“[Women’s] bodies are exquisitely sensitive to nutrient deprivation,” Scott-Dixon says. Women on a prolonged IF protocol may see a reduction in thyroid output, a decrease of estrogen, and other adverse hormonal effects. Pair that with an increase in exercise, and it could bring about menopausal-like effects (i.e. you stop getting your cycle) as well as powerful cravings.
Scott-Dixon also recommends that one group steer clear of IF: Anyone who has a history of, or tendency towards, disordered eating. “If you look at IF forums or groups, people are devoting an unhealthy amount of attention to when they get to eat, how much they get to eat, what they get to eat once they break their fast. It starts to get into a really behaviorally, emotionally weird area.” This behavior, Scott-Dixon warns, “can develop into a disordered eating kind of pattern.”
Does that mean that you need to steer clear of IF? Not exactly. It’s more of a general warning for any type of diet behavior. Counting macros and calories can lead to disordered eating just as much as intermittent fasting. So the point isn’t to avoid all potentially helpful dietary strategies, but — instead — consider how you feel and how much you stress while following an intermittent fasting style of eating. If you are stressing less and feel more in control, then great. If food and clock start to dominate your life, you might want to question if it’s helping or hurting.
Ultimately, intermittent fasting success depends a lot on your personal preferences, schedule, and how you feel when you fast.
If you find that intermittent fasting is a fit for you or something you want to try, you can do it and know it works about as well as any diet that results in a calorie deficit. A 2017 study in the Journal of the American Medical Association Internal Medicine found that obese people who followed a more-typical daily calorie restriction diet (they ate 75% of their target total every day) and those who followed an alternate-day fasting approach (they ate 25% of their target one day, then 125% the next) experienced similar mean weight loss totals over a year-long span.
Are there other health benefits of intermittent fasting?
Intermittent fasting protocols reportedly slow down aging, improve mental acuity, increase longevity, reduce chronic disease risks, and actually decrease hunger. While many of those conclusions are based on preliminary research, science to date does back up some of them.
In a 2016 study, researchers concluded that intermittent fasting prevented neuron damage in the brain. Animal studies have found an association between fasting and reduced risk of lymphoma. Four different studies have found a correlation between fasting and reduced symptoms of arthritis, and others have suggested that by reinforcing your circadian rhythms, fasting may promote longevity.
What’s fueling these benefits? A process called autophagy. In autophagy, your body essentially kills off old, diseased or otherwise incomplete cells.
“A lot of neurodegenerative diseases are characterized by this build up of crud, [old cells that need to be cleared away,” Scott-Dixon says. “Autophagy is like the cleanup crew that kind of goes in and munches everything up.”
Autophagy is triggered by fasting. When you eat, autophagy hits snooze. Your body has to redirect the energy from the autophagy process to the digestive process to break down your food.
You wouldn’t feel this process at work. But when you fast, you might feel like you have greater mental clarity and better hunger control. Martin Berkhan, who designed the Lean Gains diet, says these effects are at least partially due to chemistry. Your body releases chemicals called catecholamines after 16-24 hours of fasting to keep you from feeling famished.
One of the catecholamines that get released during a fast is adrenaline, a stress hormone that improves mental cognition. “For some people, that feels amazing,” Scott-Dixon says. “[But] You’re not tuning into the universe in any magic way.”
When you read reports of people feeling more productive and focused when they fast, it’s not just your body’s chemicals at work. “There is also a mental component to it,” Berkhan says. “If you know you’re not eating until noon, you will find other things to occupy your mind with, such as work, which also helps keep your hunger in check.”
The transition away from “I’m hungry” to “I’m thinking about something else” can take a few days. But Berkhan says that once you get used to it, the experience is liberating. He says he used to be pre-occupied with food, following a bodybuilder’s regimen of constant feeding and macro-counting, but now he no longer “feels doomed to a life of obsessive calorie counting,” so he can concentrate more on his work.
Which intermittent fasting schedule should you follow?
As we discussed earlier, there are lots of different fasting protocols. Despite some of the marketing copy you might see out there, none have been conclusively proven to be more effective than the others in a clinical study. So the question isn’t which one’s best, but which diet is best for you.
“Like with any sort of plan, if you don’t or can’t follow it, it’s a shit plan,” D’Orazio says. Finding a protocol that you’ll actually follow is more important than believing one fast period is superior to another. So if you know you can’t live without food for a whole day—or at least can’t do it and not rip your spouse’s head off—don’t do it.
Here are some options you can try.
The simplest intermittent fasting approach: 12 hours off/12 hours on
The easiest fasting protocol is probably also the least extreme: Fasting for 12 hours a day and eating during the other 12.
“If you tend not to eat that close to bed—say three hours before you sleep—all of the sudden you’re at 11 hours without eating,” says D’Orazio. Wait an hour after you get up to eat breakfast, and you’re in the clear.
The drawback here is that, as far as calorie restriction goes, 12 hours can be a long eating window. In fact, even if you’re not following an intermittent fasting protocol, it’s possible that your daily meals only span a 13- or 14-hour window. Dropping your eating period by an hour or two may not be enough to put you at an energy deficit or change your habits in a way that will produce the fat loss goals you want.
If you do opt for a 12/12 split, studies indicate that you may experience some of the health benefits of fasting, including increased insulin sensitivity. However, studies on Ramadan fasters suggest that increases in glucagon (an inducer of autophagy) happen more in the 16-hour range, so autophagy may not be maximized with a 12-hour fast.
The Lean Gains approach: 16 hours off/8 hours on
Berkhan’s Lean Gains method is designed for lifters and fitness enthusiasts who also have a real life. “It’s also easy for people with regular working hours to maintain. It’s not very hard to eat lunch at noon, when you ate something at 8-9 pm the previous day, once you get used to skipping breakfast,” says Berkhan.
The approach is favorable because a 16-hour period is long enough to trigger autophagy (as discussed above), while an 8-hour eating window may be more conducive to fat loss for some. And you can eat around your training, placing an emphasis on eating plenty of protein, controlling carbs, and scheduling meals around your workouts.
Berkhan himself, like many Lean Gains enthusiasts, doesn’t eat a full meal before training. Instead, he opts to consume some BCAAs in advance of his workout. D’Orazio suggests a different approach. He says that in order to maximize training performance and maintain muscle, someone on a 16/8 fasting split should try and eat at least one meal before their workout, and place their training session in the middle of their eating window.
Ultimately, you’ll have to find the pattern that works best for you through trial and error, taking into consideration the time of day when you can actually work out. If your job allows you to train in the middle of the day, then you could try either Berkhan or D’Orazio’s approach. But if your schedule is such that the only time you can lift is first thing in the morning, you may not have time to prepare a meal, eat, and then digest all ahead of your training.
The Warrior Diet: 20 hours off/4 hours on
In The Warrior Diet, author Ori Hofmekler took inspiration from history to design a 20-hour fast with a 4-hour feed—the idea is to mimic a hunter/gatherer lifestyle where you worked or battled all day, then chowed down at day’s end. Small snacks, like a piece of fruit or yogurt, are allowed during the day, but Hofmekler recommends eating most of your food in a giant, Viking-like feast at the end of the day. In this way, you’re not fully fasting on the Warrior Diet, but underfeeding all day, then overfeeding at night.
“Our ancestors consumed food much less frequently, and often had to subsist on one large meal per day or go for several days at a time without food,” Hofmekler’s book quotes researcher Mark Mattson as saying. “Thus, from an evolutionary perspective, human beings were adapted to intermittent feeding rather than to grazing.”
Whether or not this is historically accurate or makes sense from an evolutionary standpoint is for the anthropologists to debate. From a dietary perspective, one thing is clear: If you did limit your eating to just four hours a day, it might actually be difficult to overeat. Studies support this analysis. For example, a 2007 study found that having one meal per day was associated with a greater loss in weight and fat mass compared to eating three daily meals.
Note that we said “difficult” but not “impossible.” Roman soldiers and Vikings didn’t have high-calorie fare like Big Macs to chow down on. Fasting for even an extended period like this doesn’t give you license to eat straight-up anything and expect to lose weight.
If you find that a 20-hour fast isn’t so much a path to mental clarity as it is the road to hangrytown, then this kind of protocol can be a tough one. You might have trouble concentrating at work, or worse, find you have a short fuse and lash out at people. In that case, a warrior approach is not recommended.
Also, if gaining muscle is your focus, based on what we know about muscle protein synthesis (a key role in gaining muscle), having protein at just one meal (or maybe 2) per day within a 4-hour window is not ideal for building muscle.
Eat, Stop, Eat: Take 24 hours off of eating once per week
Brad Pilon’s “Eat, Stop, Eat” protocol calls for a weekly fast of 24 hours. During that time you can drink non-caloric beverages (think: water, coffee, and tea—all without milk or sugar, of course), but otherwise, you take a break from eating.
For some, this single, extended fast can be less disruptive. Instead of thinking about when you’re going to eat or not eat every day, you only have to consider it once a week. And the approach is helpful from a weight loss perspective too—again because of calories in/calories out.
The simple way to look at it is this: You know how, on any given day, you have a target number of calories to hit. The same is true for your total across the week. So using the earlier example, let’s say your target is 2,000 calories per day. That would mean you’d have a 14,000-calorie allotment throughout the week. If you went one entire day without eating, you could then spread the calories you saved across the other six days, meaning you could consume about 2,300 calories per day. Even though you’re eating more than you should be able to, because of the day where you eat 0 calories, this results in an overall energy deficit for the week and still lose weight.
There’s a way to structure this approach so that you can go for a 24-hour time period without eating, but not actually go an entire calendar day without eating a meal. Let’s say you finish your last meal on one day by 7 pm The following day you’d skip breakfast and lunch, then eat a little later—after the clock strikes 7. Voila! You’ve just done a 24-hour fast but still eat on consecutive days.
You might think that you’ll really struggle with hunger during such an extended time period of fasting. But gaining insight into this—what true hunger really feels like—may help you distinguish between those times when you’re really hungry, and those times when you may eat out of habit in response to something like boredom. Which is why Scott-Dixon suggests trying a fast like this at least once to change your relationship with the feelings of appetite and hunger.
“You learn you can be hungry and it’s OK, you’re not going to die,” Scott-Dixon says. “For people who are looking to lose fat, that can be very helpful.” Why? Because you’ll be better equipped to deal with those feelings on other days when you’re restricting calories or fasting for shorter periods.
The 5:2 Diet: Restrict yourself twice…and have more freedom
If you think a 24-hour fast is no problem, then why not go for two? That’s the rough concept behind the 5:2 Diet, which originated in the United Kingdom and has since been gaining in popularity. (It’s the diet that Jimmy Kimmel credits with helping him lose 25 pounds without exercise.)
If going without food for two whole days in a week sounds daunting, fear not. This is not a full-on “fast” (i.e. go completely without food) as much as it is a twice-per-week extreme restriction. That means eating about 500 calories for women and 600 for men of “fasting days.”. (The founder of the diet, Dr. Michael Mosley, has more recently stated that going as high as 800 calories is ok.)
The good news? On the other five days of the week, there aren’t many restrictions because the two days of undereating create a big calorie deficit for the week. Much like Eat Stop Eat, that allows you to eat a little more on the other five days than you “should,” meaning more flexibility. If you do the math on a 2,000-calorie per day diet, you can add up to 600 calories per day more than you would expect—or about a third of your total daily intake. You could essentially eat an extra meal on those days and be fine and still lose weight. (Remember, weight loss doesn’t happen in one day or one meal. Undereating by so much for 2 meals per week, simply enables loser calorie goals for the other 5 days.
Mosley, who is a TV personality in England that’s been compared to a “Dr. Oz crossed with Sanjay Gupta,” helped propel the 5:2 Diet’s popularity with a documentary and several books. But other than the two days per week at 800 calories or less, the fundamentals of his protocol are very similar to what you’d see in any healthy diet. The 10 elements he considers essential, which Mosley lists in this article, will sound very familiar to you: Eat protein and vegetables. Drink plenty of water. Be more physically active. Clean out the junk food from your house. Take it easy on the booze. But nothing is off limits.
But hey, basic fundamentals work—that’s why they’re the fundamentals. You could (and should) apply them to all of these approaches.
Some Helpful Intermittent Fasting Ground Rules and FAQs
If you try fasting, start slow.
Whichever fasting protocol intrigues you, give it a trial period before turning it into a commitment. Take breaks from it and notice: How do you feel? Does fasting make you have more or less energy? Were you irritable or happy? How does it affect your sleep? How did it affect your workouts? Take all of these factors into consideration to see if this kind of protocol is right to help you reach your goals.
You could start with a lower fasting period (like a 12:12) and work your way into longer durations. Gradually increasing your fasting time can help get used to some of the feelings of hunger you’ll experience. It will also help you notice and change the emotional attachment you might have to eat at certain times.
Berkhan suggests that as you’re getting accustomed to the feeling of fasting, drink coffee or other caffeinated beverages. “[It’s] quite beneficial for hunger suppression, since caffeine works better on an empty stomach.”
Which brings up a good question: What can you consume without “breaking” the fast?
You don’t have to go hungry and thirsty during your fast period: Coffee’s fine (and in fact, it can help boost autophagy). So are other zero-calorie drinks like unsweetened tea.
Supplements are OK too. Berkhan recommends 10 grams of BCAAs shortly before a workout. Now depending on the brand, that’s between 40 and 70 calories. And some studies suggest this little bit of amino acids may be enough to stop autophagy.
So you could say this counts as “breaking the fast.” But Berkhan thinks it’s worth it: “The positives—higher protein synthesis—outweighs the negatives—‘breaking the fast,’ Berkhan says.
How long should you follow a fasting plan?
Some people stay on fasting plans indefinitely after they discover the one that works for them. Berkhan is one example. He says that “once you go IF, you don’t go back. You might change or modify the protocol to better fit your needs, but you’re probably never going to go back to eating breakfast in the morning once you find that everything works better without it.”
Here’s where you could get into a bit of uncharted territory, scientifically speaking. There aren’t a ton of longer-term studies on the effects of nonstop intermittent fasting. Most studies have lasted 8 weeks or less, with many conducted during Ramadan. There have been some indications that prolonged fasting could have adverse side effects. For example, in the recent yearlong study that compared alternate-day fasting with calorie restriction, the fasting group saw increases in both HDL (“good”) and LDL (“bad”) cholesterol.
Other research—and, according to Scott-Dixon, anecdotal evidence among coaching clients—indicated that long-term fasting can depress thyroid function and reduce testosterone. For example, in the eight-week study of resistance-trained males where the group following a 16:8 fast had a sharp decrease in overall body fat, those same men also experienced a significant drop in testosterone.
“If you do [IF] for 6 months or a year, that really changes the game,” Scott-Dixon says. “We’re seeing guys now who are 20 who have the T-levels of a 70-year old,” like in this case available on Precision Nutrition’s website.
When Scott-Dixon uses fasting with clients, it’s usually with overweight clients who are suffering the effects of metabolic syndrome, a series of risk factors that can lead to heart disease and premature death. Even with these clients, Scott-Dixon says she’ll only have them fast for about 4 weeks.
For most of her non-metabolic clients, her advice is to keep the “intermittent” in intermittent fasting. “I’d suggest no more than once every two weeks for most folks,” Scott-Dixon says.
While there’s no research that tells us the “best” amount of the health benefits of fasting, a bi-weekly fast is “enough to give you the health benefits of periodic fasting, but without most of the problems.”
Are there any times when you wouldn’t want to try intermittent fasting?
D’Orazio says he wouldn’t give a fasting protocol to an athlete who works out more than once per day. It’s a fairly common practice in his work, as some clients at Complete Human Performance are triathletes who lift in the morning and do endurance training in the afternoon or evening. In order to properly fuel for these workouts, D’Orazio says this sort of person shouldn’t follow an IF protocol.
The other time when you may want to steer clear of IF? Occasions when you are experiencing a lot of stress. Fasting, or even just energy reduction, is a stressor on the body, just like exercise or lack of sleep or a really intense job. When there are lots of stressors adding up in your life, one more may not be beneficial
“I think one of the problems with any kind of diet or exercise intervention is that people consider it out of context,” Scott-Dixon says. “Even something that is “good” may not be good for YOU. We have to consider the whole picture. We can respond really well to intermittent, surprise stimuli, whether that’s an exercise challenge or a fast. What we don’t respond to as well are chronic stressors.”
Are there any times when intermittent fasting might be especially helpful?
We’ve mentioned that IF is a great way to help you “tune in” to your body and its hunger cues. Another use for intermittent fasting is during what’s politely called “hypercaloric eating”—basically, those times when you know you’re going to eat more than you should (think: you’re off visiting friends and know you’re going to have a huge meal followed by some bar-hopping later).
On this sort of day, restricting your eating window can be helpful. Just follow two guidelines: 1) Make sure the first meal you eat when you break your fast is packed with protein and veggies (they promote satiety and will help prevent you from way overdoing it), and 2) Use this technique only occasionally. It’s not meant to set up a lifestyle where you gorge yourself in the evenings. Having a big meal is not meant as a precursor to “punish” yourself with a fast. So if that’s your reasoning, then this approach is not best for you because it can lead to a bad relationship with food.