- Intermittent fasting (IF) is a dietary strategy in which you alternate periods of zero calorie consumption (fasting) with periods of normal eating. Effectively, this just amounts to skipping meals.
- IF is a powerful dietary tool because this often leads to weight loss even if you're not trying to restrict calories. However, many advocates propose that IF has additional health benefits above and beyond those attributed to calorie restriction, including benefits for mental sharpness, disease resistance, and aging.
- Current research on IF is not complete, but it may still be a promising and useful tool.
Intermittent fasting (IF) is currently in fashion, and it’s one of the most popular diet strategies on the market. Interestingly, IF is not primarily pitched at weight loss exercisers the way that most diet programs are - instead, IF is mostly pitched as a performance enhancer and life extender, and as a way of minimizing your body fat while maximizing your muscle mass for the sake of aesthetics.
This post is going to try to cover all the best information currently available on IF, what it’s good for, what it’s not good for, where the bullshit is, and much more.
What is Intermittent Fasting?
Fasting is a common theme in many cultures, including the Muslim Ramadan fast, which takes place for a month out of the year during daylight. In some religions and ascetic traditions, the ability to survive on little-to-no food is seen as a powerful sign of virtue. Fasts have both social and religious significance, and in some cases they are believed to provide health or other benefits to the virtuous.
Full fasts are the fastest way to lose weight, because you’re eating 0 calories per day. However, while fasts are a rapid way to lose weight, they aren’t ideal - here’s a post I wrote a while back on the topic of full fasts and why they never work for weight loss, and that covers a lot more on the topic.
More recently, numerous health writers have proposed the benefits of partial, or intermittent, fasting. This kind of fasting involves alternating periods of fasting and non-fasting in a regular fashion. In this way, we can reap some of the benefits of a full fast without dealing with all the same drawbacks.
One of the big benefits of IF is that it tends to focus less on the types of food eaten or even the amount of food eaten, the way that traditional diets do - by limiting your total number of meals eaten, you can cut calories even if you eat a little bit more per meal (and eat whatever you want) in the remaining meals. So, many IF advocates make this a selling point - so long as you can stick to the fast, you can be a bit less careful with your diet elsewhere.
There are a few major systems of IF currently on the market:
Time-Restricted Feeding (ie, LeanGains) - You have a small window of time during the day that you’re allowed to eat (usually 4-8 hours) and you’re not allowed to have anything that contains calories during the rest of the day.
Alternate-Day Fasting - You can’t eat at all on certain days out of the week but can eat normally on the other days. Often, this is in a 1 day on/1 day off format, or cutting out specific days routinely from the week.
5:2 Diet - In this strategy, you eat normally 5 days out of the week, but on the remaining 2 days you cut your calorie intake very low - but NOT completely. This isn’t a true fast, but functions similarly and there's not much point in being a stickler on this. One can imagine that this is a bit psychologically easier than an alternate-day fast anyway.
Fasts typically permit you to consume non-caloric beverages (unsweetened, milk-free tea or coffee, water, diet sodas, etc.) during the fast so that you can put something in your stomach, and some recommend supplementation via BCAA’s to help with muscle loss, although the research isn’t great on BCAA’s.
What are the proposed benefits of IF?
Intermittent fasting is pursued because of a host of supposed health and fitness benefits, including but not limited to:
HGH Production - Normally, the human body begins to produce a small amount of additional human growth hormone when not eating, in order to stave off muscle loss during long fasting periods. More fasting = more HGH. At the same time, more fasting = more time in a fasted state, so these two things counteract each other more than anything else. While IF is considered a “hack” to raise your HGH, chances are that any potential benefits are minimal (1, 2, 3, 4).
Insulin Sensitivity - If you have some degree of insulin resistance from high carbohydrate consumption, time spent fasting may help restore your body’s sensitivity to insulin, improving your ability to process and use carbs effectively as an energy source. This has metabolic benefits and will help protect against diabetes (5), similar to the way in which a low-carb diet functions. This is cool because you can "trick" your body into this, even while continuing to eat carbs, simply by spacing out your consumption a bit via fasting.
Long Term Health and Cancer Resistance - Studies in rodents have shown that rats on an IF protocol actually lived longer than their normally fed counterparts (6, 7, 8). These studies also show that these rats have greater neurogenesis (development of brain cells) (9), and these younger, healthier brain cells are better at learning (10) and more resistant to certain kinds of neurological disorders, including strokes (11, 12), seizures (13, 14, 15), Alzheimer’s (16, 17), and Parkinson’s (18). These benefits appear to be independent of the effects of calorie restriction (19). There was also some evidence of resistance to cancer (20, 21), although this is a more mixed bag. In short, these mice were badass. At the same time, however, these kinds of studies are very hard to replicate in humans - and it’s unlikely that humans would react the very same way, since we’re very different animals with much longer lifespans. So, while there’s some promise, we can’t really be sure what’s going on here.
Weight Loss and Leanness - IF works, as discussed above, sometimes even without any intentional caloric restriction, simply because cutting meals often leads to fewer calories eaten despite eating a bit more on the remaining meals. So, IF can be an effective weight loss and leanness strategy (22, 23).
Hunger - One would expect that hunger would increase during long periods of fast. However, evidence suggests that once you get used to the fast, hunger levels are NOT increased as much as we would expect due to the smaller number of calories consumed (29). It is unclear whether this is psychological or due to some physical effect of the fast, but it does seem to work. In the beginning, it may be rough to deal with the fasting periods, but it would seem that we quickly get used to it and hunger is suppressed during the fast. This is pitched as one of the greatest benefits of IF, because dealing with hunger is one of the biggest difficulties of many dieters.
Mental/Psychological Benefits - Many IF advocates claim that it has psychological benefits. This can include more stable energy levels, enhanced mood, more productivity, and greater concentration. It is also claimed that eating during a small window during the day means that you’re able to mentally free yourself up during your fasting periods to simply not think about food preparation or hunger, which helps to mentally compartmentalize and manage your eating. If you’re eating just one or two big meals a day, it’s much easier to manage what you’re eating versus if you’re snacking all day and not paying attention to your portion sizes. These benefits are highly individual and smack of confirmation bias/placebo effect or a kind of cult of personality around popular IF advocates, but it may be worth considering.
What is the discussion around IF?
While IF is well-studied in rodents and has some positive research in humans, it’s hard to perform the kind of long-term, large-scale trials that would really confirm serious (and unique) health benefits in humans. Thus, we can’t be sure if the existing data, which is promising, is yet accurate.
Leangains (and variations of it) is the most commonly used IF protocol, in part due to the popularity of Martin Berkhan, its creator. Berkhan has written for many years on the subject of IF and its use for physique enhancement, including references to the science of it where necessary. It doesn’t hurt that Berkhan benefits from some amazing genetics and is both jacked as hell and strong as hell, to boot.
A few years back, Bojan Kostevski did his thesis on intermittent fasting, and helpfully posted it online. It’s the most thorough overview of the research in existence, and I stole a lot of my sources from it, so I highly recommend a look if you want a bit deeper of a dive into the research.
More recently, the “leangains study” was completed. This study examined the use of a leangains-style time restricted feeding protocol and compared it up against a control group of normal dieters.
Both groups were matched for calories and macronutrients, so the intent here was to see if IF promotes benefits independent of caloric restriction. All the subjects trained throughout the study. They found minimal differences between the IF and control groups in terms of body composition, although the IF group did lose a bit more weight.
This study has some serious drawbacks, which Greg Nuckols analyzes in more depth here. Notably, diet information was based on dietary recall (trusting people to write down what they ate) which is… notoriously unreliable. Additionally, the IF group lost a bit of weight compared to the control group - this implies either that the diet data was unreliable (they ate fewer calories than they claimed) or that it was reliable, but that IF provided some kind of metabolic benefit (increasing metabolism and burning more calories). The authors of the study assume the second interpretation is more likely, but given what we know about the unreliability of dietary recall, it seems scientifically safer to assume that the first interpretation is more likely.
Another interesting takeaway is that metabolic and hormonal effects of IF seem to resemble that of a diet - more significant of a diet than the subjects were on, given their modest weight loss. In Nuckols’ analysis above, he hypothesizes that this may be because fasting sort of works to “trick” the body into thinking it’s in a more sustained calorie deficit - which may explain some of its effects.
While IF has robust evidence for a variety of positive effects in rodents, there’s less solid evidence when it comes to humans. It seems likely that IF does allow some benefit, but the primary benefit may just be that IF functions as a method of more easily restricting calories (see the psychological/mental benefits above), and that many of the “unique” benefits of IF are just the effects that can be attributed to dieting in general.
IF may or may not work well for everybody. In short, it’s no magic bullet, although it may work well for you.
Despite this, IF is currently immensely popular, particularly among “biohackers” and other lay fitness enthusiasts and self-improvement types. IF has been followed by various celebrities and athletes. IF seems to be popular among silicon valley techies, and is widely recommended by bodybuilders and physique coaches for the management of body composition, including Martin Berkhan, Andy Morgan, and Greg O’Gallagher.
I think that IF can be a very useful strategy for intuitive eating, and for many people it’s much easier to stick to than a traditional diet. However, if you’re still eating more total calories in your eating period, you’re still going to gain weight - IF doesn't significantly violate the standard CICO model. Again, IF probably works primarily via the psychological help that this structure provides in order to help you achieve a calorie deficit, the same way that any diet works.
Where’s the bullshit?
IF is not going to make you instantly jacked or solve any of your life’s problems. It’s not going to do your chores for you, make you 10x more intelligent, fix your relationship with your partner, or make your coworkers respect you more. You won’t become superhuman just because you eat a bit less. You still need to follow a good training program to gain strength and muscle or to build endurance, and you still need to cut or add calories to lose or gain weight.
There’s likely not much “magic” to it, and it’s probably not going to make you live forever or cause cancer cells to go running home to their mothers.
Like any diet, IF may not be for everyone. Calorie restriction entails negative impacts on hormones, energy, and metabolism in the longer term, and has to be carefully managed to avoid side effects. Women may not see the same results as men, particularly because their metabolisms behave a bit differently (and we often lack good research on the different effects of diet/exercise strategies across gender). If you have pre-existing health issues, a diet may not be a good idea, and you’ll probably want to check with your doctor first.
I myself have, at various points in my life, “accidentally” used IF - generally, just because I wake up, have a cup of coffee, and then work for a long time without eating much. In this way, I can often go 8+ hours, and I’m so accustomed to it that I rarely get very hungry before about 2-3pm. However, I don’t follow IF strictly in any sense, and am fine with sweeteners in my coffee plus a slice of toast or two if I remember to eat. I have just naturally taken up this schedule due to my work schedule, and the suppressed hunger throughout the day is a nice side effect. Of course, I’m certainly not winning any Mr. Olympia titles for my physique, and I still eat like a slob most of the time.
IF can be a useful tool, but you need to use it in the right context.
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