Intermittent fasting strategies and their effects on body weight
bmj.com
bmj.com
I have a belly and am what you would call "a little chubby". I don't exercise that much (once a week). I drink one black coffee in the morning.
After all these years of IF, net-net I haven't lost any weight, but I find if I stop IF (i.e. I start eating 3 meals a day), I feel sluggish. So in the absence of doing anything else, at least IF keeps my mind sharp.
p.s. the only times I've lost weight is when I've fasted once a week, cut out 50% of all carbs from diet, and starting lifting weights. I lost water weight at first, then plateaued because I gained muscle, then after that muscles did the work of burning excess calories.
I was that way for at least 3 years. Then I decided I loved food too much and added certain carbs back into my diet. These days I just do IF and nothing else, and my weight is stable.
I also enjoy food and always ate a lot (like 2 meals at lunch), and I was thin all the way up to 30 thanks to fast metabolism I guess. If I didn't start running 5 years ago my choice would be between severe cuts to my diet or obesity.
If you have the time and enjoy it, there's no better way to be able to stuff yourself silly.
Or you're suggesting exercise but of different kind?
Walking might be a better choice but no matter what you do you still need to control the intake of calories.
You should walk. It’s really good for you. But skipping the coffee break at the end will have more of an impact on your weight than the walk itself.
It’s also way easier to just not eat the calories in the first place. A bag of potato chips or a tbsp of nut butter on a rice cake is roughly the equivalent of 15 minutes of running.
Everyone should exercise and move but if it’s for weight loss diet is the way
But when you eat less, you just burn less. In the end you're constantly hungry and irritable, you go through all this crap to lose weight and lose none, which doesn't help with the mood either.
At least sport makes you feel good. Dieting without exercise seems to me more like a shortcut into nuthouse.
And I used those as examples of snacks that people might have. For people who are looking to drop a few kg, portion control is the key thing. My parents idea of a serving of pasta is “what fits in the plate we’re serving it on” - people who eat healthy and think “I’ll have the avocado salad” and don’t realise the avocado itself is the same calories as the morning roll they skipped and that’s before you add the dressing.
> In the end you're constantly hungry and irritable, you go through all this crap to lose weight and lose none, which doesn't help with the mood either.
You need to eat less than you use to lose weight, whether that’s with or without exercise. Part of losing weight is getting used to what is actual hunger and what is “craving/hormone/comfort” eating. If you don’t do that, you’ll fail even harder when you start exercising because the feelings are mega intense.
I agree that "a little cardio" (eg 30-45 minutes 2-3 times a week) can definitely be counterproductive.
I have to mix in a lot of weight lifting to actually lose weight and offset the appetite creep.
The best way to "run" is actually kind of a springy hop at about a brisk walking speed. This is what the actual pros do outside of competitions to build endurance.
I have never been overweight in my life but I lost ~10 kg in the first year and my weight has remained stable since then. I haven't measured it, but my body fat percentage is probably around 15%.
By the way I do moderate exercise every days. Walking at very slow speed for 3-4 hours or swimming for an hour. My muscle mass is always increasing, albeit slowly.
Soda? Chocolate? Sugar? Ice cream?
You must be eating something extremely calorie dense to be maintaining weight.
But I have found it successful in breaking bad habits, which results in weight loss indirectly.
For example, I had a bad habit of eating a large breakfast 1-2 hours within waking up. I was never really that hungry, but it was just something I did out of habit. Doing an IF routine made me realize that I’m not actually that hungry in the morning and can get by until 10-11am on just a coffee with milk.
The point being that rigidly sticking to IF rules is less useful than just using it as a way to reset your eating habits. (At least in my experience.)
After doing intermittent fasting for a few years, I have accidentally fasted for 24+ hours multiple times. And after you do it for a while, it makes it clear that this whole modern thing of 3 meals a day, let alone with snacking, is really just weird.
Having any high duty-cycle behavior go from un-tracked to tracked and from (largely) unconscious habitual practice to conscious practice can be a real eye opener.
When caloric intake is reduced, the body can decrease this type of expenditure without the person even realizing it.
It's a common story when people start to eat 20% less, continue the same lifestyle and lose exactly zero weight as the result. Their body didn't create 20% of energy out of nothing but it just started to waste less energy as body heat.
Anything that can influence "energy out" is by your own definition "relevant".
The body is more like a thermostat system. Environmental effects can convince your body to move the temperature/weight up or down, but for both weight gain and weight loss is a battle. Your body tries to maintain it's understood ideal weight. Your body if given a chance and you haven't convinced it to change the thermostat setting, will immediately do all it can to gain/loose weight. This is why crash diets never work. It takes sustained effort to convince your body that it's wrong about your initial weight.
These are excuses, in the fitness community, the response to "I can't lose weight" is "eat fewer calories", and its incredibly effective
Imagine a system with a background/quiescent energy consumption of 1000kCal/day.
Imagine that same system can buffer up to 500kCal for up to 24 hours store excess energy in circulation.
Imagine it converts excess energy to stored energy at an efficiency of 50%.
Assume activity correlates with marginal energy consumption but also increases in the presence of excess energy.
A system such as the one described would have very different behaviors during alternate day fasting (0kCal for 24hrs, 5000kCal for 24hrs) than consuming 2500kCal daily.
The human body is more complex than the system I just described, but it is a useful model to consider for this context.
You eat less calories, your body might start consuming less calories.
Also, there are two different pathways for using glucose in the body: aerobic and anaerobic. The aerobic one produces 15x more ATP (cellular level energy) than the anaerobic one. The anaerobic one wastes more as heat. So if for some reason you're in the second one, even though you ingest the same amount of calories, the amount of energy you have usable is much less.
So yeah, calories in calories out is true, but it's not really helpful.
This is technically true but not particularly relevant.
It's quite difficult to be in only anaerobic effort, though (and I'd say pretty ill advised since that basically means stuff like all out sprinting without warmup or cooldown).
Higher intensity effort burns more calories than lower intensity (eg [1]). It's just harder to sustain.
1- https://www.webmd.com/fitness-exercise/what-to-know-heart-ra... )
Food — not lack of exercise — fuels obesity, study finds
https://www.npr.org/2025/07/24/nx-s1-5477662/diet-exercise-o...
Some people operate with a goal of a caloric deficit of even something as small as e.g. 200kcal. But because all these things are impossible to measure accurately, a difference of just 10% beyond a daily BMR of 2000kcal isn't just a possibility; its the norm. You run for an hour; what if that burns an extra 50kcal that your Apple Watch did not account for? You eat a slice of bread which advertises it contains 80kcal; but it actually contains 100kcal [1]? You sleep poorly, which causes some mild systemic inflammation the next day, which raises your body temperature?
[1] https://health.clevelandclinic.org/are-calorie-counts-accura...
If you had a car with a broken gas gauge you would just pump until it overflowed… same idea here.
Over a month or so if your weight is stable then you are putting in as many calories as you burn. If you’re gaining weight, you’re consuming more, and if you’re losing weight, you’re consuming less.
Adjust accordingly.
Hence: Why diets exist. That is the structure. There are good ones and bad ones.
Keto, for example, can work for weight loss not because there's anything particularly interesting about the way your body absorbs carbs versus protein and fat (there are differences, but its not the biggest reason why it can work). It can work for some people because typical protein and fat food sources are less calorically dense (by volume and weight) than carb sources. You may feel full faster; so you may naturally eat fewer calories.
Similarly: IF can work for some people because most people cannot physically eat enough to consume massive calorie counts if they time-restrict the hours they're allowed to eat. It also seems to come with some well-studied metabolic effects.
You don't have to accurately track inflows and outflows, but vibe-structuring your consumption and exertion habits based on outcomes is a privilege that, sure, some people have, but is not a panacea for every body and mind. Broadly, the people who need to make change who do this will not see the change they wanted.
For instance this is something every single person who's into body building does, because you want to be in a slight caloric surplus when bulking, and then you want to get back into a slight calorific deficit when cutting.
Of course we do!
You’re thinking of this the wrong way. The goal isn’t “eat less” as you said, it’s “consume less calories (energy)”
On any given month if you gain weight or maintain when you want to lose, then you need to consume less energy next month. For 99% of people, that means reduce sofa, reduce sugar, reduce fat (all the energy dense stuff)
Feeling hungry? Drink massive glasses of water and eat literally all the vegetables you physically can get in. I have whole carrots for snacks most days. Cauliflower too. Cucumber is great. Frozen peas on a hot day. All of it, as much as you can eat.
Also note Americans are more obese than the rest of the world. Like gun violence, police deaths, healthcare and so much more - the rest of the world can fix it. Why not America.
Your body can raise or lower its temperature. It can put energy towards cell repair or cell reproduction. It can store energy as fat, or signal to burn fat, or build muscle or catabolize muscle.
It turns out the body processes different calories very differently depending on a variety of factors including: baseline genetics, time of day, menstrual cycle, prior fasting, current mineral excess/debt, gut flora/fauna biome, and the composition of previously consumed food still remaining the digestive track.
I don’t understand what’s "desperate" about IF, it’s just an easy way (for some people) to lower their calories intake. It has other benefits and some caveats but it’s one way to get healthier.
Whether it's IF, deficit cycles, periodic deep fast -- they all seem to have the same effect, which is just to give the body a break for a bit.
I don't know what to call that, but it worked and it changed my relationship with food forever (in a good way). I have since kept the weight off and switched back to eating 3 meals a day with a better understanding of how much and of what to eat.
Eat a lot more protein and way less carbs. Most meats already have the right balance of fat included, so avoid adding more. Fill up on fibrous vegetables since they don't really count towards calories and help digest all the meat you'll be eating. Drink plenty of water, sleep well, and at least hit your daily steps and heart rate targets if you're not huge on exercise. Building muscle is a good idea, but I'm lucky I've never had a problem with that and just needed to lose the body fat.
I'm now in my mid 30s and have a clean bill of health. When I was heavier I was prediabetic and my resting heart rate was 85. Now it's about 65 and blood sugar is good and doesn't spike or stay elevated all day anymore. It's good to get this stuff on track when you're still youngish.
Oh also I stopped the intermittent fasting because it was messing with my blood sugar. That was why I wrote all this and it's my argument against it.
Now and then like twice a month, skip 2/3 meals in the day. If hard, have light juices/even fruits. Think of it as giving your digestive system "rest". Don't do it when otherwise sick.
It will make you generally healthier, no drastic changes. Those require drastic measures which differ person to person.
Losing a bit of lean muscle mass along the way is not important compared to the huge health gains of losing the weight.
When it comes to long term health outcomes, losing weight is the only thing that matters.
Regardless of how you lose weight the advice is and remains:
> Eat a minimum of 0.36 grams of protein per day, per pound of LEAN* body weight. Increasing to 0.5-0.7 grams of protein per day, per pound of LEAN* body weight for older adults or when undergoing weight loss.
*LEAN is a vital detail for overweight people, they commonly miscalculate protein requirements due to this. The easiest way for overweight people to determine their requirement, is just find an "ideal body weight calculator" online, enter height and gender, and then multiply THAT figure by 0.5-0.7.
For example a man who is 6' tall and 400 lbs should eat 62 grams of protein per day MINIMUM, but during weight loss 86-120 grams of protein per day. It is common, unfortunately, to read online people in this situation miscalculate this to 280(!) grams of protein per day which is incorrect and harms their weight-loss goals.
> Protein intake should range between 1.0-1.5 grams/kg of adjusted body weight. To calculate adjusted body weight, first calculate excess weight: Excess weight = current weight — ideal body weight (IBW). Adjusted body weight = IBW + 0.25 of excess body weight. This amount generally accounts for 20% to 30% of total caloric intake.
So a slightly more complex way of calculating roughly the same thing. I'd argue that for most people getting your ideal weight is a good enough approximation, and that using your overweight/obese body fat in your protein calculation is wrong by a lot no matter which calculation you use.
https://www.ajmc.com/view/chapter-2-clinical-nutrition-guide...
You need to be getting enough protein + strength training to maintain muscle in a caloric deficit
I’ve tried all types of diets. For me, the most important for me is to save the biggest meal for late in the day. I can easily go hungry a couple of hours during the day if I know there is a filling meal coming.
I suspect IF works in a similar way.
Wouldn't the big meal be stored as fat during sleeping?
Hunger is driven by hormonal signals designed to defend a set point. Even if you consciously fast for most of the day, your brain will push you to make up the difference once you start eating. When it comes to fat loss, it still boils down to maintaining a caloric deficit — timing alone won’t keep your appetite in check for long. We’ve known this for years.
IF may have other potential benefits — better insulin sensitivity, longevity, or improved adherence for some people (since avoiding food most of the day can be psychologically easier) — but none of that is “new” anymore.
There is some evidence that there are health benefits that are specific to the fasting mode. This has mostly been studied in the context of chemotherapy, where fasting can protect against some of the side-effects of chemotherapy:
https://pmc.ncbi.nlm.nih.gov/articles/PMC5870384/
Most of this has only been studied in animals, not humans, but in animals the results were clear:
"Fasting before chemotherapy (CT) was shown to protect healthy cells from treatment toxicity by reducing the expression of some oncogenes, such as RAS and the AKT signaling pathway [2]. This reduction is mediated by the decrease of circulating insulin-like growth factor 1 (IGF-1) and glucose. In addition, starvation and calorie restriction activate other oncogenes in cancer cells, induce autophagy, and decrease cellular growth rates while increasing sensitivity to antimitotic drugs [2]."
If we assume that we have been shaped by millions of years of frequent famine, then our evolution has been shaped by famine. It is possible that our immune system simply makes the assumption that we will soon face famine, and therefore some important tasks, such as extreme autophagy, are normally postponed till the famine arrives. However, in the modern era the famine never arrives, and so we may have to induce it by artificial means.
I have experimented with very long fasts. My longest fast ever was in September of 2015 when I managed to go 12 straight days on nothing but water.
Obviously, any health benefits from that incident might have been psychosomatic, since I was expecting health benefits. But all the same, I did find some of the health benefits to be shocking and completely unexpected. Since at least 1995, and possibly 1990, I had a mole on my skin on my left arm. I wasn't worried about it, so I simply ignored it. I had it on my arm at least 20 years, maybe 25 years. I recall one morning in November of 2015 when I was in my kitchen, making breakfast, and I reached over to pour myself some coffee, and of course my arm was in my field of vision, and after a moment of thinking something was different, it occurred to me that the mole was gone. It had been there at least 20 years, and then it disappeared, at some point during the weeks after I had done the 12 day fast. I don't know when it disappeared, it just slowly faded away at some point between September and November. There was no remaining sign of it on my arm.
Again, that might have been purely psychosomatic, but it was interesting.
Longer duration trials are needed to further substantiate these findings.
The combination of Keto+IF worked so well for me, for a while my calorie tracking switched from the usual preventing eating too many calories to ensuring I was getting enough, which was certainly never on my bingo card. After a lifetime of being a slave to hunger it was liberating to suddenly feel effortlessly in control of diet and my relationship with food changed completely. Then at around 90 days my palate shifted, meaning I even lost my taste for carbie foods. If I tried a small bite of something carb-laden that I'd loved my whole life, it didn't even taste particularly good to me anymore. I also became hyper-sensitive to sugar. Sugar-soaked foods just taste poisonously over-sweetened (which they kind of are). A slice of apple now tastes as sweet as I'd ever want, like a dessert that has extra sugar-added.
In the 8th month I reached below my ideal 'dream' weight and even saw abs appear for the first time in my life! I transitioned to maintenance mode but stayed keto because being in a blood sugar controlled state felt so amazing and not just physically but also mentally and emotionally. At around a year I went from strict keto to low carb for life which I still am 8 years later. When I started that was unimaginable. I saw keto as an onerous regimen that I'd endure if it worked and stop the second I wasn't overweight. But during the journey my metabolism, palate and food preferences changed so dramatically, I was basically a different person when I arrived. Those first few months when I was rigorously tracking every calorie in an app and managing intake with measuring cups and a kitchen scale felt like a burden but were actually invaluable skill-building. After a few months all that process became automatic so I didn't need to constantly track and by six months I got to the point where I don't even think about it consciously. That early rigor helped me get so in sync with my body and able to sense where my metabolism is in its natural cycles that now I just eat when necessary and convenient for my schedule. This often ends up being IF but it's not intentional on my part, which makes me think maybe IF patterns evolved in the hunter/gatherer era as part of our natural biological rhythms. Due to habit and carb-laden factory foods I'd never been able to access those rhythms until I made the conscious effort to break the patterns I'd been raised in.
I suggest you read the FAQ of the Reddit keto forum: https://www.reddit.com/r/keto/wiki/faq. I recently posted my personal "Keys to Keto Success" learned during my 8 month weight loss 8 years ago: https://news.ycombinator.com/item?id=44685689 which includes some effective hacks I discovered. The biggest thing is to do keto rigorously. It's different than most any other diet because it's not just about reducing calories. The power of keto is 100% in cutting carbs, which is why I advise "Cheat all you want on calories but never (ever) on carbs".
For me, cutting calories on any diet was always difficult, painful and exhausting. Keto worked because I only focused on cutting carbs. Once I got my blood sugar under control by limiting carbs, my raging hunger just evaporated and the calories pretty much took care of themselves. The change was dramatic. After the first month I was no longer tracking calories to stop over-eating but instead to ensure I was getting enough :-). During the second month there was a point where I was slightly concerned I might be losing weight too fast because I was never hungry, which was certainly never something I'd imagined would be a problem for me. Of course, that's easily addressed by increasing calories a bit.
There's just one thing to watch out for as you do keto. On keto you're able to eat a lot of fats which are very filling (and delicious). This is wonderful but it's only possible because having almost no carbs puts you in a ketogenic state where instead of burning glucose (carbs) for energy as you do now, your body burns those fats for energy instead of storing them (weight gain). So, unlike a normal calorie-reduction diet, where you can slip one day and just get back on the wagon the next day, cheating on carbs during keto will convert all that delicious fat you're eating into not only pounds of weight but potentially into all the bad stored fat related things like vascular plaque etc. So, basically, keto is the one diet where cheating doesn't just "delay your weight loss another day", it changes from magically protecting you from the negative effects of eating huge amounts of fat to actively making it much worse. Eating all that luscious fat thinking you're protected by Keto but then regularly cheating on carbs "just a little" (as many do on other diets), is pretty much the worst diet imaginable. You'll turbo-charge weight gain AND all the negative effects of excess fat consumption (cholesterol, lipids, A1C, etc). Theoretically, aggressively eating keto-levels of fat but NOT being in a ketogenic state to burn it off could, given time, make you pre-diabetic and your lipid blood panels will freak your doctor out.
So, IMHO, keto is wonderful but there's just one unbreakable rule. You really, really can't cheat on keto (even a little). When I first understood this I was a little concerned but as I researched more it made complete sense. Keto isn't magic. With all these benefits of easy weight loss and healthy metabolism of course there's a potential downside to avoid. Now you know what it is - so go in with eyes wide open. For me, the stringent, "can't cheat, ever" (on carbs) aspect of keto was actually a psychological benefit. It made it 100% binary and thus easy to fully commit. I'm either really doing this or I shouldn't bother. Putting tangible health consequences on regularly "slipping just a little" actually helped me stick with keto rigorously during the first few weeks of transition and habit change (which is really the only hard part, after 30 days keto gets much easier). But you may be different, so you just need to decide if you can really commit 100% for the first ten days. If you manage that, then set your goal to finish the month. The rest really is all downhill from there. If you're as overweight as I was, after the first month of strict keto your weight loss will be so obvious people will definitely notice. In the second month, my admin told me people at the office were asking her if I was "okay", like they were worried I had cancer or was on chemo :-). So I started sharing a little more widely that I'd changed my eating habits. At least nowadays most people have heard of keto.
One thing that I'm concerned with, is longevity when a person is on keto. Do you share this concern? Do you know any reliable data source on keto diet's impact on longevity?
I have no concern because there's nothing about a low carb diet that restricts any essential nutrients. On evolutionary time scales, consuming a significant amount of carbs is a recent and unprecedented change to human diets. There are three types of macro-nutrients: protein, fat and carbs. For most of history our hunter/gatherer ancestors ate far more protein and fat than carbs, which is why they were in a ketogenic state most of the time. It's only in the last hundred years in Western democracies that carbs have become overwhelmingly dominant in human diets. In terms of long-term dietary impact, highly-processed manufactured foods and intensive factory farming (which are mostly carbs) are the massive uncontrolled experiment on the broad population that's worth being concerned about. That's the new thing for which there's no long-term data whereas keto has tens of thousands of years of proven success in humans. Going from the Standard American Diet to keto is opting out of the uncontrolled experiment and returning to what we know works. Chronic obesity and Type II diabetes are diseases of diet and they weren't common in humans until the last hundred years.
As for my personal health and longevity, keto has certainly added at least a decade to my life because before keto I was significantly overweight and had been diagnosed with pre-diabetes, high blood pressure, high triglycerides and bad HDL/LDL. I also had chronic sleep apnea as well as IBS/GERDS. I was on five prescribed medications to treat these issues. By the end of the first year on keto those serious health problems had completely resolved and I was off all five medications. To be clear, keto itself isn't some miracle cure. These problems were caused by my obesity and unhealthy diet. Keto helped me not be obese and to eat a healthier diet richer in nutrients and with far fewer manufactured and processed foods. My previous diet was making me sick.
May I ask what the differences are between the keto diet you used and the low carbohydrate diet that you are on now? Do you allow for more carbohydrates than a typical keto diet?
Today I stay low carb because it feels so much better physically, mentally and emotionally. I also know if I returned to the carb-intake I had before keto, I'd eventually gain the hundred pounds back again. Now that I'm at my ideal weight, low carb is healthy maintenance. My day-to-day diet isn't specifically planned and is based mostly on my schedule, preferences and how I feel. I only very rarely go over 100g/day and most days am well under, probably closer to 50g. I'll also often fast for a day or more simply because I get busy and it feels good. You asked about my diet but I can't answer because I don't have a "diet", my relationship with food, food preferences and taste palette have changed completely. I really and truly only eat foods I absolutely love and I eat them literally whenever I want to. I just don't like carbie foods anymore and I simply don't want to eat nearly as much or as often as I did before keto. That would have sounded unimaginable to pre-keto me, so it might sound the same to you. At this moment, it's morning where I am. I haven't eaten anything in over 12 hours and I still feel absolutely stuffed from my last meal. I'm not sure if I'll even feel like eating lunch in a few hours and may just wait until dinner.
I can still slip easily in and out of a pure ketogenic state but that's not my goal. Any conscious changes I make are driven by how I feel and my overall weight. If I'm traveling, things get hectic and I get distracted, I might occasionally gain a few pounds, so I'll make a mental note and slightly adjust my intake. It doesn't take much and the extra pounds will be gone in a few days. More importantly, if I ever start to feel lethargic or less mentally sharp, it's a sign my blood sugar levels have been too high for too long and I'll adjust. The most obvious sign I've gotten too much glucose, too quickly is starting to feel faint hunger pangs several hours later. A more severe form of that is getting a sudden sharp headache, although this has only happened twice in 8 years. Both times I intentionally ate a sugar-laden dessert knowing the inevitable consequences in advance. But when traveling to Copenhagen with family and visiting the hundred year-old shop serving the world's most legendary freshly baked hot cinnamon waffles with home-made ice cream - one makes choices. If I was still in my first year of keto or hadn't been stable at ideal weight for years, I wouldn't have had any - nothing could possibly have been worth the risk in the first year. As expected, I paid the price later that night because my body is no longer adapted to absorb that much glucose that quickly. And I felt hunger the next morning for the first time in a couple years. I just pushed through by fasting and felt fine by the afternoon. That's the accrued benefit of years of consistent adaptation to low carb. If I'd done that in my first few months on strict keto, it would have been much worse and taken several days to get back into keto. Most of the people who slip like that in the first few months never make it back into keto and end up blaming keto instead of their own choices. Hence my warning about not starting keto unless you're serious about it.
To be clear, events like that Copenhagen trip are very rare. I'm just relaying a couple notable outlier events that can happen to someone after years of experience with keto and for who weight loss as a goal is now only a distant memory. Most of the time I don't even think about any of this because it's just automatic. And none of this is relevant for someone in their first year. You need to be rigorously strict or you'll almost certainly fail at keto - feel physically awful as your blood glucose levels pogo up and down - and give up due to feeling awful. Just know that in the future, after years of adaptation, you gain some margin for occasional variance - although I rarely use it because food no longer controls me like it used to.
I can’t believe that losing 3.5 days of caloric intake would result in weight loss. In other news, water is wet.
Keeping this up is hard, many fail. Alternating normal eating with IF days is easier to do.