Intermittent fasting vs. caloric reduction – what’s the difference?

Is there any difference between fasting and caloric reduction?

Some would argue that the beneficial effect of fasting is entirely due to the caloric reduction. If true, then why is there frequently a difference between the lack of success with chronically reducing calories and the success of fasting?

Caloric Reduction has been tried innumerable times over the past few decades, and failed the majority of times. Yet fasting is often effective where simple caloric reduction is not. Why?

The short answer is that the beneficial hormonal changes that happen during fasting are prevented by the constant intake of food. It is the intermittency of the fasting that makes it more effective. This prevents the development of resistance as detailed in a previous post.

The failed caloric reduction technique advises people practice portion control, or reduce daily caloric intake – for example simply reduce 500 calories a day and expect to lose 1 pound of fat per week. Success is as rare as humility in a grizzly bear, but that doesn’t stop well meaning health professionals from recommending it. After all, who hasn’t tried to portion control strategy of weight loss? Who has been able to maintain long term weight loss? Practically nobody.

edical professionals have advised people for at least 40 years to count and restrict their calories. Calories In, Calories Out. Technically true, it turns out that the Calories Out, the part we do NOT consciously control is vastly more important than Calories In. Nevertheless, many doctors continue to advise cutting their calories and pretend that they will lose weight. When it doesn’t work, it may feel like they blame you, the patient for its failures.

Caloric reduction doesn’t work

Screen Shot 2017-01-19 at 08.52.42It has never worked. Practical personal experience confirms it. We’ve all done it. We’ve all failed to lose weight. So, whatever else you may believe, focusing on caloric reduction DOES NOT WORK for most people. But, no matter how many times I say it, I get the uncomprehending gaze of an anxious monkey.

So, what happens with focusing on calorie reduction? A detailed review can be found in the Calories series. Essentially, the body maintains its body set weight (BSW). As you start reducing calories, weight goes down, and the body compensates for this weight loss by trying to regain the lost weight. Hormonal mediators of hunger (ghrelin) increase. This means that measures of hunger and desire to eat increase. This happens almost immediately and persists almost indefinitely. This is no psychobabble that people lose their willpower over time and compliance wanes. People are really, physiologically hungrier because the hunger hormones are stimulated.

Next, the body’s metabolism starts to shut down. In response to a 30% decrease in calories, there is a roughly 30% decrease in total energy expenditure. We may start to feel tired, cold, and we have little energy for things like exercise.

As the body’s TEE decreases, the weight loss starts to slow down and then plateau. Eventually the weight starts to go back on, even as we continue to follow the diet. So the metabolic adaptations to caloric reduction is an increase in hunger and a decrease in basal metabolism. So, as you diet, you feel hungry, tired, cold and generally miserable. Does this sound familiar to any dieters? Probably sounds familiar to every dieter. This is what Dr. Ancel Keys had shown decades ago in his Minnesota Starvation Experiment. Despite its name, this was actually a caloric restriction study, with people eating 1500 calories per day, a level not far off most ‘expert’ advice today.

The worst part is that this strategy is likely to fail. The huge 50,000 woman randomized trial (Women’s Health Initiative) of the low fat low calorie diet proved to be a failure for weight loss. The problem with this strategy is that it does not address the long term problem of insulin resistance and high insulin levels. Since the insulin helps set the ‘BSW thermostat’ – the body keeps trying to regain the lost weight.

Here’s the bottom line. As you reduce calories, appetite goes up, and metabolism goes down. Yowzers. You reduce Calories In, but Calories Out goes down, too. This is likely to fail.

Intermittent fasting

As we detailed in the 27 part fasting series, the hormonal changes that happen in IF are completely different. During fasting, appetite goes down and TEE goes up. The body is trying to lose weight and helping you along. The main point is that it addresses the long term problem of insulin resistance. During caloric reduction alone, you do not get any of the beneficial hormonal adaptations of fasting.

CRaP-vs-IERDuring IF, the intermittent nature of the intervention helps to prevent the insulin resistance problem. A recent trial – The effects of intermittent or continuous energy restriction on weight loss and metabolic disease risk markers: a randomized trial in young overweight women – compares calorie reduction (CR) to IF. In this study, 107 women were randomized to two strategies. The first was a 25% continuous energy restriction (CER) – similar to the CR strategy of portion control. The second strategy was intermittent energy restriction (IER). Patients were allowed normal intake on 5 days a week, but only 25% of their usual calories on 2 days of the week – very similar to the 5:2 diet of Dr. Michael Mosley.

Let’s assume the usual caloric intake was 2000 calories per day. With CER, calories are reduced to 1500, or 10,500 calories over 1 week. With IER, weekly calories are 11,000 calories per week. Harvie1So this study effectively keeps caloric intake steady – or even favouring the CR group slightly. The basic diet was a Mediterranean style diet with 30% fat.

Over six months, what were the results? In terms of weight loss and fat loss, there were no significant differences (5.7 vs 5.0 kg weight loss, 4.5 kg vs 3.2 kg fat loss).

What happens to insulin resistance?

But the real important part of the study was the effect on insulin and insulin resistance. After all, hyper-insulinemia and insulin resistance are two key factors driving obesity and weight gain.

Harvie3Fasting provides a clear, substantial improvement in insulin levels and insulin resistance. The CR group had no improvement in their insulin resistance (IR), which can continue to provoke higher insulin levels in a vicious cycle. This keeps Body Set Weight (BSW) high and prevents successful long term weight loss.

Intermittent fasting has effectively controlled obesity for thousands of years. Portion control has only been used extensively for the last 50 years with stunning failure. Yet many continually scream at us through books, TV, and online to reduce calories. Don’t they think we’ve tried that?

But the one strategy that does help, fasting, is continually belittled as a dangerous practice akin to blood letting and voodoo. The problem with most diets is that they ignore the biological principle of homeostasis – that is the ability of the body to adapt to changing environments. If you try to keep a constant diet, the body will adapt to it. This means that successfully dieting requires an intermittent strategy, not a constant one. This is a crucial difference.

The difference is between restricting some foods all the time (CER) and restricting all foods some of the time (IER). This may be the difference between failure and success.

Jason Fung


Intermittent fasting for beginners

Top videos about calories

  1. Are all calories the same?
  2. Weight control – calories or insulin
    Q&A with Gary Taubes

Top videos about fasting

    The 2 big lies of type 2 diabetes
    The top 5 myths of fasting
  3. The 7 benefits of fasting

Earlier with Dr. Jason Fung

Insulin and Fatty Liver Disease

Fasting and Exercise

Obesity – Solving the Two-Compartment Problem

Why Fasting Is More Effective Than Calorie Counting

Fasting and Cholesterol

The Calorie Debacle

Fasting and Growth Hormone

The Complete Guide to Fasting Is Finally Available!

How Does Fasting Affect Your Brain?

How to Renew Your Body: Fasting and Autophagy

Complications of Diabetes – A Disease Affecting All Organs

How Much Protein Should You Eat?

The Common Currency in Our Bodies Is Not Calories – Guess What It Is?

More with Dr. Fung

Dr. Fung has his own blog at He is also active on Twitter.

His book The Obesity Code is available on Amazon.

The Obesity Code

His new book, The Complete Guide to Fasting is also available on Amazon.



  1. Lewis Owen
    I just love how the "Calorie Restriction as Primary" acronym is CRaP. How did people not see this method was full of it from the start? It's CRaP. Had to say it. Smile and don't eat CRaP.
  2. Sally
    I just love 'I get the uncomprehending gaze of an anxious monkey.' Who writes this stuff?
    Also love clear concise explanation of how intermittent fasting works; thank you.
  3. Han
    What's TEE?
    Reply: #7
  4. Di
    You wrote: "Over six months, what were the results? In terms of weight loss and fat loss, there were no significant difference although the fasting tended to do better (5.7 vs 5.0 kg weight loss, 4.5 kg vs 3.2 kg fat loss)."

    I get that insulin resistance went down and that's good, but what does it mean that there's no significant difference in weight and fat loss? Does that mean that a more long term fasting diet will start to work better for weight loss and outstrip CRaP? Because it reads like there's no difference and therefore either strategy will work contrary to the point you made before this. I'm confused.

    Also what is TEE?

  5. Diana
    I think that there are a couple of things happening with people who fast as to people who CRaP. The people who fast change their body set weight, so when they go back to normal eating they don't put the weight back on or at least their hormones aren't driving them to consume more to put the weight back on through both hunger and metabolism. People who use CRaP, don't reduce their set weight and so they eventually go back to their original weight. Their metabolic rate has been lowered and some hormone is telling them the are hungry so they get back to their BSW. The body wants to get back to the BSW. Whereas with the IM fasting group, their metabolic rate hasn't been lowered.
    To top it off, is the insulin functioning. The CRaP group are still consuming food and maintaining higher levels of insulin, whereas the fasting group are not. The amount of insulin in your body drives fat gain. When fasting, you are not producing as much insulin and your insulin functioning (i.e.insulin resistance) gets better. I tell my kids that when I fast, my body uses up all the sugar in my liver and my pancreas and then my blood. So essentially the people who fast, end up with better insulin functioning, so when they go back to normal eating, their insulin does a better job, so they don't need to produce as much. Not to mention their are a whole host of other benefits. I know it is far more complicated than how I have explained it, but that is my understanding of it.

    I have no idea what TEE is. Maybe it is in part of the calorie series he was referring to earlier in the post.

  6. ramona
    TEE= total energy expanditure.
  7. Tyrker
    TEE is Total Energy Expenditure. The total amount of energy you expend or "burn." Measured - oh, the irony! - in calories or kilocalories.
  8. Anthony
    You don't list any sources for the research you are basing your article on. Your links only refer to someone else's blog, often lacking proper references as well.
  9. Esmira Poladova
    I've been doing daily intermittent fasting coupled with a LCHF diet. But I'm confused on whether I should still reduce calories during my feeding window in order to lose body fat, or would my body interpret that as starvation and lower my basal metabolic rate?
  10. Laura
    So what about daily intermittent fasting, like doing 1 meal a day? I still don't understand whether that counts as calorie reduction or fasting...
    If my TDEE is around 1500 kcals, and I eat 1 keto meal a day (varying between 900-1500 kcals), will this slow my metabolism? I know that eating below your BMR combined with eating those calories spread out over the entire day will slow your metabolism, but is a shorter fast (like OMAD) enough to "make up" for this tiny deficit in calories and keep your metabolism up?
    I ask because most info online i can find focusses on the difference between calorie reduction and longer fasts, and don't mention shorter ones (like 23:1, OMAD)
  11. Paz
    You are actually citing the Minnesota Starvation Experiment as proof of this starvation mode BS? Have you even read the study? I can't believe the irony of this, how you can even think that study applies to regular dieters at all, people who are OVERWEIGHT. The men in that study were EMACIATED, THAT'S why their metabolism was 30% lower. Good job for being deliberately misleading. You are either a dishonest snake or you believe in some pseudoscience woo woo. Diets fail because people don't follow them, and they gain weight again because they go back to their old eating habits, not because their metabolism slows down. Utter nonsense. Obese and overweight people, even those who have lost some weight, actually have a higher metabolism than people of a normal weight because they have more mass. This is just the physical law of conservation of energy. A fat person going on a diet for 2 weeks and slowing their metabolism by 30% would break physics. In other words IT'S NOT POSSIBLE.

    Just as a personal anecdote, I lost 35 lbs from a normal weight, I wasn't even overweight when I started, entirely through caloric restriction. And I have since kept it off. By CONTINUING to restrict my calories at the level of my TDEE. And out of curiosity I increased my calories to see what would happen and lo and behold, I gained weight. I decreased them again and the weight came back off again. Calories in calories out works if you actually DO IT.

    I have nothing against intermittent fasting and I am considering adopting this way of eating in the future but if what you're saying is true, you shouldn't have to blatantly lie to people to convince them to adopt your ideology.

  12. Mathew Abonyi
    It's important not to lose sight of the value of Caloric Restriction by this comparison.

    CR shouldn't be conflated with reducing from 50000 calories a week to 10000. That isn't CR; that's just common sense. You'll lose weight regardless. To reach your optimal weight, you need your body to expect to burn fat.

    CR is a -maintenance- regimen. It forces your body to reduce its daily metabolic stress. It is not primarily lauded for weight loss; people who champion it for this are trying to be the stick-thin 50 year old in a 20 year old's body, before they went through the steps to get there.

    Intermittent Fasting is a -weight control- regimen. It forces your body to more readily use your fat stores (i.e. lose weight), as we naturally did in pre-civilisation. You can use it for maintenance too but it is far more dramatic at burning fat than CR.

    Caloric Restriction (as in the Michael Mosley documentary) that is practised in Los Angeles and around the southern West Coast, is one of the longest human studies in extreme CR. It is not a weight loss method; these are people doing it for years and decades, during which their body has reconditioned to a decreased energy expenditure and thus less stress on the body from the metabolic process. It stands to reason -- if you don't use your tools as much, they don't wear out as fast.

    I agree that calorie counting is generally moronic and misleading in popular culture. If I eat 1500 kcal of fat and sugar over four meals a day, I will still be fat. If I eat the same amount in a single sitting every other day, I will lose weight. (I tried this, too, since everybody is a little different.) But why? It's about the body's expectation to store, i.e. insulin and glucose control, and what it is used to doing. I have a brain and can trick my body's natural adaptive cycle. It can only adjust to what it has received recently, not what I intend to shovel into my face unexpectedly.

    Whatever the study and on whatever individuals, weight loss to a healthy BMI should be more ideally achieved by Intermittent Fasting than by Caloric Restriction, for most of the reasons mentioned above.

    That is not to say that reducing calories won't get you there, but it isn't the healthiest or easiest way to do so, and the chance of rebounding (the "98% failure") is extremely high. You'd have an easier and quicker time coming off of heroin.

    Personally I've done both. Keto IF was the most weight I ever lost (24 lbs in 2 months), compared to a sushi CR diet in LA (18 lbs in 2 months). IF was very easy, too, in that it felt natural. Water is your best friend on IF.

    I bet you, though, that CR is much better at maintenance and has far greater anti-ageing benefits when comparing two people of the same age, optimal weight, and diet.

    Both have theoretical roots in our pre-civilisation habits as human beings. Neither are wrong, but do avoid the square peg round hole issue here. Just because there are idiots out there, does not mean what they are saying has no purpose.

  13. Giles
    If Dr Fung is correct about intermittent fasting being so superior to calorie restriction for weight loss, then why is it that virtually every randomized clinical trial conducted to date has found no difference between these strategies for weight loss, body composition or cardiometabolic risk factors?

    It may be that the advantages of intermittent fasting over calorie reduction for weight loss do not manifest over such short-term studies - and that IF has other benefits. However, until such benefits are demonstrated, we would be wise to keep a more open mind and not necessarily buy into the current hype around IF.

  14. Adam
    Having read one of Dr. Fung's books already and currently enjoying the benefits of intermittent fasting, I was looking for an article to share with a friend who is currently engaged in calorie restriction dieting to get right down to the differences between the two. However, as I see the world a certain way because of my readings, so does my friend see the world their way from their readings and personalized medical guidance. In that context the tone of this article makes it useless for conveying compelling information because it needles and belittles people who believe as my friend does and therefore has too great of a potential to inspire defensiveness. Unfortunate. I'll have to look elsewhere for something a little more matter of fact.
  15. Sandman
    This is psuedoscientific b.s. End of the day it's simple physics. If you're on a caloric defecit you lose weight and fat. You could do IF and eat 2000kcal and your maintenance is 1600kcal, you're not magically going to lose weight.

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