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Water fast Vs Intermittent fast Vs Calorie cutting - whats the difference biologically?

Answered on October 09, 2019
Created January 13, 2017 at 10:26 AM

I've been reading a lot about fasting but I'm still trying to understand the science behind it.  I have a number of questions about the differences between these diets and how they work.  It would be great if someone could clear up my misunderstandings! 

I was reading a book about fasting which discussed how much better for you fasting is for weight loss than calorie cutting - becuase of the hormonal changes involved and the result is that you are less likely to regain weight aftewards and shift your set point upwards.  

But the 5:2 allows the person to have 500 calories a day, so isn't that just extreme calorie cutting rather than fasting, or is the amount of calories here really important?  Also I read that it can take 36 - 48 hours to reach ketosis in order for your body to go into fat burning mode - on a water fast you would get here after a couple of days, but on an IF you would never get here because you'd go back into re-feed.  Also on 5:2 500 cals counts as a fast day, then you could do a 500 cal fast for n days, like a water fast, so 500 cals for 10 days.  Would that have the same benefits of fasting, or is that just extreme calorie cutting?

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on October 09, 2019
at 10:55 PM

First, there are multiple types of nutrition interventions that are called intermittent fasting. Any fast (always drink water during fasts) that is 24 hours or greater is technically a true fast and in the scientific literature, including intermittent fasts of 24 hours or greater in your nutrition routine is called intermittent fasting. Fasts of less than 24 hours such as 16:8 (16 hour fast + 8 hour eating window) or 12:12 are technically called time restricted eating.

Here is a very detailed article specifically on Intermittent Fasting but here is a brief overview. Intermittent Fasting Facts and Myths

There is a good deal of scientific evidence in animal models (ex. rodents) that intermittent fasting reduces insulin and leptin, increases insulin and leptin sensitivity, reduces body fat, increases ketone levels, reduces resting heart rate, reduces blood pressure, reduces inflammation, increases the resistance of the brain and heart to stress, and is beneficial in the treatment and prevention of cancer (ref). Furthermore, intermittent fasting in animals can delay onset and progression of neurodegenerative disorders such as Alzheimer’s, Parkinson’s and Huntington’s disease (ref).

However, the scientific evidence in humans is limited to short term studies mainly on markers of metabolic syndrome. But the evidence does indicate that it most likely reduces measurable markers of metabolic syndrome. This includes weight loss, reduced fat mass, improved cholesterol levels, reduced inflammatory markers, reduced insulin levels, improved insulin sensitivity, and increased ketone production (ref, ref, ref, ref, ref).

Therefore, intermittent fasting may be beneficial for people with metabolic syndrome and chronic inflammatory diseases.

It is starting to become accepted that intermittent fasting will reduce the risk of cancer, reduce the risk of cancer recurrence and improve the results of conventional cancer treatments; however, there is no direct evidence in humans proving this is true. Does that mean we shouldn’t implement intermittent fasting for these purposes…no, but we cannot communicate any potential cancer preventing benefits of intermittent fasting in humans as absolute fact. What we can do is say that it is definitely possible and here is the available evidence for why this is the case:

Animal Studies: There is evidence in animals that intermittent fasting can reduce cancer risk and relapse as well as improve conventional cancer treatments (ref, ref). But does this apply to humans…we cannot say for sure although it is possible.


Human Studies: As previously mentioned, there are no long term intermittent fasting studies in humans. That means there are no studies looking at direct cancer development, survival, and relapse. However, we can infer possible benefits for cancer risk reduction by looking at short term human intermittent fasting studies and how they impact cancer risk via improvements in markers and conditions that are known risk factors for cancer. These include: Obesity, Diabetes, Inflammation, Insulin, IGF-1, Leptin & Adiponectin.

As we have already discussed, some, but not all, of the intermittent fasting studies in humans show improvements in the above-noted markers (ref, ref, ref, ref, ref, ref); so, does this mean intermittent fasting reduces the risk of cancer in humans? MAYBE, but we can’t say for sure.

That being said, it may not be appropriate for all people including but not limited to the following groups:

Athletes in active competition or training (or people that train like athletes): as we know, athletes need more calories and nutrients than the regular population and implementing intermittent fasting during training and competition times may have a negative impact on recovery and performance. That being said there are ways to implement this with athletes who are at risk of certain chronic diseases that intermittent fasting may reduce the risk of. However, it is strongly suggested that an athlete work with a qualified nutrition practitioner in such situations to determine the best approach since there may be other options that have similar benefits without the possible negative impacts on performance (ex. ketogenic diet, cyclical ketogenic diet, time-restricted eating).

Infants, children, adolescents, and teenagers (i.e. people who are still growing): These people need sufficient calories, minerals, and nutrients even more than the rest of the population because they are growing. Nutrition status in growing populations is a confirmed factor relating to the proper growth of the body as a whole, not just the structural body but other aspects such as the immune system.

People who are chronically stressed: Chronically stressed individuals experience chronic activation of what is called their hypothalamic pituitary adrenal axis (HPA axis). Activation of this pathway is what releases the stress hormone cortisol. Chronic activation of this pathway and chronic release of cortisol is well known to disrupt the immune and endocrine systems, increasing the risk of many chronic diseases. OK, so we all know that chronic stress is bad for us, but fasting is technically also a stressor that the body must adapt to, and while many of these adaptations are beneficial, they may negatively impact and already over-stressed HPA axis. And yes, there is some evidence in humans that intermittent fasting can elevate cortisol levels (ref).


Older populations: The main concern here is that as we age, muscle mass is an extremely important factor in longevity, and maybe more importantly, quality of life. Improperly implemented intermittent fasting can impact muscle mass and obviously vitamin and mineral status, both very important factors to maintain health as we age. That being said, the older population is also at the highest risk for the chronic diseases that intermittent fasting may reduce the risk of…so, it can be used but it must be done properly and ideally with the guidance of a nutrition practitioner.

This information is specific to intermittent fasting.  Stay tuned for a post on time restricted eating.

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