IT’S SCIENCE (MAYBE?) – Research on Intermittent Fasting

IT’S SCIENCE (MAYBE?) – Research on Intermittent Fasting

A blog on supporting research for Intermittent Fasting

For those reading this who are unfamiliar with the concept of Intermittent Fasting (IF) please refer to blog 1 as an introduction to the topic of IF and why we do not eat Mondays and have not for some time.

To begin, we need to address the ‘evidence’ behind IF. The research is really in it’s infancy. Although this is not a new concept (we have likely been intermittent fasting as a species for our entire existence), the research is still young – growing, but young. There is a lack of high quality trials with a noted void in comparing the various methods of IF. Further, much of the research thus far is from animal studies, which we know are not perfect at predicting human response. A lot of IF research is done on groups already afflicted with a certain condition (ie obesity) to evaluate their response (for example this study on type 2 diabetes and IF compared to traditional caloric restriction approaches). This does not mean it is entirely transferable to a healthy, active population. John Berardi brings up another excellent point that much of the research is compared with ‘normal eating’. But any “diet” when compared to ‘normal’ North American eating will come out smelling like roses as to be blunt, our ‘normal’ diet sucks!

My thoughts on this are two fold. First, it can be difficult to obtain ethics approval for studies on ‘potentially dangerous’ outcomes. I would assume despite the fact that IF is not potentially dangerous for most, ethics approval to run IF studies could be a barrier to proper research (and subjects likely aren’t lining up to join studies on not eating!). Second, funding. Research is not cheap. In fact, the majority of published research is funded by those with a vested interest in the outcomes of said research. A lot of nutrition based research is funded by food companies. This is NOT NECESSARILY A BAD THING! Most people will demonize “big corporations” for bias however without their funding a lot of great research would never be performed. In the case of IF however, my guess is that food companies don’t jump at the opportunity to fund trials of people not eating!


This is a common argument against IF where people will present this research article as evidence. This may surprise you but I agree! However, I only agree based on the specific outcome of this research paper – that it is not superior to other methods of weight loss for decreasing body weight (or other physiological measures such as glucose homeostasis etc). If you read the actual article you will find that the review compares forms of intermittent fasting with continuous energy restriction (aka constant dieting) and found that “apart from a possible decrease in the drive to eat, likely associated with ketosis or other factors concomitant with energy restriction, this work found no evidence that IF, as applied in the clinical trials hereby reviewed reduced adaptive responses to energy restriction relative to effects of continuous energy restriction.” That’s great! So both fasting, and always dieting work! BUT, for us, and everyone else that enjoys food, fasting may be more palatable (pun intended) then constantly restricting calories. In fact, prolonged caloric restriction is the ONLY proven nutrition method of weight loss. Diet methods all have overall caloric restriction as the backbone of their method and IF is the exact same. The other facet of IF is that aside from the fast, you eat your normal (presumably healthy) diet. There are no other alterations to your daily intake, you are just adding a fasting period. In general, as this study shows, Intermittent fasting may be an option for achieving weight loss and maintenance.


False (kind of). PROLONGED fasting will have an effect on your metabolic rate. There are studies however showing that even 3 day fasts do not lower your metabolic rate. A second study evaluating the feasibility of alternate day fasting had people fast every other day for 22 days and still saw no decrease in their resting metabolic rate. Your metabolic rate also doesn’t appear to change if you start skipping breakfast, or eat numerous meals per day as opposed to two large meals. In reality, metabolic rate is based on the energetic costs of keeping ourselves alive. Food actually does not have as much to do with your metabolism as you would think. It is more closely tied to your lean mass. The more you have, the faster your ‘metabolism’. A second factor is exercise and movement. So, want to quicken your ‘metabolism’? Go work out! Stop worrying so much about calories.


Well it turns out (again dependent upon outcomes measured) that short-term food deprivation does not significantly impair cognitive function, even with a 24 hour fast.  A study of 21 women was tested skipping one meal, 2 meals, fasting 24 hours, and eating normally.  There was no change between the 4 conditions for attention, focus, reaction time, or memory.  The only difference was with 24 hours of food deprivation the subjects performed poorly on a low load tapping task.  Long term deprivation can decrease cognitive function. The threshold period prior to cognitive dysfunction based specifically on fasting does not seem to have been established.


Not really. In fact, one of the great aspects of fasting is that you end up utilizing fat as an energy source. Brad Pilon does a great job of explaining this to the everyday population and I will borrow some of his explanation here. When we eat, we ingest glucose which gets stored away as something called glycogen in our muscles and liver to be used as energy later. The glycogen in your muscle can only be used for that same muscle while liver glycogen supplies your other organs, and other muscles with energy. In fasting, your body generally uses fat and sugar stored in your liver for energy. Even a few consecutive days of fasting with no exercise has little effect on muscle glycogen content meaning it is reserved for energy needs of exercise.  (Read Brad’s excellent book Eat Stop Eat for further commentary on these mechanisms – really it’s an easy and great read that we highly recommend).


This is one of the primary benefits of fasting. Free fatty acids begin to be released by your body fat through a complicated process called lipolysis to be used as fuel essentially as soon as you are done burning the calories you consumed during your last meal. By 12-14 hours you burn predominately body fat and after 24 hours your fat burned as a fuel increases by over 50%.


To us, the common sense approach exists here. Fasting is obviously not recommended prior to long length endurance events, or the training of elite athletes (specifically with more then one workout in a day). Studies of fasting athletes and exercise during Ramadan (fasting from sunup to sundown) show many do worse for a period while their bodies adjust to fasted training. For some, they never really take to it. It does seem to depend on the type of exercise as well as the individual. Research on Ramadan shows a negative effect on exercises requiring more intense but longer efforts like a 200 or 400 metre run, repeated short intense runs like in soccer, repeated power explosive movements like jumping, some types of strength and work capacity as well as psychological effects like a negative effect on how athletes perceive their performance. Conversely, fasting does not seem to effect sprint performance, single explosive movements (like 1 jump), non maximum lower repetition weight training, overall aerobic capacity and agility. None of this is definitive and in fact most ‘negative effects’ are minimal.  Again it depends on the individual and because the research is done on Muslims during Ramadan, most have had a small breakfast and typically are dehydrated as opposed to continually drinking water during an IF protocol so it is not entirely apples to apples. Further, some older research does show performing aerobic exercise fasted will encourage loss of lean mass as it too will be used as a fuel source which muddies the waters further. In general, if performance is the top variable of your workout (meaning it is more important than body composition), fasting on your exercise day may not be appropriate. For the general population however, it is not generally frowned upon. John Berardi from Precision Nutrition has an excellent resource (free) in which he performed a ‘self-experiment’ with different types of intermittent fasting and took many objective measures pre and post. He speaks about how he felt, how he functioned, and how he performed against his goals over a 6 month period. During his one day fast, John used his fasting day as a ‘full recovery day’ meaning he did not work out which served his purposes well.


The caveat? You likely must be involved in some sort of resistance training on an ongoing basis. Long term caloric restriction on its own will cause lean mass loss (not a surprise), however with resistance training you will preserve your muscle mass. It is well established during a fast you tend to use fat as your primary energy source. As long as you are using your muscles they will not waste away with IF. In fact, up to 72 hour fasts do not appear to cause muscle breakdown.


Yes it does have an impact on your hormone profile. Brad Pilon’s book goes into this topic extensively, however to summarize some findings:

  • Short term fasts will not harm leptin levels, but long term fasts will
  • IF will not harm testosterone levels, but long term fasts will (but they then rebound)
  • Fasts up to roughly 72 hours will not increase cortisol (but on the good side – losing body weight will lower cortisol which IF does help with)
  • Both IF and long term caloric restriction improve insulin sensitivity (again weight loss with or without exercise significantly improves insulin sensitivity)
  • IF helps decrease blood glucose levels (and increase glucagon levels)
  • Fasting increases catecholamines (epinephrine and norepinephrine) levels. This may be why some feel more alert when fasting in the short term.
  • It increases Growth Hormone (more later)


Women tend to burn more fat in general then men and are more insulin sensitive. Women also have higher amounts of free fatty acids in their blood compared to men after longer fasts of 40-72 hours and women actually continue to burn fat even after the fast is broken according to Brad Pilon. I know you are thinking “That sounds great!”, BUT we have to be cognizant of topics like RED-S or relative energy deficiency where women may lose their menstrual cycles, and essentially enter a ‘self preservation’ mode. In fact, women who are already very lean or who are extremely active may not be well suited to 24 hour fasts.  An interesting study looked at fasting with women who had 25% body fat, who had good outcomes versus 19% body fat who had bad outcomes from fasting 72 hours. Some research noted in Eat Stop Eat even suggests lean women may be at a higher risk of developing neuroendocrine and follicular phase reproductive abnormalities when nutrition is completely withdrawn for 3 days. To be fair, at the 3 day mark men also have a significant decrease in testosterone which also affects reproduction so this is generally too long. One other note, it goes without saying you SHOULD NOT follow an IF protocol if pregnant or breastfeeding.


FASTING INCREASES CELLULAR ‘CLEANSING’: Autophagy is the degradation of damaged materials within the cell. Think of it like the ‘maintenance system’ of your body. It’s your clean up so you can grow and repair. Fasting is a signal to turn up autophagy. Eating generally turns it off. This upregulation of autophagy is one of the areas of research behind fasting and neuronal diseases like Alzheimer’s and Huntington’s. This is also one of the hypotheses for regulating the aging process.

FASTING MAY HELP NEURODEGENERATION: Again on rats and mice. When on IF they seem to exhibit less neuronal dysfunction and degeneration and fewer clinical symptoms in models of Alzheimer’s disease, Parkinson’s disease and Huntington’s disease.

FASTING DECREASES INFLAMMATION: In short, we know that fasting reduces many blood markers of inflammation. Don’t believe me? Check out these studies here and here. In general, I shouldn’t need to enlighten you as to why whole body inflammation is a terrible thing. We do know that excess fat is correlated with chronic low grade inflammation while exercise is associated with a decrease in low grade inflammation, so a simple recipe to lessen inflammation? Eat less, eat healthier, decrease body and visceral fat, and exercise more!

INCREASE GROWTH HORMONE (GH): GH has been thought of as ‘the fountain of youth’ by some. By others, its demonic (as it is banned in professional sports). GH is secreted by exercise, sleep, sex hormones, and fasting! That’s right, a short term fast can have a substantial increase in GH which in turn has an increase in fat burning (this is one of the mechanisms by which we do not lose lean muscle while fasting). Eating can actually prevent GH release in the short term. Overeating can have a rapid suppression of it. Prolonged high levels of GH can have some negative effects that we will not go into for the purposes of this blog, but short term spikes can be a good thing. Adding exercise to fasting will increase it even more.


I’m not kidding. Again, most of this research is on yeast, worms and mice, however it now seems to hold true for primates. Animals tend to age slower and live longer when they consume fewer calories. IF is a method to obtain this.


Much of the research on people that skip breakfast is marred by other unhealthy habits. Most stereotypical breakfast skippers lead unhealthy lifestyles, but that does not necessarily mean that skipping breakfast itself is unhealthy.


Again, primarily we are attracted to its simplicity. Additionally the longer you fast you will use fat (and later ketones) to preserve your blood sugar and body protein stores. With longer fasts your body gets used to a permanent fat burning physiology involving a down regulation of hormones and enzymes responsible for carbohydrate burning so you end up with a decreased sensitivity to insulin which can carry over to when you eat. This won’t happen during a 24 hour fast.



Here is a free fairly in depth review article on the proposed molecular mechanisms and clinical applications of IF from 2015.


The evidence for IF is by no way concrete. Many facets of IF are not well understood, and the research is still in its infancy. In general, you can sum up much of intermittent fasting by a process called Hormesis which means that you can have a positive effect from exposure to low doses of an otherwise negative agent. In this case, devoiding yourself of calories for a period. We feel there is a robust enough base of research for us to try our own self experimenting on intermittent fasting as the ultimate test is how we actually feel! To see our personal experiences, stay tuned for blog 3.

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