Fast, fasting.

Intermittent fasting, medium duration fasts and long fasts.

What happens during fasting.

A nutrition and health post.

If you’re wondering then the link with the title picture is related to the lent fast which should be happening round about now with some practitioners.

This is the first post in a series of 3 in which i want to explore the subject of fasting with some observations from my own experience, some points about religious and cultural fasting, the medical/physiological side : what actually happens during fasting and then what i can only describe as fasting’s dark side….deliberate starvation.

In this series of posts i find that i’m slowly constructing a better personal  understanding of nutrition and health/disease and at the same time i’m exploring different ways of expressing my own thinking on the subject.  In a previous post i related a few of my own experiences with fasting and then offered an insight into what my writing brain does when it’s taken out for a walk : this post is kind-of the opposite in that i’m dipping more into the historical practice, the physiology, and some of the known medical case history of fasting .

Before i begin though i quickly want to express a personal view about features of health and nutrition writing that i now find i’m continuously kicking against and questioning .  The best way i can put this across for now is what i see as a use of words that is almost like naked marketing, for instance talking about ‘healthy’ grains or fruit, or vegetables or whatever….it’s not so much the foodstuff that is being talked about but the sheer hubris and ignorance of calling something ‘healthy’ just because it fits in with a currently accepted view.

I see this most at the vegetarian/vegan end of the diet spectrum where it seems to be an unchallenged axiom that vegetables are unquestionably and always ‘good’ and that to eat them is somehow not just automatically ‘good’ or healthy but where ‘good’ also becomes ‘moral’ or , using a religious term…..holier/more pure than you (holier than thou).    A quick look at food marketing in the supermaket shows how expressions like ‘healthy’ are almost always used to cynically market foodstuffs that are anything but…..where often for example a useful level of fat is reduced and replaced with sugar….food marketing and it’s primary targets are something i want to cover in a future post.

Visitors to my blog will probably realise that i’m just over a year into having changed my dietary habits from an unthinking and demonstrably unhealthy western diet ; high to very high sugar intake and high overall carbohydrate intake plus a high use of seed oils, to initially a low carbohydrate and ketogenic approach.  Last year, a few months in and i also discovered that i’d fallen into the practice of intermittent daily fasting , which seemed to just work well for the way i live and later still i deliberately experimented with moderate duration fasts.   During all of that i think i began to see that it would be easy to fall into one of the diet ‘camps’ or ‘tribes’….the thinking does seem remarkably closed and cult-like at times.  For a while i was definitely in the LCHF/Keto tribe as i’d had personal success with that approach and where my basic level of nutritional physiology and biochemistry did seem to tally with most of what i read and watched about LCHF/Keto : most of it still does by the way.

The danger i think, and this goes back to what i was saying at the start, is that i now don’t think that a ‘one size fits all’ approach to food and nutrition is in any way right or good.  I do think there is a useful consensus and common ground that even the 2 extremes of the food-ist claimants could accept….that the high sugar intake diet that i was unthinkingly doing every day is nothing but harmful….from there almost any change away from high sugar , and i would add high seed oil intake, has to be a better choice.  Whether it’s a better dietary choice to then become a nancy vegetarian or a blood soaked meat eater i’m really not so sure and maybe it isn’t even an either/or choice as much as just eating ‘real’ food anyway.  Today i’m also drifting back towards a higher carbohydrate intake sometimes and to support other activities….long distance hiking for instance where it might just be a useful dietary strategy….maybe.


By the end of last year i was beginning to think that a combination of weekly moderate term fasts of say 36-48 hour duration coupled with a more ‘normal’ eating pattern during the rest of the week…but also having some days as OMAD (one meal a day) or 2MAD (2 meals) might be a better approach for me : but define ‘better’ you might say and/or better than what ?.

Now we come to it….what do i think of or what do i think i mean when i put ‘better’ in a sentence about something that i am doing ?.   My answer maybe should now be more careful, considered and precise, that i consider it better for example to not have regular and frequent spikes of blood glucose and hyper-insulinaemia , better not to live with a sugar driven inflammatory state, better to be within a normal weight range rather than obese, better to have a normal and untreated blood pressure and much better to have reduced risks of disease.   Having given it all a lot more work i now think that fasting might have better outcomes for me in terms of my perceived health goals but might not be right for other people.

Talking about fasts and fasting then…..what happens.


For the purposes of this post and to define my own terms i regard intermittent fasting (or time restricted feeding) as having a fasted duration of 12-23 hours, medium or moderate duration fasts as being from 1 to 3 days and longer fasts as being anything beyond 3 days.   Also in my definition i’m talking about no solid or liquid food intake but certainly allowing for clear fluid intake : water, black coffee and in my case Bovril,  plus or minus electrolytes (salt and minerals).    My main purpose here today is to set out what physiological processes take place at roughly what times after a last meal is taken, i use the term ‘after last meal’ (ALM) as a kind of time stamp in this regard.  In this example i’m going to start off with day zero and hour zero as being around 6pm which is how i do it myself and where i give estimates of caloric expenditure and basal metabolic rate (BNR) i’m doing so on my own numbers, so lets begin :

Day zero, hour zero, final meal is taken at around 1800 hrs (6 pm) , lets say that this is a ‘normal’ meal and not specifically low carbohydrate but contains all 3 primary nutrient groups as per standard dietary guidelines.

0-6 hours.  Blood sugar and blood insulin both peak and then slowly drop away to baseline by about 6 hours ALM.   This assumes that the person has a normal insulin response and is not in a pre-diabetic or diabetic (Type 2 DM) state.   The action of insulin on the available blood sugar will have been to allow some of the glucose to be taken up by muscle cells and the liver as glycogen which is essentially the dry stored version of glucose.   In an average adult around 100 gms of glycogen can be stored in the liver and around 400 gms in skeletal muscle.  Excess glucose that hasn’t been taken up as glycogen is stored in special cells called adipoctyes (fat cells) as triglycerides , once again under the action of insulin. Cortisol, regarded as one of the ‘stress’ hormones should be at it’s lowest in the evening and melatonin at it’s highest at around this time.

6-12 hours or overnight. Once sugar (technically blood glucose) drops to baseline and insulin to zero in the normal insulin sensitive individual, blood glucose is then maintained by utilizing stored glycogen under the influence of glucagon which simply put works in the opposite way to insulin.  One important fact of physiology is that glycogen stored in muscle isn’t released into circulation but only used in each cell where it is stored : circulating glucose which the brain partially relies on for fuel overnight is derived from glycogen stored in the liver by the process of gluconeogenesis.    Remember also that resting state basal metabolic rate during sleep is still maintained at around 60-70 % of normal due to normal physiological activity and of those the human brain has the highest individual use of fuel.     Overnight, cortisol rises and melatonin falls.

By the morning of day 1 or 12 hours plus ALM blood glucose should be at normal levels, insulin at baseline and the body will have an overall glycogen deficit from what has been used overnight.  The hormone Grehlin usually peaks for the first time just before normal expected ‘breakfast’ time in most people creating hunger : Grehlin levels then drop away but tend to rise and peak just before ‘normal’ mealtimes.

If the fast is broken at this stage, say 12 hours, then the cycle simply repeats with initially new available glucose being used to directly supply circulating blood glucose and replenish glycogen stores..and once again the excess glucose is stored as fat.  Here i deliberately haven’t talked about the second sugar (sucrose) molecule ie fructose as that has an entirely different metabolic pathway and mostly always has to be stored as fat and primarily in the liver itself.

12-24 hours ALM.  This is when things start to get interesting and relevant to fasting practices because the common 16 : 8 pattern of IF falls in the middle of this time frame.  Physiologically it’s also where some other things are really starting to happen thus : first, that there now begins a gradual transition from glycogen metabolism to fat metabolism and therefore the onset of fat oxidation and it’s fuel by-product , ketone….in short , ketosis slowly kicks in at around 14-16 hours.   Secondly that with insulin at baseline other processes that insulin blocks now start to come into play : one of these is the sodium retaining effect of insulin is lost and this is where some people can start to experience the diueretic effect as sodium is excreted in the kidney and takes a lot of water with it.  Also….as insulin and other nutrient signalling is reduced or shut off then autophagy may begin in some cells : to explain that one needs a separate post but for now think breakdown and consumption of broken down intracellular debris.

We can say that then what will happen physiologically during IF and/or something like an OMAD (one meal a day routine) , usefully that the effects of insulin will be hugely reduced resulting in the reversal of mandatory fat storage, that the inflammatory processes caused by high sugar consumption will be temporarily stopped and that sodium/water loss will happen….although not immediately apparent in the first period of either fasting or intentional dietary ketosis this will be the mechanism that can quickly lead to a spontaneous reversal of inflammatory conditions and hypertension.  Just within this period and usefully within a 23 hour fast (OMAD) routine there will be some mobilisation and oxidation of stored fat resulting in some net weight loss although much of the weight loss at this point will be due to water loss and depleted glycogen…..this might account for the apparent small muscle mass loss as muscles deplete their glycogen store (can be as much as 400gms).

24-30 hours ALM. This is the usual time frame in which fasting moves into greater effects as the process of autophagy becomes more widespread across more tissue types and even other forms of cellular repair may begin under the influence of new factors : one of these is BDNF or brain derived neurotrophic factor which is the key component of brain cell repair and growth….essential for learning.   Subjectively this is consistent with what many practitioners of fasting experience and report which is firstly the increase of mental clarity and/or feeling of calm and secondly the noticeable side effects noted earlier : higher water loss (pee more often) , the dry metallic taste of ketone bodies on the breath, often a peripheral cooling effect and in some people a paradoxical increase in energy.

30-48 hours ALM or longer duration fasting.  Up until recently a fast of 64 hours is the limit of my personal experience but what iv’e been doing a lot of are sequential fasts of 30-48 hrs ; as frequently as once per week.  What we know of the physiology of this period is that ketosis increases, in fact it has been shown to steadily increase up to about day 5 of a fast and then level off.  Also when measured using continuous glucose monitoring, blood glucose can paradoxically rise and the probable mechanism is a cortisol/stress response.

Beyond about 48 hours we are into long duration fasts and there does seem to be some evidence that longer fasts might have some benefits in treating disease if such a long fast can be tolerated : there is one extreme example* of a very long fast voluntarily undertaken to treat morbid obesity but also we have to consider that death from starvation has been observed at around the 60 day mark** and that there has been at least one ‘doctor’ imprisoned for killing her patients (victims) by fasting/starving them.  Even a quick look at the opinion piece at the end of the Wiki page about fasting shows that it was obviously highly biased against fasting used as a medical practice.

All of this brings us onto a question then ‘what is the best or most effective duration for fasting’ and my answer, even for myself, is ‘it depends’ in that it would depend on my intentions.  As it is i think that a 30-48 hour fast is the sweet spot for me because i should have within that a 30 hour or so period of active fat oxidation…..but rather than spinning this post out i think i will continue in a follow-up post.

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