The Complete Guide to Fasting by Jason Fung (review)
Considering the massive amount of research and interest in the idea of fasting, not a lot has been written for the general population on the topic.
Brad Pilon’s 2009 e-book Eat Stop Eat was a great, though fairly concise, resource on the mechanisms and benefits of fasting.
Martin Berkhan’s LeanGains blog had a cult following for a while in the bodybuilding community.
Then in 2013, Jason Fung emerged onto the low carb scene with his epic six-part Aetiology of Obesity YouTube Series in which he detailed a wide range of theories relating to obesity and diabetes.
Jason’s key points are:
- simply treating Type 2 diabetes with more insulin to suppress blood glucose levels while continuing to eat the diet that caused the diabetes is futile,
- people with Type 2 diabetes are already secreting plenty of insulin, and
- insulin resistance is the real problem that needs to be addressed.
Jason’s Intensive Dietary Management blog has explored a lot of concepts that made their way into his March 2016 book, The Obesity Code. However surprisingly, given that Jason is the fasting guy, the book didn’t talk much about fasting.
my experience with fasting
I have benefited personally from implementing an intermittent fasting routine after getting my head around Jason’s work. I like the way I look and perform, both mentally and physically, after a few days of not eating. I also like the way my belt feels looser and my clothes fit better.
Complete abstinence is easier than perfect moderation.
I recently did a seven day fast and since then I’ve done a series of four-day fasts, testing my glucose and blood and breath ketones with a range of different supplements (e.g. alkaline mineral mix, exogenous ketones, bulletproof coffee/fat fast and Nicotinamide Riboside) to see if they made any difference to how I feel and perform, both mentally and physically.
Fasting does become easier with practise as your body gets used to accessing fat for fuel.
I love the mental clarity! My workout performance and capacity even seem to be better when I’ve fasted for a few days.
My key fasting takeaways are:
- Fasting is not that hard. Give it a try.
- You can build up slowly.
- If you don’t feel good. Eat!
The more I learn about health and nutrition, the more I realise how critical it is to be able to burn fat and conserve glucose for occasional use. We get into all sorts of trouble when we get stuck burning glucose.
Our body is like a hybrid car with a slow-burning fat motor (with a big fuel tank) and high octane glucose motor (with a small fuel tank). If you’re always filling the small high octane fuel tank to overflowing, you’ll always be stuck burning glucose and your fat burning engine will start to seize up (i.e. insulin resistance and diabetes).
Reducing the processed carbs in our diet enables us to lower our insulin levels and retrain our body to burn fat again. But nothing lowers insulin as aggressively and effectively as not eating.
Even though lots of Jason’s thoughts on fasting seem self-evident, his blog elucidating them has been very popular, perhaps because the concept of fasting is novel in the context of our current nutritional education.
We’ve been trained, or at least given permission, to eat as often as we want by the people that are selling food or sponsored by them.
Jason’s angle on obesity and diabetes comes from his background as a nephrologist (kidney specialist) who deals with chronically ill people who are a long way down the wrong track before they come to his office. Jason also talks about how he had tried to educate his patients about reducing their carbs, however, after eating the same thing for 70 years, this is just too hard for many people to change.
Desperate times call for desperate measures!
Many of these patients come to him jamming in hundreds of units a day of insulin to suppress blood glucose levels, even though their own pancreas is still likely secreting more than enough insulin.
Rather than continuing to hammer more insulin to suppress the symptom (high blood glucose), the solution, according to Jason, is to attack the ultimate cause (insulin resistance) directly.
Jimmy Moore is well known to most people that have an interest in low carb or ketogenic diets. Whether you agree with his approach, it’s safe to say that low carb and keto would not be as popular today without his role.
Meanwhile, Jason talks about trying to educate people about reducing the processed carbs from their diet not working, not because of the science but more due to people not being able to change their eating habits after 70 years.
the Complete Guide to Fasting Review
You’ve probably heard by now that Jason has teamed up with Jimmy to write The Complete Guide to Fasting which captures Jason’s extensive thoughts on fasting from the blog along with Jimmy’s n=1 experiences and wraps them up in a cohesive comprehensive manual with a colourful bow.
Jason and Jimmy both sent me a copy of their new 304-page book, The Complete Guide to Fasting, to review (thanks guys). So here goes…
Similar to The Obesity Code, TCGTF is a compilation of ideas that Jason has developed on his Intensive Dietary Management blog. Blogging is a great way to get the ideas together and thrash them out in a public forum. Some people love to read the latest blog posts and debate the minutiae, however, most people would rather spend the $9 and sit down with a comprehensive book and get the full story.
Unlike The Obesity Code, TCGTF is a bright, full-colour production with great graphics that will make it worth buying the hard copy to have and to hold.
TCGTF did originally have the working title Fasting Clarity as a follow on from Jimmy’s previous Cholesterol Clarity and Keto Clarity. However, other than Jimmy’s discussion of his n=1 fasting experiences, TCGTF is predominantly written in Jason’s voice building from his blog, so it wouldn’t be appropriate for it to have become the third in Jimmy’s Clarity series.
What is similar to Jimmy’s clarity series is that it’s easy to read and accessible for people who are looking for an entry-level resource. This book will be great for people who are interested in the idea of fasting. It is indeed the complete guide to fasting and is full of references to studies, however, it doesn’t go into so much depth as to lose the average reader with scientific detail and jargon.
The book covers:
- Jimmy’s n=1 experience with fasting,
- Dr George Cahill’s seminal work on the effects of fasting on metabolism, glucose, ghrelin, insulin, and electrolytes,
- the history of fasting over the centuries,
- myth-busting about fasting,
- fasting in weight loss,
- fasting and diabetes, physical health, and mental clarity,
- managing hunger during a fast,
- when not to fast, and
- when fasting can go wrong.
The book is complete with a section on fasting fluids (water, coffee, tea, broth) and a range of different protocols that you can use depending on what suits you. What did seem out of place are the recipes for proper meals. Apparently, the publisher insisted they include these to widen the appeal (If you don’t like the fasting bit you’ve still got some new recipes?)
Overall, the book will be an obvious addition to the library (or Kindle) of people who are already fans of Jason and/or Jimmy and want a polished, consolidated presentation of all their previous work with a bunch of new material added.
TCGTF will also be a great read for someone who is interested in learning more about fasting and wants to start at the beginning. TCGTF is the most comprehensive book on the topic of fasting that I’m aware of.
My additional 2c…
I thought I’d take this opportunity to give you my 2c on some of the issues related to fasting that aren’t discussed in the book.
Target glucose levels
In the book, Jason does discuss relaxing target blood glucose levels during fasting. This makes sense for someone taking a slew of diabetic medications, particularly injected insulin. They’re probably not going to continue the journey if they end up in a hypoglycaemic coma on day one.
The chart below shows the real-life glucose variability for someone with Type 1 Diabetes on a standard diet. With such massive fluctuations in glucose levels, it’s impossible to target ideal blood glucose levels (e.g. Dr Bernstein’s magic target blood glucose number of 4.6 mmol/L or 83 mg/dL).
If your glucose levels are swinging wildly due to a poor diet coupled with lots of medication, your glucose levels are simply going to tank when you stop eating. Hence, a safe approach is to back off the medication, at least initially, until your glucose levels have normalized.
Being married to someone with Type 1 Diabetes, I have learned the practical realities of getting blood glucose levels as low as possible while still avoiding dangerous lows. My wife Monica doesn’t feel well when her blood glucose levels are too low, but neither does she feel good with high blood glucose levels. Balancing insulin and food to get blood glucose levels as low as possible without experiencing lows requires constant monitoring.
The chart below shows how scattered blood glucose levels can be even if you’re fairly well controlled.
Ideally, you want the average blood glucose level to be as low as possible while minimising the number of hypoglycaemic episodes (i.e. below the red line). If you can’t reduce the variability you just can’t bring the average blood glucose level down. The last thing you want is to be eating to raise your blood glucose levels because you had too much blood glucose-lowering medication.
Pretty much everyone agrees that it’s not smart to be eating nutrient-poor hyperpalatable food while dosing with industrial levels of insulin to manage blood glucose levels. High levels of exogenous insulin just drive the sugar that is not being used to be stored as fat in your belly, then your organs, and then in the more fragile places like your eyes and the brain.
Jason’s perspective is that people who are chronically insulin resistant and morbidly obese are likely producing more than enough insulin. The last thing they need is exogenous insulin which will keep the fat locked up in their belly and vital organs. Dropping insulin levels as low as possible using a low insulin load diet and fasting coupled with reducing medications will let the fat flow out.
fasting to optimise blood glucose levels
In the long run, neither high insulin nor high glucose levels are optimal.
Once you’ve broken the back of your insulin resistance with fasting, you can continue to drive your blood glucose levels down towards optimal levels.
One of the most popular articles on the Optimising Nutrition blog is how to use your glucose meter as a fuel gauge which details how you can time your fasting based on your blood glucose levels to ensure they continue to reduce.
Your blood glucose levels can help calibrate your hunger and help you to understand if you really need to eat. I think this is a great approach for people whose main issue is high blood glucose levels and who aren’t ready to launch into longer multi day fasts.
In a similar way, a disciplined fasting routine can help optimise blood glucose levels in the long term. The chart below shows a plot of Rebecca Latham’s blood glucose levels over three months where she used her fasting blood glucose numbers AND body weight to decide if she would eat on any given day.
While there is some scatter in the blood glucose levels, you can see that regular fasting does help to reduce blood glucose levels over the long term.
Once you’ve lost your weight , broken the back of your insulin resistance and stopped eating crap food, you may find that you still need some exogenous insulin or other diabetic medication to optimise blood glucose levels if you have burned out your pancreas.
The TGTF book covers off on several fasting regimens such as intermittent fasting, 24 hours, 36 hours, 42 hours and 7 to 14 days. One concept that I’m intrigued by, similar to the idea of using your glucose meter as a fuel gauge, is using your bathroom scale as a fuel gauge.
The reality, at least in my experience, is that we can overcompensate for our fasting during our feasting and end up not moving forward toward our goal.
If your goal is to lose weight I like the idea of tracking your weight and not eating on days that your weight is above your goal weight for that day.
Again, Rebecca Latham has done a great job building an online community around the concept of using weight as a signal to fast through her Facebook group My Low Carb Road – Fasting Support.
The chart below shows Rebecca’s weight loss journey through 2016 where she initially targeted a weight loss of 0.2 pounds AND a reduction of 0.25 mg/dL in blood glucose per day. After three months, she stabilized for a period (during a period when she had a number of major family issues to look after). She is now using a less aggressive weight loss goal as she heads for her long-term target weight at the end of the year.
The chart below shows the fasting frequency required to achieve her goals during 2016. Tracking her weight against her target rate of weight loss has required her to fast a little more than one day in three to stay on track.
Eating quality food is part of the battle, but managing how often you eat is also an important consideration. After you’ve fasted for a few days, you can easily excuse yourself for eating more when you feast again. And maybe it’s OK to enjoy your food when you do eat rather than tracking every calorie and trying to consciously limit them.
The obvious caveat is that there are a lot of other things that influence your scale weight such as muscle gain, water, GI tract contents etc, but this is another way to keep yourself accountable over the long term.
FAST WELL, FEED WELL
Fasting is a key component of the metabolic healing process, but it’s only one part of the story.
Fasting is like ripping out your kitchen to put in a new one. You have to demolish and remove the old stovetop to put the new shiny one back in. You don’t sticky tape the new marble benchtop over the crappy old Laminex. You have to clean out the old junk before you implement the new, latest, and greatest model.
In fasting, the demolition process is called autophagy, where the body ‘self eats’ the old proteins and aging body parts. The great thing about minimising all food intake is that you get a deeper cleanse than other options such as fat fast, 500 calories per day or a protein-sparing modified fast (PSMF).
But keep in mind that it’s the feast after the fast that builds up the shiny, new body parts that will help you live a longer, healthier, and happier life.
“Fasting without proper refeeding is called anorexia.”
Even fasting guru Valter Longo is now talking about the importance of feast / fast cycles rather than chronic restriction. In the end, you need to find the right balance of feasting/fasting, insulin/glucagon, mTOR / AMPK that is right for you.
In TCGTF, Jason and Jimmy talk about prioritising nutrient-dense, natural, unprocessed, low carb, moderate protein foods after the fast. I’d like to reiterate that principle and emphasise that nutrient density becomes even more important if you are fasting regularly or for longer periods.
In the long term, I think your body will drive you to seek out more food if you’re not giving it the nutrients it needs to thrive. Conversely, I think if you are providing your body with the nutrients it needs with the minimum of calories I think you will have a better chance of accessing your own body fat and reaching your fat loss goals.
optimising insulin levels AND nutrient density
It’s been great to see the concept of the food insulin index and insulin load being used by so many people! In theory, when people reduce the insulin load of their diet they more easily access their own body fat and thus normalizes appetite.
Some people who are very insulin resistant do well, at least initially, on a very high fat diet. However, as glycogen levels are depleted and blood glucose levels start to normalise, I think it is prudent to transition to the most nutrient dense foods possible while still maintaining good (though maybe not yet optimal) blood glucose levels.
The problem with doubling down on reducing insulin by fasting combined with eating only ultra-low insulinogenic foods is that you end up “refeeding” with refined fat after your fast.
While lowering carbs and improving food quality is the first step, I think that, as soon as possible you should start focusing on building up your metabolic machinery (i.e. muscles and mitochondria). A low carb nutrient dense diet is part of the story, but I don’t see many people with amazing insulin sensitivity that don’t also have a good amount of lean muscle mass which is critical to ‘glucose disposal’, good blood sugar levels and metabolic health.
This recent IHMC video from Doug McGuff provides a stark reminder of why we should all be focusing on maximising strength and lean muscle mass to slow aging.
The chart below shows a comparison of the nutrient density of the various dietary approaches. Unfortunately, a super high-fat diet is not necessarily going to be as nutrient-dense and thus support muscle growth, weight loss, or optimal mitochondrial function as well as other options.
The chart below (click to enlarge) shows a comparison of the various essential nutrients provided by a high fat therapeutic ketogenic dietary approach versus a nutrient dense approach that would suit someone who is insulin sensitive.
I developed a range of lists of optimal foods that will help people in different situations with different goals to maximise the nutrient density that should be delivered in the feast after the fast. The table below contains links to separate blog posts and printable .pdfs. You can find it here.
Jason had a “robust discussion” with Steve Phinney over the topic of ideal protein levels recently during the Q&A session at the recent Low Carb Vail Conference.
To give some context again, Phinney is used to dealing with athletes who require optimal performance and are looking to optimise strength. Meanwhile Jason’s patient population is typically morbidly obese people who are on kidney dialysis and probably have some excess protein, as well as a lot of fat that they could donate to the cause of losing weight.
I also know that Jimmy is a fan of Ron Rosedale’s approach of minimising protein to minimise stimulation of mTOR. Jimmy and Ron are currently working on another book (mTOR Clarity?). Protein also stimulates mTOR which regulates growth which is great when you’re young but perhaps is not so great when you’ve grown more than enough.
The typical concern that people have with protein in a ketogenic context is that it raises blood insulin in people who are insulin resistant. ‘Excess protein’ can be converted to blood glucose via gluconeogenesis in people who are insulin resistant and can’t metabolise fat very well.
Managing insulin dosing for someone with Type 1 Diabetes like my wife Monica is a real issue, though she doesn’t actively avoid protein. She just needs to dose with adequate insulin for the protein being eaten to manage the glucose rise.
The chart below shows the difference in glucose and insulin response to protein in people who have Type 2 Diabetes (yellow lines) versus insulin sensitive (white lines) showing that someone who is insulin resistant will need more insulin to deal with the protein.
As well as insulin resistance, these people are also “anabolic resistant” meaning that some of the protein that they eat is turned into glucose rather than muscle leaving them with muscles that are wasting away.
People who are insulin resistant are leaching protein into their bloodstream as glucose because they can’t mobilise their fat stores for fuel. They are dependent on glucose and they’ll even catabolise their own muscle to get the glucose they need if they stop eating glucose.
While it’s nice to minimise insulin levels, I wonder whether people who are in this situation may actually need more protein to make up for the protein that is being lost by the conversion to glucose to enable them to maintain lean muscle mass. Perhaps it’s actually the people who are insulin sensitive that can get away with lower levels of protein?
As well as improving diet quality which will reduce insulin and thus improve insulin resistance, in the long term it’s also very important to maintain and build muscle to be able to dispose of glucose efficiently and also improve insulin resistance.
In TCGTF Jason talks about the fact that the rate of the use of protein for fuel is reduced during a fast and someone becomes more insulin sensitive. He goes to great lengths to point out that concern over muscle loss shouldn’t stop you trying out fasting (which is a valid point).
A big part of the magic of fasting is that you clean out some of your oldest and dodgiest proteins in your body and set the stage for rebuilding back new high-quality parts. But the reality is that you will lose some protein from your body during a fast (though this is not altogether a bad thing). 
Bodybuilders often talk about the “anabolic window” after a workout where they can maximise muscle growth after a workout. Similarly, one of the awesome things about fasting is that you reduce your insulin resistance and anabolic resistance meaning that when at the end of your fast your body is primed to allocate the high-quality nutrients you eat in the right place (i.e. your muscles not your belly or bloodstream).
In the end, I think optimal protein intake has to be guided to some extent by appetite. You’ll want more if you need it, and less if you don’t.
I think if we focus on eating from a shortlist of nutrient-dense unprocessed foods we won’t have to worry too much about whether we should be eating 0.8 or 2.2 g/kg of lean body mass.
However, avoiding nutrient-dense, protein-containing foods and instead “feasting” on processed fat when you break your fast will be counter-productive if your goal is weight loss and waste a golden opportunity to build new muscle.
are you really insulin resistant?
Insulin resistance and obesity is a continuum.
Not everyone who is obese is necessarily insulin resistant.
If you are really insulin resistant, then fasting, reducing carbs, and maybe increasing the fat content of your diet will enable you to improve your insulin resistance. This will then help with appetite regulation because your ketones will kick in when your blood glucose levels drop.
However, if you continue to overdo your energy intake (e.g. by chasing high ketones with super high fat, low protein diet), then chances are, just like your body is primed to store protein as muscle, you will be very effective at storing that dietary fat as body fat.
I fear there are a lot of people who are obese but actually insulin sensitive who are pursuing a therapeutic ketogenic dietary approach in the belief that it will lead to weight loss. If you’re not sure which approach is right for you and whether you are insulin resistant, this survey may help you identify your optimal dietary approach.
optimal ketone levels
Measuring ketones is really fascinating but confusing as well.
“Don’t be a purple peetone chaser.”
Carrie Brown, The Ketovangelist Podcast Ep 78
Urine ketones strips have limited use and will disappear as you start to actually use the ketones for energy.
In a similar way blood ketones can be fleeting. Some is better than none, but more is not necessarily better. As shown in the chart of my seven day fast below I have had amazing ketones and felt really buzzed at that point but since then I haven’t been able to repeat this. I think sometimes as your body adapts to burning fat for fuel the ketones may be really high but then as it becomes efficient it will stabilise and run at lower ketone levels even when fasting.
If your ketone levels are high when fasting then that’s great. Keep it up. They might stay high. They might decrease. But don’t chase super high ketones in the fed state unless you are about to race the Tour de France or if you want your body to pump out some extra insulin to bring them back down and store them as fat.
The chart below shows the sum of 1200 data points of ketones and blood glucose levels from about 30 people living a ketogenic lifestyle. Some of the time they have really high blood ketone levels but I think the real magic of fasting happens when the energy in our bloodstream decreases and we force our body to rely on our own body fat stores.
the root cause of insulin resistance is…
So we’ve worked out that large amounts of processed carbs drive high blood glucose and insulin levels which is bad.
We’ve also worked out that insulin resistance drives insulin levels higher, which is bad.
But what is the root cause of insulin resistance?
I think Jason has touched on a key component in that, as with many things, resistance is caused by excess. If we can normalise insulin levels, then our sensitivity to insulin will return, similar to our exposure to caffeine or alcohol.
However, at the same time, I think insulin resistance is potentially more fundamentally caused by our sluggish mitochondria that don’t have enough capacity (number or strength) to process the energy we are throwing at them, regardless of whether they come from protein, carbs, or fat.
A low carb diet lowers the bar to enable us to normalise our blood glucose levels. However, the other end of the spectrum is focusing on training our body and our mitochondria to be able to jump higher. In the long term, this is achieved through, among other things, maximising nutrient-dense foods and building lean body mass through resistance exercise.
- The Complete Guide to Fasting is, as per the title, the complete guide to fasting. It’s the most comprehensive guide to the nuances of fasting out there and there’s a good balance between the technical detail, while still being accessible for the general public.
- Fasting can help optimise blood glucose and weight in the long term, with a disciplined regimen.
- Fasting makes the body more insulin sensitive and primes it for growth. When you feast after you fast, it is ideal to make sure you maximise the nutrient density of the food you eat as much as possible while maintaining reasonable blood glucose levels.
- Understanding your current degree of insulin resistance can help you decide which nutritional approach is right for you. As you implement a fasting routine and transition from insulin resistance to insulin sensitivity you will likely benefit from transitioning from a low insulin load approach to a more nutrient-dense approach.