Table of Contents
Our bodies have the amazing ability to be incredibly flexible, both physically and metabolically.
We can see this in humans who have pulled over 1000 pounds off the floor, power jumping several feet in the air, and running a marathon in less than 2 hours.
Humans are truly amazing!
While this physical flexibility is amazing, it’s not the reason why we have become the dominant species on this planet. We didn’t need to be able to lift 1000 lbs, jump many feet in the air or run at incredible speeds to survive as long as we have.
What allowed us to become the dominant species on Earth?
Our flexible metabolism!
Yes our brains are the primary driver to our continued success as a species but we needed to fuel those massive brains. Despite having to adapt to all kinds of different food environments, we have somehow managed to find a way to fuel the same human brain.
Some humans lived in environments that did not have the ability to support fruit and vegetable growth and survived mostly on animal foods.
Other humans lived in environments that contained fruits and vegetables year around and they consumed more plant foods.
Some humans lived by the ocean and ate an abundance of seafood, but others were completely landlocked and ate virtually no seafood.
Some humans consumed dairy, and others didn’t.
Some humans ate a lot of rice, some ate a lot of tubers, while others ate a lot of fruit.
Some humans cherished honey, others cherished animal fat.
We are all the same species but our bodies can adapt our metabolisms to what is available to consume.
Human metabolism has historically been incredibly flexible.
Yet today many people in the Western world are losing that flexibility.
Why?
Their diets never change and they are rich in calories.
The Impact of Modern Diets on Metabolism
Around 60% of many Western diets are processed foods which are high in fat, carbs, and calories (Ultra-Processed Food Consumption Among US Adults From 2001 to 2018, 2022).
Combine this with the increasing sedentary nature of Western individuals (Luke, n.d.) and it results in a caloric excess resulting in increased body fat.
When the human body has excess body fat, is consistently taking in excess calories, and those calories are rich in fat and carbs, the body needs to make some drastic choices to survive.
The deadly piece of this equation has to do with the excess carbs. I AM NOT DEMONIZING CARBS I AM JUST STATING HOW THE HUMAN BODY WORKS, CARBS ARE NOT BAD!
When we eat carbs they will be digested and enter the bloodstream as something called glucose. The amount of glucose in the blood needs to be controlled because if it gets too high or too low you could die. So when you eat an excess of carbs the body needs to do something to lower its glucose. It essentially has two options to do so:
- Burn the glucose immediately for energy.
- Store it so it can use it later when it needs it.
The body can store glucose in 3 places:
- Within your muscle.
- Within your liver.
- Convert the glucose to fat and store it as body fat.
Now some glucose can be used immediately to fuel your body’s muscles and brain but since most people are not moving a whole bunch the amount immediately burnt after consuming carbs is going to be small in comparison to how much most people are consuming.
In addition, since most people are not moving their bodies they have low muscle mass and the glucose stores in what muscle people do have is already full. While the liver can store some glucose the amount it can store is small, especially when compared to what you could store in your muscles.
So the body is left with only one option, take the glucose and store it as body fat.
Now here comes the amazing part of the human body…
When this scenario happens over and over again for years on end your metabolism “forgets” how to tap into its fat stores to provide energy when it needs it because it has an overabundance of carbs that it is constantly having to deal with. This is what is meant by the terms “metabolic inflexibility”, the metabolism has lost its ability to burn fat.
(Note this is the common scenario many people find themselves in today, but you can also be metabolically inflexible in the other direction and lose the ability to burn carbs. This can be seen in individuals who are on a low carb or ketogenic diet for a long period of time.)
Why Fasting Can Be Difficult for Some
Now let’s say someone who has lost the ability to burn fat hears about the amazing benefits that something like intermittent fasting can provide for them, what do they do?
They hop on the fasting bandwagon…all aboard!
They start fasting and adopt an 8 hour eating window and a 16 hour fasting window.
Then they hit the struggle bus.
Why?
Because during their fasting period their body is looking for fuel and it can’t find any.
But wait, the average person has loads of excess body fat, there is plenty of fuel on their bodies, what is wrong?
That body fat is only good if the metabolism knows how to use it, but since the average person in this case has lost the ability to burn their body fat the body has no way to access it. After 16 hours of not eating their stored glycogen is starting to run low and their body is worried that if it doesn’t eat soon death is around the corner.
After white knuckling the 16 hours of not eating this individual’s body is telling them to get all the calories they can and fast because we have a real emergency on our hands.
The result?
In the 8 hour eating window they manage to consume MORE calories than they would have if they just stuck with their normal eating patterns and somehow end up in a worse spot than they were before. In addition they are probably still relying on the highly processed high carb high fat high calorie foods as before so their body is right back on the carb burning bandwagon. The fast has done nothing to improve their metabolic flexibility!
Research Insights: Metabolic Inflexibility and Fasting
This isn’t just anecdote, a recent study provided evidence that metabolically inflexible individuals will overeat when practicing fasting (Impaired Metabolic Flexibility to Fasting Is Associated With Increased Ad Libitum Energy Intake in Healthy Adults, n.d.).
In the study researchers put 44 people in a whole-room indirect calorimetry chamber for 24 hours. This is a special room that can measure the amount of carbs and fats someone is burning. Normally to measure how many carbs and fats someone was burning they would have to be wearing a mask connected to a machine, but these rooms allow them to move about just like you were in any other room while at the same time measuring your metabolism as you do….pretty cool.
Participants spent 24 hours in the room under two different conditions. In the first condition they consumed an energy-balanced diet consisting of 50% carbohydrates, 30% fat, and 20% protein. In the second condition they fasted the entire time they were in the chamber. After their time in the chamber they had unlimited access to a vending machine with various foods in it and researchers tracked what they got from the vending machine.
The participants who burned more carbs under the fasting condition and were unable to tap into their body fat stores (meaning they were more metabolically inflexible) ate more calories after their time in the chamber than those that burned less carbs and more fats. This was only true in the fasting intervention, when the same participants spent 24 hours in the chamber while eating the energy-balanced diet they did not consume any more calories than the participants who were more metabolically flexible.
The authors theorized that because the participants were unable to tap into their fat stores to fuel their bodies while fasting their bodies perceived that there was some kind of energy emergency happening and they needed to eat more once the fast was over.
So is all hope lost for metabolically inflexible people? Are they destined to rely on carbohydrates?
No, not at all.
In fact, if they continued this fasting period and were not given a vending machine full of food their bodies would begin to tap into their fat stores. However, those first few days without food would be pretty miserable.
In fact we hear stories about this all the time from people who go Keto and get what is called the
“Keto Flu” where they feel low energy, lethargic, have headaches, and overall just feel pretty crappy. (As a side note some of what people experience as the Keto Flu may also be due to electrolyte depletion as their body sheds a whole bunch of water.) Part of the reason why some people feel so crappy when first going Keto is because they are metabolically inflexible and despite eating enough calories and having enough calories stored as body fat, their body has lost the ability to tap into the metabolic pathway to burn fat.
The take away from this blog post is this.
Strategies to Improve Metabolic Flexibility
If you are coming from years of eating a diet composed of highly processed foods and are trying to jump into a fasting protocol as the first means to improve your health it may not be the most pleasant experience and could be counterproductive for your goals.
If you can survive this period of time while your body learns how to tap into its body fat again for energy you will make it through to the other side unharmed. However I am not a fan of trying to push through tough dietary changes, especially when pushing through may involve a week or more of your effort. People have enough stress and pressures in their lives and we can take an alternative approach to improving metabolic health that will be much less stressful on you.
Instead of jumping into a fasting protocol that stresses your body I would much rather first transition you away from the processed food diet and take some time to learn how to eat a more unprocessed diet. This has 2 benefits.
First it will help you become more metabolically flexible because the unprocessed diet will be lower in calories and lower in carbohydrates.
Second, it will be a better diet for you when it comes to incorporating fasting (if you still want to) because it will provide you with a better regulation of appetite and blood sugar. Eating an unprocessed whole foods diet will trump a diet comprising processed foods with fasting.
For more information about how to construct a more whole foods unprocessed diet check out this video.
The Role of Exercise in Metabolic Flexibility
In addition to switching your diet to a more unprocessed whole foods diet I would also suggest you start adding movement and exercise into your lifestyle. Remember, the biggest storage depot for carbohydrates is in your muscles, so the more you can move your muscles and the more muscle mass you have the better your ability to handle carbohydrates and improve your metabolic flexibility. By moving your body throughout the day, for example by getting 7-10 thousand steps a day, you will allow your body to utilize the carbs stored in your muscle. By adding strength training 2-4 times a week you will then build more muscle and have more room to store carbohydrates.
See this video on how to build a movement routine and this blog post on how to construct your own strength training program if you need help.
As an added bonus tip on the exercise front you could consider fasted aerobic training as the icing on the cake when it comes to improving metabolic flexibility.
Why?
If you wake up in the morning after sleeping for 8 hours your body is in a pretty decent fasted state and by then doing some easy aerobic exercise (for example keeping intensity to the point where you can exercise only breathing through your nose) your body will be more likely to tap into fat as its primary fuel source. While this is not a MUST DO or something I would suggest when starting out, it is a great way to force your body to learn how to utilize fat as a fuel again.
Incorporating Fasting Safely: A Step-by-Step Approach
After spending several months moving over to a whole foods unprocessed diet and getting a good movement and exercise routine under your belt if you then want to try to do some fasting you will be in a much better position to do so. You would have improved your metabolic flexibility enough where an intermittent fast won’t be a suffer fest or result in your over eating later in the day. Still I would not jump into a longer fast right out of the gate, instead I would slowly build up to it.
If you have been eating within a 12 hour window for the past few months (meaning there were 12 hours where you weren’t eating) then I would push those 12 hours to 14 hours and try that for a few weeks. If you feel terrible not eating for 14 hours then push it back to 13 hours and see how you feel. If you still feel terrible at 13 hours and find yourself incredibly hungry you probably have more work to do from a metabolic flexibility standpoint. Continue to focus on your whole foods diet and moving your body while exercising a few days a week.
If doing a 14 hour fast is a piece of cake go ahead and try 16 hours. I personally wouldn’t recommend going longer than 16 hours on a consistent basis. You probably are not going to get many additional benefits by doing so and you start to run into an issue of getting in enough calories, protein, and micronutrients in such a short eating window. However if occasionally you want to try and do longer fasts once a quarter or so that will be perfectly fine.
Key Takeaways for Successful Fasting
Fasting can be a powerful tool and certainly has its health benefits. However diving in head first after years of eating a processed diet likely causes you to lose your metabolic flexibility and can make fasting a miserable experience that may actually not serve you in reaching your goals. Luckily you can improve your metabolic flexibility through some less extreme measures that will also serve you in reaching your goals. Transitioning to a whole foods unprocessed diet, moving your body, and strength training will improve your metabolic flexibility making incorporating fasting much easier and pleasant.
If you are looking to make your transition to a healthier version of yourself more pleasant subscribe to my newsletter using the form below. Each week I dispense valuable information that is actionable and can make the transition to a healthier lifestyle less painful and more successful. It is all free and comes in various forms, written, long form, video, you name it there will be something there that resonates with you and your goals!
References
Impaired metabolic flexibility to fasting is associated with increased ad libitum energy intake in healthy adults. (n.d.). PubMed. Retrieved August 12, 2024, from https://pubmed.ncbi.nlm.nih.gov/38650517/
Luke, D. A. (n.d.). Declining rates of physical activity in the United States: what are the contributors? PubMed. Retrieved August 12, 2024, from https://pubmed.ncbi.nlm.nih.gov/15760296/
Ultra-processed food consumption among US adults from 2001 to 2018. (2022, January 11). PubMed. Retrieved August 12, 2024, from https://pubmed.ncbi.nlm.nih.gov/34647997/