Isn't metabolic flexibility more a function of activity levels?
Endurance Training
Endurance Training and Sports tend to promote more body fat being used as fuel. That in part due to the fact their intensity level is lower.
The lower your intensity level, the greater the percentage of ketones/body fat burned and the lower you percentage of muscle glycogen used for fuel.
Thus, your sleeping fuel source is predominately ketones/body fat. An activity like walking is also predominately burn ketones/body fat.
As this chart show...
Metabolic Efficiency: Becoming A “Better-Butter-Burner
The Crossover Concept
The crossover concept describes the relation.ship between exercise intensity and the use of fat and carbohydrate for energy...
...aerobic exercise without a lot of available carbohydrates, your body adapts by increasing its workforce of enzymes that metabolize fat...
Glucose Dependent Individual
Research shows that individual on a the Western High Carbohydrate Diet are glucose dependent. Their energy system is more dependent on glucose for all activities.
While endurance training increased their ability to utilize more ketones/body fat, they continue be glucose dependent when on a Western High Carbohydrate Diet.
Keto Dependent
The Ketogenic Diet enable individual to utilize ketones/body fat for fuel. However, it appears that individual are more ketone dependent; don't access glycogen as well.
The Good News
The good new for Endurance Athletes on a Ketogenic Diet is they aren't going to run out of ketones.
As the article stated, "... most trained runners are carrying about 1,500 – 2,000 calories of carbohydrates and 80,000-plus (!) calories of fat. (Even the leanest runners.)"
Thus, Keto Adapted Endurance Athletes utilize more ketones/body fat for fuel and preserve muscle glycogen. They are less likely to "Bonk" (run out of muscle glycogen) or they are able to go farther before they do.
The Bad News
It appears that Keto Adapted Athlete are not able to access muscle glycogen as rapidly and effectively. Thus, they may not be able to access and utilize the Glycolytic Energy System for more intensive run that require for up 2:5 minutes of moderately high intensity.
With that said, Rachael Gregory's "
A Low-Carbohydrate Ketogenic Diet Combined with 6-Weeks of Crossfit Training Improves Body Composition and Performance" research indicates that a well Keto Adapted Athlete can perform well in the Glycolytic Energy System.
In a email discussion with Al Caimpa, he stated the same and went into some interesting detail on it.
So, the topic is still open for discussion.
Metabolic Flexibility
Metabolic Flexibility amount your energy system operating like a Hybrid Car.
Metaphorically speaking, with low level activities the car runs off electricity (ketones).
In high level activities (accelerating to pass another car) the car run off gas (glucose).
The most effective diet that programs the body to utilize either ketones or muscle glycogen, dependent on the intensity level of the activity is...
Intermittent Fasting
Dr Mike T. Nelson is one of the leading experts with Intermittent Fasting. Nelson's research has determined that Intermittent Fasting essential turns you into a Hybrid Car.
Your choice of fuel will depend far and away more on activity levels than anything else.
Definitely
However, sometime the fuel used can be change by modifying the activity, as with...
"Cluster Set Hypertrophy Training"
Most Hypertrophy Training revolves around the Glycogen Energy System.
Cluster Sets allow an individual to mainly utilize the Phosphagen Energy System and minimize the Glycolytic Energy System.
I'm not clear on the phrase "metabolic flexibility". Isn't that just another way of saying you don't have any metabolic disorders?
No
Metabolic Flexibility means being able to use the right tool (right energy system) for the right job (activity), as in the Hybrid Car analogy
Kenny Croxdale