The human body is an infinitely complex machine, but like even a simple mechanical device it requires a few basic inputs to operate. In simplest terms, you can think of your body like an engine in which the process of internal combustion is replaced by cellular respiration to produce movement. Just like the engine in your car (unless you drive an electric car, in which case I’m guessing your lack of animal protein consumption has left you frail and barely able to stand to read this blog) your body requires fuel to operate.
Instead of 87 octane though, your body runs on organic substances, and unlike your car which can barely seem to survive the 405, you body has developed unique survival adaptations over millennia that actually allow it to run on multiple types of fuels. Your body’s ability to switch between these fuels and burn them efficiently during different types of activity can unlock both your performance and physique goals.
The two primary fuel sources you body runs on are glucose and fat. Glucose is found in carbohydrate while fat can come from ingested food or your own adipose tissue. Metabolic flexibility is therefore the body’s ability to switch between these two fuel sources depending on availability and need.
*Although the body can burn protein for energy this is an inefficient process and near last resort.
Metabolic flexibility is a survival adaptation developed over the course of human evolution. Imagine the pre-agricultural or Cave man era (also know as starving Freshman year in the dorms). When food is plentiful, the body runs mostly on carbohydrate, but when food is not available, rather than starving, the body is able to burn its existing adipose tissue to survive. Ramen noodles don’t count as food so I’m not sure where they fit into the energy systems for cave men and college freshman. Although a survival adaption, metabolic flexibility in our overfed modern society provides the means to “hack” our biology to improve both performance and appearance!
Taking It back To Freshman Bio Class
How Does The Body Fuel Itself?
Glycolysis (Carb fueled metabolism) (fed state)
When you ingest carbohydrate your body breaks down this macronutrient into a simple sugar called glucose. Glucose is fuel for your body that is converted into ATP, the energy source for your cells. Too much glucose is poisonous for the body so once it enters the blood stream, the body produces a hormone called insulin that signals the body to absorb the glucose. Glucose is either converted to ATP for immediate energy in a process called glycolysis, or stored in muscles as glycogen. Excess glucose is stored as body fat, which is why those donuts give you a donut around your mid section.
Ketosis (Fat fueled metabolism) (fasted state)
When carbohydrate is lacking from the diet as in the lean times of the pre agricultural era, or on popular ketogenic diets such as the Atkins, the body will switch from metabolizing carbohydrate for fuel to fat instead. This fat based metabolism produces acids called ketones, hence the name for the process.
How Does The Body Expend Energy?
Depending on the activity, the body relies on three major energy pathways.
ATP – Creatine Phosphate (CP)- Intense activity of 0-10 seconds (think one rep max lift)
Glycolic or Anaerobic – 10-120 seconds in duration (think a shorter metcon or sprint)
Oxidative or Aerobic – 120 sec + (Longer metcons or “cardio”)
CrossFit workouts are a unique blend of all three energy systems, which is important to note in fueling a particular training session.
Which Energy System Requires Which Fuel?
When a person is metabolically flexible, they can readily switch between glycolysis and fat burning. The ATP/CP and glycolic energy systems are best fueled by glucose from carbs, while the more moderate, aerobic energy system can be fueled by fat. As humans always exist in a moderate aerobic state, not only does metabolic flexibility mean that you use fat as fuel when you head out on a run, but also at “rest” just sitting on your couch your body will metabolize fat, some of which comes from your own adipose stores. Metabolic flexibility is one reason that the fitter individuals you see around you can seem to get away with eating junk food now and then; their bodies literally burn it differently.
What Is Meant By A Fed vs. Fasted State?
A fed state is one in which the body has adequate stores of glycogen in the muscle to perform high intensity activity. In this state the body will be running primarily off of glucose from carbohydrate for fuel. A fasted state on the other hand is one in which the absence of glycogen stores means that the body will burn primary fat for fuel. A fed vs. fasted state is important for which type of workout you choose and how you fuel it. A metabolically flexible individual will perform high intensity anaerobic activities in a fed state, while training aerobically in a fasted state.
How Do You Eat To Promote Metabolic Flexibility?
Eating to promote metabolic flexibility is thankfully simple and intuitive. As an athlete you must ask; what energy system will I primarily be using for this training session? If you are going to do some lifting followed by a metcon then it is best to throw some carbs into your pre and post workout meal for best performance and recovery. On the other hand, if you are going out for a run or rowing a 5k, then fat can be the primary fuel for your system.
Basic Macronutrient Distribution for Metabolic Flexibility:
Carbohydrate – 20/40 grams pre and post workout for anaerobic training
Protein – .7-1.0 gram per pound of bodyweight
Fat – 60 – 80 grams
*Combine 20/40 grams of carbs with 20/40 grams of protein pre- and post-workout on anaerobic training days
How Do You Train To Promote Metabolic Flexibility?
Focusing each day’s training on a specific energy pathway will help to promote metabolic flexibility. Alternate training days between anaerobic work (metcons and lifting) and aerobic work (rowing, running, hiking, “cardio” etc.) On the night before and prior to training on anaerobic days, incorporate carbohydrates into your diet as rocket fuel for you intense workouts. The night before and during the day of aerobic training cut back on the carbs and incorporate more fat and protein into your meals to complete these workouts in a “fasted” state that will burn fat for fuel.
A great strategy for this is “fasted cardio” which is best undertaken in the morning prior to eating. Try waking up and going for a run on an empty stomach, but make sure to hydrate. As you have not eaten since the night before your body will be in a fasted state meaning that your it will quickly burn through the remaining glycogen in your muscles and move onto burning fat.
*A note for Athletes with multiple training sessions per day – If you are a competitive athlete you may need to complete multiple workouts across energy systems throughout the day. If this is the case it is still possible to achieve metabolic flexibility specific to each workout throughout the day, but it is best to build this flexibility through the simplified alternating training day system first and then add complexity to your training. As an athlete the ability to switch between metabolizing fat or carbohydrate throughout the day will be useful for competitions involving multiple workouts in the same day. Switching between glycolysis during events and fat based metabolism at rest will help with recovery between workouts. If completing multiple workouts in the same day it is best to complete aerobic work in the morning in a fasted state, and then reload the body with carbohydrate for more intense sessions later in the day.
Benefits of Metabolic Flexibility
- Increased fat loss – once your body learns to burn fat for fuel, it will do so at rest, so while you sit on the couch or walk down the street you body will be burning away at your adipose tissue
- Better recovery between workouts – The ability to switch to ketosis when at rest results in better and faster recovery and repair between workout days and even between workouts in the same day.
- Increased insulin sensitivity – Being metabolically flexible leads to greater insulin sensitivity, which means that the body will more efficiently use carbohydrate as fuel for workouts.
- Better Anabolic Response to meals – Your body will more efficiently utilize nutrients to build muscle