Of all the fierce debates raging in the world of fitness and nutrition, there seems to be one that stands out above all else: Carbohydrates or Fat for Fuel?!

Individual beliefs on this debate vary and are generally based on personal goals. The hardcore athletes among us look at athletic performance first and will do whatever it takes to improve it. Even if that means slamming refined carbohydrates all day for cheap and easy energy. Inflammation and free radicals be damned!

The other side of the debate is where most of the nutritionists hang out. Nine times out of ten, when a person reaches out to a nutritionist for help, they’re struggling. Their health may be in decline or already in a pretty bad place. It’s the nutritionist’s duty to put health and wellness in front of all other goals. Six-pack abs and bulging biceps be damned!

So, which group do I side with? Both… duh.

Here’s the thing, while countless people get caught up arguing about the little intricacies and differences in these varying approaches, I tend to step back and take in the view from 30,000 feet. If our bodies have two primary sources of energy… why should we have to choose?! In a perfect world, fat and carbohydrates would be interchangeable because our bodies could efficiently burn either. When our bodies shift from one fuel source to the other, it would be undetectable to us… in a perfect world.

Well, my friends, the world can be perfect. That is the beauty of Metabolic Flexibility. Let’s dive in…

I want to touch on a few things to give a brief overview of the science behind all this…
Your body needs energy to perform each and every function. This means your body is always creating energy within its cells. It does this using two major pathways…
  1. Aerobic Pathway
  2. Anaerobic Pathway
There are also two primary sources of fuel in the body…
  1. Glucose (carbohydrates)
  2. Lipids (fats)
Glucose is broken down much faster than lipids because it does not need oxygen to create energy. Lipids do need oxygen to create energy. Think of it this way…
  • Aerobic = with oxygen
    • Burns Fat
    • The aerobic pathway takes a bit longer than the anaerobic pathway to create energy. Lipids can’t be used for energy without conversion. The aerobic pathway uses oxygen to convert lipids into something called ketones. Only then can they be utilized by the body for energy.
  • Anaerobic = without oxygen
    • Burns Glucose
    • The anaerobic pathway is the fastest way to get instant energy. It does not use oxygen because glucose can be converted into energy even in the absence of oxygen.

Certain activities require immediate energy. These activities are considered “glycolytic” meaning that they require glucose to fuel. A few examples: sprinting, Olympic lifting, high-intensity interval training, etc. etc.. Anything that requires fast, all-out bursts of maximum strength. In these moments, the body cannot afford to wait for lipids to be converted into ketones, so it uses glucose.

 Other activities do not require immediate energy. A few examples: walking, yoga, light jogging, certain body building exercises, etc., etc.. The body can take its time creating energy, so it will use oxygen to convert lipids (and stored body fat) into ketones and use those for fuel.

Sounds like a pretty great system, right?! But there’s a slight catch. Here’s where things get tricky…

Generally speaking, as long as there is excess glucose in the body, it will never burn fat! Even for the low-intensity workouts! Burning glucose is easier and takes less time, so the body just does it. The reason for this is that most Americans have had a constant surplus of glucose (thanks to carbohydrates) their entire lives. Their bodies have never had to burn fat, so they suck at it, and just don’t do it.

This is called, “metabolic inflexibility.” This means your body only knows how to use one metabolic pathway for fuel. In this case, the Anaerobic Pathway (glucose). Unfortunately, almost every person I’ve ever worked with is living in a state of metabolic inflexibility by the time they get to me. They have trained their bodies to understand that the supply of glucose is never ending!

If you’ve spent any amount of time researching low-carb Paleo or the Ketogenic Diet, you may have heard the term, fat adapted. A fat adapted individual can efficiently burn fat for fuel. They have trained their body to do this using specific nutrition protocols, sometimes combined with certain types of aerobic training.

You may be asking, “why do we need to train our bodies to do this?” Well, because most of our bodies are broken and malfunctioning. Somewhere along the line, our bodies simply forgot how to burn fat. It sounds ludicrous but (for most of us) it’s the sad truth. Once upon a time, our bodies actually preferred burning fat for fuel. By once upon a time I mean, since the day we were born. Why? Because nature intended it to be that way!

Exclusively breastfed babies remain in a constant state of mild Ketosis, meaning they are only burning fat for fuel. That’s why mother’s milk is extremely high in fat and extremely low in carbohydrates (not counting fiber of course). It’s not until the well-meaning adults come along and introduce refined carbohydrates that said babies gets placed squarely on a path towards metabolic inflexibility. Perfect little baby bodies don’t even know simple carbohydrates exist until they’re introduced to them by well-meaning adults. That’s when the snowball effect begins. Thanks to the standard Western Diet, these babies grow up eating more and more carbohydrates, each and every day, year after year. All throughout life, there is such a massive supply of carbohydrates that the body says “screw it” and starts burning glucose exclusively for all its energy needs. Even though that it is a very inefficient way for the body to operate. But why should you truly care about this?! Simple…
  • Metabolic Inflexibility leads to insulin resistance
  • Insulin resistance leads to Type 2 Diabetes

Do I have your attention? Good!

So, how can we “fix” our bodies and switch from a state of metabolic inflexibility into a state of metabolic flexibility? Simple… discipline!

By switching to a low-carbohydrate diet for a period of time (without cheating), we are sending a message to our body that the glucose supply has dried up. This leaves the body with two choices:
  1. Die
  2. Find an alternate source of fuel

I promise you, your body will choose option 2. Fair warning, this process may not be fun, but it is more than worth it for the long-term health benefits!

When you first switch to a low-carb diet, your body has no clue what it is doing. Be patient. The body will have to re-learn how to burn fat as an efficient fuel source. You have to give your body time to adjust! Don’t be alarmed if you feel a little rough the first few days. You may have experienced this if you have ever made an attempt at following a very low-carb diet. Some people actually refer to it as “The Keto Flu.” The good news is, you only feel crappy until your body learns to burn fat. The degree of said crappy feelings will vary significantly from person to person, based on the number of carbohydrates the individual was consuming daily before making the switch to low-carb.


I recommend 30 days of following a low-carb, Paleo, diet, consuming less than 50 grams of net carbohydrates per day. You can do anything for 30 days, right?!

If you’re an athlete, I also recommend you continue your workouts during this period. It might suck, and your performance may temporarily suffer, but your body will adjust, and the exercises will help you burn off excess glucose in the system even faster. (Speaking of faster, actual fasting is probably the quickest way to remove glucose from the body, if you’re into that sort of thing!)

In my experience, after a successful fat adaptation period, athletes actually report improved performance long-term! Their system is no longer unnecessarily stressed and stunted by metabolic inflexibility.

Eventually, once you have trained your body to burn fat, you will have achieved what is called, “Metabolic Flexibility.” This means, your body can effortlessly switch between burning glucose and burning fat for fuel, depending on the situation. It will only burn glucose when it is absolutely necessary! The rest of the time, it will burn fat. Being able to switch fuel sources based on the demand placed on the body is the true meaning of fat adaptation. This is exactly how our bodies are designed to operate! In fact, our body is capable of creating its own glucose for glycolytic activities even if we do not ingest any carbohydrates. This process is called gluconeogenesis. Certain functions in the body require glucose, so our body has built-in processes for creating it. Unless you’re competing in an Iron Man, there’s simply no reason to overload the body with harmful practices such as “carb-loading” for any physical activity.

If your primary goal is to burn fat, you’ll want to make a conscious effort to make sure that most of your training is aerobic, not glycolytic. The best way to know if the training you are doing is glycolytic is to track your heart rate. The best explanation of this is by Dr. Phil Maffetone and his 180 Formula. 180 minus your age is your Max heart rate to make sure that you are using the aerobic pathway (with oxygen, burning fat) for energy. For example, I am 30 years old, so my max heart rate would be 150.

As long as you follow this rule, and do not engage in activities that spike your heart rate above your number (180 – Your Age = Your Max Heart Rate), you can be confident that you are burning fat for fuel, not glucose!

Becoming fat adapted and achieving metabolic flexibility require time, patience, and discipline. Three things that seem harder and harder to come by in our current social climate of “instant gratification.” But I assure you, what you give is what you get. Your body will reward you generously for taking the time to get it back to where it started. You know, before all the refined carbohydrates… Tisk, tisk.