Can You Still Burn Fat When Eating Carbs?

When I first ventured into the sport of endurance racing I was thrown into the low-carb high-fat model of fueling exercise. I landed on this model by pure accident. I had zero background or knowledge on how to fuel any type of exercise and came across the low-carb high-fat model purely because I was listening to a podcast.

As I normally do with the things I am trying to learn more about, I dove deep down the rabbit hole. The benefits to using fat as your main fuel source of endurance training seemed endless. The claims made by the people I was consuming information from not only said it would help me in my goal of running a 14+ mile obstacle course race but it would do amazing things for my health.

If I switched to eating a boat load of vegetables, some amount of protein, eliminated processed foods, and minimized all carbohydrates I would be able to run forever without needing to fuel and pretty much eliminate my chance of ever ending up with any kind of chronic disease out there.

Sounds like a win all around!

I was sold, I wanted to become a fat burner!

I swore off all carbs, including fruits, potatoes, squash, rice, and every packaged food known to man. I ate mostly above ground veggies, and had a small serving of protein with each meal.

Coming from a standard American diet rich in processed foods this dietary switch obviously improved my health, but I also saw my fitness improving at the same time…it was working!

While I was going through this dietary switch I was starting to ramp up my running as well. I learned how doing long slow distance running trained your body to utilize fat which complimented the diet I was eating.

I also learned about fasting, and how skipping meals forced your body to utilize fat rather than carbs.

I made it my goal to become the best fat burning endurance athlete I could.

I was able to accomplish that goal in 2016 when I finished the Vermont Spartan Beast in 6 hours and 44 minutes. The race was 17 miles long, contained 30 obstacles, and had about 7000 ft of elevation gain. I did it consuming nothing but nut butters and water.

Shortly thereafter though is when I realized my health was in the shitter, yeah I could burn fat like no one’s business, but I felt the worst I had ever felt in my life.

Fast forward to 2023.

I have now spent the better part of a year eating as many calories and carbs as I can possibly fit in my stomach. I am eating close to and sometimes over 400 grams of carbs a day. I rarely fast, I eat 3 large meals a day. While I still primarily eat whole foods, I am actively seeking out higher carb whole foods rather than higher fat whole foods.

I feel the best I have ever felt in my life. All subjective and objective markers of health are in the optimal range. I am the strongest I have been in my entire life.

If I was to share my diet with anyone in the low carb high fat space they would instantly call me a “carb burner” and say I have no ability to burn fat. They would probably also say that I am on my way to developing diabetes with all the carbs I am eating.

Well, I have been getting regular blood tests for several years now and never had a marker of metabolic health come back out of the optimal range. I have also strapped a continuous glucose monitor (CGM) to myself for several months multiple times and never seen any kind of pattern in my blood sugar that was cause for alarm. By all health metrics I can measure it appears that I can eat around 400g of carbs a day and be perfectly healthy.

But what about my ability to burn fat during exercise? By shoving down all these carbs each day have I eliminated my ability to burn fat? Am I now a “carb burner”?

I wanted to put this all to the test on myself. Luckily I have a metabolic cart that I can use to test exactly how much fat and how much carbs I am burning during exercise, so I decided to perform an experiment on myself!

I would pick an exercise and intensity that I could easily repeat under various conditions and measure the amount of fat and carbs I was burning. I would hold as many variables as possible constant and only shift the food I was/was not eating. For the exercise modality I chose to row 5000m on a Concept 2 rower. This would allow me to not only measure my heart rate during exercise but also the wattage I was producing so I could make sure intensity was as consistent as possible. It also allowed me to control the conditions which I was exercising under as much as possible.

Next I picked 6 different conditions to row the 5000 meters under. I knew I wanted to test both fed and fasted states, but I also wanted to test whether eating low carb or high carb the day before affected things. In addition, I thought it would be interesting to see if taking exogenous ketones would make a difference. I landed on 6 different conditions to run my test under.

Condition 1: Normal diet day before, 100g of carbs in meal before the 5k

Condition 2: Normal diet day before, done first thing in the morning, about 12 hours after dinner the night before

Condition 3: Normal diet the day before, done 15.5 hours after eating dinner the night before

Condition 4: Low carb the day before (less than 50g), done first thing in the morning about 12 hours after dinner the night before

Condition 5: Normal diet the day before, 100g of carbs in the meal before the 5k, plus 2 shots of HVMN Keto IQ (20g of ketones)

Condition 6: Low carb the day before (less than 50g), done about 15 hours after dinner the night before, 2 shots of Keto IQ (20g of ketones)

Normal Diet: 150g of protein, 350-400g of carbs, 90g fat

But before we dive into the results…first we must go down the nerd chute!

I am sorry if you find this stuff incredibly boring, in which case you can stop reading here and wait for the 3rd post in this series to see the results. However I think understanding the physiology, even at a high level, helps you not only make sense of the results, but also helps dispel the dogma around being a fat burner and being a carb burner.

With that said, if you want to understand how we burn fat, carbs, and ketones when exercising let’s dive in!

As you probably know, the food we eat is composed of three different types of macronutrients: protein, fats, and carbs. We can eliminate protein from the discussion about energy production because except in very rare cases, it is purely a mechanism for building tissue in your body. In RARE and EXTREME scenarios our bodies can take protein and convert it into energy (glucose) but your body really wants to avoid doing that because that means it is catabolizing itself, and that’s not good…no one wants to be a protein burner, because that means you are a muscle burner!

With protein out of the picture we can focus on carbohydrates and fats.

The carbohydrates and fats you consume are what provide nearly all the energetic needs of the body. It’s what allows your heart to beat, your brain to think, your eyes to read this blog, and your muscles to contract for exercise.

When you eat carbohydrates your stomach breaks them down and they enter your bloodstream as something called glucose. Glucose is a chain of six carbon molecules and six water molecules.

Glucose is then stored in the body in two places, your liver and your muscle.

When you consume fat the stomach breaks those down and releases them into the bloodstream as free fatty acids. These fatty acids are stored as triglycerides in the muscle or in adipose tissue, aka body fat. Triglycerides are basically 3 long chains of carbon atoms.

https://www.sciencedirect.com/topics/engineering/triglycerides

Here is a graphical representation of how carbs and fats are broken down and stored in the body

Metabolic States of the Body

Our muscles cannot use glycogen or triglycerides directly during exercise. An exercising muscle needs what’s called adenosine triphosphate or ATP. So how do we take glycogen and triglycerides and turn them into ATP?

It basically comes down to breaking up the carbon atoms that make up the glycogen and triglycerides. As we break down the bonds between these carbon atoms ATP will be released.

https://www.mdpi.com/1660-4601/18/9/4963

Since the muscle has two sources to choose from to create ATP from, glucose and triglycerides how does the muscle decide which one to use?

It all comes down to how fast the muscle needs that ATP, and whether there is enough oxygen available.

Triglycerides can produce a lot of ATP, but the process takes time and comes at a cost. We need oxygen present in order to break down triglycerides and there are many more steps required in taking a triglyceride and turning it into ATP.

Glycogen can be broken down with or without oxygen and can be done quickly but it produces much less ATP per molecule and when broken down without oxygen it produces hydrogen ions which cause the burning sensation in your muscle and causes fatigue.

In a healthy person, the body will choose the fuel it uses wisely. If the intensity at which you are exercising is low and there is plenty of oxygen around it will use more triglycerides to produce ATP because it has the time and the oxygen. Plus it produces more ATP and no pesky hydrogen ions.

Two things happen when the intensity of the exercise increases though. The demand for ATP goes up which means we need to produce more of it at a faster rate, and less oxygen is available due to it being utilized more by all the working muscles. To deal with this the body starts breaking down glycogen without oxygen. It can do this quickly to satisfy the demand but now it also needs to deal with the cost which is the hydrogen ions building up in the muscle. We won’t discuss how the body deals with the hydrogen ions in this post, just know that it’s a byproduct of this process and can cause fatigue in the muscle.

The muscle also has to make a choice as to where it is going to source the triglycerides and glycogen it’s going to use to create ATP.

In the graph below we see four sources of fuel, 2 from within the muscle, and 2 from outside the muscle. Muscle glycogen are carbs stored within the muscle, and muscle triglycerides are fat stored within the muscle. Plasma glucose is partly contributed by carbs that have been released from the liver into the blood and plasma FFA (free fatty acids) are fats released from adipose tissue.

Notice at 25% of VO₂Max (which would equate to a walk for most people) very little of the fuel will come from carbs (glycogen or glucose), most comes from your body fat. At 65% of VO₂Max, (which would be a moderate intensity jog for most people) we will have more than half our energy coming from carbs and the rest fat. As we start to approach near max intensity at 85% VO₂Max more energy comes from carbs than fat.

The other thing to notice is the location from which the carbs and fats are coming from. When at 25% of VO₂Max most of the energy is coming from the fat and carbs in your blood. This is because again the muscle has the luxury of time to pull the energy from the blood rather than the muscle. As intensity increases, more and more energy will come from within the muscle itself because it needs to access that energy quickly.

https://www.nature.com/articles/s42255-020-0251-4

There is a tradeoff to accessing the energy within the muscle. Energy within the muscle is able to be accessed quickly which is advantageous when exercise intensity is high, however the amount of energy stored within the muscle is finite. We have at most around 6000 calories of energy stored within the muscle, while outside of the muscle we have a virtually unlimited amount (due to body fat).

https://www.nature.com/articles/s42255-020-0251-4

If you are still with me at this point, congrats!

But we have one more thing to throw into the mix…ketones!

Up until this point we have discussed how fat and carbs fuel exercising muscle, however there is a 3rd fuel source we can consider…ketones.

Up until very recently, the only way to use ketones was for the body to produce them. Ketones are used by the body in times of food scarcity and are a beautiful evolutionary survival fuel.

In times when food was scarce our muscles would continue to burn fat and carbs as I have described above. However without any food coming in at some point our stored carbohydrates would start to get low. There are certain cells in the body (namely the brain and red blood cells) that can’t utilize free fatty acids as their fuel so we need to have some glucose around for these cells.

In order to conserve its precious glucose stores the body evolved a way of producing a 3rd fuel source called ketones. In order to create ketones the body takes free fatty acids, which as we talked about is virtually unlimited, sends it to the liver and the liver releases ketones. These ketones can be used by virtually any cell to produce ATP, even the ones that can’t burn free fatty acids directly! Our cells will preferentially burn ketones when they are present as opposed to burning glucose and therefore preserving our glucose stores until we can find more food.

At this point you have a basic understanding of how an exercising muscle takes the foods we eat and turns that into fuel it needs to perform exercise.

To summarize…

• Fats and carbs provide all the energy we need
• Fats get stored as triglycerides in the muscle and in adipose tissue (body fat)
• Carbs get stored as glycogen in the liver and in the muscle
• The muscle cannot use triglycerides and glycogen directly it needs ATP
• ATP can be created from triglycerides if oxygen is present
  • It produces a large amount of ATP
  • It can take a long time to convert triglycerides into ATP
  • Oxygen is required to convert triglycerides into ATP
• ATP can be created from glycogen whether oxygen is present or not
  • The creation of ATP from glycogen without oxygen will produce far less ATP and also created hydrogen ions which increases muscle fatigue
• The intensity of exercise dictates which fuel is used
  • As intensity increases the muscle will be forced to create ATP from glycogen without oxygen
  • We can satisfy ATP demand but it comes at the cost of muscle fatigue from hydrogen ions
• Intensity also dictates where the triglycerides and glycogen will be sourced from
  • At a low intensity it can use fuel from the bloodstream
  • At a high intensity it will use fuel from within the muscle, but supplies are limited
• Ketones provide a 3rd backup fuel source when glycogen stores state to become depleted

In the next post we will discuss how we can manipulate what fuel gets burned both through diet and technology. It turns out the context in which we are exercising plays a big role in fuel utilization. After that discussion we can get to the results of my experiment which will demonstrate how this physiology is working in real life! To be the first to know when these two posts are released join my newsletter using the form below and I will email you as soon as they are available.