How Exercise Influences Your Ability To Burn Fat And Carbs

It seems like everyone is talking about how to become a better “fat burner” and I completely understand why.

When you look at the typical Western Diet, carbohydrates make up a giant portion of most people’s food intake.

Between fruits, vegetables, and grains 75% of the USDA’s My Plate recommendations are carbohydrates. Now that is not particularly a bad thing IF the carbohydrates are whole foods (and you tolerate them).

https://myplate-prod.azureedge.net/sites/default/files/2020-12/MyPlate_blue.png

The problem is that those things are not whole foods, they are processed foods. A prime example is pizza.

Would you consider pizza a vegetable?

I hope not.

But the US Congress does…because it has tomato sauce on it.

The consequence of eating mostly processed foods is a dependence on carbohydrates to correct for crazy blood sugar swings that occur when you consume highly processed foods.

When you enter this vicious cycle of constantly needing carbohydrates to power yourself throughout the day you have become what’s known as a “carb burner”.

To break this vicious dependence on carbohydrates you will hear many people say that you need to become a better “fat burner”.

One approach to becoming a better fat burner is to eat a “low carb diet” where you lower the amount of carbohydrates you are consuming to the point where your body has no other option but to start burning more fat. While this approach does certainly make you a better fat burner and eliminates your dependency on carbohydrates. it is often taken to the point where the person can no longer consume any carbohydrates without wanting to crawl back in bed and take a nap.

For most people the ideal state is not to be reliant on just carbohydrates or just fat, but instead be able to use both fat and carbohydrates depending on the demands placed on their body.

This is what is known as being metabolically flexible.

Obviously diets play an important role in being metabolically flexible, drift too far in one direction, either by consuming too many processed carbs or by eliminating all carbs and only consuming fat (and protein), and you lose the ability to use the other fuel source.

However, diet is not the only factor that can influence your ability to tolerate and utilize carbohydrates and fats. The other major influence on this process is movement and exercise.

Despite what many in the low-carb space say, you are never burning exclusively fat or carbs, it is always a mixture of both. The ratio of which you burn is what can change.

As you can see in the image below, as the intensity of our movement increases, fat oxidization declines and carbohydrate oxidization increases. At some point you will make the switch from burning mostly fat to mostly carbohydrates, this is known as your crossover point.

https://pubmed.ncbi.nlm.nih.gov/7928844/

In the image above this person’s crossover point occurred at roughly 70% of the aerobic power. The thing to remember is that everyone has a different crossover point and that it can fluctuate based on training and diet.

The average person off the street is going to have a crossover point that is a lot lower because they are a carb burner, while an elite marathoner will have a crossover point that is much higher because their body is more metabolically flexible and their training (and diet) has built the metabolic machinery to burn fat at higher intensities.

If you are someone who’s crossover point is much lower and you are dependent on carbs, one thing you can do from an exercise point of view to become better at burning fat and move that crossover point further out is to do fasted aerobic exercise. Depending on your training level that might be as simple as getting up in the morning and going for a walk. Due to not having eaten in 8-12 hours, insulin, a hormone that you can think of as a fuel switch, should be low and that will allow your body to tap into stored fat. In addition, the intensity of the exercise needs to be easy, if you can do it breathing through your nose, you know you are at the right intensity. Those two things combined, low insulin and low intensity, will allow the exercise to be mostly fueled by fat.

Overtime, by doing this fasted aerobic exercise, you will find that you can start increasing the intensity and feel ok. At first after the walk you might be seriously hungry, or feel very tired. With time though your walk might feel like nothing, just another walk in the park. At that point you might want to go for longer walks, or maybe try an easy bike ride. Once that becomes too easy, try an easy jog. The fact that you can do harder or longer bouts of activity without needing to eat during or immediately after means you are moving that crossover point further out and becoming more metabolically flexible.

Perhaps the best way to become more metabolically flexible via exercise is to strength train. Why is strength training the king of making you more metabolically flexible?

When done right, strength training will add lean mass, aka muscle, and muscle is a huge store of carbohydrate. The body can only store carbohydrates in two places, the liver or your muscle. The liver can only store about 13% of the total carbohydrate in your body, the rest goes into muscle^[1]. Not only does the act of strength training use carbohydrates to fuel the muscle contractions, it also contributes to building additional muscle to store more carbohydrates.

How does this relate to metabolic flexibility?

Imagine you are someone with a low amount of muscle mass who is not strength training. You consume your favorite carbohydrate (sweet potato, fruit, rice, maybe a piece of birthday cake), and your body now needs a place to put this carbohydrate. Since you have very little muscle and you have not worked out your blood sugar spikes, and your body rushes to lower it sending it crashing down and then you are searching for your next carbohydrate fix an hour or two later.

Now imagine you are strength training and have built more muscle, and 6 months later you have more muscle mass and consume the same carbohydrate. Since you have exercised, you have depleted your muscle stores of carbohydrates and you have more muscle mass on your body, your body can take that carbohydrate and put it into the muscle to help fuel your next workout.

This keeps blood sugar lower, avoids the blood sugar crash, and therefore keeps you from entering the vicious cycle of becoming carb dependent.

On the other hand exercise can also make you metabolically inflexible as well.

WHAT?!?!?!

How can exercise actually hurt you?

Well, too much of a good thing is not always good for you.

Getting some sunlight is good. Getting too much can cause you to burn.

Some fasting can be good, too much can leave you depleted.

An occasional alcoholic drink from time to time can be fine for some people, drinking too much all the time is going to be bad for you.

Exercise follows a similar pattern.

Too much exercise intensity too frequently can actually make you less tolerant to carbohydrates. In a recent study scientists increased the amount of high intensity exercise participants performed over the course of three weeks. They started out doing 36 total minutes of high intensity exercise spread out across a week. On week two that bumped up to 90 minutes. On week three that number jumped to 152 minutes! By the end of the third week the participants tolerance to an oral glucose tolerance test was less than their baseline measurements^[2]!

The bar graph below represents the area under the curve of the glucose response of the athletes to the oral glucose (aka carbohydrate) tolerance test. This represents how long it takes for their glucose to come back down after a spike. Notice the difference between baseline training (BL) and excessive training (ET).

https://www.cell.com/cell-metabolism/fulltext/S1550-4131(21)00102-9

In the same study researchers attached a continuous glucose monitor to elite endurance athletes and control matched them with participants that were training less than 7 hours a week and measured glucose in free living conditions for 2 weeks. Researchers found that both the control group and the endurance athletes had the same mean glucose levels. However, the endurance athletes spent more time outside the normal range, experiencing greater times of high glucose and low glucose levels!

https://www.cell.com/cell-metabolism/fulltext/S1550-4131(21)00102-9

When it comes to harnessing both carbohydrates and fat to fuel our bodies throughout the day, a combination of both diet and exercise is going to help you promote optimal metabolic flexibility. Today, so many people rely solely on diet to help promote better metabolic flexibility by eliminating carbohydrates. However, I find that with most people, never eating carbohydrates again is going to be a hard thing to do for the rest of their life. In addition, the elimination of carbohydrates can in fact make you less metabolically flexible, eventually taking you further away from your goals.

Instead of relying on just manipulating your diet to improve your metabolic flexibility you can also use your exercise to help. To improve your ability to use fat you can do some easy fasted aerobic exercise, anything from walking to an easy run will work. To improve your ability to use carbohydrates you should strength train in order to build more muscle mass and have more room to store carbohydrates. With those two pieces in place you should be able to consume a mixture of both fat and carbohydrates in your diet without being reliant on one or the other.

It should come as no surprise that a combination of both aerobic training and strength training is what our ancestors did on a near daily basis. Long slow walking was done daily when gathering food and hunting. Picking up, squatting down, throwing, jumping, crawling were all staple movements that worked our ancestors’ muscles and allowed them to build and maintain optimal muscle mass.

It is no surprise that they could go long distances fasted, burning mostly fat and then when they found the beehive, chow down on a massive amount of honey, and be no worse off. Our ancestors were metabolically flexible because they ate whole unprocessed foods and moved their bodies. The flexibility they built helped them be more adaptable and hence survive no matter what food was available to them.

By following my suggestions above I am hoping to eliminate some of the modern ancestral mismatches that make us metabolically inflexible and help you reach your health, wellness, and performance goals. For more information on how eliminating ancestral mismatches can help you reach your goals, use the form below to sign up for my newsletter where I provide you with actionable steps you can take on a weekly basis.

1. (2018, February 10). Fundamentals of glycogen metabolism for coaches and … – NCBI – NIH. Retrieved June 10, 2021, from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6019055/ ↑
2. (2019, May 16). Controlled study links processed food to increased calorie …. Retrieved June 11, 2021, from https://www.eurekalert.org/pub_releases/2019-05/cp-csl050919.php ↑