My wife told me this blog post is long…she’s right, it is! If you don’t want to read it all I wanted to summarize all the main points up front.
- We are always burning a mixture of fat and carbs, even those people who are eating a low carb diet
- Chronic low carb diets can inhibit your ability to tap into carbs at higher intensities
- Chronic overeating, especially with carbohydrates, can inhibit your ability to tap into fat even at lower intensities
- Acute dietary intake will also affect fat and carb utilization during exercise. If you eat carbs before exercise your body will burn carbs even if the intensity is low
- The top of training you are doing will also bias your fat vs carb utilization. Chronic high or low intensity training will bias carb and fat utilization respectively
- Low respiratory function will cause you to burn more carbs due to the inability to get oxygen into the muscle
One more thing before we jump into everything….
I want to clarify this entire blog post by saying I present a lot of research in this post, and research does not look at the individual. Each way of eating and exercise can affect people in vastly different ways and research cannot predict individual outcomes. If eating a ketogenic diet, or low carb, or doing chronic endurance training improves your performance and goes against the research, that is great! I realize there are LOTS of factors that go into performance like sleep, stress, digestion, mental health, and much, much more. It may be that your specific way of eating or training improves one of these other factors resulting in improved performance…in which case I say SCREW THE RESEARCH AND YOU DO YOU!
With that out of the way let’s jump in!
In the previous blog post in this series we learned about how the body can utilize three different types of fuels during exercise, fats, carbs, and ketones.
As we discussed in that blog post, many people, especially in the low carb space, like to categorize people as either a fat burner or a carb burner. Many of these people also hold the opinion that being a fat burner is good and a carb burner is bad (regardless of your goal).
This is a very black and white way of thinking, and as you know I am not a fan of black and white thinking because it’s rarely ever beneficial or true, and this is especially true in the case of energy metabolism.
One of the takeaways from last week’s post was that we are almost always burning a mixture of fats and carbs. We already know that no one can be exclusively a “fat burner” or exclusively a “carb burner”, and that even the most “fat adapted” humans we have studied are still burning some amount of carbs during exercise.
The most well known study demonstrating this is referred to as the FASTER study (Volek & Phinney, 2015) in which the athletes in the low-carb arm followed a ketogenic diet (very low carb diet) for a year straight. While they burned a massive amount of fat during a treadmill run at 65% of their VO2Max they still burned around 0.5 grams of carbs per minute.
This study brings us to our first hack for manipulating your energy metabolism…your diet.
Looking at the diets of the high carb and low carb group in the FASTER study we can see the high carb group was eating an average of nearly 500 grams of carbs a day while the low carb group was eating around 80 grams a day. Like I mentioned above, they did this for a year and we can see how much of a difference there is between the two groups in the amount of fat and carbs they burnt.
What we don’t know is what their fat and carb metabolism looks like when exercising at a high intensity, remember this treadmill run was done at 65% of their VO2Max, which is moderate intensity at best.
Another study (The Effects of a Ketogenic Diet on Exercise Metabolism and Physical Performance in Off-Road Cyclists, 2014) took 12 male cycling athletes, had them eat a ketogenic diet (50g of carbs or less) or a mixed diet eating about 480g of carbs per day. After eating this diet for 4 weeks researchers had them perform a 105 minute exercise session. For the first 90 minutes they exercised at 85% of their lactate threshold, but for the last 15 minutes they exercised at a maximal intensity.
The nice part about this study is they used a cross over design, so after the first 4 weeks eating either the mixed or ketogenic diet they had them spend another 4 weeks eating the opposite diet and repeat the test. By doing this it allowed researchers to look at how each person performed on both diets and compare the results.
Just as we saw in the FASTER study when the intensity was moderate, for the first 90 minutes of the test, the athletes on the ketogenic diet burn more fat than the mixed diet. However once intensity started to ramp up at the 90 minute mark the amount of fat burned in the ketogenic group was no longer statistically significant when compared to the mixed diet.
The same pattern was observed in an opposite manner when looking at the amount of carbs burned. The ketogenic group burned less carbs for a greater duration but it was not statistically significant when compared to the mixed diet group. When intensity started to ramp up around the 90 minute mark we saw the mixed diet group burning a statistically significant amount more of carbs than the ketogenic group.
The ultimate outcome for most athletes though is not the amount of fat or carbs you do or do not burn during exercise, its performance. And when we look at the performance metrics measured during the test we can see that the mixed diet produced a greater wattage at max workload and a greater wattage at their lactate threshold. This would theoretically translate into a greater performance during competition.
We also notice that the ketogenic diet had a better VO2Max and a greater VO2 at their lactate threshold. While we shouldn’t dismiss those results, oxygen consumption is not a predictor of performance.
Well except in a very specific scenario…
The shift in VO2 at lactate threshold when on a ketogenic diet as we see in the results of this study can be seen as a performance benefit, depending on the sport. First, let’s understand what this means.
Lactate threshold is the point at which lactate starts accumulating rapidly at a near exponential pace. Lactate is a by-product of metabolizing carbohydrates very rapidly. In the lactate graph from this study we see that when on a ketogenic diet this gets shifted to the right in the graph.
What this tells us is that when on a ketogenic diet we are altering our fat metabolism relying on fat as a predominant fuel source for a longer period of time at a higher intensity. This can have its benefits in that you can conserve your carbs at a higher intensity. This is particularly beneficial when we start to look at long endurance events of several hours. However if your event is under in 30 minutes then being able to conserve your carbs serves no benefit because you are at no risk of running out anyways.
This brings us to the overarching concept as it relates to diet and fuel utilization. Your body is going to adapt to the conditions you place it under. Whether that adaptation is beneficial or not depends completely on your goals.
If you restrict carbohydrates and continue to exercise under this condition your body is going to begin to adapt in a way that makes you rely on fat, and potentially ketones, to fuel that exercise. The reliance on fat may come with some benefits or it may not. If you want to run 100 miles it’s going to be beneficial, allowing you to utilize your carbohydrates more efficiently. If you are going to run a mile all out it will likely be detrimental as you have down regulated the use of carbohydrates and in prioritized fat and therefore your body may not be able to generate ATP as fast resulting in a slower mile time.
The body can also adapt in the other direction as well. What if you are overfeeding yourself on carbohydrates and never allowing your body a chance to utilize fat?
You guessed it! Your body is going to adapt in a way that you prioritize burning carbohydrates all the time. A prime example of this is looking at the fat utilization of people who have type 2 diabetes during exercise and comparing them to people who don’t have type 2 diabetes.
Type 2 diabetes can be developed in combination with several factors, but one of those factors is an overconsumption of calories. In today’s world that means a high amount of carbohydrates when compared to the amount of activity an individual is doing. This results in a chronically high blood sugar level, even in a fasted state. Since blood sugar needs to be tightly regulated (or else you can die) the body is always trying to burn off that blood sugar. This then results in individuals with type 2 diabetes burning more carbohydrates and less fat even at rest when compared to someone without type 2 diabetes.
To demonstrate this, one study (Kelley, n.d.) looked at three groups of men and women who had type 2 diabetes and had them perform three different interventions. The first group modified their diet to lose weight and undertook a 16 week exercise program. The second group just did the 16 weeks of exercise. The third group just modified their diet for weight loss. Researchers then compared several outcomes between the three groups, one of which was how much fat they burned in a fasted state and during an oral glucose tolerance test.
The two exercise groups were instructed to exercise 4-6 times a week for at least 30 min per session at an intensity of 60–70% of their maximal heart rate for the first 4 weeks. During weeks 5–8, exercise sessions were increased to 40 min at the same intensity. During weeks 9–16, exercise sessions were continued at 40 min and the intensity increased to 75% of maximal heart rate.
The two groups that modified their diet were eating 500-1000 calories less per day and eating a “low fat diet” with less than 30% of total calories coming from fat. (This would imply a majority of their energy was coming from carbs.)
After the 16 week study fat oxidation after an overnight fast went from 38% before the interventions to 52% after the interventions! Furthermore this study found that the greater the improvement in fat oxidation after an overnight fast the better the improvements in insulin sensitivity. In English, in these type 2 diabetics, the more their ability to burn fat after an overnight fast increased, the bigger the improvement in their type 2 diabetes!
But what about fat utilization of type 2 diabetics during exercise?
Another study (van Baak, n.d.) looked at 8 male type 2 diabetics and 8 male obese subjects without type 2 diabetes. The researchers matched the men so that all characteristics (age, height, body fat, muscle mass, etc) were the same except that one group had type 2 diabetes. They then had the men perform 60 minutes of cycling at 50% of their VO2Max (relatively easy intensity) and looked at how much fat they burned.
As you can see from the chart below the men with type 2 diabetes burnt less fat both at rest and during exercise.
Your ability or inability to burn fats or carbs that I have been describing thus far is what is known as metabolic flexibility. We can become metabolically inflexible in either direction. We can restrict carbs for a long period of time and lose our ability to burn carbs when we need to, or we can restrict fat/over consume carbohydrates to the point where we lose our ability to burn fat when we need to.
The other topic related to diet and fat metabolism is acute dietary intake.
While chronic diet intake will bias your fuel utilization in both the fed and fasted state, someone who is metabolically flexible can see dramatic differences in fuel utilization depending on whether they are fed or fasted and if they are fed what the contents of their food intake looks like in the 24 hours prior to exercise.
Someone who is metabolically flexible can burn more fat just by eating lower carbs leading up to the exercise bout. If the same person then performed the same exact exercise but consumed more carbs leading up to the exercise, they would burn more carbs. This is because the body is going to prioritize carbs if it has an abundance of them available. It needs to dispose of the carbs regardless of whether you are exercising or not, it can’t leave them floating around in the bloodstream, so it’s beneficial for it to prioritize burning them over its fat because it’s literally a life or death situation. It has been shown that ingesting 50-100g of carbs in the hours leading up to exercise can reduce the amount of fat someone burns by 30-40% (Modulation of Carbohydrate and Fat Utilization by Diet, Exercise and Environment, n.d.).
The other factor you can control that can dictate your ability to burn fat and carbs is the type of training you are doing. Just like your body can adapt due to the diet you chronically eat, it can also adapt to the type of training you are doing.
If you are doing an ultra-marathon and your training requires to run long distances at a low intensity, fat is going to be your best fuel source, you have an unlimited supply of it and the intensity is low enough that the body can produce enough ATP fast enough to get the job done. The body is not dumb, it’s going to upregulate your ability to burn fat since that is the signal you are sending it.
Researchers have shown that if you take untrained men and women and have them walk or jog at a low intensity 3 days a week you can see significant improvements in fat oxidation rates (Kindermann, n.d.).
On the other hand if someone does exclusively high intensity short duration exercise the body will adapt in the other direction, prioritizing its ability to burn carbs because it can’t burn fat quick enough to provide the ATP it needs.
The one final factor I want to bring up when it comes to fat versus carb metabolism and how we can influence it is our respiration. If you were to hook yourself up to a metabolic cart and breathe nice and slow and relaxed you should notice a decent amount of fat being burnt. If you then hyperventilate, you will notice your carb utilization go way up…without changing a single thing about your diet or exercise! This is due to the fact that we need oxygen to burn fat, and if we remove that oxygen via hyperventilation we can no longer burn fat because we lack the oxygen to do so.
This simple experiment demonstrates the importance of our respiratory system in our fat and carb oxidation. If you have strong respiratory function you can utilize fat more efficiently. I won’t go into a massive amount of detail on this topic because I have already done so in this blog post. The short of it though is that exercise in itself will improve your respiratory system and is part of the reason why exercise can improve fat metabolism.
Finally there are a couple of other factors that can influence your fat and carb utilization but the rest are mostly outside of your control. Here is a quick table to get a better idea of what other factors can play a role if you are interested.
Whether you are metabolically inflexible and can’t burn fat or you are metabolically inflexible and can’t burn carbs you are hindering your performance. For optimal performance under a variety of conditions you want to maintain the ability to burn both fuel sources. Once you have achieved metabolic flexibility then you can learn to utilize your training and diet to help you reach the performance goals you are after. In the final piece to this blog post we will leave the research and science behind (you are welcome!) and look at my n=1 experiment on my own fat versus carb utilization and how it impacts my performance!
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The Effects of a Ketogenic Diet on Exercise Metabolism and Physical Performance in Off-Road Cyclists. (2014, June 27). NCBI. Retrieved March 16, 2023, from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4113752/
Kelley, D. E. (n.d.). Enhanced fat oxidation through physical activity is associated with improvements in insulin sensitivity in obesity. PubMed. Retrieved March 17, 2023, from https://pubmed.ncbi.nlm.nih.gov/12941756/
Kindermann, W. (n.d.). Effects of one year aerobic endurance training on resting metabolic rate and exercise fat oxidation in previously untrained men and women. Metabolic endurance training adaptations. PubMed. Retrieved March 17, 2023, from https://pubmed.ncbi.nlm.nih.gov/20432193/
Modulation of carbohydrate and fat utilization by diet, exercise and environment. (n.d.). PubMed. Retrieved March 17, 2023, from https://pubmed.ncbi.nlm.nih.gov/14641041/
van Baak, M. (n.d.). Impaired oxidation of plasma-derived fatty acids in type 2 diabetic subjects during moderate-intensity exercise. PubMed. Retrieved March 17, 2023, from https://pubmed.ncbi.nlm.nih.gov/11118013/
Volek, J. S., & Phinney, S. D. (2015, 11 02). Metabolic characteristics of keto-adapted ultra-endurance runners. Science Direct. https://www.sciencedirect.com/science/article/pii/S0026049515003340#f0020