Metabolic Flexibility and Performance Part 1/2
What really limits your performance - lack of fat or lack of carbohydrates? In this article, we explore the concept of metabolic flexibility and what it means in practice for those who train. Drawing from both personal experiences and research, we compare fat and carbohydrate metabolism and examine what actually dictates energy usage during exercise. The picture is more nuanced than the debate often suggests. While the body can be trained to use more fat, intensity sets the boundary. For most performances, it's not the ideology around diet that determines the outcome, but the understanding of which energy source is needed when the pace accelerates.
The Background to the Debate
On June 5, 2016, I embarked on a journey from Treriksröset to Smygehuk. Along with my team of three other people, Jennie Almström, Oscar West, and Tommy Ivarsson, we had meticulously planned in the months leading up to this — and the day had finally arrived. The project Sverigetrampet involved cycling from Sweden's northernmost to southernmost point, relying solely on water, salt, and stored fat reserves. A journey of over 2000 kilometers. To learn more about the challenge/project, click here:
Amidst the many internet trolls who voiced their opinions, the adventure sparked discussions as a monumental proof that LCHF was superior to other diets. It's intriguing how we interpret information presented to us. There's a good expression for this in English: narrative fallacy — when we look for a connection that aligns with our preconceived notions. We all have biases on various issues; it's our basic, human way of attempting to relate things. The world is complex, and being critical of everything is challenging, especially when lacking fundamental knowledge on the subject.
In this two-part article series, I aim to provide a nuanced view of the two "camps": fat adaptation and carbohydrate fueling. The debate continues to rage, and navigating the jungle of anecdotes is certainly not easy. To avoid being just another article on the no-side, I'll approach the issue from a scientific standpoint and describe the state of research in both fields.
The picture is from my Strava file after the fifth day of the Sverigetrampet, after about 120 hours of fasting. I was really struggling here... The first 25 km of the day included two really tough climbs. Each of these ascents were just slightly more challenging than a climb up Hallandsåsen. Afterward, I wasn't feeling particularly cocky, but I was confident that the rest would go smoothly. Having made it this far, it was just a matter of pushing on.
What is the purpose of training?
“The role of training is to accumulate adaptations in the muscle and other body organs/systems to achieve specific characteristics that underpin success in the athlete’s event via a series of systematic and periodized stimuli involving the interaction of nutrition and exercise” – Re-Examining High-Fat Diets for Sports Performance: Did We Call the ‘Nail in the Coffin’ Too Soon?
The quote emphasizes that all forms of training aim to prepare you for your upcoming performance. It might sound obvious, but it's worth repeating to keep the big picture in mind. Should an elite athlete with 25 training hours per week train in the same way as a woman juggling a career and family? — Absolutely not!
What is Metabolic Flexibility?
In your muscles, the cells work efficiently to produce adenosine triphosphate (ATP), which ultimately causes your muscles to contract. This happens through two main processes where primarily fats and carbohydrates are broken down. Fundamentally, these can be divided into aerobic processes and anaerobic processes. Aerobic processes extract ATP using oxygen, while anaerobic processes extract energy without oxygen. We'll explore a small calculation example in part 2 that describes this in more detail. The source of energy depends on several factors: intensity, duration, training status, and — perhaps most interestingly for many reading here — diet.
This is where metabolic flexibility comes into play. It's well-known that our fat reserves are abundant, almost unlimited when it comes to energy storage. However, our carbohydrate reserves are our Achilles' heel, with about 400 g stored in the body.
Figure 1. The image demonstrates how quickly glycogen in the muscles depletes based on intensity. The percentages indicate the intensity performed in relation to VO2max.
The Impact of Energy Supply
In the study below, see link, the cyclists performed the same experiment on three different occasions. The Y-axis in the figure shows intensity in watts and the X-axis shows how much time was remaining in the test. All participants completed all three tests, and the image displays the group's averages. The participants did not know which test they would perform on each occasion — in the research world, this is known as a randomized controlled experiment.
The test setup involved cycling for 2 hours at approximately 55% of their maximum watt (around 200 W), followed by a so-called time trial (TT) where a specific amount of work had to be performed. This standard setup is used to eliminate as many other variables as possible. The total amount of work was calculated using the formula: Total amount of work = 0.75 Wmax 3600 kJ.
The different test drinks were as follows:
- P, Placebo. Flavored water throughout the entire test
- G, Glucose in the form of maltodextrin. Single carbohydrate source at a dose of 108 g/h
- GF, Glucose and fructose in a 2:1 ratio where the glucose was maltodextrin at the same dose: 108 g/h
Figure 2. The figure above illustrates how intensity is influenced depending on whether (P) or which energy (G/GF) is provided.
The results indicated that when participants consumed only water (P), the test duration was significantly longer (67 min and 2 s), with an average wattage of 231 W. When provided with maltodextrin (G), the test lasted 60 min and 41 s, averaging 254 W. Finally, when given maltodextrin and fructose (GF), the test was 56 min and 7 s, with an average wattage of 275 W. This study is one of the foundations for the formula of Umaras sports drink, “Sport” (http://umarasports.com/produkt/sport/).
Using an optimal sports drink can enhance capacity by 19% better compared to water alone, and also 8% better than using just maltodextrin or other single carbohydrate.
Limitations of Relying Solely on Fat Metabolism
Our fat oxidation capacity is primarily governed by two factors:
- Relative intensity
- Maximum oxidation capacity
Relative intensity
The biggest bottleneck in relying solely on fat burning is that our relative intensity significantly influences which energy substrate is used. What does that mean in practice?
Figure 1 and Figure 2 illustrate how the fat oxidation capacity rises up to 60–65% and then decreases to almost negligible levels as the intensity nears 80–85% of VO2max.
It's important to highlight that this is about relative intensity, not speed. Participants with a VO2max over 65 ml/kg/min — particularly well-trained individuals — exhibit the same curve relationship as athletes with lower (<65 ml/kg/min) oxygen uptake capacity. A higher oxygen uptake capacity doesn't necessarily mean you can perform at a higher relative intensity. A higher VO2max means you can ingest more oxygen for fuel burning. This benefits both fat burning and carbohydrate burning.
The researchers further write:
During the last 15 minutes of exercise, fat oxidation rates were 0.84 g/min in the trained and 0.33 g/min in the untrained group. The finding that at the same relative intensity trained individuals (i.e., individuals with a higher VO2max) have greater rates of fatty acid oxidation can be explained by the fact that the trained individuals are exercising at a higher absolute work rate.
Maximum Oxidation Capacity
Image 9.3 illustrates how energy expenditure in cycling has a linear relationship with increased consumption at higher intensity. Image from "Nutrition for Higher Education – Fifth Edition, page 171".
So, how is it decided what to use? We'll explore this from different perspectives since diet and different periodization concepts can be interesting for personal customization. What primarily dictates this is relative intensity. The table below is included as a visual aid.
Ketogenic Diet or High-Fat Diet (LCHF)?
Between 1980 and 2016, various high-fat diet variations have been studied to determine if and how they might enhance performance. Broadly speaking, the studied variations can be summarized as:
- Ketogenic diet over time. Discussed below
- LCHF, Low Carb High Fat. Covered in part 2
- Periodized variant with 5–7 days of LCHF followed by carbohydrate loading before performance. Covered in part 2
Few topics evoke emotions like diet, and my intention here is not to criticize LCHF or the ketogenic diet in its various forms. Following LCHF offers several benefits, and many people report how advantageous it's been for them in different ways, particularly for weight management. However, regarding performance, there is currently no evidence that a ketogenic diet leads to improved performance in most sports.
Ketogenic Diet
There are a few exceptions, such as competitions where energy supply is very inaccessible, or specific individuals who respond unusually well to this type of diet. A frequently cited example is: The human metabolic response to chronic ketosis without caloric restriction: preservation of submaximal exercise capability with reduced carbohydrate oxidation.
In that study, five well-trained cyclists underwent three weeks of a ketogenic diet. The setup was that they first underwent a test week with a diet based on 57% energy (E%) carbohydrates, 15% E protein, and the rest fat. The participants were then tested to exhaustion at an intensity of 63% of their VO2max. After that, a ketogenic diet intervention was initiated where carbohydrate intake was limited to a maximum of 20 g/day (still energy balance and 15% E protein). At the end of the third week, the participants were tested again with the same setup at 63% of VO2max.
Figure 3. Cycling test at an intensity of 62–63% of VO2max until exhaustion.
The results in Figure 3 indicate there wasn't a notable difference between the initial and final test as a group. However, one participant performed significantly better after the dietary intervention, extending their time by about an hour in the second test. This single deviation slightly elevated the average, important to recognize as not reflective of a broad effect.
Other intriguing findings from the study include the fact that muscle glycogen was reduced by half during the intervention but no more than that. It decreased from 140 mmol/kg (wet weight) to 76 mmol/kg. This is significant because muscle glycogen stores never become completely depleted but are drained to varying degrees. Here, experience and dietary history are crucial factors. We'll revisit this in the next article when we discuss glycogen sparing in practice. Additional interesting data is summarized here:
“Furthermore, in both trials, at the cessation of exercise, muscle glycogen depletion was observed in type 1 fibers with a fourfold reduction in its contribution to fuel use in the LCHF trial. Blood glucose contribution to fuel use was diminished threefold, while gluconeogenic contributions from glycerol released from triglyceride use, as well as lactate, pyruvate, and certain amino acids, prevented hypoglycemia during exercise and facilitated glycogen storage between training sessions. Lipid oxidation increased to compensate the fuel substrate for the exercise task.”
What does this mean in practice?
Much suggests that even with moderate intensity (63% of VO2max), the body's ability to burn fat and ketones can be significantly improved through various dietary interventions. Next week, I will explore more about the studies conducted on this and the practical insights you can use in your training. As a little teaser, I recommend watching this short video clip with Louise Burke, where she shares her perspective on the subject.