Metabolic Flexibility and Performance Part 2/2

This is part 2 of the article series "Metabolic Flexibility and Performance". If you haven't read part 1, you can find it here. In this part, we bring metabolic flexibility from theory into everyday life: how different dietary approaches impact performance when the going gets tough, and how you can use the "train low" strategy without getting stuck in rigid dogma. The essence is that the body can quickly improve in fat burning but often at the expense of being able to utilize carbohydrates when you need them the most. The conclusion: periodization can be smart – but the foundation is still sufficient energy, the right intensity at the right session, and a plan you can actually stick to.

This will be a relatively long article, so grab a coffee and dive in with us.

We begin with studies examining the impact of a more traditional high-fat diet (LCHF diet) with over 60E% from fat and under 25E% from carbohydrates on athletic performance. Some studies were excluded because they did not test with a sufficiently high fat intake. We then explore how performance is affected by a temporary LCHF period (5-7 days) followed by carbohydrate loading for 1-3 days to see its effect on athletes.

Finally, we focus on something that deserves more attention: an individualized approach to nutrition. Here, we delve into periodization and how you can maximize your potential by incorporating some form of “train low.” Let's get started!

The variation in protocols makes it challenging to determine the effects of LCHF, but overall, the results indicate that at high intensity, there is no advantage with LCHF—in most cases, the effect is even negative for performance. However, during time to exhaustion at lower intensities and with suboptimal energy intake, LCHF can maintain performance, and in some studies, even improve it.

What our dedicated sports performance researchers gather from these studies is that it takes a relatively short time (5-7 days) to adapt the body to become better at burning fat. This insight has led them to now study how short periods of an LCHF diet, followed by carbohydrate loading, can provide the best of both worlds. Train low, Compete High.

The study highlighted a little extra in the review is one from 2006. It was designed to mimic the demands of a typical cycling race. Participants could choose their intensity, but at specific stretches, they performed sprints to simulate accelerations. These consisted of both 1 km sprints at an intensity above 90% of maximum capacity, as well as 4 km speed-ups at 78–84% of max capacity. The results are shown in the figure above where white dots/squares represent those who underwent a fat adaptation period and the black ones are the high-carbohydrate athletes.

In summary, the overall completion time didn't differ between the cyclists, although there was a trend that those who did not follow a low-carb high-fat (LCHF) diet finished the test faster (153 min 10 s against 156 min 53 s). Interestingly, participants who followed the LCHF diet had lower average watts, especially during the more intense 1 km speed-ups. The likely explanation comes in the next section.

PDH is an enzyme that's crucial for the citric acid cycle and our ability to convert glycogen into energy for movement. Reduced activity of this enzyme is linked to decreased utilization of stored energy. During truly high-intensity sprints and tempo increases, this is where LCHF makes us less metabolically flexible. We simply become less adept at switching to rocket fuel when the pace picks up.

“This study provided evidence of glycogen ‘impairing’ rather than ‘sparing’ in response to adaptation to an LCHF diet and a robust explanation for the impairment of key aspects of exercise performance as a result of this dietary treatment.”

I'm not aiming to criticize LCHF as a dietary approach, but regarding how it affects athletic performance, there isn't much to suggest it would be positive in most competitive contexts. Before I explore how we can increase our metabolic flexibility, I want to conclude this section with a good quote:

”It is important to consider insights from research and athlete testimonials to identify different scenarios in which one approach might offer advantages over another or to explain divergent outcomes (Table 5), rather than insist on a single ‘truth’ or solution. Indeed, although there is a continual cry to rid sports nutrition of ‘dogma’ [4], it would seem counterproductive if new ideas were as dogmatic as the old beliefs they seek to replace. This author and others continue to undertake research to evolve and refine the understanding of conditions in which low carbohydrate availability can be tolerated or actually beneficial [58, 59].”

The aim is never to disregard anecdotal evidence. Some perform really well on a fat-rich diet, but overall the majority perform better on a carbohydrate-rich diet and become more metabolically flexible — meaning they're better at utilizing stored glycogen when needed. In Table 5, which the quote refers to, there are a few scenarios where LCHF might be functional.

If we set aside the carbs vs. fat debate and focus on optimizing "train low," we're not changing WHAT we eat, but WHEN we eat.

Running double training sessions is a practice commonly undertaken by both elite athletes and dedicated fitness enthusiasts. It's a great strategy to "accidentally" train certain sessions with reduced carbohydrate stores to enhance the stimulation of various aspects, including mitochondrial growth and capillarization (growth and division of blood vessels).

When you have multiple sessions per day, training with half-empty energy stores is almost inevitable. It's about selecting the right intensity to maximize the benefits of your sessions. A demanding interval session as the second or third of the day is rarely ideal for quality, especially if they're close together or if recovery has been suboptimal.

Advantages:

• An easy way to enhance stimulus/signaling for adaptations like mitochondrial growth.
• A study from 2008 showed that athletes who did double sessions every other day improved as much as those who did one session per day — despite the fact that the quality of the second session could be lower. Training volume was the same between the groups.

Disadvantages:

• Relative intensity in the sessions may decrease.
• The immune system can become compromised, increasing the risk of illness.
• Recovery time may be longer.

Many long sessions are planned to be done either on an empty stomach or with little to no energy. What are the pros and cons?

Advantages:

• Increases the stimulation of fat oxidation at low intensity.
• Less damage-related stress on ligaments and muscles as the duration can often be kept shorter—which can be positive for avoiding overload injuries. You get a strong stimulus from shorter sessions.

Disadvantages:

• Relative intensity decreases.
• The session becomes shorter and recovery takes longer.
• Can be difficult to schedule if weekly volume is high without affecting the quality of intense sessions. It is best done with careful planning.

Benefits:

• Essentially all positive as long as the session is short (30–60 min) and the intensity is controlled.
• A morning session can boost metabolism throughout the day (see our article on 24 h fasting). If you have a good breakfast afterwards, these sessions shouldn't pose any problems.

Drawbacks:

• If the session becomes too long or too intense, it can impact the ability for more intense training later in the day.

In a study from November 2016, participants engaged in a high-intensity session in the evening and did NOT replenish carbohydrates immediately after the session but instead went to sleep "empty." The replenishment occurred only in the morning, after an hour of endurance training.

Benefits:

• Participants following the "sleep low" protocol performed better in the subsequent 20 km time trial than those who ate evenly distributed throughout the day. They maintained a more consistent intensity curve throughout the test.

Drawbacks:

• Going to bed without replenishing carbohydrates can impair sleep quality.
• The immune system may be negatively affected, similar to the impact of double sessions.
• Both sleep and the immune system are crucial for a long-term plan, so this must be considered in the planning.

Skipping breakfast and avoiding energy intake during a workout are variations of controlled fasting. I personally tackled the Sverigetrampet with 15 days of fasting, and there are many studies on fasting in different forms. For a deep dive, I refer you to our article here.

Advantages:

• Significant stimulation for increased fat burning during low-intensity sessions.
• Can be effective for weight loss as you eliminate a day's caloric intake.

Disadvantages:

• We respond very differently to fasts; it's not for everyone.
• Training intensity decreases, the immune system is at greater risk, and sleep may suffer.
• Your surroundings might suffer from one or two unnecessary outbursts due to irritability. Execute with caution and preferably during base training, not in the competition period.

All these methods to periodize carbohydrates for enhancing your metabolic flexibility are like the icing on the cake. The key question is: Does this improve my performance? Or more importantly: Is it worth the effort?

In previous articles, I've mentioned that if you have time to train 10–12+ hours per week, the benefits of excessively experimenting with your diet become questionable. It can raise the risk of overtraining, and you might lose focus on the overall picture. Maintain consistency and enjoy the journey. All the tips above carry risks and demand extra effort from you.

I often refer to the African runners, where up to 70E% of their diet is carb-rich, yet they achieve, on average, 30% higher fat oxidation than Western runners. Consistency and high weekly mileage naturally lead to optimizing your fat burning. Metabolic flexibility is about using fat when the pace is easy and switching to carbohydrates when it gets intense. Feel free to experiment — but always keep your eyes on the goal.

When it comes to sports performance, there's an abundance of research on various approaches, though much remains to be explored. In these two articles, I've explored how sports performance changes through everything from ketogenic diets, LCHF, LCHF with carb loading, and periodization of macronutrients — without altering what you eat, but rather when you eat.

In summary (a really good word), there is minimal evidence to suggest that a high-fat diet in different variations would enhance performance as long as the competition can be sustained with energy, or if you can provide energy during competition (e.g., cycling). Focusing excessively on a low-carb diet can often hinder your capacity and should be evaluated on an individual basis to decide if it's worthwhile.

At the same time, there may be benefits to fasting or low-glycogen sessions. As long as you refuel shortly after, you can reap the benefits of both approaches.