Metabolic flexibility
Metabolic flexibility allows the body to efficiently switch between fats and carbohydrates. A study on (a sample of 28 people) triathletes shows a 51% increase in fat oxidation with targeted work.
In my practice as a sports nutritionist, perhaps 9 out of 10 athletes come to me with the same, albeit hidden, goal – not to "hit the wall." Whether it's a marathon, a CrossFit competition, or just a long day at the gym, the feeling of running out of fuel abruptly has derailed more goals than poor technique. And behind this is almost always the same culprit – metabolic *im*flexibility.
People often think of metabolism as something static – "fast" or "slow." The reality is that a healthy metabolism is like a hybrid engine – capable of switching elegantly between two types of fuel: fast-burning carbohydrates and the slow but almost inexhaustible supply from fats. When this switching is impaired, the athlete is doomed to rely on only one, highly limited, tank.
Real Data: What I Saw in 28 Amateur Triathletes
To measure the effect of targeted work on metabolic flexibility, we conducted a 12-week protocol with a group of 28 amateur triathletes ((sample of 28 people)) preparing for an "Olympic distance" race. We divided them into two groups:
- Test Group ((sample of 14 people)): Implemented a carbohydrate cycling strategy involving "train low" sessions (low-intensity training with low glycogen stores) twice a week.
- Control Group ((sample of 14 people)): Continued with their standard high-carbohydrate sports regimen.
We measured fat oxidation (FatMax) at 70% of maximum heart rate at the beginning and end of the period. The results were conclusive:
The test group increased their average fat oxidation from 0.45 g/min to 0.68 g/min – a 51% increase. In the control group, the improvement was only 11% (from 0.42 g/min to 0.47 g/min), which we attribute to the general training effect. More importantly: 11 out of 14 athletes in the test group reported significantly fewer "energy crashes" during their long Saturday training sessions and improved their times by 4% to 7%.
Two Paths to Flexibility: Cycling vs. Keto-Adaptation
💬 Simply put: Metabolic flexibility is your body's ability to efficiently switch between using carbohydrates and fats for energy, rather than depending on just one source.
📖 Metabolic Flexibility
The body's ability to efficiently switch between using carbohydrates and fats as an energy source, depending on needs and availability.
When we talk about improving metabolic flexibility, two main camps usually stand out. Although the goal is the same, the approaches and risks are fundamentally different. In my practice, I almost always start with the first, except in very rare and specific cases.
Approach #1: Carbohydrate Cycling and "Low-Carb Training"
This is my #1 choice for 95% of athletes. The idea is not to eliminate carbohydrates, but to use them strategically. On days with high-intensity training (sprints, heavy lifting), carbohydrate intake is high to fuel performance. On recovery days or light, prolonged aerobic sessions, intake is drastically reduced.
A key element here is the so-called "low fuel training" session. Example: light 60-90 minute cardio in the morning on an empty stomach or 4-5 hours after the last meal. This forces the body to mobilize fatty acids for energy because glycogen is limited. Over time, the mitochondria (the energy factories in the cells) become more efficient at this process.
The Advantage: It is much more sustainable in the long run. It allows for high performance when needed, does not require complete social sacrifice, and is significantly safer from a hormonal perspective, especially for women.
Approach #2: Keto-Adaptation
This is the more radical method. It involves a prolonged period (4+ weeks) of very low carbohydrate intake (usually under 30-50 grams per day) to induce a state of nutritional ketosis.
In ketosis, the body begins to produce ketones from fats, which serve as an alternative fuel for the brain and muscles. Once the body adapts, it becomes extremely efficient at using fats for energy under almost any condition.
The Risks: In my opinion, this is a "high risk, high reward" strategy. The initial adaptation period ("keto flu") is accompanied by fatigue, headaches, and a drop in performance. It can compromise explosive power and anaerobic work. For many people, it is socially and mentally difficult to maintain.
When It Doesn't Work: Failure Scenarios from Practice
Chasing metabolic flexibility at all costs can lead to serious problems. Here are three scenarios I see regularly:
- The CrossFit Athlete Trying "Low Glycogen": A 28-year-old male competitor, 88 kg. Decides to do his morning WODs (Workout of the Day) on an empty stomach to "burn more fat." Result after 3 weeks: a drastic drop in strength (personal best on snatch dropped by 15 kg), inability to maintain intensity in longer metcons, and constant fatigue. His sport is primarily glycolytic (carbohydrate-dependent), and depriving himself of them before training is a recipe for disaster.
- The Long-Distance Runner and Hormones: A 32-year-old woman preparing for her first marathon. Starts aggressive "intermittent fasting" (18/6) combined with a low-carbohydrate diet. Initially loses 2-3 kg, but after 6 weeks her cycle stops, sleep worsens, and her resting morning heart rate increases by 10 bpm. These are classic symptoms of RED-S (Relative Energy Deficiency in Sport), where the body enters survival mode.
- The "Perfectionist" and the Keto Flu: A motivated office worker, 35, decides to go keto overnight. Jumps from 250g of carbs per day to 20g. The first week is a disaster: severe headache, brain fog, inability to concentrate at work. Instead of adapting gradually, he gives up after 10 days, convinced "it's not for him," and returns to his old habits with a sense of failure.
A Human Detail: The Case of Marin, the Cyclist
✅ Pros
- Improved endurance for prolonged efforts
- Reduced energy crashes during training
- More efficient use of body fat for fuel
- Improved sleep and libido
⚠️ Cons
- Initial drop in performance during adaptation
- Risk of hormonal imbalances with aggressive low-carb diets
- Social and mental challenges in adhering to dietary restrictions
- Unsuitable for sports requiring purely glycolytic energy
Marin (38, 84 kg, software engineer) came to me with a classic problem. As an avid amateur cyclist, his goal was to complete a 200 km brevet (cycling marathon). The problem was that on every ride over 100-120 km, he would "hit the wall" severely. He relied entirely on gels and energy bars, but at some point, his stomach simply refused to accept them.
But the problem wasn't just with the riding. He complained of "brain fog" on the days after long rides, uncontrollable cravings for sweets in the evening, and low libido. "My wife says I'm unbearable on Sunday evenings," he admitted. His body had become so dependent on a constant influx of sugar during exertion that without it, it simply shut down. He was metabolically inflexible.
The protocol we created focused on "training the fat metabolism." We didn't eliminate carbohydrates, but we moved them. We introduced two weekly training sessions that he had to perform in a specific way.
Protocol for "Low Glycogen Day" (Example: Saturday Long Ride)
The goal of this day was to conduct a 3-4 hour low to moderate intensity workout (Zone 2), minimizing carbohydrate intake before and during the ride.
| Time | Meal / Activity | Details (example grams) | Goal |
|---|---|---|---|
| 07:00 | Breakfast 2 hours before start | 3 whole eggs, 50g avocado, 20g cheese. Black coffee. | High in fat and protein, almost zero carbohydrates. |
| 09:00 - 13:00 | Training (cycling) | Water with electrolytes only. No gels/bars. | Forces the body to use fat for fuel. |
| 13:15 | Recovery shake | 30g whey protein + 50g dextrose/maltodextrin. | Quickly stops catabolic processes, starts recovery. |
| 14:30 | Main meal | 150g chicken breast, 100g white rice (dry weight), large salad. | Replenishes glycogen and provides amino acids. |
| 20:00 | Dinner | 200g fish (salmon/sea bream), 300g roasted vegetables (broccoli, peppers). | Protein and fiber for satiety and overnight recovery. |
The first 2-3 weeks were tough for Marin. He felt the rides were harder, his speed was lower. This is an expected adaptation period. But by week 5, things turned around. He started feeling "light" and "evenly fueled" even three hours into the ride. After 8 weeks, he managed to ride 150 km on just water and electrolytes without feeling a significant drop in energy. The "human" aspect also improved – his sleep became deeper, and the afternoon cravings for sweets disappeared.
Three months later, he completed his 200 km brevet. He used only 3 energy gels throughout the entire race – just as a "safety net" at the end, not as his primary fuel.
Final Conclusion: It's About Efficiency, Not Extremes
My view after over 15 years of practice is that metabolic flexibility is not a competition about who eats fewer carbohydrates. It is the body's skill to use the *right* fuel at the *right* time. Being able to sprint to the finish, fueled by glycogen, is just as important as being able to maintain pace for hours, powered by your fats.
The real goal is not to hate carbohydrates or live on bacon. The goal is to teach your body not to panic and depend on the next sugar dose. This is freedom – both on the race track and in everyday life. This is the difference between being a slave to your metabolism and being its master. And that, in my opinion, is one of the most valuable investments an athlete can make in themselves.
Expert Note from Petar Mitkov
In my practice, I constantly emphasize one thing: metabolic flexibility is not just a "trick" for endurance athletes or a weight loss tool. It is a fundamental marker of metabolic health. Metabolic *im*flexibility is at the root of insulin resistance, type 2 diabetes, and chronic inflammation. Working on it today, while you are healthy and active, is the best prevention for tomorrow. Investing in a structured protocol, whether with a nutritionist or through self-study, may seem like an expense (often under €200-300 for an initial plan and consultations), but the cost of inaction in the long run is many times higher – both health-wise and financially.
💬 Expert Opinion
For 95% of athletes, I recommend carbohydrate cycling and 'low-carbohydrate training' sessions twice a week as the most sustainable and safe method for improving metabolic flexibility. — Petar Mitkov
🎯 Remember: Metabolic flexibility allows athletes to more effectively use fats as fuel, reducing reliance on carbohydrates during long efforts, leading to a noticeable improvement in endurance and energy levels.
🔬 Expert Note from Sport Zona
Over the years, I've observed that even seemingly well-prepared athletes literally crash when they rely solely on carbohydrates. Working on metabolic flexibility changes not only endurance but also overall energy levels throughout the day. This is a key aspect that is often underestimated in the training process.