Just surviving or fuelled?
Just because you’re surviving doesn’t mean you’re performing optimally.
You can run a marathon on a keto diet — it’ll just take you a while.
Your body will always find a way to survive. However, if you want it to perform, recover, and actually adapt — you need to understand how it uses fuel.
This is where metabolic flexibility comes in. It is your body’s ability to use the right fuel at the right time.
What “Metabolic Flexibility” Actually Means
In simple terms, metabolic flexibility is your body’s ability to use fat and carbohydrate fuels at different rates, depending on what you’re doing and how hard you’re doing it.
At low intensity → your body burns a higher ratio of fat.
At moderate-to-high intensity → carbs (glycogen and glucose) become the dominant fuel.
At rest and recovery → your metabolism recalibrates to replenish both.
When this system works well, you can climb, sprint, and recover efficiently — without crashing, bonking, or feeling like your body can’t keep up with your goals.
Your muscles are constantly deciding whether to pull from the fat tank or the carbohydrate tank. Both tanks are always in use and your body doesn’t actually flip a “fat-burning switch.”
As Dr. Asker Jeukendrup puts it:
“There is no switch that turns fat metabolism on or off. Your body uses both fuels all the time — the balance just shifts with intensity.”
— MySportScience, The Myth of Switching to Fat Metabolism
So, when you train and fuel right, your body becomes more efficient at switching between those tanks. When you restrict or underfuel, you lose that adaptability — and that’s when performance starts to tank.
Survival vs. Performance Nutrition
Let’s be real — most people eat to survive, not to perform. Sometimes this is intentional, and sometimes it’s not.
They grab food when they’re starving, skip breakfast, and run on caffeine and adrenaline. This might keep you alive, but it’s not going to help you climb mountains, recover between sessions, or stay sharp all week. It is not optimizing your nutrition.
Survival Nutrition looks like:
Skipping meals or running on caffeine
Eating reactively — grabbing food when you’re already starving
Hoping recovery just “happens”
Restricting foods and calling it discipline
Performance Nutrition looks like:
Fueling before, during, and after training
Planning meals around effort — not aesthetics
Using food as strategy, not reward
Building recovery right into your plan
What’s True, and What’s Twisted about Fat Adaptation (AKA Here’s where a lot of confusion starts)
Endurance training naturally improves your body’s ability to use fat as fuel. That’s part of what makes you more efficient at long, steady efforts — your mitochondria adapt, your enzymes upregulate, and your body learns to preserve glycogen.
Somewhere along the way, “fat adaptation” started being sold as a shortcut to performance — train fasted, go low-carb, burn more fat. The funny thing is just because you burn more fat doesn’t mean you perform better.
When you restrict carbs or consistently train low, you might see higher fat oxidation during sessions — but you also limit your top-end capacity and slow your recovery. Burke et al. (2021) found that athletes on low carbohydrate high fat diets burned more fat — but were less efficient. This means they needed more oxygen for the same effort and had reduced ability to perform at high intensities which translates to having slower climbs and weaker finishes, and feeling like crap.
That’s why Uphill Athlete emphasizes that forcing fat adaptation beyond what normal training achieves doesn’t improve performance — and often backfires, especially for women.
The trade-offs outweigh the benefits:
You reduce your body’s ability to access carbohydrates during high-intensity efforts
You blunt your training response (less adaptation for the same workload)
You recover slower and increase the risk of hormonal disruption, fatigue, or injury
The key message:
You don’t need to force fat adaptation by cutting carbs. Your body adapts when you train — and it adapts better when you fuel properly.
A special place for carbohydrate
Carbohydrate restriction can lead to Low Energy Availability (LEA) even when total calorie intake looks “adequate.” LEA means your body isn’t getting enough usable energy after training to keep all its systems running smoothly. You might be eating, but once your workout calories are “spent,” there’s not enough left to cover recovery, hormones, immunity, and focus. Research from the International Olympic Committee (Mountjoy et al., 2018) and sports nutrition experts like Burke and Stellingwerff shows that Low Energy Availability isn’t just about total calories, it can also associated with chronic low carbohydrate availability which can trigger the same metabolic and hormonal disruptions. This can be especially true in female athletes.
Here’s how that plays out physiologically:
When carbs are low, muscle glycogen stores stay depleted. Your body interprets that as an energy shortage, even if you’ve eaten enough protein or fat
This perceived deficit can trigger energy conservation responses — reduced metabolic rate, hormonal shifts, increased fatigue, poorer recovery
Over time, that limited carbohydrate availability can directly lead to LEA symptoms (impaired performance, menstrual disruption, suppressed immunity, etc.), especially in endurance and high-volume athletes.
A few Real-World Scenarios
If you’re training long and slow (Zone 2):
You’re mostly burning fat — keep steady snacks on board.
If you’re hitting sprints, climbs, or intervals:
Your body flips to carbs — pre-fuel and sip carbs throughout.
If you’re working outdoors all day:
You’re constantly switching between fuels — eat often, hydrate, and don’t wait for hunger.
If it’s a rest or recovery day:
Eat normally. Refueling is part of adaptation, not optional.
See you out there!
References:
Burke, L. M., Ross, M. L. R., Garvican-Lewis, L. A., Welvaert, M., Heikura, I. A., Forbes, S. G., Mirtschin, J. G., Cato, L. E., Strobel, N., Sharma, A. P., Hawley, J. A., & Phillips, S. M. (2018). Low carbohydrate, high fat diet impairs exercise economy and negates the performance benefit from intensified training in elite race walkers. The Journal of Physiology, 596(15), 3671–3691. https://doi.org/10.1113/JP275173
Burke, L. M., Stellingwerff, T., & Morton, J. P. (2019). A framework for periodized nutrition for athletics. International Journal of Sport Nutrition and Exercise Metabolism, 29(2), 141–151. https://doi.org/10.1123/ijsnem.2018-0305
Jeukendrup, A. E. (2024, March 7). The myth of switching to fat metabolism. MySportScience. https://www.mysportscience.com/post/the-myth-of-switching-to-fat-metabolism
Mountjoy, M., Sundgot-Borgen, J., Burke, L., Ackerman, K. E., Blauwet, C., Constantini, N., Lebrun, C., Lundy, B., Melin, A. K., Meyer, N. L., Sherman, R., Tenforde, A. S., Klungland Torstveit, M., & Budgett, R. (2018). International Olympic Committee (IOC) consensus statement on Relative Energy Deficiency in Sport (RED-S): 2018 update. British Journal of Sports Medicine, 52(11), 687–697. https://doi.org/10.1136/bjsports-2018-099193
Uphill Athlete. (2024). Fueling the distance: The science and practice behind fat adaptation. https://uphillathlete.com/nutrition/fat-adaptation/