After completing 17 Ironman distance triathlons, I have become good friends with fatigue. Over the past few years I have become much better at tolerating fatigue. Winning my age group at IM Chatt ('17), IMWI ('18) and IMLP ('21) and qualifying for the Ironman World Championship six times has proved to me that becoming a better long-distance triathlete is all about consistency, patience and strength.
Over the past few years, there's been a great amount of talk over fat burning as a metabolic approach to delay fatigue and to improve endurance performance. Carbohydrate fuels are limited in the body and glycogen (stored carbs) depletion is a major cause of fatigue during high duration activities. Therefore, by increasing the reliance on fat for fuel (because fat fuel supplies are unlimited in the body), endurance athletes can delay fatigue through increased fat burning. To increase fat oxidation during exercise, athletes often resort to fasted training, a high-fat/low carb diet or a high amount of very low-intensity ("fat burning zone) training.
With so much hype over fat burning to delay fatigue, many athletes are blaming poor race day performances on not being metabolically efficient (aka not being a fat-burning machine) and in turn, are spending a lot of time and effort on manipulating training and dietary methods to increase the body's capacity for fat oxidation.
Fatigue is certainly a limiter to endurance performance. It affects your physical and mental ability to perform as it's the point when you can no longer maintain a given output of force. Too much training fatigue leaves you unable to perform well the next day. Minimizing fatigue on race day means faster times due to less of a slow down. Interestingly, there are many causes of fatigue - from neurological to mechanical. Fatigue is a product of many factors and the mechanisms that cause fatigue are specific to the task being performed. Many times, fatigue is mental - it's a feeling or a decline in mental function.
With no single cause of muscle fatigue, the primary definition leads us to believe that fatigue is the inability to continue a task - whether it's finish a workout or a race, the body and/or mind become too exhausted to continue the task at a given effort.
This brings me to my point - is metabolic inefficiency to blame?
While fatigue can result due to depleted energy (carbohydrates), becoming more fat-adapted does not necessarily improve the capacity of your muscles, nervous system and cardiovascular system to work better. Being fat adapted does not mean that your muscles will no longer be imbalanced, it does not improve your terrain management skills or that your movement patterns become more economical.
Not fueling appropriately will not make you a better athlete. Intentional fat adaptation will not improve your performance if you are not training in a way that enables you to become a stronger, more resilient and fitter athlete. Not eating before a workout doesn't mean you are changing your physiology through training. Becoming fat adapted doesn't guarantee that your body composition will change.
Instead of blaming fatigue (or lack of endurance) on not being a great fat burner, be consistent with your training, fuel and eat smart, be patient and build up slowly.
Endurance refers to the ability of a muscle to undergo repeated contractions against a set resistance for a period of time.
Building endurance and becoming better at resisting fatigue takes time. It's a very slow and gradual adaptation. Naturally, overtime your body will become more efficient with how it uses fuel. You do not have to force it through fasted training, a high fat diet or a significant amount of low intensity training.
Consistent training at various intensities (to stress different metabolic systems) will induce major adaptations in your skeletal, nervous and cardio systems to improve your ability to better perform endurance exercise training. The stronger you get, the less you fatigue and the faster you will be.