The Red Flags
Although fat is the primary fuel source in fasted, aerobic workouts, depending on the workout intensity or duration, the body will select the best fuel sources (fats, carbs and protein) for energy production.
In non-fasted endurance training, protein contributes approximately 5 percent of energy. However, in a fasted state, the amount of protein breakdown in muscles is double. Breaking down muscle tissue leads to a decrease in resting metabolic rate, a reduction in strength, poor performance and can lead to injury.
Training in a fasted state to delay or avoid bonking may sound like a good idea, but research warns it’s a major physiological stressor for the body. Athletes who train under fueled experience elevated cortisol levels, deep fatigue, poor recovery, abdominal fat storage and systemic inflammation.
Break the fast by eating just enough to bring cortisol levels down and allow the body to access carbs and free fatty acids so you are physically able to hit top-end efforts in training.
Athletes tend to underestimate caloric needs and sacrifice carbs in our carb-phobic world. Training under-fueled can signal restricted eating and may lead to disordered eating or a full-blown eating disorder.
Relative energy deficiency in sport (RED-S) or low energy availability is rampant in the athletic population from novice to pros, and any time an athlete restricts food to improve body composition or performance, it signals an alarm.
Finally, research suggests the consequence of negative energy availability among female athletes comes at a higher price than male counterparts. Not to suggest fasted training is appropriate for male athletes, but females are negatively affected by their hormonal makeup.
In the female monthly cycle, there is the follicular and luteal phase (low and high hormone, respectively). In the luteal or high hormone phase (days 15-28), both estrogen (anabolic) and progesterone (catabolic) are elevated.
Estrogen promotes fatty acid oxidation and spares glycogen. Therefore, the female athlete is an efficient fat burner since this occurs monthly for 35+ years.
Progesterone dampens the body’s ability to store glycogen, so in the high hormone phase, the body instinctively leans on fat over carbs for fuel. During the high hormone phase, fueling needs change depending on the intention of the session.