Changes that occur when training fasted
Training fasted may be beneficial for athletes. Sports that rely upon endurance or muscular endurance fitness may lend themselves well to athletes who complete training WITHOUT prior ingestion of carbohydrates.
In essence, the athlete participates in training having NOT prepared in the manner typically recommended i.e. eating protein + carbohydrates ~3 hours prior to training; then 30-50g carbohydrates ~1 hour prior to training; then consume carbohydrates as needed during training.
The scenario in which the athlete has not consumed nutrition prior to training may actually BENEFIT the athlete long-term. The additional ‘stress’ of training fasted may give rise to metabolic and physiological adaptations that serve to produce performance benefits to the athlete come competition (which SHOULD be completed with elevated carbohydrate/glycogen stores – see HERE). Athletes that immediately fall into this bracket include: cyclists, runners, swimmers. Beyond that, there may be a case for other athletes, such as team sport athletes like football players, to conduct some training fasted.
To clarify, I’m not recommending always training fasted. One might conduct sessions Tuesday & Thursday morning in the fasted state; sessions Wednesday evening, Saturday & Sunday completed having eaten beforehand. In addition, conducting resistance training fasted may not be optimal for those who have body composition or physique aspirations, where muscle hypertrophy is necessary. However, some resistance training in the fasted state may also have its own unique benefits. Generally speaking, however, carbohydrates consumed before and after resistance training would be recommended.
Changes when training fasted
There’s evidence to suggest that training in the fasted state leads to an increase in glycogen storage capacity, suggesting that training without having consumed prior carbohydrates leads to an increase in the bodies glycogen stores during recovery periods. This would be beneficial, some competition, to the athlete, in that they’re ability to store glycogen is enhanced.
In addition, THIS paper found a greater increase in VO2 max after fasted endurance training, when compared to fed training. This is not always the case. (Interestingly, this finding wasn’t due to an increase/improvement in fat oxidation, as other studies have suggested)
Caveat 1: men and women may respond differently to fasted & fed training. THIS paper suggests men respond better to fasted training.
Caveat 2: untrained vs trained individuals may respond differently/acquire different adaptations from fasted and fed training
Fasted training appears to promote increase in Fatty acid binding protein (FABP) and CD36, two proteins responsible for transferring the fatty acid into the cell (read HERE) and therefore paramount in fat metabolism. In addition, fasted training may also increase CPT1, responsible for transferring fatty acid CoA into the mitochondria for oxidation. In essence, fasted training appears to increase the effectiveness and efficiency of the fat burning supply chain – the machinery involved in oxidizing fat to produce energy during exercise becomes more productive.
Alterations in the normal local environment of the muscle with training – changes in AMP/ATP ratio, increase in H+ ions, decreased glycogen stores, increased Reactive Oxygen Species (ROS) etc. – activate a number of proteins kinases (Bartlett et al. 2012). The activation of these protein kinases then trigger a cascade of changes, which eventually results in changes to the number and size of mitochondria, and therefore one’s exercise capacity. Other important signalling kinases, that are relevant to the endurance athlete participating in fasted training, include: AMPK & p38 MAPK, and it is thought they have a beneficial effect on the tumour suppressor protein: p53. Read more HERE
There may be an increase in lactate production when exercising in a fasted state, as the Abstract HERE suggests. Interestingly, this found blood sugar levels to remain remarkably stable, despite a long period of fasting followed by exercise.
This quote sums up nicely the skeletal muscle gene response that also occurs with fasted training:
“… rate limiting metabolic proteins are upregulated by the cumulative effect of short term increases in gene transcription in response to each exercise bout… Carbohydrate intake would blunt these adaptations” (De Bock et al. 2008)
Training fasted produces a range of benefits to the athlete. Even though the intensity of that particular training session may be slightly lower than it would’ve been had the athlete eaten before, specific adaptations occur (due to the athlete’s ‘fasted’ nutritional status) that benefit performance come competition day. Increases in glycogen storage capacity, mitochondrial biogenesis, and the pathways involved in fat oxidation are just some of the potential factors involved in improved performance after conducting some training sessions fasted. These adaptations would perhaps not occur in an athlete that always trained in the FED state.