Source: Journal of Applied Physiology
Title: Beneficial metabolic adaptations due to endurance exercise training in the fasted state
Authors: Van Proeyen K, Szlufcik K, Nielens H, Ramaekers M, Hespel P.
The purpose of this study is to investigate if consistent training in a fasted state can stimulate adaptations in muscle cells to facilitate energy production via fat oxidation. The study involved 20 young male volunteers who were physically active but did not consistently engage in cycling or running based endurance training. After preliminary testing, the participants were matched according to exercise capacity and energy intake. They were subsequently split randomly into either the fasted training group (F) or the regular group (CHO).
The participants then underwent a 6-week supervised training program coupled with a dietary control regimen. The training consisted of 1-1.5 hours of cycling at about 70% VO2 max for 4 days a week. Group F trained in a fasted state while the CHO group ingested sufficient carbohydrates before and during the training session.
At the end of the 6-week program, both groups experienced a similar increase in VO2 max and performance in a 60-min simulated time trial. Subsequently, a metabolic measurement was made. This involved a 2 hour constant-load exercise bout in the fasted state at 65% pre-training VO2max. It was found that in group F, exercise-induced intramyocellular lipid (IMCL) breakdown was enhanced in type I fibers and tended to be increased in type IIa fibers. This result was not replicated by the CHO group. Furthermore, it was found that baseline muscle glycogen content in group F was increased in the post-test but net glycogen breakdown was unchanged. Lastly, lower blood lactate levels during the post-test indicate that more of the pyruvate formed entered the Krebs cycle. This clearly demonstrates that endurance training in the fasted state is more effective at increasing muscular oxidative capacity at an identical duration and intensity of endurance training.
On normal days, I work out twice a day with the first workout in a fasted state and usually consisting of some form of steady state endurance training like running or cycling. Hence the results of this study have reinforced my belief that training in a carbohydrate-restricted state is more beneficial for endurance sports in terms of muscular adaptations for fat oxidation. It would be interesting to continue finding more research related to fasted-state training adaptations related to resistance training or even high intensity interval trainings.