Beneficial metabolic adaptations due to endurance exercise training in the fasted state

Abstract

Training with limited carbohydrate availability can stimulate adaptations in muscle cells to facilitate energy production via fat oxidation. Here we investigated the effect of consistent training in the fasted state, vs. training in the fed state, on muscle metabolism and substrate selection during fasted exercise. Twenty young male volunteers participated in a 6-wk endurance training program (1-1.5 h cycling at ∼70% Vo(₂max), 4 days/wk) while receiving isocaloric carbohydrate-rich diets. Half of the subjects trained in the fasted state (F; n = 10), while the others ingested ample carbohydrates before (∼160 g) and during (1 g·kg body wt⁻¹·h⁻¹) the training sessions (CHO; n = 10). The training similarly increased Vo(₂max) (+9%) and performance in a 60-min simulated time trial (+8%) in both groups (P < 0.01). Metabolic measurements were made during a 2-h constant-load exercise bout in the fasted state at ∼65% pretraining Vo(₂max). In F, exercise-induced intramyocellular lipid (IMCL) breakdown was enhanced in type I fibers (P < 0.05) and tended to be increased in type IIa fibers (P = 0.07). Training did not affect IMCL breakdown in CHO. In addition, F (+21%) increased the exercise intensity corresponding to the maximal rate of fat oxidation more than did CHO (+6%) (P < 0.05). Furthermore, maximal citrate synthase (+47%) and β-hydroxyacyl coenzyme A dehydrogenase (+34%) activity was significantly upregulated in F (P < 0.05) but not in CHO. Also, only F prevented the development exercise-induced drop in blood glucose concentration (P < 0.05). In conclusion, F is more effective than CHO to increase muscular oxidative capacity and at the same time enhances exercise-induced net IMCL degradation. In addition, F but not CHO prevented drop of blood glucose concentration during fasting exercise.

New strategies in sport nutrition to increase exercise performance

“…In this regard, we and others have collectively observed that reducing endogenous and/or exogenous CHO availability during short-term (e.g. 3-10 week) endurance training increases mitochondrial enzyme activity and protein content [55,59,60] increases both whole body [55] and intramuscular lipid oxidation [61] and in some instances, improves exercise capacity [62,63]. These data have therefore led to the innovative "train-low (or smart), compete-high" model surmising that athletes deliberately complete a portion of their training programme with reduced CHO availability so as to augment training adaptation but yet always ensure high CHO availability prior to and during competition in an attempt to promote maximal performance [64].…”

“…As such, many challenges remain as to how best to periodise train-low into an elite athlete's training programme without increasing the risk of the aforementioned maladaptive responses. At present, research studies examining the efficacy of train-low strategies have largely adopted fasted training protocols [60], protein only sessions [70], training twice per day models [62] reducing CHO intake in the post-exercise period [71] and most recently, both sleeping and training (i.e. sleep low-train low models) on the subsequent morning with reduced CHO intake [21,72].…”

High-fat diet overrules the effects of training on fiber-specific intramyocellular lipid utilization during exercise

“…Furthermore, the fasting endurance-training program also stimulated net IMCL degradation in type IIa muscle fibers during exercise (64). It is reasonable to conclude that exercising with lowcarbohydrate availability is an effective strategy to promote fat oxidation rate by enhanced IMCL breakdown in endurance exercise (37,64,70). This conclusion is supported by studies showing increased ␤-HAD activity as well as protein content or elevated activity of citrate synthase and succinate dehydrogenase following training in a carbohydrate-depleted state (26,37,48,64,70).…”

“…We also demonstrated that consistent training in F can increase membrane-bound fatty acidbinding protein and FAT/CD36 protein content in muscles more than an identical training program with ample carbohydrate intake (CHO) during the training sessions (16a). Furthermore, the fasting endurance-training program also stimulated net IMCL degradation in type IIa muscle fibers during exercise (64). It is reasonable to conclude that exercising with lowcarbohydrate availability is an effective strategy to promote fat oxidation rate by enhanced IMCL breakdown in endurance exercise (37,64,70).…”

“…It is reasonable to conclude that exercising with lowcarbohydrate availability is an effective strategy to promote fat oxidation rate by enhanced IMCL breakdown in endurance exercise (37,64,70). This conclusion is supported by studies showing increased ␤-HAD activity as well as protein content or elevated activity of citrate synthase and succinate dehydrogenase following training in a carbohydrate-depleted state (26,37,48,64,70).Another classical nutritional intervention to stimulate energy turnover via fat oxidation is the administration of a HFD, which increases energy provision via fat oxidation both at rest and during exercise (32,38,43,51). However, previous studies have consistently used isocaloric fat-rich diets when investigating the effects of a HFD on exercise metabolism (8, 9, 25, 29, 32-34, 38, 43, 58).…”