Interval Training and Substrate Utilization

Interval Training and Substrate Utilization

February 13, 2012

3FU3L_CrossFit_CostaMesa
One of the primary arguments from the traditional endurance community is that logging long training hours will enable an athlete to utilize substrates (fat and carbohydrate) more efficiently. The argument seems to go that during the last few miles of a marathon or triathalon, the athlete who has trained their body to most efficiently use the available energy will have an advantage. A recent study gives us good reason to think that running high-intensity intervals will translate to submaximal work as well; by which we mean the type of work you'd find in an endurance event. However, what we're talking about here isn't "sprint training" - it's High Intensity Interval Training (HIIT), specifically: 4 minutes of work at 90% intensity, followed by 2 minutes of rest (2:1 work to rest). This isn't an inconsequential amount of work - we're talking about hour long training sessions here, thrice weekly; nothing compared to the kinds of mileage the traditionalists would have us believe are necessary, but still a reasonable amount of work. In this particular study, besides the expected adaptation to the training ( power output increased by 21%, and VO2 peak increased by 9% in an exhaustion test), it was found that substrate utilization - the body's ability to use FAT and CHO - was increased in a 1 hour test at 60% intensity (far closer to the submaximal level one would need to maintain over the course of multiple hours in an endurance event). Resting glycogen content even went up by 59%! Check it out:
Appl Physiol Nutr Metab. 2008 Dec;33(6):1112-23. doi: 10.1139/H08-097. High-intensity aerobic interval training increases fat and carbohydrate metabolic capacities in human skeletal muscle. Perry CG, Heigenhauser GJ, Bonen A, Spriet LL.   High-intensity aerobic interval training (HIIT) is a compromise between time-consuming moderate-intensity training and sprint-interval training requiring all-out efforts. However, there are few data regarding the ability of HIIT to increase the capacities of fat and carbohydrate oxidation in skeletal muscle. Using untrained recreationally active individuals, we investigated skeletal muscle and whole-body metabolic adaptations that occurred following 6 weeks of HIIT (~1 h of 10 x 4 min intervals at ~90% of peak oxygen consumption (VO2 peak), separated by 2 min rest, 3 d.week-1). A VO2 peak test, a test to exhaustion (TE) at 90% of pre-training VO2 peak, and a 1 h cycle at 60% of pre-training VO2 peak were performed pre- and post-HIIT. Muscle biopsies were sampled during the TE at rest, after 5 min, and at exhaustion. Training power output increased by 21%, and VO2 peak increased by 9% following HIIT. Muscle adaptations at rest included the following: (i) increased cytochrome c oxidase IV content (18%) and maximal activities of the mitochondrial enzymes citrate synthase (26%), beta-hydroxyacyl-CoA dehydrogenase (29%), aspartate-amino transferase (26%), and pyruvate dehydrogenase (PDH; 21%); (ii) increased FAT/CD36, FABPpm, GLUT 4, and MCT 1 and 4 transport proteins (14%-30%); and (iii) increased glycogen content (59%). Major adaptations during exercise included the following: (i) reduced glycogenolysis, lactate accumulation, and substrate phosphorylation (0-5 min of TE); (ii) unchanged PDH activation (carbohydrate oxidation; 0-5 min of TE); (iii) ~2-fold greater time during the TE; and (iv) increased fat oxidation at 60% of pre-training VO2 peak. This study demonstrated that 18 h of repeated high-intensity exercise sessions over 6 weeks (3 d.week-1) is a powerful method to increase whole-body and skeletal muscle capacities to oxidize fat and carbohydrate in previously untrained individuals.


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