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About the study

ADAPTATION TO LOW CARBOHYDRATE HIGH FAT DIET IS RAPID BUT IMPAIRS ENDURANCE EXERCISE METABOLISM AND PERFORMANCE DESPITE ENHANCED GLYCOGEN AVAILABILITY
Louise M Burke, Jamie Whitfiel, Ida A. Heikura, Megan L.R. Ross, Nicolin Tee, Sara F. Forbes, Rebecca Hall, Alannah K.A. McKay, Alice M. Wallett and Avish P. Sharma

The study investigates substrate utilisation during exercise after brief (5-6 d) adaptation to a ketogenic low-carbohydrate (CHO), high-fat (LCHF) and similar washout period. Thirteen world-class male race walkers completed economy testing, 25-km training and a 10,000 m race (Baseline), with high CHO availability (HCHO), repeating this (Adaptation) after 5-6 d LCHF (n = 7; CHO: <50 g d-1, protein: 2.2 g kg-1 d-1; 80% fat) or HCHO (n = 6; CHO: 9.7 g kg-1 d-1; protein: 2.2 g kg-1 d-1). Adaptation race was undertaken after 24-hr HCHO and pre- race CHO (2 g kg-1), identical to Baseline race. The poster is designed to bring out the methodology, results and key takeaways from the study in the best possible way

Key takeaways from the study

  1. Core characteristic of keto- adaptation can be achieved in a much shorter timeframe than previously considered. Brief (5-6 d) adaptation to LCHF in elite athletes increases exercise fat oxidation to rates previously observed with medium (3-4 wk) or chronic (>12 month) adherence to this diet, with metabolic changes being washed out in a similar timeframe 
  2. Acute restoration of endogenous CHO availability (24 h HCHO diet, pre-race CHO) only partially restores substrate utilisation during a race warm-up. 
  3. Acute restoration of CHO availability fails to overturn the impairment of high- intensity endurance performance previously associated with LCHF adaptation, potentially due to the blunted capacity for CHO oxidation.  
  4. Substantial increases in fat utilisation are associated with an increased oxygen cost of exercise, particularly in high-intensity (>70% VO2peak) domains. Whereas the aerobic reserve can accommodate this trade-off during moderate intensity exercise, oxygen availability may become a limiting factor for energy production at higher intensities, contributing to a reduction in performance. 
  5. In well-trained athletes robust increases in fat oxidation occur in as little as 5-6 days of a non-ketogenic LCHF diet however, this has not been established with elite athletes and a ketogenic LCHF diet. 
  6. Due to the prolonged nature of the exercise, both groups displayed a decrease in RER over the course of the 25-km walks, which was associated with a significant increase in both absolute and relative VO2and a gradual reduction in CHO and increase in fat oxidation 
  7. Following the 6-d adaptation to LCHF and a 1- day HCHO diet designed to enhance skeletal muscle glycogen availability, fat oxidation rates at Stages 2 and 4 were reduced by 40% respectively, relative to the economy test performed following LCHF adaptation, but remained elevated compared to the first Pre-Race test performance performed at Baseline.
  8. Despite the lack of a statistically significant change in maximum aerobic capacity (relative VO2peak), all 6 athletes in the HCHO group improved performance by an average of 5.7 (5.6)%. In contrast, 6 of 7 LCHF athletes displayed impaired performance with a mean race time that was 2.2 (3.4)% slower, despite a small but significant increase in relative VO2peak 
  9. Adaptations that substantially increase fat oxidation during exercise in response to the LCHF occur in elite athletes in as little as 5-6 d, reaching the same rates as observed in endurance athletes who have adhered to this diet for medium (3-4 wk) and long-term periods (>12 wk)
  10. Muscle retooling to enhance fat oxidation with brief exposure to a ketogenic LCHF is maintained in the face of acute increases in muscle CHO availability, but is reversed by 5 d of chronic high CHO diet; 
  11. The economy of exercise at intensities relevant to real life endurance events is reduced by brief adaptation to a ketogenic LCHF when fat oxidation is maximised 
  12. Within 5-6 d of adherence to a ketogenic LCHF, rates of fat oxidation during exercise reaches a mean of ~ 1.43 g min-1 during a graded economy test and a prolonged training session at speeds that are relevant to the 50-km race walking event 
  13. Blood glucose levels are maintained at lower, albeit euglycemic, concentrations during exercise while βHB are elevated above 1 mmol L-1 at rest and during exercise. 
  14. Following a washout period is must before going on a ketogenic diet 
  15. Although studies of adaptation to ketogenic LCHF diets have failed to directly investigate the mechanism(s) underpinning enhanced fat utilisation, it is likely that similar upregulation of fat availability, mobilisation, transport and mitochondrial uptake within the muscle is responsible. 
  16. Muscle retooling is maintained for at least 24 hr of reintroduction to HCHO availability and muscle glycogen restoration.
  17. We were able to examine substrate use (Figure 6) and exercise economy at two chosen treadmill speeds during a graded economy test and the pre-race warm up, 24 hr apart. In the case of Race 1, we found tight replication of substrate use at the Stage 2 and Stage 4 treadmill speeds which relate to the pace required for the 50-km and 20-km race walking events on the current Olympic program, respectively. During the Race 2 warm up, acute restoration of CHO availability increased rates of CHO oxidation and reduced the contribution of fat oxidation to energy production compared with the previous day‟s economy test undertaken under full keto- adaptation conditions. However, this represented an intermediary shift in substrate use, with CHO oxidation failing to be fully restored to Baseline rates 
  18. Examination of changes in CHO metabolism in keto-adapted athletes is less well developed, although there is consistent evidence of a reduction in muscle glycogen oxidation 
  19. Ketogenic-LCHF diet for longer periods (i.e. > 6 months) display impaired glucose tolerance during an oral glucose tolerance test as a result of decreased GLUT4 and insulin receptor substrate 1 (IRS1) protein content 
  20. It contributes to new knowledge by showing that gross changes in substrate utilisation and metabolism during exercise in response to keto-adaptation are reversed by 5 d of reintroduction of a HCHO diet. 
  21. Acute restoration of muscle CHO availability is unable to rescue the impaired performance of endurance exercise undertaken at high relative and absolute intensities otherwise associated with the ketogenic LCHF diet  
  22. Exposure of CHO to oral receptors enhances performance of higher-intensity endurance exercise when endogenous CHO availability is low, with an enhanced central motor drive potentially being the main influencing mechanism 
  23. The authors theorised that an impairment of CHO oxidation secondary to fat-adaptation prevented the contribution of this important pathway to the fuel cost of the high-intensity workloads. 
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