Adaptation to a low carbohydrate high fat diet is rapid but impairs endurance exercise metabolism and performance despite enhanced glycogen availability

Abstract

Brief (5-6 days) adaptation to a low carbohydrate high fat diet in elite athletes increased exercise fat oxidation to rates previously observed with medium (3-4 weeks) or chronic (>12 months) adherence to this diet, with metabolic changes being washed out in a similar time frame. r Increased fat utilisation during exercise was associated with a 5-8% increase in oxygen cost at speeds related to Olympic Programme races. r Acute restoration of endogenous carbohydrate (CHO) availability (24 h high CHO diet, pre-race CHO) only partially restored substrate utilisation during a race warm-up. Fat oxidation continued to be elevated above baseline values although it was lower than achieved by 5-6 days' keto adaptation; CHO oxidation only reached 61% and 78% of values previously seen at exercise intensities related to race events. r Acute restoration of CHO availability failed to overturn the impairment of high-intensity endurance performance previously associated with low carbohydrate high fat adaptation, potentially due to the blunted capacity for CHO oxidation.

A 2 Week Cross-over Intervention with a Low Carbohydrate, High Fat Diet Compared to a High Carbohydrate Diet Attenuates Exercise-Induced Cortisol Response, but Not the Reduction of Exercise Capacity, in Recreational Athletes

“…At the same time, heart rate was higher and perceived exertion equal between both test days on the LCHF diet. The lower RER after 2 days on the LCHF diet observed in our study is in agreement with other findings suggesting that increased fat oxidation can occur within days during low carbohydrate availability [18,28]. Studies suggest that prolonged adherence to a LCHF diet enhances the breakdown, transport, and oxidation of fat in skeletal muscle [29].…”

“…The reduced work output after following the LCHF diet for 2 days is in line with previous studies and can be explained by decreased CHO oxidation, even though fat oxidation rates may already be increased after short term adaptation [18]. This was emphasized by the higher rates of perceived exertion.…”

“…The wash-out period of 2 weeks between the interventions was based on previous findings regarding adaptation time to a LCHF diet [17] and turning back to baseline after CHO loading [18]. Further studies are warranted to explore the time-course of this reversal of effects.…”

“…A wash-out period, consisting of their habitual diet, of at least two weeks was applied between both diets. Two weeks seemed long enough, as changing to a LCHF diet already leads to adaptations within 5 days [17], and changing back to a HC diet, leads to 'baseline levels' after again 5 to 6 days [18]. An upper respiratory tract symptoms (URTS) questionnaire was filled out before the intervention and 2 weeks after each diet ended ( Figure 1).…”

High‐fat versus high‐carbohydrate diets for optimal higher intensity endurance exercise performance: and the winner is…

“…In this issue of The Journal of Physiology , Burke and colleagues address several of these questions in world‐class race walkers (Burke et al . 2021). After training and following their standard HCHO ketogenic diet, undergoing baseline exercise tests and a 10 km race, these athletes self‐selected to follow either a LCHF or a HCHO diet for 5 days, with exercise training and food consumption meticulously controlled.…”