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

Louise M Burke,Nicolin Tee,Ida A Heikura,Sara F Forbes,Megan L R Ross,Avish P Sharma,Jamie Whitfield,Alice M Wallett,Alannah K A Mckay,Rebecca Hall

doi:10.1113/jp280221

Abstract

Key points 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.Increased fat utilisation during exercise was associated with a 5–8% increase in oxygen cost at speeds related to Olympic Programme races.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.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. We investigated substrate utilisation during exercise after brief (5–6 days) adaptation to a ketogenic low‐carbohydrate (CHO), high‐fat (LCHF) diet 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 days’ LCHF (n = 7; CHO: <50 g day−1, protein: 2.2 g kg−1 day−1; 80% fat) or HCHO (n = 6; CHO: 9.7 g kg−1 day−1; protein: 2.2 g kg−1 day−1) diet. An Adaptation race was undertaken after 24 h HCHO and pre‐race CHO (2 g kg−1) diet, identical to the Baseline race. Substantial (>200%) increases in exercise fat oxidation occurred in the LCHF Adaptation economy and 25 km tests, reaching mean rates of ∼1.43 g min−1. However, relative V˙O2 (ml min−1 kg−1) was higher (P < 0.0001), by ∼8% and 5% at speeds related to 50 km and 20 km events. During Adaptation race warm‐up in the LCHF group, rates of fat and CHO oxidation at these speeds were decreased and increased, respectively (P < 0.001), compared with the previous day, but were not restored to Baseline values. Performance changes differed between groups (P = 0.009), with all HCHO athletes improving in the Adaptation race (5.7 (5.6)%), while 6/7 LCHF athletes were slower (2.2 (3.4)%). Substrate utilisation returned to Baseline values after 5–6 days of HCHO diet. In summary, robust changes in exercise substrate use occurred in 5–6 days of extreme changes in CHO intake. However, adaptation to a LCHF diet plus acute restoration of endogenous CHO availability failed to restore high‐intensity endurance performance, with CHO oxidation rates remaining blunted.

Highlights

According to current sports nutrition guidelines, the performance of sustained high-intensity endurance sports is best supported by conditions of high carbohydrate (CHO) availability (Thomas et al 2016), defined as matching the finite body CHO stores to event-specific muscle and central nervous system fuel needs (Burke et al 2018a)
There were no significant differences between groups for energy and macronutrient intake characteristics for the Baseline and Restoration phases, and each group was able to achieve the main targets for these phases: energy availability (EA) of ß40 kcal (168 kJ) kg−1 fat free mass (FFM) day−1, protein intake of ß2 g kg−1 body mass (BM) day−1, and CHO intake of 9–9.5 g kg−1 BM day−1 contributing ß65% of energy
There is clear evidence from both intervention (Phinney et al 1983; Burke et al 2017; Shaw et al 2019; 2020) and cross-sectional studies (Volek et al 2016; Webster et al 2016) that adherence to a ketogenic LCHF diet leads to substantial (e.g. 200–250%) increases in capacity for fat oxidation during exercise, even in well-trained endurance athletes who achieve this characteristic as an outcome of their training (Hawley et al 2018)

Summary

Introduction

According to current sports nutrition guidelines, the performance of sustained high-intensity endurance sports is best supported by conditions of high carbohydrate (CHO) availability (Thomas et al 2016), defined as matching the finite body CHO stores to event-specific muscle and central nervous system fuel needs (Burke et al 2018a). Contemporary views around everyday nutritional support for the high-volume endurance training necessary to prepare for such events promote HCHO availability, at least for key workouts in which high-intensity/quality performance or race simulation is desired (Burke et al 2011, 2018a). A deliberate contrast to these guidelines is the revived interest in adaptation to low CHO, high fat (LCHF) diets, due to observations that athletes can achieve substantial increases to their already enhanced capacity for oxidising fat during exercise, including an increase in the exercise intensity at which maximal rates of fat oxidation occur (for review, see Burke, 2020). Rigorously controlled studies from our group have reported detriments to the real-life performances of high-intensity endurance events in elite athletes following 3–4 weeks of a LCHF diet, in comparison to training/racing with HCHO availability (Burke et al 2017). 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

Objectives

Methods

Results

Conclusion