High fat diet improves metabolic flexibility during progressive exercise to exhaustion (VO 2 max testing) and during 5km running time trials

Abstract

Recently we reported similar performances in both progressive tests to exhaustion (VO 2 max) and 5km running time trials (5KTT) after consuming low-carbohydrate, high-fat (LCHF) or high-carbohydrate, low-fat (HCLF) diets. Accordingly, we tested the null hypothesis that the metabolic responses during both tests would be similar across diets. In a randomized, counterbalanced, cross-over design, seven male athletes (VO 2 max: 61.9 ± 6.1 mL/kg/min; age: 35.6 ± 8.4 years; height: 178.7 ± 4.1 cm; mass: 68.6 ± 1.6 kg; body fat: 5.0 ± 1.3%) completed six weeks of LCHF (6/69/25% energy carbohydrate/fat/protein) and HCLF (57/28/15% energy carbohydrate/fat/protein) diets, separated by a two-week washout. Substrate utilization and energy expenditure were measured during VO 2 max tests and 5KTTs. The LCHF diet markedly increased fat oxidation and reduced carbohydrate oxidation, with no associated impairment in either the VO 2 max tests or the 5KTTs. Following the LCHF diet, athletes generated 50% or more of their energy requirements from fat at exercise intensities up to 90% VO 2 max and reached the crossover point for substrate utilization at ~85% VO 2 max. In contrast, following the HCLF diet, carbohydrate provided more than 50% of the total energy consumption at all exercise intensities. During the 5KTT, ~56% of energy was derived from fat following the LCHF diet whereas more than 93% of the energy came from carbohydrate following the HCLF diet. This study provides evidence of greater metabolic flexibility following LCHF eating and challenges the popular doctrines of “carbohydrate dependence” for high intensity exercise and the role dietary macronutrients play in human performance.

Low carbohydrate high fat ketogenic diets on the exercise crossover point and glucose homeostasis

“…First, amongst the highest rates of fat oxidation (1.5 g/min) yet measured were recorded in the Olympic-class racewalkers following adoption of a LCHF diet for just 5 days ( Burke et al, 2017 ; Burke et al, 2021 ). These data are very similar to those measured in recreational but well-trained runners who had adapted to the LCHF diet for 6 weeks ( Prins et al, 2019 ; Prins et al, 2023b ). While these rapid adaptations may be partially explained by elite-level fitness ( Rothschild et al, 2022 ), recent analyses have demonstrated that respiratory quotient and substrate oxidation changes occur in 4 days in male endurance athletes on an isocaloric LCHF diet ( Prins et al, 2019 ), within 7 days ( Hall et al, 2016 ) and 14 days in moderately trained athletes ( Cipryan et al, 2018 ) and overweight/obese subjects ( Buga et al, 2021 ; Hall et al, 2021 ), as biochemical changes consistent with increase fat oxidation (i.e., reduced glucose and insulin load; increase ketones) occur upon the first 24 h following reduced carbohydrate intake (<50 g/day) ( Hengist et al, 2022 ).…”

“…Since Prins et al (2019) performed VO 2 max tests in their subjects when eating either the LCHF or HCLF diets for 6 weeks, they were able to study the effects of this dietary change on the crossover point during progressive exercise to exhaustion [( Prins et al, 2023b ); Figure 3 ]. Interestingly, Figure 3 (left panel) shows that when eating the HCLF diet subjects did not show a definitive crossover point.…”

“…When subjects ate the LCHF diet, they showed progressive, exponential changes in carbohydrate oxidation with increasing exercise intensity as predicted in Figure 1 . Figure 4 includes the data for peak rates of fat oxidation measured in that study ( Prins et al, 2023b ) as well as data from Achten and Jeukendrup ( Achten and Jeukendrup, 2003 ) and the most recent study of Burke et al ( Burke et al, 2021 ). Whereas the data from Prins et al (2023b) are from well-trained recreational athletes who adapted to the LCHF diet for 6 weeks, the data from Burke et al ( Burke et al, 2021 ) are from elite Olympic class racewalkers who followed the LCHF diet for just 5 days before resuming their more usual HCLF diet for another 5 days whereafter they were re-tested.…”

“…* p < 0.05, ** p < 0.01, *** p < 0.001, significant difference between LCHF and HCLF. This is reproduced from reference ( Prins et al, 2023b ). The orange box in the left panel indicates high rates of carbohydrate oxidation at low exercise intensities with no obvious crossover point when following the HCLF diet.…”

Low and high carbohydrate isocaloric diets on performance, fat oxidation, glucose and cardiometabolic health in middle age males

“…Countering this evidence is a growing body of data demonstrating that extended habituation to a low-carbohydrate high-fat (LCHF) diet can shift the "cross-over" set-point in favor of greater fat oxidation, even at much higher intensities [(< 85% VO 2max ); (27,28)], and dramatically increase the rates of peak fat oxidation at moderate intensities (i.e., 60% VO 2max ) (29). Rates of fat oxidation during exercise across these LCHF studies are amongst the highest yet measured (24,25,28,(30)(31)(32) even though they were measured during progressive exercise to exhaustion (e.g., minutes), rather than more prolonged exercise (e.g., hours). These studies opened key questions of whether performance-equivalence would still hold (i) if exercise intensity was increased (i.e., running trial 800-1,609 m) and (ii) if high-intensity interval sessions would facilitate more muscle glycogen depletion in those habituated to LCHF which would become more apparent as the number of intervals increased.…”