Dietary nitrate supplementation with beetroot juice (BR) has received widespread attention as an ergogenic aid. However, recent evidence in well-trained cyclists has not consistently reported improved cycling economy or performance. The present study examined the effects of acute and chronic BR supplementation on V˙O2 during submaximal running and 1500-m time trial (TT) performance of elite distance runners. Eight male 1500-m runners (V˙O2peak, 80 ± 5 mL·kg·min; 1500-m personal best, 3:56 ± 9 s) participated in this study. In a randomized, double-blind, crossover design, subjects supplemented with BR or a nitrate-free BR placebo (PL) for 8 d separated by at least 1 wk. On days 1 (acute) and 8 (chronic), subjects ingested 210 mL of BR (19.5-mmol nitrate) or PL and completed a submaximal treadmill run and 1500-m TT on an indoor 200-m track. Plasma nitrate increased from 37 ± 15 to 615 ± 151 μM (acute) and 870 ± 259 μM (chronic) after BR supplementation. There were no V˙O2 differences between conditions at 50%, 65%, and 80% V˙O2peak (acute PL, 4194 ± 90 mL·min; chronic PL, 4216 ± 95 mL·min; acute BR, 4192 ± 113 mL·min; chronic BR, 4299 ± 92 mL·min). The 1500-m TT was unaffected by acute or chronic BR supplementation (acute PL, 4:10.4 min:s ± 2.5 s; chronic PL, 4:11.4 min:s ± 2.7 s; acute BR, 4:10.7 min:s ± 1.5 s; chronic BR, 4:10.5 min:s ± 2.2 s). However, two subjects improved their TT performance after acute (5.8 and 5.0 s) and chronic BR supplementation (7.0 and 0.5 s). Acute and chronic BR supplementation did not reduce running V˙O2 or improve 1500-m TT performance of a group of elite distance runners, but two responders to BR were identified.

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: 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 (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.

The AMP-activated protein kinase (AMPK) is a heterotrimeric complex with central roles in cellular energy sensing and the regulation of metabolism and exercise adaptations. AMPK regulatory β subunits contain a conserved carbohydrate-binding module (CBM) that binds glycogen, the major tissue storage form of glucose. Research over the past two decades has revealed that the regulation of AMPK is impacted by glycogen availability, and glycogen storage dynamics are concurrently regulated by AMPK activity. This growing body of research has uncovered new evidence of physical and functional interactive roles for AMPK and glycogen ranging from cellular energy sensing to the regulation of whole-body metabolism and exercise-induced adaptations. In this review, we discuss recent advancements in the understanding of molecular, cellular, and physiological processes impacted by AMPK-glycogen interactions. In addition, we appraise how novel research technologies and experimental models will continue to expand the repertoire of biological processes known to be regulated by AMPK and glycogen. These multidisciplinary research advances will aid the discovery of novel pathways and regulatory mechanisms that are central to the AMPK signaling network, beneficial effects of exercise and maintenance of metabolic homeostasis in health and disease.

This study aimed to examine the effects of 7 d of beetroot juice (BRJ) supplementation on SkM contractile characteristics and function. Methods Recreationally active males (n = 8) underwent transcutaneous electrical muscle stimulation of the vastus lateralis for the evaluation of contractile characteristics before and after 7 d of BRJ supplementation (280 mL·d−1, ~26 mmol NO3−). An additional group of individuals (n = 8) followed the same supplementation protocol but underwent SkM biopsies pre- and post-supplementation for the determination of proteins associated with calcium handling via Western blotting, and the ratio of reduced/oxidized glutathione (GSH:GSSG), an indicator of cellular redox state, via high-performance liquid chromatography (HPLC). Results After supplementation, there was no change in maximal voluntary force production (602 ± 50 vs 596 ± 56 N) or electrically induced tetanic contractions. By contrast, force production was increased at 10 Hz electrical stimulation (41.1% ± 2.3% vs 37.6% ± 2.4% of peak force, P < 0.05), as was peak twitch tension (164.0 ± 12.5 vs 136.5 ± 7.2 N, P < 0.01) and maximal rates of force development and relaxation (3582.8 ± 382.3 vs 2575.7 ± 196.2 and −2752.4 ± 423.9 vs −2104.4 ± 249.0 N·s−1, respectively, P < 0.05). Despite these measurements implicating a change in calcium handling, the content of associated proteins (SERCA1a, SERCA2a, dihydropyradine receptor, ryanodine receptor, and calsequestrin) and the GSH:GSSG ratio were unaltered by BRJ. Conclusion BRJ supplementation increases force production at low-stimulation frequencies; however, in human SkM, this is independent of changes in redox stress or the expression of protein targets associated with calcium handling....