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Hunter S. Waldman, Brandon D. Shepherd, Brendan Egan, and Matthew J. McAllister

quantities of the D - and L -β-hydroxybutyrate (β-OHB) enantiomers, in a mineral salt form, which produce slight to modest elevations in plasma D -β-OHB concentrations (<1.0 mM; Evans et al., 2018 ; O’Malley et al., 2017 ; Rodger et al., 2017 ; Stubbs et al., 2017 ). Conversely, of the various ketone

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Pedro L. Valenzuela, Javier S. Morales, Adrián Castillo-García, and Alejandro Lucia

Carbohydrates represent a main muscle energy source during exercise—particularly when performed at high intensities—thus ensuring a high availability of this substrate during exertion, which is of major importance to maximize performance. 1 Ketone bodies (acetoacetate, acetone, and β-hydroxybutyrate

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David M. Shaw, Fabrice Merien, Andrea Braakhuis, Daniel Plews, Paul Laursen, and Deborah K. Dulson

loading and supplementation ( Burke et al., 2011 ). However, recent scientific inquiry has focused on the metabolic and performance effects of an additional energetic substrate, ketone bodies (KB; Evans et al., 2016 ; Pinckaers et al., 2016 ). Ketone bodies predominantly consist of the D-enantiomer of β-hydroxybutyrate

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Grant M. Tinsley and Darryn S. Willoughby

Low-carbohydrate and very-low-carbohydrate diets are often used as weight-loss strategies by exercising individuals and athletes. Very-low-carbohydrate diets can lead to a state of ketosis, in which the concentration of blood ketones (acetoacetate, 3-β-hydroxybutyrate, and acetone) increases as a result of increased fatty acid breakdown and activity of ketogenic enzymes. A potential concern of these ketogenic diets, as with other weight-loss diets, is the potential loss of fat-free mass (e.g., skeletal muscle). On examination of the literature, the majority of studies report decreases in fat-free mass in individuals following a ketogenic diet. However, some confounding factors exist, such as the use of aggressive weight-loss diets and potential concerns with fat-free mass measurement. A limited number of studies have examined combining resistance training with ketogenic diets, and further research is needed to determine whether resistance training can effectively slow or stop the loss of fat-free mass typically seen in individuals following a ketogenic diet. Mechanisms underlying the effects of a ketogenic diet on fat-free mass and the results of implementing exercise interventions in combination with this diet should also be examined.

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Estelle V. Lambert, Julia H. Goedecke, Charl van Zyl, Kim Murphy, John A. Hawley, Steven C. Dennis, and Timothy D. Noakes

We examined the effects of a high-fat diet (HFD-CHO) versus a habitual diet, prior to carbohydrate (CHO)-loading on fuel metabolism and cycling time-trial (TT) performance. Five endurance-trained cyclists participated in two 14-day randomized cross-over trials during which subjects consumed either a HFD (>65% MJ from fat) or their habitual diet (CTL) (30 ± 5% MJ from fat) for 10 day, before ingesting a high-CHO diet (CHO-loading, CHO > 70% MJ) for 3 days. Trials consisted of a 150-min cycle at 70% of peak oxygen uptake (V̇O2peak), followed immediately by a 20-km TT. One hour before each trial, cyclists ingested 400 ml of a 3.44% medium-chain triacylglycerol (MCT) solution, and during the trial, ingested 600 ml/hour of a 10% 14C-glucose + 3.44% MCT solution. The dietary treatments did not alter the subjects’ weight, body fat, or lipid profile. There were also no changes in circulating glucose, lactate, free fatty acid (FFA), and β-hydroxybutyrate concentrations during exercise. However, mean serum glycerol concentrations were significantly higher (p < .01) in the HFD-CHO trial. The HFD-CHO diet increased total fat oxidation and reduced total CHO oxidation but did not alter plasma glucose oxidation during exercise. By contrast, the estimated rates of muscle glycogen and lactate oxidation were lower after the HFD-CHO diet. The HFD-CHO treatment was also associated with improved TT times (29.5 ± 2.9 min vs. 30.9 ± 3.4 min for HFD-CHO and CTL-CHO, p < .05). High-fat feeding for 10 days prior to CHO-loading was associated with an increased reliance on fat, a decreased reliance on muscle glycogen, and improved time trial performance after prolonged exercise.

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Samuel G. Impey, Kelly M. Hammond, Robert Naughton, Carl Langan-Evans, Sam O. Shepherd, Adam P. Sharples, Jessica Cegielski, Kenneth Smith, Stewart Jeromson, David L. Hamilton, Graeme L. Close, and James P. Morton

acids (NEFA), glycerol, β-hydroxybutyrate, insulin, and amino acids were analyzed as previously described ( Impey et al., 2016 ). RNA extraction and analysis and reverse transcriptase quantitative real-time polymerase chain reaction (rt-qRT-PCR) Muscle samples (∼20 mg) were immersed and homogenized in 1

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Brianna J. Stubbs, Pete J. Cox, Tom Kirk, Rhys D. Evans, and Kieran Clarke

. , Nally , R. , Kearns , R. , Larney , M. , & Egan , B. ( 2018 ). Effect of acute ingestion of β-hydroxybutyrate salts on the response to graded exercise in trained cyclists . European Journal of Sport Science, 18 ( 3 ), 376 – 386 . doi:10.1080/17461391.2017.1421711 10

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Ed Maunder, Deborah K. Dulson, and David M. Shaw

keto-adaptation for endurance performance: disentangling the effects of two distinct metabolic states . Sport Med . 2020 ; 50 ( 4 ): 641 – 656 . PubMed ID: 31820376 doi:10.1007/s40279-019-01246-y 10.1007/s40279-019-01246-y 3. Achanta LB , Rae CD . β-Hydroxybutyrate in the brain: one molecule

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Petter Fagerberg

, lowered insulin, triiodothyronine (T3), growth hormone, insulin-like growth factor 1 (IGF-1), leptin, glucose, and increases in cortisol and β-hydroxybutyrate production ( Loucks & Thuma, 2003 )—all at EA < 30 kcal/kg FFM, which closely resembles resting metabolic rate ( Loucks et al., 2011 ). These

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Romain Meeusen and Lieselot Decroix

, Van Cutsem, Decroix, et al., 2017 ; De Pauw, Roelands, Van Cutsem, Marusic, et al., 2017 ). Other Brain Fuels Ketone bodies such as acetoacetate and β-hydroxybutyrate (βHB) are synthesized in the liver from fatty acids when carbohydrate levels are low. Ketones act as a back-up fuel for the brain when