Despite a hundred-plus year history, there has been renewed interest in research describing the effects of lowering dietary carbohydrate on health and exercise performance (Volek, Noakes and Phinney, 2015). Approaches have included training in the fasted state, training with reduced glycogen stores after prior exercise, and restricting carbohydrate (CHO) intake during recovery (Bartlett et al., 2015). A very low-CHO high-fat (VLCHF) diet, also known as the ketogenic diet, is a nutritional approach that restricts daily carbohydrates to 20-50 g/d, replacing the majority of those reduced calories with fat, while maintaining low to moderate quantities of protein (Feinman et al., 2015). In persons habituated to typical Western diets, the VLCHF diet has been shown to lower basal glucose and insulin levels, increase fat oxidation rates, and upregulate an alternative energy source in the form of hepatic ketone bodies (Paoli et al., 2015). These ketogenic diet-induced metabolic changes drive the body to preferentially use fat and ketones as their primary fuel sources (Volek et al., 2015), and might be considered advantageous, particularly for prolonged exercise, since access to the extensive quantity of fat stores implies a steady source of energy flow, unlike our limited endogenous CHO stores from muscle and liver glycogen (Yeo et al., 2011; Volek et al., 2015).
Initial exposure to low CHO diets can reduce resting muscle (and presumably liver) glycogen stores within the first several days. This reduction of muscle glycogen stores without notable increase in the capacity for fat utilization is associated with a lowered capacity for exercise performance (Burke and Hawley, 2002). Indeed, a short term ketogenic diet (1-3 days) has been shown to reduce both prolonged submaximal (~70% of maximal oxygen uptake ([VO.sub.2max])) (Starling et al., 1997; Pitsiladis and Maughan, 1999) and high-intensity supramaximal exercise (Langfort et al., 1997; Lima-Silva et al., 2013). Beyond 5-7 days of exposure to the ketogenic diet, fat oxidation during submaximal exercise is enhanced (Paoli et al., 2015), with maximal performance benefits postulated to take up to several weeks or months (Volek et al., 2015).
CHO availability and muscle glycogen content are primary components of metabolism and exercise performance (Bergstrom et al., 1967), with skeletal muscle increasingly reliant on CHO as a fuel source as exercise rises in intensity (van Loon et al., 2001). Thus, it stands to reason that restriction of endogenuous CHO availability, forcing greater reliance on fat as the primary fuel source, would adversely affect strenuous exercise performance, such as that needed for high-intensity interval training (HIIT) (Buchheit and Laursen, 2013a). However, no studies to date have examined the effects of long-term reductions in CHO intake on HIIT performance. Therefore, the purpose of this study was to examine the effects of altering from one's HD to a VLCHF diet over 4 weeks on cardiorespiratory and metabolic responses during HIIT.
Eighteen moderately trained males were recruited for this study, before being assigned to 2 groups: a very low-CHO high-fat diet group (VLCHF; N = 9) and a habitual mixed Western diet group (HD; N = 8). Inclusion criteria consisted of being male and between the age of 18 and 30, and actively engaged in regular (at least 3 sessions/week) non competitive exercise activities, mostly of a low-intensity endurance nature. At baseline, both groups were similarly matched for age (23.8 [+ or -] 2.4 vs. 23.8 [+ or -] 1.8 years), body height (1.80 [+ or -] 0.07 vs. 1.83 [+ or -] 0.04 m), body mass (83.2 [+ or -] 17.7 vs. 83.7 [+ or -] 9.4 kg), body fat (13.7 [+ or -] 7.7 vs. 16.1 [+ or -] 7.4%) and self-reported exercise activity per week (6.4 [+ or -] 2.7 vs. 6.2 [+ or -] 1.9 h, VLCHF vs. HD, respectively). All participants were free from any known diseases, and currently not taking medications or dietary supplements. None of the participants had experience with the VLCHF diet before the study. One participant in the HD group withdrew due to his inability to comply with the performance test schedule. Participants were informed of the nature of the study and written informed consent was obtained prior to study commencement. The experimental protocol was approved by the local Ethics Committee and conformed to the principles outlined in the Declaration of Helsinki.
Overview of study design
At baseline (PRE), after 2 weeks (MID) and 4 weeks (POST) of the controlled experiment, participants attended the exercise physiology laboratory for testing. A maximal incremental treadmill test (GXT) was performed at PRE and POST, while an HIIT session was performed at all time points. Tests were separated by 48 h.
All sessions were conducted in the morning, at least 3 h after their last meal and in a thermally-controlled laboratory room (21[degrees]C, 40% relative humidity). Each participant performed their laboratory sessions during similar morning hours ([+ or -] 30 min). Body mass and composition were determined using a bioelectrical impedance analyser (InBody770, Seoul, Korea) and a capillary blood sample was drawn from a finger for the measurement of [beta]-hydroxybutyrate (FreeStyleOptium Neo, Oxon, UK) before all exercise interventions. Participants were advised not to participate in vigorous activity 24 h before the laboratory visits. Apart from the prescribed dietary intervention in the case of the VLCHF group or maintaining HD, participants were asked to perform 3-5 sessions per week of non-supervised training, primarily endurance-based running sessions or exercise according to interest (e.g. team sports, hiking, in-line skating), to record their training heart rate (HR) using a HR monitor (Polar410, Oy, Finland), and to keep an exercise diary.
Dietary intervention and recording
Participants in the HD group were asked to maintain their habitual dietary intake without restriction. The VLCHF diet adhered to by the VLCHF group was defined as consisting of up to 50 g/d of CHO (Feinman et al., 2015). Neither diet included a specific calorie or energy goal. Detailed dietary advice, consisting of meal planning recipe suggestions were provided by a dietician before the study, and assistance was provided at any time during the intervention as required. A handbook was given to participants that contained recipe examples, food lists and guides for counting macronutrients. All foods and quantities consumed were recorded daily beginning 5 days before the intervention period (www.kaloricketabulky.cz). Alcoholic beverages were restricted for the intervention period. Dietary supplements were not permitted for the 1 month before and during the intervention period, while caffeine beverages were restricted before the laboratory sessions.
Exercise testing sessions
In order to determine the maximum aerobic capacity ([VO.sub.2max]) and the minimal running speed required to elicit [VO.sub.2max] (v[VO.sub.2max]) participants performed a graded exercise test (GXT) to volitional exhaustion. The treadmill (LodeValiant, Groningen, The Netherlands) GXT protocol started at 7.0 km/h and increased by 1.5 km/h every 4 min with inclination remaining at 1%. Expired air was continuously monitored for analysis of [O.sub.2] and C[O.sub.2] concentrations during the GXT by the use of a breath-by-breath system (ZAN600Ergo; Oberthulba, Germany). Determination of [VO.sub.2max] was made based on the highest average [O.sub.2] consumption measured over a 30s period. v[VO.sub.2max] was determined as the speed upon which a plateau in the [VO.sub.2] first occurred or [VO.sub.2] did not increase more than 2.1 ml/kg/min despite an increasing running speed (Kohn, Essen Gustavsson and Myburgh, 2011). Gas-exchange measurements were also used to quantify the second ventilatory threshold ([VT.sub.2]). [VT.sub.2] was defined as the second increase in ventilation (VE) with increase in both VE/[VO.sub.2] and VE/[VCO.sub.2]. HR was measured using a chest belt (Polar-Electro; Finland). Fat oxidation was calculated from indirect calorimetry measurements using stoichiometric equations from RER (Jeukendrup and Wallis, 2005). The average RER of the last 2 min of each 4 min interval was used for this calculation, and the highest value was considered as the [Fat.sub.max].
The HIIT session involved a 10 min warm-up at 60% v[VO.sub.2max], followed by 5 high-intensity repetitions consisting of 3 min at 100% v[VO.sub.2max], separated by 1.5 min passive recovery (work to rest ratio, 2:1). For the HIIT session, the treadmill was set at 1%, and lasted a total of 34 min. This long interval HIIT protocol (Buchheit and Laursen, 2013a) was chosen...