Considerations of Low Carbohydrate Availability (LCA) to Relative Energy Deficiency in Sport (RED-S) in Female Endurance Athletes: A Narrative Review

he purpose of this narrative review is to identify health and performance consequences associated with LCA in female endurance athletes. The intake of carbohydrates (CHO) before, during, and after exercise has been demonstrated to support sport performance, especially endurance activities which rely extensively on CHO as a fuel source. However, low energy availability (LEA) and low carbohydrate availability (LCA) are common in female athletes. LEA occurs when energy intake is insuf cient compared to exercise energy expenditure, and LEA-related conditions (e.g., Female Athlete Triad (Triad) and Relative Energy De ciency in Sport (RED-S)) are associated with a myriad of health and performance consequences. The RED-S model highlights 10 health consequences and 10 performance consequences related to LEA. The independent effect of LCA on health and performance has been under-researched, despite current CHO intake being commonly insuf cient in athletes. It is proposed that LCA may not only contribute to LEA but also have independent health and performance consequences in athletes. Furthermore, this review highlights current recommendations for CHO intake, as well as recent data on LCA prevalence and menstrual cycle considerations. A literature review was conducted on PubMed, Science Direct, and ResearchGate using relevant search

insuf cient in athletes. It is proposed that LCA may not only contribute to LEA but also have independent health and performance consequences in athletes. Furthermore, this review highlights current recommendations for CHO intake, as well as recent data on LCA prevalence and menstrual cycle considerations. A literature review was conducted on PubMed, Science Direct, and ResearchGate using relevant search terms (i.e., “low carbohydrate/energy availability”, “female distance runners”). Twenty-one articles were identi ed and twelve met the inclusion criteria. The total number of articles included in this review is 12, with 7 studies illustrating that LCA was associated with direct negative health and/or performance implications for endurance-based athletes. Several studies included assessed male athletes only, and no studies included a female-only study design. Overall, the cumulative data show that female athletes remain underrepresented in sports science research and that current CHO intake recommendations and strategies may fail to consider female-speci c adaptations and hormone responses, such as monthly uctuations in estrogen and progesterone throughout the menstrual cycle. Current CHO guidelines for female athletes and exercising women need to be audited and explored further in the literature to support female athlete health and performance. Keywords: carbohydrates (CHO); relative energy de ciency in sport (RED-S); low energy availability (LEA); low carbohydrate availability (LCA); female athletes; menstrual cycle 1. Introduction Over the past 50 years, female athlete participation has been on the rise, with the Tokyo Olympics in 2020/2021 being the rst ever gender-balanced Olympics [1]. However, research focused on the female athlete has not yet corrected for this increased participation in both recreational and elite-level sports. For example, in a review of 1382 exercise and sports science articles published from 2011–2013, 39% of study participants identi ed as female, and only 4–13% of the articles incorporated a female-only design in their studies [2]. An updated review of six major journals, including a total of 5261 sports science articles Nutrients2023,15, 4457.

Nutrients2023,15, 4457 2 of 14 published from 2014–2021 illustrated that 34% of all participants reviewed identi ed as women and 6% of articles included women-only study designs [3]. This highlights the need to balance sports science research on female and male athletes. Relatedly, the importance of acute carbohydrate (CHO) intake has primarily been studied in male athletes and recommendations have been extrapolated to female athletes. However, it may be necessary to account for sex-based differences in determining CHO intake guidelines based on sex and sport type. In a 2022 publication, researchers conducted an audit of 937 research studies evaluating acute CHO intake recommendations to assess if there was evidence to support the use of these strategies among female athletes [4]. Only 11% (n= 197 of 937 studies) of the participants in these studies were women, and of these studies, only 31% utilized suf cient methodologies to de ne menstrual status [4]. The persisting nutritional recommendations for female athletes emphasize the primary goal of obtaining suf cient energy availability (EA) and hydration. Once caloric needs have been met, the composition of EA can then be considered, including CHO, protein, and fat availability [5]. This lack of high-quality research using female-speci c study designs directly impacts our ability to provide high-quality, population-speci c nutritional recommendations for athletes. Given that the prevalence of low EA (LEA) is fairly common in female endurance ath- letes, a growing area of concern is that athletes are not meeting their CHO recommendations. These athletes may experience an additive effect of low carbohydrate availability (LCA) in addition to the health and performance consequences associated with LEA.Figure illustrates the proposed theoretical model of LCA, adapted from Mountjoy et al., 2014, to demonstrate the potential independent effect of LCA on health and performance conse- quences associated with LEA [6]. The purpose of this narrative review is to evaluate the current evidence that gives rise to the consideration of LCA and the associated health and performance consequences in female endurance athletes.Nutrients 2023, 15, x FOR PEER REVIEW 2 of 15 identified as female, and only 4–13% of the articles incorporated a

health and performance conse- quences associated with LEA [6]. The purpose of this narrative review is to evaluate the current evidence that gives rise to the consideration of LCA and the associated health and performance consequences in female endurance athletes.Nutrients 2023, 15, x FOR PEER REVIEW 2 of 15 identified as female, and only 4–13% of the articles incorporated a female-only design in their studies [2]. An updated review of six major journals, including a total of 5261 sports science articles published from 2014–2021 illustrated that 34% of all participants reviewed identified as women and 6% of articles included women-only study designs [3]. This highlights the need to balance sports science research on female and male athletes. Relatedly, the importance of acute carbohydrate (CHO) intake has primarily been studied in male athletes and recommendations have been extrapolated to female athletes. However, it may be necessary to account for sex-based differences in determining CHO intake guidelines based on sex and sport type. In a 2022 publication, researchers conducted an audit of 937 research studies evaluating acute CHO intake recommendations to assess if there was evidence to support the use of these strategies among female athletes [4]. Only 11% (n = 197 of 937 studies) of the participants in these studies were women, and of these studies, only 31% utilized sufficient methodologies to define menstrual status [4]. The persisting nutritional recommendations for female athletes emphasize the primary goal of obtaining sufficient energy availability (EA) and hydration. Once caloric needs have been met, the composition of EA can then be considered, including CHO, protein, and fat availability [5]. This lack of high-quality research using female-specific study designs directly impacts our ability to provide high- quality, population-specific nutritional recommendations for athletes. Given that the prevalence of low EA (LEA) is fairly common in female endurance athletes, a growing area of concern is that athletes are not meeting their CHO recommendations. These athletes may experience an additive effect of low carbohydrate availability (LCA) in addition to the health and performance consequences associated with LEA. Figure 1 illustrates the proposed theoretical model of LCA, adapted from

prevalence of low EA (LEA) is fairly common in female endurance athletes, a growing area of concern is that athletes are not meeting their CHO recommendations. These athletes may experience an additive effect of low carbohydrate availability (LCA) in addition to the health and performance consequences associated with LEA. Figure 1 illustrates the proposed theoretical model of LCA, adapted from Mountjoy et al., 2014, to demonstrate the potential independent effect of LCA on health and performance consequences associated with LEA [6]. The purpose of this narrative review is to evaluate the current evidence that gives rise to the consideration of LCA and the associated health and performance consequences in female endurance athletes. Figure 1. Proposed theoretical model; low carbohydrate availability (LCA) has independent health and performance consequences, illustrated in the Relative Energy Deficiency in Sport (RED-S) model due to low energy availability (LEA) (* Psychological consequences can either precede LCA or be the result of LCA) (adapted from Mountjoy et al., 2014 [6]). 2. Low Energy Availability The following equation is traditionally used in sports science literature to define energy availability (EA): EA (kcal/kg fat-free mass (FFM)/day) = energy intake (EI; kcal) − Figure 1. Proposed theoretical model; low carbohydrate availability (LCA) has independent health and performance consequences, illustrated in the Relative Energy De ciency in Sport (RED-S) model due to low energy availability (LEA) (* Psychological consequences can either precede LCA or be the result of LCA) (adapted from Mountjoy et al., 2014 [6]). 2. Low Energy Availability The following equation is traditionally used in sports science literature to de ne en- ergy availability (EA): EA (kcal/kg fat-free mass (FFM)/day) = energy intake (EI; kcal) exercise energy expenditure (EEE; kcal), normalized to fat-free mass (FFM; kg) [7]. It is often recognized that reduced or subclinical EA ranges from 30–45 kcal/kg FFM/day, which can serve as a tolerable range for athletes seeking to lose weight as part of a diet or exercise program that is short in duration [7]. Typically, LEA is de ned as less than 30 kcal/kg FFM/day and illustrates an unsafe energy balance for optimal body function

often recognized that reduced or subclinical EA ranges from 30–45 kcal/kg FFM/day, which can serve as a tolerable range for athletes seeking to lose weight as part of a diet or exercise program that is short in duration [7]. Typically, LEA is de ned as less than 30 kcal/kg FFM/day and illustrates an unsafe energy balance for optimal body function [7]. Physiological responses to the dose and duration of LEA may vary depending on sex, sport type, and genetics [7]. LEA is more common in female athletes compared to male ath-

Nutrients2023,15, 4457 3 of 14 letes [8]. LEA is associated with unfavorable health and sport performance consequences, as illustrated by LEA-related conditions such as Female Athlete Triad (Triad) and Relative Energy De ciency in Sport (RED-S) [8,9]. 2.1. Female Athlete Triad (Triad) The Female Athlete Triad (Triad) was proposed by the American College of Sports Medicine (ACSM) in a 1992 position statement and updated in 2014 [9,10]. The Triad demon- strates an interrelated condition of (1) LEA with or without eating disorder/disordered eating (ED/DE), (2) osteoporosis, or low bone mineral density (BMD), and (3) functional hypothalamic amenorrhea, or menstrual irregularities. Prospective studies to date illustrate a causal relationship between LEA and menstrual disturbances [9–14]. Additionally, cur- rent ndings show a strong correlation between Triad and negative bone health outcomes, such as increased bone stress injuries and decreased BMD [6,7,9,11,14–18]. 2.2. Relative Energy De ciency in Sport (RED-S) The International Olympic Committee (IOC) introduced the conceptual model of Relative Energy De ciency in Sport (RED-S) in a 2014 position statement [6]. This con- ceptual model, which includes consequences of LEA, was updated in 2018 [6,8]. The RED-S conceptual model outlines 10 health consequences (e.g., menstrual function, bone health, hematological, psychological, endocrine, immunological, metabolic, growth and development, cardiovascular, gastrointestinal) and 10 performance consequences (e.g., decreased endurance performance, increased injury risk, decreased training response, im- paired judgement, decreased coordination, decreased concentration, irritability, depression, decreased glycogen stores, decreased muscle strength) due to LEA [6,8]. RED-S can affect both female and male athletes but is more prevalent in female athletes and the current literature estimates show that 80% of elite and pre-elite female athletes have at least one symptom of RED-S, compared with 47.7% of male Olympic-level athletes [19,20]. Further- more, 37% of elite and pre-elite female athletes demonstrated two or more symptoms of RED-S, compared to 15.9% of male athletes [19,20]. 2.3. The Role of LCA in LEA When individuals do not consume enough CHO, compared to the requirements for their body composition and physical activity or exercise levels, it can lead to LCA. LCA is a separate concern, independent of LEA, though they

pre-elite female athletes demonstrated two or more symptoms of RED-S, compared to 15.9% of male athletes [19,20]. 2.3. The Role of LCA in LEA When individuals do not consume enough CHO, compared to the requirements for their body composition and physical activity or exercise levels, it can lead to LCA. LCA is a separate concern, independent of LEA, though they often occur simultaneously [7]. LCA is de ned as a diet with low CHO intake before, during, or after exercise, which reduces the amount of available CHO for the body to use due to low endogenous (glycogen stores) and/or low exogenous (CHO and glucose intake) levels of CHO [21,22]. Most intervention studies assessing LEA in athletes are also accompanied by a considerable reduction in CHO intake (approximately 25–60% reduction in CHO, depending on the magnitude of LEA) [16,23–25]. Therefore, many intervention studies on LEA result in concurrent LCA [20,26–28]. This is con rmed by several studies which demonstrate that LCA is common in female athletes who are also at risk for LEA [20,26–28]. CHO consumption before, during, and after exercise has been illustrated to bene t sport performance, especially in endurance activities which rely extensively on CHO as a fuel source and substrate for oxidative pathways [29]. CHO substrates provide a greater yield of ATP, or energy, per volume of oxygen delivered to the mitochondria compared to lipids and proteins [29]. Currently, there is no standardized equation or methodology to determine an individual's CHO availability status as a majority of studies have simply assessed CHO intake. This is a growing area of concern as it has been illustrated that LCA can lead to decreased glucose utilization, hypometabolism, muscular fatigue, impaired fat storage mobilization, decreased performance, and decreased growth hormone production [20,22]. As such, it is important to understand the role of CHO for female endurance athletes and elucidate the health and performance consequences of LCA.

Nutrients2023,15, 4457 4 of 14 3. CHO Background 3.1. Physiological and Biochemical Role of Carbohydrates Under normal conditions, endogenous CHO are stored primarily as glycogen in the skeletal muscle (~500 g) and the liver (~100) [30,31]. The main function of liver glycogen is to maintain blood glucose concentration since the glycogen stored in skeletal muscles cannot be released as glucose due to the lack of glucose 6-phosphatase [30]. Therefore, muscle glycogen is the local energy substrate for exercise, and exercise training can increase the glycogen storage capacity in skeletal muscles, albeit nitely [30,31]. Reduced glycogen content in the skeletal muscle can increase insulin sensitivity, which, in turn, emphasizes the importance of rapidly restoring glycogen content [30]. Muscle glycogen degradation in the myocytes provides the necessary energy for muscle contraction by activating glycogen phosphorylase and debranching enzymes for glycogenolysis [31]. Glycogen phosphorylase activation is dependent on a number of factors to closely regulate the energetic requirements of the working muscles [31]. The allosteric binding of adenosine monophosphate (AMP) and inosine monophosphate (IMP) activates glycogen phosphorylase, which allows the enzyme to be responsive to the energy state of the cell. Furthermore, muscular contractions also increase cytosolic calcium release and an adrenaline-mediated increase in cyclic AMP (cAMP), which activates phosphorylase kinase (PK) and subsequently glycogen phosphorylase [31]. Muscular contractions also activate AMP-activated protein kinase (AMPK) via an increase in cellular AMP/ATP ratios, which results in an inhibition of ATP-utilizing pathways and activation of ATP-generating pathways [31]. Therefore, AMPK promotes glucose uptake, glycolysis, and fatty acid oxidation while inhibiting glycogen and protein synthesis [31]. In mice models, a lack of AMPK notably impaired muscle contraction and reduced voluntary wheel running, which demonstrates the essential role of AMPK in endurance exercise [31]. An increase in calcium concentration in the cells, via phosphorylase kinase activation, stimulates glycogen degradation [31]. The rate of muscle glycogen degradation depends on exercise intensity and train- ing status; as exercise intensity increases, muscle glycogen is depleted at an increasing rate [30,32]. Exercise greater than 70% of VO2max utilizes muscle glycogen as the major carbohydrate source. Well-trained individuals improve their ability to

calcium concentration in the cells, via phosphorylase kinase activation, stimulates glycogen degradation [31]. The rate of muscle glycogen degradation depends on exercise intensity and train- ing status; as exercise intensity increases, muscle glycogen is depleted at an increasing rate [30,32]. Exercise greater than 70% of VO2max utilizes muscle glycogen as the major carbohydrate source. Well-trained individuals improve their ability to metabolize glucose and fat at a much higher capacity compared to untrained individuals [30]. High-intensity activity (e.g., repeated sprinting) can rapidly decrease glycogen stores contained within the active muscle cells, despite the total activity time being relatively brief [32]. However, endurance athletes often train for hours at a time, which causes a marked decline in muscle glycogen over time, especially at exercise intensities greater than 70% VO2max [30,32]. Over time, muscle glycogen degradation progressively decreases due to the nite storage of CHO within skeletal muscles [31]. Another critical factor for glycogen metabolism is CHO availability before exercise as increases in the rate of muscle glycogen degradation during exercise are exponentially related to muscle glycogen concentrations before exercise [31]. 3.2. Metabolism of CHO Exercise performance typically decreases due to an impaired availability and, hence, ability to use CHO as fuel, which is necessary for exercise at higher intensities and recom- mended for longer durations such as endurance training [33]. Few studies have attempted to de ne CHO availability; however, one study calculated CHO availability as the difference between dietary CHO intake and the amount of CHO oxidized during exercise beyond the requirements of normal activity [23]. Nonetheless, LCA ranges have not been established based on these criteria for CHO availability, and many studies continue to report CHO intake, using low CHO intake to de ne intake below current recommendations [34–43]. LEA, mediated by reduced CHO availability, affects leptin and triiodothyronine (T3) concentrations, which, in turn, can suppress the resting metabolic rate (RMR) [44]. In two conditions of identical LEA (with one achieved from dietary restriction and the other one achieved from exercise), CHO availability was 57% higher in women who reduced EA via

mediated by reduced CHO availability, affects leptin and triiodothyronine (T3) concentrations, which, in turn, can suppress the resting metabolic rate (RMR) [44]. In two conditions of identical LEA (with one achieved from dietary restriction and the other one achieved from exercise), CHO availability was 57% higher in women who reduced EA via

Nutrients2023,15, 4457 5 of 14 exercise compared to reduced EA via dietary restriction [45]. In this same study, leptin levels decreased by 50% in response to dietary restriction, and this difference is proportional to the reductions in CHO intake [45]. Restricted EA conditions, due to reduced energy intake, of 10 kcal/kg LBM/day, 20 kcal/kg LBM/day, 30 kcal/kg LBM/day, reduced CHO availability by approximately 80%, 60%, and 40%, respectively [23]. In this study, CHO availability was de ned as the difference between CHO intake and the amount of CHO oxidized during exercise beyond the requirements of normal activity [23]. These data demonstrate that there is a dose-dependent effect of LEA on CHO availability and that dietary restriction is detrimental to the use of CHO during activity [23]. Additionally, it has been demonstrated that skeletal muscle alters its fuel source in response to LEA conditions, oxidizing less CHO and more fat during exercise [45]. This may have signi cant effects on athletic performance due to CHO having a faster rate of energy production, compared to fatty acids, to fuel activity. Since glucose metabolized from muscle glycogen cannot enter systemic circulation, CHO availability in the muscles can be adequate while CHO availability systemically is low [5]. Future research should attempt to classify LCA due to low CHO intake based on exogenous (CHO and glucose intake) and endogenous (glycogen stores) availability [22,46]. LCA is common in female athletes, especially those at risk for LEA [20,26–28]. Despite no current recommendations for CHO intake before activity speci c to female athletes, it is suggested that consuming a high CHO snack (e.g., granola bar, dried fruit, pretzels, and/or fruit juice) 3–4 h before exercise can help mitigate the effects of reduced gluconeogenesis rates during the luteal phase [5]. 3.3. Current CHO Recommendations The current recommended amount of CHO for athletes is 55–75% of totalcalories [5,43] . However, more speci c recommendations vary from 5–12 g CHO/kg BW/day for athletes, depending on training activities [33]. Table intake provided by various organizations and governing bodies. It is important to empha- size the incorporation of small CHO-rich snacks or sports

This review evaluates the impact of low carbohydrate availability on health and performance in female endurance athletes.