How Does the Dietary Intake of Female Field-Based Team Sport Athletes Compare to Dietary Recommendations for Health and Performance? A Systematic Literature Review

Field-based team sports present large energetic demands given their intermittent high-intensity nature. Current evidence suggests that the dietary intake of female athletes may be insufficient to meet such demands, resulting in negative consequences for athletic performance and health. The primary aim of this review was to therefore assess the adequacy of dietary intake of female field-based team sport athletes when compared to dietary recommendations. A systematic search of databases, including PubMed, Web of Science, SPORTDiscus, and OpenGrey, was performed from the earliest record available until July 2020, obtaining an initial total of 2588 articles. To be included within the final review, articles were required to provide a quantitative assessment of baseline dietary intake specific to the target population. A total of 20 studies (n = 462) met the full eligibility criteria. A majority reported that the dietary intake of female field-based team sport athletes was insufficient in overall energy (2064 ± 309 kcal·day−1), carbohydrate (4.3 ± 1.2 g·kg·day−1), and iron intake (13.6 ± 6.2 mg·day−1) when compared to recommendations. Future research is required to establish why female team sport athletes consistently demonstrate deficient dietary practices, and to explore the potential negative consequences of this.


Introduction
Field-based team sports are characterized by intermittent high-intensity activity, followed by periods of low-to-moderate active recovery or passive rest [1][2][3][4][5]. The exact characteristics of each sport vary from those that are more strength and power-focused (American football and rugby) to those with greater emphasis on endurance (soccer, field hockey, lacrosse, Australian rules, and Gaelic football) [4,5]. The gameplay of all field-based team sports places demands on both aerobic and anaerobic systems of energy production, leading to the depletion of muscle glycogen stores [6]. Therefore, nutritional strategies to ensure glycogen stores are sufficient to meet the energy costs of training and competition are highly recommended to delay the onset of fatigue, optimize performance during gameplay, and support physiological adaptation and recovery [6][7][8][9].
Despite the broad publication of dietary recommendations for athletes [8][9][10], female field-based team sport athletes have repeatedly been shown to consume diets that are insufficient in energy intake during competition [11,12] and training periods [13,14], largely explained by a failure to meet recommendations for carbohydrate intake [11][12][13][14][15][16]. Such inadequate dietary intake in female athletes has not only been associated with decreased athletic performance, but also a multitude of acute and chronic health issues ranging from menstrual dysfunction [17][18][19] and suboptimal bone health [17][18][19] to increased risk of injury and illness [19]. The concept of low energy availability (LEA), whereby an individual has inadequate energy intake relative to their energy expenditure (EE) [20,21], has recently been identified to occur frequently, not just in weight-sensitive and endurance-based athletes, but also those that compete in team sports [22,23].
It has been suggested that measuring adherence to macronutrient recommendations [9,24] may support the monitoring of energy status and identification of LEA risk among female athletes [25]. Given the prevalence of micronutrient deficiencies with LEA [22], and the existing risk of deficiency among female athletes [26,27], it may be pertinent for micronutrient intake to also be explored in this context. From a total of 64 studies investigating the dietary intake of team sport athletes captured in previous narrative [5] and systematic reviews [28], only 29.7% (n = 19) included female athletes and 14.1% (n = 9) took part in field-based team sports. An increasing number of dietary intake observations in female field-based team sport athletes now exist [12,[14][15][16][29][30][31][32], which have not yet been subject to review. Given the distinct physiological requirements of field-based team sports [1][2][3][4][5], and the increased requirement to address issues of dietary inadequacy and LEA within female athletes [17][18][19]21,27], a systematic review focusing specifically on this population is warranted.
Therefore, the primary aim of this review was to assess the adequacy of dietary intake in female field-based team sport athletes when compared to dietary recommendations for maintenance of general health [33,34] and optimal sporting performance [8][9][10].

Protocol Registration
All methods of the review were conducted in accordance with the preferred reporting items for systematic reviews and meta-analyses (PRISMA) guidelines [35]. The review protocol was registered with the international prospective register of systematic reviews (PROSPERO) before the formal screening of search results against eligibility criteria (registration number: CRD42020197673) [36]. The PICOS (participants, intervention, comparison, outcome, study design) criteria applied to the review are outlined in Table 1. Table 1. Participants, intervention, comparison, outcome, and study (PICOS) criteria.

Parameter Description
Population Female field-based team sport athletes Intervention/exposure Baseline/habitual dietary intake Comparison Dietary intake in comparison to consensus recommendations Outcomes Meeting/not meeting recommendations Study design Cross-sectional, longitudinal, and randomized controlled trials

Search Strategy
A comprehensive search strategy designed to identify both commercially published articles and sources of grey literature was developed with assistance from an experienced health science librarian. A systematic search of databases, including PubMed (MEDLINE), Web of Science (core collection including conference proceedings), SPORTDiscus, and OpenGrey (grey literature database), was performed by one author (MR) from the earliest record available until July 2020. Examples of search terms used included: "dietary intake", "dietary assessment", "field-based sport", and "team-sport athlete". The full list of search terms used is detailed in Table 2.

Concept Keywords
Dietary Intake "nutrient requirement*" OR "dietary intake*" OR "daily food*" OR "food intake*" OR "dietary assessment" OR "dietary requirement*" OR "sports nutrition" OR "sport nutrition" OR "food diary" OR "food frequency" OR "macronutrient" OR "nutrient needs" OR "dietary needs" OR "nutrient intake" OR "RDA" Field-Based Team Sport Rugby OR Soccer OR Football OR "Australian Rules" OR Hurling OR Camogie OR Hockey OR "Field Hockey" OR Lacrosse OR "Gaelic*" OR "GAA" OR "field sport*" OR "field-sport*" OR "field-based sport*" OR "field based sport*" OR "field-sport athlete*" OR "field sport athlete*" OR "team-sport" OR "team sport*" OR "team-based sport*" OR "team based sport*" OR "team-sport players" OR "team sport players" OR "team-sport athlete*" OR "team sport athlete*" OR "invasion-team sport*" OR "invasion team sport*" OR "invasion sport*"

Eligibility Criteria
All original research, including observational, cross-sectional, and randomized control trials, which provided a measurement of total energy (MJ/day, kcal/day) and macronutrient intake (carbohydrate, fat, and protein) were considered for inclusion. Studies that did not report a baseline assessment of dietary intake and studies that only provided a qualitative assessment of dietary intake were excluded. Only articles published in English were included; unpublished theses, conference posters, and abstracts were considered if all other inclusion criteria were met. Each study included was required to have reported data specifically regarding female athletes that competed in field-based invasion team sports. Field-based invasion team sports were defined as games played on a field/pitch by two opposing teams, with the primary objective of invading the oppositions territory to score points [37] (e.g., field hockey, soccer, lacrosse, rugby). Data reported in an aggregated format, whereby the specific target population was indistinguishable from others, were excluded. All age divisions (i.e., youth and adult) and competitive levels (i.e., amateur and professional) that met the above criteria were accepted. A full list of the inclusion and exclusion criteria that were applied is detailed within Table 3.

Study Selection Process
Studies were initially screened based on title and abstract content by two authors (MR and COC). Duplicates and studies clearly unrelated to the review topic were removed at this stage. All articles that progressed from the title and abstract review were retrieved for full-text review. Each full text was screened against the inclusion and exclusion criteria outlined in Table 3 by two authors (MR and COC). Any disagreement surrounding article eligibility between the two independent reviewers at this stage was reviewed by additional authors (DK and NNC). Finally, the reference list of all articles that passed full-text review and were deemed eligible for data extraction were checked by two authors (MR and COC) and manual searches were conducted using Google Scholar to ensure no relevant articles were overlooked. The selection and exclusion of studies for each stage are reported in Figure 1.

Quality Assessment
The quality of each full-text article that was deemed eligible for data extraction was assessed independently by two authors (MR and COC). Any disagreement surrounding the quality rating allocated to each study at this stage was reviewed by additional authors (DK and NNC). Studies were reviewed using the Academy of Nutrition and Dietetics quality criteria checklist [44], which permits the assessment of relevance and validity, with the allocation of either a positive, neutral, or negative quality ranking. Questions that referred to study group comparisons, methods for handling withdrawals, and the use of blinding and intervention descriptions were not applied as all studies included were either observational or cross-sectional in design. Therefore, the key criteria to achieve a positive rating were a thorough outline of study procedures, evidence of an inclusion/exclusion criteria applied, use of methods to reduce selection bias, and reporting of potential confounding factors on the outcome being measured (dietary intake).

Quality Assessment
One study received a positive quality rating [32] whilst all remaining studies received a neutral rating [11][12][13][14][15][16]25,[29][30][31][38][39][40][41][42][43][45][46][47]. No studies received a negative rating, therefore quality assessment did not eliminate any articles obtained. Studies that only received neutral ratings were mostly affected by failure to specify an inclusion/exclusion criterion for the study population, and by extension, failure to provide sufficient information to prove the study participants were a representative sample of the target population. In addition, few studies accounted for other factors that could affect the outcome (dietary intake), for example, risk/prevalence of disordered eating within the population and the potential impact this could have on the values reported.
Only two studies [16,29] provided comparisons of macronutrient intake between training, recovery, and match days. One investigation compared the macronutrient intake of Australian rules football players (n = 23) [16] between their main and light training days and a recovery day and identified no significant differences in intake across all macronutrients. The other compared macronutrient intakes of professional soccer players (n = 13) [29] between heavy and light training days, a recovery day, and a match day, only identifying a significant difference in relative fat intake, which was higher on the heavy training day in comparison to the light training day.

Discussion
This review aimed to assess the adequacy of dietary intake in female field-based team sport athletes when compared to dietary recommendations for maintenance of general health [33,34] and optimal sporting performance [8][9][10].
Despite this, two studies [25,29] identified dietary intake in female soccer players that fell below the low energy availability cut-off of <30 kcal·kg FFM −1 ·day −1 [18,19,53] and from the two that provided comparisons of energy intake between training, competition, and recovery days [16,29], and no differences were observed despite increased energy demands. This provides more objective evidence to highlight that energy intake may be insufficient to meet needs and female field-based athletes may be at risk of the negative consequences associated with energy deficiency. Chronic energy deficiency can lead to insufficient glycogen stores and loss of fat-free mass, which can compromise performance through premature reductions in physical capacity and decreases in muscular strength and power [56,57]. From a health perspective, a long-term negative energy balance is likely to result in adaptations to reduce energy expenditure, prevent weight loss, and promote survival [55]. Unfortunately, such adaptations commonly result in impaired menstrual function and suboptimal bone health in female athletes and an increased tendency for injury and illness [17][18][19]21,27]. Increases in injury and illness rates can in turn have a further negative impact on sporting performance and training adaptation due to the prevention of consistent and high-quality training [58,59]. Risk of LEA has previously been associated with missing >22 days of training during the previous year due to illness and stress fractures [23].
The reason behind the observed lower energy intake in female field-based team sport athletes relative to their high energy demands remains unclear. However, it is likely to be multifaceted in nature. It has been suggested that athletes may inadvertently experience periods of low energy availability and compromised energy intake when the intensity and volume of training are high [55,60]. Under such conditions, athletes may simply be unaware of the energy cost of exercise [55], have a reduced number of eating occasions due to a demanding training schedule [61], and/or suffer from appetite suppression in response to high-intensity exercise [62,63]. Poor nutrition knowledge has previously been observed in female field-based team sport athletes [64,65] and this may also contribute to the lower energy intakes observed.
Given the intermittent high-intensity nature of field-based team sports, whereby players cover large total distances per game (6-10 km) [66][67][68], resulting in significant glycogen depletion [5], the maintenance of maximal performance in players is highly reliant on carbohydrate intake as a fuel source [4,69,70]. Across the course of a single soccer match glycogen depletion has been shown to occur and results in a reduction in distance covered, and speed of running during the second half of a match [71,72]. By extension, this could also impair players' ability to perform repeated sprints, which is a performancedetermining factor in intermittent team sports [73,74]. Soccer players that consumed a high-carbohydrate diet pre-match (65% TDEI) performed 30% more high-intensity runs than those who consumed a low-carbohydrate diet (30% TDEI) [75].
Deficiencies in carbohydrate intake and subsequent glycogen availability are thought to accentuate the stresses and negative consequences of energy deficiency and low energy availability [76]. Chronic low carbohydrate intake is related to a reduced capacity to use carbohydrate as a fuel, increased muscle breakdown, and impaired immune function [77]. Deficits as small as 10% in glycogen replenishment may lead to decreases in performance during subsequent training and/or competition and should be avoided [78]. Preservation of glycogen status has shown to diminish the hormonal impairments observed during periods of LEA [79], highlighting the importance of optimal carbohydrate intake in addition to meeting overall energy needs. Although this review highlights the poor carbohydrate intake in female field-based team sport athletes, the reason for this requires further investigation. A recent meta-analysis on male soccer players revealed a significant decrease in carbohydrate intake between the periods of 2000-2009 and 2010-2019 [80]. This may be a result of warnings about excessive carbohydrate intake leading to unwanted weight gain in the general population, and the more recent media-driven promotion of carbohydraterestricted higher protein diets [76,80]. Interestingly, 47% of athletic and recreationally active females have reported previous adherence to a carbohydrate-restricted diet [23] and may explain the low carbohydrate intake observed in female field-based team sport athletes.

Fat Intake of Field-Based Team Sport Athletes
Average intake of fat ranged from 21-37.5% of TDEI [11][12][13][14][15][16]25,[29][30][31][32][38][39][40][41][42][43][45][46][47], with a minority of studies [12,30,31,40,43] reporting intakes that exceeded the recommended range of 20-35% [9]. In such scenarios, excessive fat consumption may compromise the overall consumption of carbohydrate (similarly to protein intake) as previously displayed within male soccer players [83]. From the limited studies that compared fat intake between training, recovery, and competition days [16,29], an increase in relative fat intake on a heavy training day in comparison to a light training day was observed with no significant changes in carbohydrate intake [29]. Dietary patterns that are high in fat but low in carbohydrate have been repeatedly shown to compromise high-intensity exercise performance [85] and would likely have a particularly detrimental impact on the performance of a field-based team sport athlete given their heavy reliance on carbohydrate as a fuel source [4,6].

Micronutrient Intake of Field-Based Team Sport Athletes
A majority of studies [11,13,15,16,42,[45][46][47] reported intakes of calcium that meet recommendations of 700 mg·day −1 for females aged 19-50 years [33], whereas few [13,42,45,46] recorded intakes of iron that meet recommendations of 14.8 mg·day −1 for females aged 11-50 years [33]. The risk of iron deficiency in female athletes has been highlighted previously [26,27], and a combination of dietary inadequacy, declines in nutritional status due to heavy physical activity, and blood losses during menstruation periods are thought to contribute towards this [86,87]. The prevalence of iron deficiency among women competing in a variety of sports has previously been reported in the ranges of 25-35% [88], and 59% of the Swedish female national soccer team were found to be iron deficient before the 2003 FIFA women's world cup [89]. If such iron deficiencies lead to iron deficiency anemia and the related decreases in circulating hemoglobin concentration, this would likely compromise exercise performance through a decrease in aerobic capacity [90]. Chronic iron deficiency anemia may also comprise an athlete's general health due to fatigue, cognitive impairment, and suppressed immune system function [91]. Studies reported dietary intake of vitamin D that met recommendations of 10 µg·day −1 (400 IU·day −1 ) [34]. Despite most meeting the recommendation, 33-42% still appear to be deficient, highlighting dietary intake of vitamin D as a crude measure of vitamin D status [92]. Nonetheless, the low dietary intake of vitamin D, as observed in female field-based team sport athletes, may still be indicative of a requirement for vitamin D status to be assessed given the negative impact deficiency can have on bone density and stress fracture risk [26]. Micronutrient deficiencies have been reported to be accentuated by excessive exercise, restricted eating practices, and conditions of low energy availability [22,93,94], which, in conjunction with the observations presented, further highlights the requirement for micronutrient status to be regularly monitored among female field-based team sport athletes.

Sub-Group Comparisons
National/professional-level female field-based team sport athletes displayed similar dietary intake in comparison to their varsity/collegiate-level counterparts and therefore little difference was observed based on performance level. The average calorie intake of youth athletes (2304 ± 470 kcal·day −1 ) was higher than the average for adults (2018 ± 257 kcal·day −1 ), which could lead to greater energy deficits in adult athletes given their proportionally greater size and subsequent higher energy expenditure. The discrepancy in calorie intake between groups seemed to be primarily the result of greater carbohydrate intake in youth athletes (5.4 ± 1.7 g·kg·day −1 ) when compared to adults (4.1 ± 1.0 g·kg·day −1 ). This has been previously observed in male soccer players, whereby youth/junior players have displayed a greater % TDEI from carbohydrate [95] and higher carbohydrate intake overall [80], in comparison to adult/senior players. The reason for this remains unclear; however, the mean age of youth athletes included in this review (15.6 ± 2.0 years) may bring into question whether or not higher carbohydrate intakes were fulfilled on a self-determined and autonomous basis or instead were simply a consequence of support teams'/parental control over dietary intake. It is also important to acknowledge that the dietary intake data of adolescents can be particularly prone to misreporting [96]. Findings in reference to dietary intake observed across multiple time-points were equivocal, with three studies [31,32,39] reporting a lack of significant differences between phases in hockey and lacrosse players, while two studies [25,45] reported greater energy, carbohydrate, and protein intake during pre-season in comparison to in-season [25] and post-season [25,45] in soccer players. Unfortunately, all of the longitudinal investigations included [25,31,32,39,45] were limited to varsity/collegiate-level athletes and between-day observations for each phase were not reported. It would therefore be ambiguous to assume that the higher dietary intakes observed during pre-season periods [25,45] were meaningful attempts at nutritional periodization.

Limitations
The heterogeneity of studies included, and the limited reporting of health and performance outcomes prevented a meta-analysis from being performed and limited this review to a narrative synthesis. A large majority of field-based team sport athletes included within this review were female soccer players and findings may be less relevant to field-based sports where only small sample sizes were captured. Many of the studies included relied on prospective food records, which are known to influence usual intake, and are prone to both under and over-reporting [61,97]. A previous meta-analysis reported a mean bias of 19% underreporting (600 kcal·day −1 ) when comparing self-reported methods to doubly labelled water techniques [54]. Many of the studies included within this review highlighted the potential for underreporting within their data and utilized a broad range of methods to measure both energy intake and energy expenditure, limiting the extent of comparison that can be made. This, however, speaks to a larger issue that standardized techniques to measure energy intake, energy expenditure, and energy availability in free-living athletes are yet to be determined, and all are subject to a degree of error [55]. However, underreporting and error alone cannot refute the substantial body of evidence presented by this review that indicates that female field-based team sport athletes' diets are deficient in both overall energy and carbohydrate intake. Future research would benefit from adopting standardized techniques to measure dietary intake with longitudinal observations of professional/elite female field-based team sport athletes and reporting of between-day differences of both energy intake and expenditure. Future investigations should also aim to capture factors that might influence dietary intake, such as nutrition knowledge [98,99], and assess both the risk factors and negative health outcomes of low energy availability that may also be present, using validated tools for both [100][101][102].

Conclusions
This review identified that in comparison to dietary recommendations for health and sports performance, female field-based team sport athletes present diets that are insufficient in overall energy, carbohydrate, and iron intake. When interpreted in the context of the high energetic demands of field-based sports, female athletes may be viewed as a particularly high-risk group for low energy availability and its associated health implications. Future research is required to establish the reason behind current dietary practices observed and to explore the potential negative consequences athletes might experience as a result. Based on this review's findings, interventions to promote greater adherence to dietary recommendations in female field-based team sport athletes are recommended to prevent the potential negative health consequences and performance impairments related to inadequate dietary intake. These interventions should measure the influence of improvements in dietary intake on a broad array of health and performance outcomes as this review highlights that such information is currently lacking.

Funding:
The principal investigator is a recipient of a president's doctoral scholarship which includes a tuition fee waiver and a monthly stipend from their academic institution. This research received no other funding.
Institutional Review Board Statement: Not Applicable.