Soccer is a high-intensity intermittent sport that is played over a 90-minute period, which is split into 2 45-minute halves. During a match, professional youth soccer players may cover distances that are comparable to their adult counterparts (i.e., 9 km ). The metabolic demands of soccer are further complicated by the unpredictable pattern of activities that match play requires, including sprinting, walking, backward jogging, turning, and the inclusion of technical actions. In addition to the reported benefits that optimized dietary intake has in soccer players (2,16,31), youths have additional energy demands attributable to growth and development (14,38); therefore, it stands to reason that if the performance and the physical development of the youth soccer player are to be optimal, youth players should receive similar, or even more, nutritional support than their older counterparts.
At present, there are no data that exist concerning the energy cost of a training week in young soccer players that compete within the United Kingdom. This is somewhat surprising as the inclusion of parallel activity data will help to contextualize the nutritional information collected (27). Nevertheless, as similarities have been found to exist in movement demands between youth and senior teams during soccer match play (18), then previously reported energy expenditures of 6 MJ per game in senior sides (36) are likely to reflect the energy cost of soccer match play in youth players. Moreover, it is not uncommon for professional youth players to be involved in a number of soccer teams at both country and club levels (i.e., youth, reserves, A team), and consequently, they might play multiple matches within a 7-day period (with the likelihood of additional technical training also). Because diets lacking carbohydrate content have been found to impair glycogen restoration after training and match play (2) and considering the negative effects of such diets on postexercise immune function (31), it is crucial that the nutritional practices of youth soccer players are optimized in a manner that enables growth and development, preparedness for subsequent exercise, and the prevention of illness.
Studies that have incorporated players from senior soccer teams have routinely indicated less than optimal nutritional regimes; particularly with respect to total energy intake (28) and the energy distribution of macronutrients (27,28). For example, it is recommended that soccer players consume a diet that consists of about 60-70% of total energy intake from carbohydrates (28,29); however, the analysis of diets from senior squads during the competitive period seldom reveal carbohydrate contributions to the total energy yield that exceed 55% (range: 43-54% [7,27,28]). This is a trend that also appears to be present in the few studies that have analyzed the diets of younger soccer players competing outside of the United Kingdom (6,19,21,32), with these studies reporting greater than recommended energy contributions from fats rather than from carbohydrate sources (6). Given the variation that has been found to exist in the dietary practices of different countries (35), it remains to be elucidated whether such trends are present within soccer players competing within the United Kingdom because previous authors have only examined the diets of youth players from outside the United Kingdom (4-6,21,32,34).
Therefore, the aim of this study was to investigate the dietary and activity regimes of professional soccer players who played for the youth department team (Under 18s) of a professional club based within the United Kingdom during a 7-day period within the first half of their 2009/2010 competitive season. Based upon the findings of previous authors, it is hypothesized that the dietary habits of the sampled soccer players will be inadequate to meet recommended values and will also fail to meet the demands imposed by their involvement in soccer training and match play.
Experimental Approach to the Problem
This study investigated the dietary habits and activity profiles of young professional UK-based soccer players in one week during the competitive season. From these data comparisons were made to published and recommended values and also between energy intake and energy expenditure.
Ten male players (age: 17 ± 1 years, height: 1.72 ± 0.01 m, mass: 67.5 ± 1.8 kg, estimated maximal aerobic capacity: 57.8 ± 0.9 ml·kg−1·min−1) who played in outfield positions for the youth department of a Championship soccer team, which reflects the second tier of professional soccer within the United Kingdom, participated in this study. Before any form of participation, appropriate written informed consent was obtained for all players after approval was granted from a University ethics committee. Participants were recruited on the basis that they had regularly been involved in the training and competitive schedule of the youth department team for at least 12 months before the start of the study.
Dietary Assessment and Analysis
Dietary intake was assessed throughout the month of October in the first half of the 2009/2010 competitive season using self-reported diet sheets that were similar in terms of required information and measurement units to those previously published (6,27). A series of lectures were given to the players and the club's coaching staff by the lead investigator (who was also responsible for the athletic development of the individuals involved; Certified Strength and Conditioning Specialist: National Strength and Conditioning Association) regarding the specific nature of the information required. Iglesias-Gutiérrez et al. (21) proposed that the nutritional practices typically observed within the family environment are disturbed in dietary studies that are undertaken in training centers (e.g., [24,32]); consequently, the data presented in this study were determined using previously published methods that incorporate the use of universal household measures (e.g., cup, teaspoon, etc. ) in foods prepared within the home environment. Information concerning: time of day, food description (including cooking methods and brand names), amount prepared, amount remaining, volume and type of fluid consumed were recorded. Additionally, players were required to record the use of any supplements that were taken. A 7-day duration of data collection was used because this represents the time frame associated with valid nutritional information (3) and to ensure that data collected were of high ecological validity, no attempts were made to influence the player's diets.
Upon return of the diet sheets, food records were carefully reviewed, and each player attended an interview with the authors to clarify any ambiguous information. Commercially available software was used to analyze the diet records (CompEat version 5.8.0; Nutrition Systems, United Kingdom) and as per the recommendations of Deakin (11), a single researcher performed all dietary analyses so that potential variation in the interpretation of dietary records was minimized.
To estimate average daily energy expenditure, players were also asked to record their activity patterns throughout the same 7-day period analyzed for dietary intake. According to Leenders et al. (25), daily energy expenditure can be considered the sum of basal metabolic rate (BMR), energy expenditure during physical activity, and the thermic effect of food (TEF). The BMR was calculated according to the Harris-Benedict equation (equation 1 ), which has previously been validated (33). Energy expenditure on rest days (EER) was calculated according to equation 2; a value of 1.58 was assigned to physical activity level as per previous research that has used a similar standard and age of adolescent soccer player (6). The 7-day period examined in this study was incorporated within the maintenance phase of the player's competitive season; requiring in addition to the 1 match played, participation in approximately 9 hours of training per week (consisting of 4 predominantly tactical-based sessions that were 2 hours in duration, with the remaining time dedicated to resistance training). To minimize the effects of interindividual variation over the analyzed period, each player participated in all of the training sessions and 1 90-minute match against opponents similar in league position. Players were encouraged to rest upon cessation of training and competition.
On nonrest days energy expenditure was corrected for the specific durations of training and match play that were performed (equations 3 and 4; modified from ) as per the Compendium of physical activities (1). The TEF was calculated as the sum of the individual thermic effects that each macronutrient contributed to the individualized average energy intake using the known values of 7, 2.5, and 27.5% for carbohydrates, fats, and proteins, respectively (23) (equation 5).
Energy expenditure on training days (EET) is given by
Energy expenditure on match days (EEM) is given by
TEF is given by
where n is the exercise duration in hours, %CHO is the average daily energy intake from carbohydrates (%), %PRO is the average daily energy intake from protein (%), %FAT is the average daily energy intake from fat (%), and TOTAL is the average daily energy intake (kcal).
Statistical analysis was carried out using SPSS software (Version 16.0; SPSS Inc., USA). All data are presented as mean ± SEM and the level of statistical significance was set at p ≤ 0.05. The reliability of dietary intake data was assessed using the percentage of relative standard error (SEM ÷ Mean) with values of <20% being considered reliable. A 7-day average for total energy intake (kcal), total energy expenditure (kcal), energy deficits (energy intake - energy expenditure), macronutrients (% total energy intake, g, g·kg−1), micronutrients (% reference nutrient intakes, mg, μg), the ratio of energy intake to expenditure, and fluid intake (L) were determined for each participant. A paired sample t-test was used to assess energy balance (energy intake vs. energy expenditure), whereas a 1-way repeat measures analysis of variance was used where data contained >2 time points. Mauchly's test was consulted and Greenhouse-Geisser correction was applied if the assumption of sphericity was violated. Significant main effects were further investigated using multiple pairwise comparisons with Bonferroni confidence interval adjustment. One-sample t-tests were used to identify whether the recommended values of Recommended Nutrient Intakes (9) had been achieved. Retrospective power analyses were performed using commercially available software (G*Power version 3.0.10), and the sample size used was sufficient for 80% statistical power to be achieved for the identification of differences between energy intake and expenditure.
Estimates of nutritional intake are considered reliable because relative standard error values did not exceed 10 and 20% for any of the analyzed macronutrients and micronutrients, respectively. Mean daily energy intake was 2,831 ± 164 kcal (11.9 ± 0.7 MJ), which represented 42.3 ± 2.9 kcal·kg−1 body mass (BM). The daily average macronutrient intakes are summarized in Table 1.
Intakes of specific micronutrients expressed relative to the individualized activity corrected RNI values are presented in Figure 1. Fiber was the only micronutrient that was significantly lacking in the analyzed diets (67% of RNI, p < 0.001, effect size [ES] = 0.992, observed power > 0.8) because all other micronutrients met or exceeded the recommended values. Mean daily fluid intake was 3.2 ± 0.3 L.
The mean daily energy deficit of 788 ± 174 kcal was calculated (Intake, Expenditure: 2,831 ± 164 kcal, 3,618 ± 61 kcal, p = 0.001, ES = 0.833, observed power > 0.8) from the difference between daily energy intakes and the summation of daily BMR data (1.760 ± 28 kcal), energy consumption from the TEF (258 ± 14 kcal), and average energy expenditures associated with activities during the training week that included participation in 1 match, 4 days of training (including 3 light days and one heavy day) and 1 rest day. Figure 2 illustrates the daily energy intake and expenditure data for each individual and Figure 3 illustrates that mean energy expenditure varied with exercise intensity in activities performed during the training week (main effect of activity: F(2,18) = 1181.716, p < 0.001, ES = 0.992, observed power > 0.8). The ratio of mean energy intake to mean energy expenditure was 78 ± 5%.
The primary finding of this study was that the dietary practices of professional youth soccer players who competed on behalf of a UK-based Championship club were inadequate to meet the demands imposed by their training and competition. Specifically, a deficit between mean daily energy intake and energy expenditure was observed, and the contribution of energy from carbohydrate sources was depressed as a consequence of an elevated consumption of fats. Moreover, despite the consumption of the majority of micronutrients meeting the recommended values, the intake of fiber did not.
The aim of this study was to investigate the dietary and activity practices of youth soccer players who competed on behalf of a professional club within the United Kingdom. Previous authors that have analyzed the diets of players from around the World have consistently reported that energy intake is less than optimal (4-6,19,24,34); for example, energy deficits in excess of 800 kcal·d−1 have been observed in professional youth players competing in Italy (6). Considering that a mean daily energy deficit of 788 ± 174 kcal·d−1 was calculated in the present study (Figure 2), it appears that youth soccer players from the United Kingdom also have suboptimal nutritional practices in regards to energy intake that are comparable in magnitude to previous literature. Using previously published equations (29), the energy deficit reported in this study would contribute to a mean weekly weight loss of 0.10 ± 0.02 kg per player, which over time may have considerable impact upon the growth and development, performance, and recovery of the individual. Unfortunately, changes in player mass in the period either preceding or after this study were unavailable to substantiate this assumption. Nevertheless, this is the first study to document the dietary profiles of professional male youth soccer players competing within the United Kingdom using subject numbers similar in magnitude to studies which have previously been published (7,32); however, some caution should be exercised when interpreting the outcomes because of the small number of subjects used.
The contribution of carbohydrate metabolism to soccer match play has resulted in diets that are high in carbohydrate intake (i.e., 60-70%) being recommended for soccer players (28,29); however, in reality published research of soccer player's diets seldom reveal carbohydrate contributions to energy intake that equal this amount (6,7,19,21,27,28,32); with values as low as 43% having been observed (37). In this study, carbohydrate intake represented 56 ± 1% of total energy intake; consequently, based upon previous research that has examined the role of carbohydrate consumption in the restoration of muscle glycogen stores (8) it can be postulated that prolonged periods of insufficient energy being provided from carbohydrates, combined with the demands of training and match play that youth players routinely face, will undoubtedly impair physiological adaptation and performance during subsequent training or competition because of suboptimal recovery. An increased consumption of carbohydrate-containing beverages, breads, cereals, fruits, dairy products, and starchy vegetables will increase the carbohydrate contribution to the total energy yield and may also, in the case of the latter items in this list, have a concomitant effect upon the alleviating the low levels of dietary fiber observed in this study (i.e., 67% of RNI).
Fat is a necessary component of any balanced diet because of its involvement in biological processes and availability as an energy source. In the present study the total energy intake from fat represented 31 ± 1%, which marginally exceeds the upper recommended limit of 30% of total energy intake (8) but falls within the range of values previously observed in professional soccer players from countries outside the United Kingdom (4,22,37). Similarly, cholesterol intake also exceeded the recommendation of no more than 300 mg·d−1 (14), being 346 ± 45 mg·d−1. Elevated proportions of energy derived from fat sources compromises the total energy yield available from other macronutrients such as carbohydrates. However, given the caloric deficit observed in this study, it would be unwise to solely recommend a reduction in total fat intake; alternatively, a player's primary focus should be to increase the consumption of carbohydrates (and protein), as this would have a concomitant effect of decreasing the proportion of energy derived from total fat intake, which only marginally exceeds recommended values.
Adult athletes are encouraged to consume greater amounts of protein relative to their sedentary counterparts (1.2-1.7 vs. 0.8 g·kg−1·d−1 ) because of the role that this macronutrient has in the maintenance and gain of lean muscle mass after muscle-damaging exercise (39). It appears that the individuals analyzed in this study were meeting the recommendations for adult soccer players concerning protein intake because 1.7 ± 0.1 g·kg−1·d−1 were reported to be consumed which represented 16 ± 1% of mean daily energy intake and the proportions previously reported in soccer players (4,6). Although specific recommendations for the protein intake required by youth soccer players is currently lacking, it has been recommended that at least 1.6 g·kg−1·d−1 of protein is consumed to elicit a positive nitrogen balance in adolescent soccer players (4). Consequently, the youth soccer players involved in this study should seek to maintain the consumption of recommended proportions of protein when seeking to optimize the restorative effects of this macronutrient after exercise.
Relatively few studies have addressed the micronutrient content of diets of youth soccer players (4,21), and we believe that we are the first to do so regarding the vitamin and mineral content of the diets of professional male youth players that compete within the United Kingdom. In this study, all vitamins and minerals but not fiber (Figure 1) met or exceeded activity corrected RNI values from dietary intake alone, as surprisingly, only 1 player reported the use of any supplement-type product (i.e., Echinacea supplement which did not contain any other additional vitamins or minerals). We observed a mean fiber intake of 16 ± 1 g·d−1, which is representative of previous authors examining the diets of French adolescent soccer players (4). Because of the benefits that diets high in fiber can have upon health (20), it is recommended that players should seek to increase their consumption of whole grain foods, nuts and seeds, fruits, and vegetables (13).
Soccer players are advised to consume 2.5 L of fluid throughout the day in addition to the 0.15-0.25 L that is recommended every 15 minutes during intermittent exercise (10). Such practices aim to prevent or delay the reduction in bodyweight that exceeds the critical 2% threshold, which has been found to impair both physical and skilled performances in soccer players (30). In this study, professional youth soccer players consumed 3.2 ± 0.3 L·d−1. Considering that exercise accounted for approximately 2 hours per training day, which based on current recommendations results in an overall contribution of 1.2-2.0 L of fluid being consumed during exercise, this implies that habitual fluid intake was inadequate in this group of soccer players. Consequently, increased consumption of carbohydrate-containing sports beverages may be of benefit to this group of players because consuming such drinks will increase the volume of fluid intake while also increasing the contribution of carbohydrates to the total energy yield.
The validity of dietary recall studies have previously been questioned on the grounds that the duration of such studies may not reflect long-term nutritional practices (27). That said, it has previously been reported that a 7-day data collection period improves the reliability and validity of data collected when compared to a single days intake (3,26). In addition to the 7-day data collection period used in this study, players and management staff attended a series of lectures that took place before data collection, where an in-depth description of the required dietary information was given along with actual examples of how the ambiguity of such information could affect the outcome of the analysis procedures. Moreover, upon collection of the dietary and activity records, attempts were made to clarify any ambiguous information by means of interviews with the individuals involved. Consequently, the reliability of the estimates for analyzed macro and micronutrients (assessed using relative standard error) were considered statistically acceptable. Furthermore, in comparison to those studies that have examined the eating habits of soccer players during periods of intense training and/or in unfamiliar environments (e.g., [24,32]), the data presented in this study demonstrates high ecological validity as no attempts were made to influence the dietary and activity practices of the players involved.
Considering that total energy expenditure must equate to total energy intake if energy balance is desired, then it is important that data are collected concerning both the activity and dietary habits of those individuals being examined. In the present study, we aimed to increase the applicability of the nutrition data by estimating energy expenditure according to the type and duration of the exercise performed by modifying published methods (6). We provide the first data concerning the estimated energy expenditure of professional male youth soccer players that compete within the United Kingdom in a variety of activities performed throughout the training week (Figure 3). Our results show an estimated mean energy expenditure of 3,618 ± 61 kcal·d−1, which is representative of data collected using comparable methods in youth soccer players from a Spanish first division club (21). Moreover, the ratio of mean energy intake to mean energy expenditure was 78 ± 5%, which is representative of previously published values that have estimated energy expenditure using laboratory based techniques (12).
Quantification of energy expenditure throughout the competitive week is relatively limited within soccer literature; this is somewhat surprising considering the importance of such data in interpreting the results of dietary analyses. We estimated energy expenditure through the summation of daily BMR data, which were calculated using the Harris-Benedict equation (17), energy consumption from the TEF, and average energy expenditures associated with activities during the training week. However, it has been documented that underreporting is an issue that may compromise the integrity of dietary analysis studies, especially those incorporating adolescents (6). To alleviate the influence of under-reporting, it has been proposed that a minimum level of energy expenditure that equates to 1.1 × BMR can be used as a threshold to represent true habitual intake (15). In this study, it is unlikely that underreporting accounted for the differences observed in energy intake and expenditure, as the minimum level of energy expenditure equates to a threshold of 1,936 ± 31 kcal·d−1, which is significantly less than that was reported to be consumed (i.e., mean daily energy intake; 2,831 ± 34 kcal·d−1, p < 0.001). Moreover, Figure 2 illustrates that the majority of players sampled consumed inadequate energy relative to their energy expenditure.
This study provides the first information concerning the dietary and activity habits of professional youth soccer players that compete within the United Kingdom; consequently, such data can be used as a bench mark for future research within similar populations from the same country. In support of previous research that has examined the nutritional practices of youth players from outside the United Kingdom, our data show that the dietary habits of the sampled players were inadequate to meet the demands imposed by their training and competition. Ensuring that sufficient calories are consumed (in a manner that optimizes the energy contribution from carbohydrates, proteins and fats) while also encouraging the intake of fiber through the consumption of whole grains, nuts, seeds, fruits, and vegetables, will ultimately enhance the potential for growth and development, preparedness for subsequent exercise, and the prevention of illness in the professional youth soccer player.
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