Differences in Diet Assessment and Body Composition among Young Spanish Elite Footballers: Morning Training vs. Evening Training

1. Introduction

In football, clubs' youth academy teams usually have several groups per category, each of them often having different training schedules. Human beings exhibit circadian rhythms and diurnal variations in their physiology and the time of day they exercise, typically identifying as either morning or evening chronotypes [1]. There is still controversy regarding training timing; some authors have observed improvements in muscular adaptations and energy utilization during morning exercise [2,3,4,5,6], while muscular function has been favoured in other studies when exercise is performed in the afternoon or evening [7,8,9,10]. In this regard, research exploring the potential effects of Exercise Timing of the Day (ETOD) on the body composition and diet of young athletes is not fully documented across multiple domains of applicability in daily life, justifying its assessment. For example, data regarding multimodal exercise regimens, psychological response, and potential differences in physiological response by ETOD [11,12] are scarce, and almost non-existent for healthy, exercise-trained individuals. Moreover, strictly controlled nutritional intake with adequate protein to promote recovery and exercise adaptation is often absent in ETOD interventions [1].

The time of day is a factor that influences the optimization of sports performance. Schedules are usually adjusted according to convenience or availability and resources. However, few studies have specifically investigated the effect of early morning versus afternoon-evening training on fatigue indices in performance. This is important from a sports perspective because circadian rhythms, player readiness, and alterations in sleep or meal patterns can affect performance [13].

The growth development and talent detection of an athlete can be determined through different measurement methods, with anthropometry being the most commonly used [14]. Anthropometry refers to the various measurements of the size and proportions of the human body [14,15,16]. One institution involved in disseminating anthropometry is the International Society for the Advancement of Kinanthropometry (ISAK), which was founded as an organization whose scientific and professional work is related to kinanthropometry [17]. Furthermore, anthropometric techniques stand out among various composition assessment methods due to their low cost, good reproducibility, and ease of application [15]. In fact, skinfold measurements and their sum seem to be the least affected by control factors (hydration intake and daily activity) compared to other measurement methods [18].

A balanced diet is recognized as an important factor in aiding the proper functioning of our bodies and thus maximizing training effectiveness. Something crucial for athletes is meeting their energy needs, which we refer to as energy balance: when energy intake equals energy expenditure. This relationship influences sports performance and makes it vital that during physical activity, the organism's needs are met, including energy requirements, adequate intake of macro and micronutrients, and optimal hydration in accordance with the training plan, competition cycles, and other factors such as exposure to cold, heat, high altitude, injuries, medications [19], or as in the case of our study population, the time of day at which training occurs [9]. Also, the group we are discussing is mostly young individuals still in development, who require adequate energy intake to ensure proper growth and development [20].

Given the scarcity of scientific research examining potential differences in the time of day (TOD) of training on body composition and diet in young elite athletes, who often have different training schedules due to the demands of their sport, the main objective of this study was to descriptively analyse potential differences in body composition and energy and nutrient consumption between the morning training (MT) group and the evening training (ET) group.

The aim of this study was to analyse differences in anthropometric characteristics, estimation of energy expenditure and consumption, as well as intake of macronutrients and micronutrients in elite youth soccer players from the Spanish league with different training schedules (morning and evening). Consequently, the initial hypotheses were Hypothesis 1 (H1): The time of day of training will reflect differences between the morning training group and the evening training group in terms of body composition. Hypothesis 2 (H2): There will be differences in nutrient intake due to different mealtimes between both groups.

2. Materials and Methods

Study Design. This is a descriptive, cross-sectional, and non-experimental study of anthropometric characteristics, estimation of expenditure and energy intake, and nutrient intake estimation in professional male soccer players under 19 years of age, from the youth category of the Spanish league.

Participants. Participants belonged to two teams from the same Spanish club, both in the youth category. A total of 41 players participated in this study, consisting of 17 players from one team and 24 players from the other. Inclusion criteria for the study were as follows: (a) being a healthy individual with medical authorization for federated sports practice; (b) belonging to one of the two teams in the youth category of the Spanish league; (c) being federated in football; (d) training at different times, either in the morning or in the afternoon-evening; (e) measurements taken in March, during the competitive phase. Exclusion criteria for the study were: (a) not being of the corresponding age for the youth category, 16-18 years old. All players were previously informed of the objectives and methods of the research, signing the informed consent document before starting the study. For underage players, legal guardians were informed, and they authorized the minor's participation through the corresponding informed consent.

Procedure. Evaluations were conducted during each team's training schedule. On the day of evaluation, it was necessary for players not to have performed high-intensity exercises, training, or stretching on the same day. Therefore, anthropometric measurements were taken before training. All players were familiar with the test procedures, as they regularly performed them as part of their routines. The various questionnaires administered to the players were explained beforehand, and any doubts were resolved to clarify and standardize their completion.

Anthropometric Measures. Anthropometric measurements were taken following ISO 7250-1:2017 [21] and the International Society for the Advancement of Kinanthropometry (ISAK) standard [17]. The following measurements were taken: two basic measures (body mass and height), six skinfold thicknesses (triceps, subscapular, supra-iliac, abdominal, thigh, and calf), and three circumferences (arm relaxed, waist, and hip). An inextensible metal tape CESCORF (CESCORF, Porto Alegre, Brazil) was used to measure circumferences, and a Holtain mechanical calliper (HOL-98610ND) with an accuracy of 0.2 mm was used for skinfold measurements. All anthropometric measurements were taken two or three times, depending on whether the technical measurement error (TEM) between the first two measurements was greater than 5% in skinfolds and 1% in other measurements, taking the mean or median, respectively, for subsequent analysis. This ensures that the collected data are precise and consistent, which in turn guarantees the validity of the research results and allows for accurate interpretation of observed differences between measurements. The room temperature where the measurements were taken was standardized at 24°C. Body composition was determined using equations described in the consensus document of the Spanish Group of Kinanthropometry of the Spanish Federation of Sports Medicine [22], following the four-component model (MM, FM, BM, and residual mass (RM)). The following equation was used: Faulkner [23] to calculate FM expressed as a percentage. Two health indicators were calculated: waist-hip ratio and BMI.

Questionnaires. 24-Hour Recall Questionnaire (R24h): It retrospectively collects and quantifies the intake of foods and beverages consumed during the 24-hour period prior to or on the day before the interview. It allows estimating the energy and nutrients provided by the subject's diet. On this occasion, three R24h were conducted for each player, on two different weekdays and one weekend day, as this improves accuracy. The DIAL program version 3.15 2021 [25] was used to assess the R24h, allowing the translation of foods into nutrients through its analysis.

Activity Record (24 hours): In this questionnaire, the player describes how many hours per day they spend on activities of different intensities described in the questionnaire. With this information, we can estimate energy expenditure in 24 hours using metabolic index (METs) units.

Statistical Analysis. The normality of the variables was analysed using the Kolmogorov–Smirnov test with the Lillieforts correction, and homoscedasticity was analysed with the Levene test. For the comparisons between groups of continuous variables, the nonparametric Mann-Whytney U test was used to determine differences in anthropometric and nutrient intake variables, depending on each group's training schedule. A significance level of p < 0.05 was established to determine statistical significance. Quantitative variables were expressed with the mean value, minimum, maximum, and standard deviation (SD). Statistical analyses were performed using the SPSS statistical package version 28.0.0.0.

3. Results

Table 1 shows a comparison of the anthropometric characteristics by training group. Statistically significant differences were not found between MT and ET groups for all the characteristics, except for BMI (p= 0.004) and supra-iliac skinfold (p= 0.026).

Regarding macronutrient intake, as shown in Table 2, neither group had an energy intake (EI) that matched their total energy expenditure (TEE), with the MT group having the highest total energy expenditure and ET group the highest energy intake. Statistically significant differences were found between the MT and ET groups for total energy expenditure (TEE). No statistically significant differences were found between de MT and ET groups with respect to the macronutrient intake nor lipid profile. With respect to the caloric profile, % Lipid was higher in ET group (p = 0.042).

Micronutrient intake means were compared between groups and against recommended dietary intakes (RDIs) for the general Spanish male population accordingly (Table 3 and Table 4).

Regarding vitamin intake, only B12 showed statistically significant differences between the MT and ET groups (p = 0.001), with higher values in the MT group. Both groups exceeded the RDI for all vitamins, except for vitamin E which both groups met at least two-thirds of the RDI.

As for minerals, calcium (p = 0.049) and chloride (p = 0.032) showed statistically significant differences between the MT and ET groups, with higher intake in ET. Both groups exceeded the RDI for all minerals, except for calcium and iodine which both groups met at least two-thirds of the RDI.

4. Discussion

The aim of this study was to analyse differences in anthropometric characteristics, estimation of expenditure and energy consumption, as well as intake of macronutrients and micronutrients in elite youth soccer players from the Spanish league's youth category with different training schedules (morning and evening).

Considering football players grouped by morning or evening training, anthropometric characteristics did not show significant differences between both groups. Regarding estimated body fat percentage, values were around 11.5% in both groups, which align with findings from various studies conducted using DXA, where the range observed falls between 8-13% [26,27].

Regarding Total Energy Expenditure (TEE), results showed statistically significant differences between the morning training group and the evening training group, with TEE being much higher in the morning training group (5385.86 kcal/day) compared to the evening group (3771.21 kcal/day). These results are consistent with another study where the morning training group showed an increase in TEE consistent with prescribed exercise, while the evening group had a muted increase in TEE [28].

However, no significant differences were observed in energy intake, with the mean intake being 3108 kcal/day for both groups. This poses a risk for the morning training group and could be explained by activities performed apart from training in individuals with morning versus evening habits. This energy imbalance could lead to low energy availability in players, common in young athletes, as several studies show, where the prevalence of Low Energy Availability (LEA), defined as energy levels below 30 kcal·kg FFM-1·day-1, was 56% and 47.6% respectively [29,30].

Carbohydrate intake is low in both groups, with no significant differences between them. Both the morning and evening training groups deviate significantly from the Mediterranean diet pattern (MD), which recommends 55-60% of energy intake in the form of carbohydrates. The average carbohydrate intake in youth players from this club is 40% of total energy consumption. This represents poor efficiency for performance and potential poorer recovery due to inadequate replenishment of glycogen stores used during sports practice. In this regard, research indicates that the importance of carbohydrates in football has been recognized since the early 1970s. Players starting the game with higher muscle glycogen reserves, by consuming sufficient carbohydrates, achieved higher movement intensities and were able to maintain better total distance covered between rest times than those who started with low reserves [31].

When comparing fat consumption between both groups, statistically significant differences were found, with both groups consuming above 37%. The evening training group had a higher fat intake, ingesting 37.8% of total energy in the form of fats (134 grams on average versus 124 grams on average for the morning group). Players training in the evening consumed a greater amount of saturated fatty acids (SFAs); in this regard, another study observed an increase in lipid and saturated fatty acid intake when dinner was shifted to later hours or when the chronotype is evening [32].

Body lipid levels are under circadian control, involving numerous "peripheral clocks." Fat-rich foods are considered potent chronodisruptors because they act by modifying the expression of circadian or "clock" genes, which activate or deactivate other factors leading to physiological changes in cells over 24-hour periods [33]. Therefore, a high-fat diet causes circadian desynchronization with metabolic disturbances that are detrimental to athletes' health [34].

Protein consumption in this population exceeds the recommended intake by the Mediterranean diet of 10-15% of total dietary energy in the form of proteins, with consumption around 20% in both groups. In this regard, higher protein intakes seem to enhance training adaptations [35]. These intake levels can be easily achieved through a varied diet, if energy intake is sufficient to meet training demands [36].

Regarding vitamin D, both groups showed excess consumption of this nutrient when compared to the Recommended Daily Allowance (RDA). In this case, the morning training group would have a higher risk, as they are more exposed to sunlight than the evening training group, so the recommendation could be to consume a lower amount of vitamin D in this case, as reducing calcium and vitamin D intake in the diet is effective in treating adverse effects of excess consumption [37].

5. Conclusions

In this descriptive study conducted on young elite soccer players with different training schedules, the following conclusions can be drawn: (1) No significant differences were found in body composition between the group training in the morning and the group training in the evening, except for the supra-iliac skinfold; (2) Significant differences were observed in total energy expenditure, with the morning training group exhibiting significantly higher expenditure compared to the evening training group. However, there were no differences in energy intake between groups, indicating a risk of low energy availability (LEA) in the morning training group; (3) Total intake of lipids and saturated fatty acids was higher in the evening training group, potentially explained by the shift in intake to later hours; (4) Further studies are needed to determine the effects of different training schedules, particularly on young elite athletes, and how they affect various health and performance parameters such as diet, training, and rest. Regarding the latter, a study on the chronotype of these players, as well as their circadian rhythms and sleep-wake patterns, would be important.