Relationship between fat-free mass index and nutrient intake in protein supplement user among Japanese collegiate soccer athletes

Ryunosuke Takahashi; Yukiko Kobayashi; Takako Fujii; Shino Kuwabara; Yuko Segawa-Yoshimoto; Wataru Aoi; Masashi Kuwahata

doi:10.20463/pan.2024.0021

Phys Act Nutr > Volume 28(3); 2024 > Article

Takahashi, Kobayashi, Fujii, Kuwabara, Segawa-Yoshimoto, Aoi, and Kuwahata: Relationship between fat-free mass index and nutrient intake in protein supplement user among Japanese collegiate soccer athletes

Original Article

Physical Activity and Nutrition 2024;28(3):036-042.

Published online: September 30, 2024

DOI: https://doi.org/10.20463/pan.2024.0021

Relationship between fat-free mass index and nutrient intake in protein supplement user among Japanese collegiate soccer athletes

Ryunosuke Takahashi¹, Yukiko Kobayashi^2,^*, Takako Fujii³, Shino Kuwabara², Yuko Segawa-Yoshimoto², Wataru Aoi², Masashi Kuwahata²

¹The Institute of Health and Sports Science, Chuo University, Tokyo, Japan

²Graduate School of Life and Environmental Sciences, Kyoto Prefectural University, Kyoto, Japan

³Department of Sports and Medical Science, Kokushikan University, Tokyo, Japan

*Corresponding author : Yukiko Kobayashi Graduate School of Life and Environmental Sciences, Kyoto Prefectural University 1-5, Shimogamo-Hangicho, Sakyo-ku, Kyoto 606-8522, Japan. E-mail: yukicoba@kpu.ac.jp

Received June 20, 2024 Revised August 16, 2024 Accepted September 13, 2024

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/2.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

[Purpose]

Protein supplements are widely used by athletes, prompting the scrutiny of their impact on low energy availability. This study investigated whether habitual protein supplement use is linked to physical characteristics and nutrient intake in Japanese university soccer athletes. In addition, an attempt was made to examine the differences in physical characteristics and nutrient intake according to muscle mass in protein supplement users using the fat-free mass index (FFMI), which reflects muscle mass.

[Methods]

A dietary survey, physical activity survey, and measurement of the physical characteristics of 38 Japanese collegiate soccer players were conducted.

[Results]

The habitual protein supplement utilization rate among the participants was 50%, and the participants exhibited an FFMI of 19.2 ± 1.3 kg/m². Significant disparities between the two groups were noted in exercise energy expenditure (p = 0.023); carbohydrates (p = 0.039); copper (p = 0.003); and vitamins B1 (p = 0.016), B2 (p = 0.040), B6 (p = 0.003), C (p = 0.012), and D (p = 0.033), with higher levels observed in the protein-consuming cohort compared to the non-consuming cohort. Protein supplement users (n = 15) were stratified into two groups based on the median FFMI of the entire population (18.9 kg/m²). While the energy balance ratios were comparable between the groups, the high FFMI group tended to exhibit elevated energy intake (p = 0.169), consumption (p = 0.076), and carbohydrate consumption (p = 0.092), compared to the low FFMI group.

[Conclusion]

These findings suggest that adequate carbohydrate and protein intakes are crucial for preserving or augmenting muscle mass in young Japanese soccer athletes, including those consuming protein supplements.

Keywords: fat-free mass index, dietary intake, protein supplement, Japanese soccer players, relative energy deficiency in sport

INTRODUCTION

Athletes consistently strive to augment muscle mass for enhanced athletic performance. Muscle, the largest organ in the body, exhibits metabolic activity. Muscle proteins undergo cyclic breakdown and synthesis. Given that muscle mass changes are dependent on myofibrillar protein content, maintaining adequate nutritional intake, including protein intake, is crucial for athletes [1-3]. However, the International Olympic Committee introduced the concept of “Relative Energy Deficiency in Sport” (REDs) in 2014 [4], with a revised iteration published in 2023 [5]. REDs explicitly states that diminished energy availability (EA) suppresses various physiological functions, including energy and bone metabolism, thereby affecting performance [6-11]. Low EA, which was initially understood to focus on female athletes in 2014, has improved understanding, suggesting that it can also be applied to male athletes, with further evidence-building being conducted before the 2023 revision. The estimated prevalence ranges widely from 15% to 70% in men and from 23% to 79.5% in women [5]. Japan is no exception, with an estimated prevalence of approximately 50% [12], highlighting the need for nutritional interventions to improve EA.

Since the onset of the COVID-19 pandemic, the market for protein powder has expanded rapidly in Japan, and the number of users may increase further. The prevalence of protein supplement consumption among athletes has surged recently, and it is necessary to verify whether the intake of protein supplements contributes to muscle maintenance and enhancement in addition to improving EA. In a recent report, a large meta-analysis of randomized controlled trials evaluating the effect of protein intake on lean body mass proposed a dose-response relationship between protein intake and muscle mass gain, and an appropriate range of protein intake [13]. However, this meta-analysis was conducted with the implementation of resistance training, and similar large-scale studies on non-resistance training are anticipated. Furthermore, only a few studies have focused on elucidating the relationship between protein supplement intake, muscle mass, and diet in endurance ball players, such as soccer players, and data on Japanese soccer players in particular are currently lacking. This study investigated the physical characteristics and nutrient intake of young Japanese soccer players and compared them between protein supplement users and non-users. In addition, we explored the characteristics of low muscle mass protein supplement users using the fat-free mass index (FFMI), which reflects muscle mass.

METHODS

Participants

Thirty-eight male students from a university soccer club (Upper League) in Tokyo were selected as the study participants. The participants were asked to complete oral and written surveys, and informed consent was obtained from all participants. After excluding individuals with injuries or missing data, 30 participants were included in the final analysis. This study was approved by the ethics committee of the Kyoto Prefectural University (No. 207).

Dietary survey

Participants were instructed to document their meals on two non-game days within the study period (October 2023) using the approximate quantity method and capture images with a smartphone. During photography, the participants were asked to include their student ID alongside the meal to aid in portion size determination. Nutrients and other intakes were analyzed from the collected records and photographs using specialized nutrition calculation software (Eiyou Plus ver. 1.0; Kenpakusha, Tokyo). Daily intakes and energy density-adjusted values were calculated for 22 items: energy; protein; fat; carbohydrate; potassium; calcium; magnesium; phosphorus; iron; zinc; copper; retinol activity equivalent; vitamin D, K, B₁, B₂, B₆, B₁₂, and C; alpha-tocopherol; folic acid; and salt equivalent. Additionally, data on the frequency and quantity of protein supplement consumption were gathered and incorporated into nutrient and other intake analyses. However, the nutrient content of supplements other than proteins, such as multivitamins, was not included in the analysis.

Physical activity survey

The physical activity survey was conducted over two days on the same day as the dietary survey. Behavioral data were collected at approximately 5-minute intervals from waking to bedtime using recording forms. Physical activity intensity was determined based on the Physical Activity Recording Standards for Health Promotion 2013 [14] and the revised METs table for physical activity [15]. The exercise intensity and time duration of training at their club were determined by analyzing videos of the training sessions. Daily energy expenditure (kcal/day) was calculated using the formula: daily energy expenditure = body weight × 1.05 × Σ(exercise intensity × time). Participants engaged in club activities six days a week, with some undertaking independent training.

Physical characteristics

Physical characteristics were assessed using data from routine measurements conducted in September 2023. Physical attributes of the participants were evaluated using a body composition analyzer (InBody; InBody Japan, Tokyo, Japan) by bioelectrical impedance analysis. Measured parameters included body weight, body fat percentage, fat mass, fat-free mass (FFM), body water content, and body water percentage. Height was determined using a stadiometer, and body mass index (BMI) was calculated using the formula: BMI (kg/m²) = weight (kg)/height (m) × height (m). The FFMI was calculated using the following formula: FFMI (kg/m²) = FFM (kg)/height (m) × height (m).

Statistical analysis

Normality was assessed using the Shapiro-Wilk test. The non-paired t-test was used to compare normally distributed data, whereas the Mann-Whitney U test was used to compare non-normally distributed data between groups. Pearson’s correlation coefficient was used to assess the correlation for normally distributed data, whereas Spearman’s rank sum correlation coefficient was used for non-normally distributed data. Statistical analysis was performed using SPSS Statistics (version 24.0; IBM, Armonk, NY, USA), with the significance level set at 5%.

RESULTS

Physical characteristics and nutrient intake of participants

Average age of participants was 20.4 ± 1.3 years. Table 1 presents the physical characteristics, energy expenditure, and nutrient intake of the participants. BMI ranged from 18.5 to 25.0 for all participants. Average FFM of the participants was 59.6 ± 6.8 kg, with a median of 57.9 kg. Average FFMI was 19.2 ± 1.3 kg/m², with a median of 18.9 kg/m². Average available energy was 35.1 ± 9.0 kcal/kg FFM, with 32.3% of participants having a distribution of <30 kcal/kg FFM. The nutrients that deviated the most from the Japanese Dietary Reference Intake [16] were fats (57.9%, upward deviation), calcium (78.9%, downward deviation), retinol activity (63.2%, downward deviation), and vitamin D (76.3%, downward deviation).

Comparison of physical characteristics and nutrient intake with and without protein supplement use

The habitual protein supplement utilization rate among participants was 50%. Table 2 illustrates the physical characteristics and nutrient intakes related to protein supplement utilization. Significant disparities between the two groups were noted in exercise energy expenditure (p = 0.023); carbohydrates (p = 0.039); copper (p = 0.003); and vitamins B₁ (p = 0.016), B₂ (p = 0.040), B₆ (p = 0.003), C (p = 0.012), and D (p = 0.033); with higher levels observed in the protein-supplement-consuming cohort compared to the non-consuming cohort. Similar findings were evident for the energy-adjusted intake. FFM, FFMI, energy expenditure, energy intake, protein intake, and protein intake per kilogram of body weight were higher in the protein-supplement-consuming cohort than in the non-consuming cohort; however, these discrepancies were not statistically significant.

Comparison of nutrient intake by FFMI among protein supplement users

Fifteen protein supplement users were selected and divided into two groups based on the median FFMI of the entire population: nine in the upper group and six in the lower group. Comparing each item in both groups (Table 3), the energy consumed for exercise in the upper group was significantly higher than in the lower group (p = 0.005). The energy balance ratios of the two groups were similar; however, neither energy intake (p = 0.169) nor energy expenditure (p = 0.076) were significantly higher in the upper group compared to that in the lower group. When nutrient intake was adjusted for energy, the upper group carbohydrate intake tended to be greater than the lower group carbohydrate intake, with a significance level below 10% (p = 0.092). However, no significant difference was observed in protein intake between the two groups (Figure 1).

Relationship between FFMI and protein and carbohydrate intake of participants

Single correlation analysis was performed to examine whether FFMI was associated with dietary protein and carbohydrate intake (Table 4). Significant positive correlations were found between FFMI and intake of protein (r = 0.448; p = 0.013) and carbohydrates (r = 0.590; p = 0.001). Consistent results were observed for the energy density-adjusted nutrient intake.

DISCUSSION

In athletes, maintaining or increasing fat-free muscle mass is a challenge for enhancing athletic performance. This study investigated whether protein supplement use is linked to the physical characteristics and dietary intake of high-lev-el college soccer players in Japan. These results suggest that the habitual protein supplement utilization rate among the participants was 50%, and sufficient carbohydrate and protein intake is crucial for the maintenance or enhancement of muscle mass.

Half of the participants consumed protein supplements. According to a survey by the Japan Sports Association, 66.2% of athletes belonging to college athletic teams, such as the participants in this study, used protein supplements [17]. Protein supplement users had a significantly higher energy expenditure from exercise, a component of the available energy calculation, than non-supplement users. This is presumably due to independent training outside club activities, suggesting that protein supplements are used by athletes with higher activity levels. Additionally, protein supplement users had significantly higher intakes of carbohydrates, copper, and vitamins than non-users. FFMI, energy expenditure, energy intake, protein intake, and protein intake per kilogram of body weight were also higher in protein supplement users than in non-users; however, the differences were not statistically significant, suggesting large individual differences.

In our investigation of the association between dietary intake, protein supplement utilization, and muscle mass, 40% of athletes exhibited FFM or muscle mass in the lower half of the total subject cohort despite the incorporation of protein supplements. Consequently, a comparison of the nutrient intake between the upper and lower strata was conducted based on the median FFMI of the entire subject pool. The results indicated comparable energy balance ratios between the upper and lower groups, both demonstrating higher energy intake and expenditure in the upper group, albeit insignificantly. Further scrutiny revealed similar protein intake across both groups, whereas carbohydrate intake was notably elevated in the upper group, although the significance remained below 10%. This finding implies that adequate carbohydrate consumption by protein supplement users contributes to muscle accretion. Conversely, sustaining muscle mass may pose challenges for protein supplement users with adequate protein intake but deficient carbohydrate intake. Notably, these findings underscore the importance of elevating energy rather than protein intake to augment lean body mass, thereby corroborating the existing literature [18]. Furthermore, the correlation analysis of FFMI with protein and carbohydrate intake showed significant positive correlations. This result further emphasizes the contribution of carbohydrate and protein intake to the increase and maintenance of muscle mass.

The limitations of this study include the small number of participants and cross-sectional design, precluding the establishment of a clear causal relationship. The measurement of activity in this study was performed using a time study survey and video analysis of the training sessions. This was performed to avoid the risk of damage or loss of activity meters. Although these methods may be strictly limited to a complete picture of all physical activities, we believe that we can encompass physical activity in club training, voluntary training, and daily life. Additionally, soccer players may have different activity levels depending on their position. However, the number of participants in this study was small, and it was not possible to stratify the data according to position. This is an interesting finding, because protein intake may differ on the basis of position. In the future, it will be necessary to increase the sample size and examine the data based on position. Specific protein supplement consumption amounts were not assessed. Nevertheless, little information is available regarding protein supplement utilization and its association with dietary intake status and FFMI in young Japanese soccer players. Furthermore, the data were successfully collected from the same sports competitions. These findings contribute to the foundational understanding of the relationship between muscle mass and nutritional intake during endurance ball games.

In conclusion, this study investigated whether protein supplement use is linked to physical characteristics and dietary intake in young Japanese soccer players. These results suggest that the habitual protein supplement utilization rate among the participants was 50%, and sufficient carbohydrate and protein intake is crucial for the maintenance or enhancement of muscle mass. When administering protein supplements to meet protein requirements, it is imperative to assess carbohydrate intake to avoid perpetuating low EA of athletes. These findings should help establish a basis for nutritional supplementation strategies for muscle maintenance in athletes and enhance the strategic use of protein supplements.

Acknowledgments

We thank the members of the university soccer club for their cooperation in conducting this study. This work was supported by Urakami Foundation for Food and Food Culture Promotion, Japan. All authors have no conflicts of interest.

Figure 1.

Comparison of energy intake and expenditure, carbohydrate and protein intake by FFMI among protein supplement users in young Japanese soccer players.

Fifteen protein supplement users in young Japanese soccer players divided into two groups based on the median FFMI (fat-free mass index) of the entire population: 9 in the upper group and 6 in the lower group. White boxes indicate FFMI upper group; light gray boxes, FFMI lower group. Non-paired t-test. The center line of the boxplot indicates the median, the crosses indicate the mean, the top of the box indicates the 75th percentile, and the bottom of the box indicates the 25th percentile. Error bars indicate ± SD.

Table 1.

Participants characteristics.

n=30	mean ± SD
BMI, kg/m²	21.6 ± 1.4
Body weight, kg	66.8 ± 7.8
FFM, kg	59.6 ± 7.2
FFMI, kg/m²	19.2 ± 1.3
Exercise energy expenditure, kcal/day	777 ± 348
Energy availabity, kcal/kg FFM	37 ± 9
Total energy expenditure, kcal/day	3186 ± 547
Energy intake, kcal/day	2977 ± 677
Energy intake/expenditure	0.94 ± 0.20
Dietary protein, g/day	131.0 ± 34.2
Dietary protein, E%	17.8 ± 3.4
Dietary protein, g/kg bodyweight	1.96 ± 0.41
Dietary fat, g/day	112.3 ± 46.8
Dietary fat, E%	33.5 ± 9.5
Dietary carbohydrate, g/day	370.0 ± 110.3
Dietary mineral intake/day
K, mg	3461 ± 1002
Ca, mg	633 ± 326
Mg, mg	364 ± 105
P, mg	1541 ± 441
Fe, mg	11.8 ± 3.7
Zn, mg	15.3 ± 5.0
Cu, mg	1.7 ± 0.5
Dietary vitamin intake/day
Retinol activity equivalent, µg	640 ± 321
Vitamin D, µg	13.3 ± 13.3
Alpha-tocopherol, mg	10.8 ± 5.3
Vitamin K, µg	446 ± 227
Vitamin B₁, mg	2.2 ± 0.9
Vitamin B₂, mg	2.2 ± 1.1
Vitamin B₆, mg	2.7 ± 1.3
Vitamin B₁₂, µg	9.8 ± 13.4
Folic acid, µg	456 ± 154
Vitamin C, mg	161 ± 104
Salt, g/day	11.3 ± 2.6

BMI, body mass index; FFM, fat-free mass; FFMI, fat-free mass index.

Table 2.

Comparison of physical characteristics and nutrient intake with and without protein supplement use in Japanese young soccer players.

n=30	Protein supplement		p
	User	non-User
	n=15	n=15
BMI, kg/m²	22.0 ± 1.3	21.2 ± 1.4	0.114
Body weight, kg	68.9 ± 8.6	64.8 ± 6.6	0.149
FFM, kg	61.5 ± 7.8	57.6 ± 6.3	0.146
FFMI, kg/m²	19.6 ± 1.2	18.9 ± 1.3	0.134
Exercise energy expenditure, kcal/day	918 ± 335	636 ± 309	0.023^*
Energy availabity, kcal/kg FFM	36 ± 8	38 ± 10	0.404
Total energy expenditure, kcal/day	3342 ± 553	3030 ± 513	0.120
Energy intake, kcal/day	3096 ± 643	2857 ± 710	0.342
Energy intake/expenditure	0.93 ± 0.14	0.96 ± 0.25	0.696
Dietary protein, g/day	140.7 ± 37.1	121.3 ± 29.0	0.121
Dietary protein, E%	18.4 ± 4.1	17.2 ± 2.7	0.356
Dietary protein, g/kg bodyweight	2.0 ± 0.4	1.9 ± 0.4	0.255
Dietary fat, g/day	106.1 ± 49.5	118.5 ± 44.6	0.478
Dietary fat, E%	30.3 ± 10.2	36.7 ± 8.0	0.065
Dietary carbohydrate, g/day	411.1 ± 126.0	328.8 ± 75.4	0.039^*
K, mg/day	3655 ± 1021	3265 ± 978	0.294
Ca, mg/day	668 ± 294	599 ± 362	0.575
Mg, mg/day	394 ± 114	335 ± 90	0.127
P, mg/day	1525 ± 473	1558 ± 421	0.842
Fe, mg/day	12.8 ± 4.2	10.9 ± 2.9	0.156
Zn, mg/day	16.3 ± 5.7	14.3 ± 4.1	0.272
Cu, mg/day	2.0 ± 1.5	1.5 ± 0.4	0.003^*
Retinol activity equivalent, µg/day	621 ± 315	658 ± 337	0.757
Vitamin D, µg/day	18.4 ± 15.6	8.2 ± 8.4	0.033
Alpha-tocopherol, mg/day	11.3 ± 6.8	10.3 ± 3.2	0.596
Vitamin K, µg/day	473 ± 156	419 ± 285	0.527
Vitamin B₁, mg/day	2.6 ± 1.0	1.8 ± 0.6	0.016^*
Vitamin B₂, mg/day	2.6 ± 1.8	1.8 ± 0.7	0.040^*
Vitamin B₆, mg/day	3.4 ± 1.4	2.0 ± 0.6	0.003^*
Vitamin B₁₂, µg/day	11.1 ± 18.2	8.5 ± 6.1	0.604
Folic acid, µg/day	490 ± 152	421 ± 153	0.223
Vitamin C, mg/day	208 ± 110	114 ± 75	0.012^*
Salt, g/day	11.1 ± 2.8	11.5 ± 2.5	0.712

Value are mean ± SD.

^* p<0.05. Non-paired t-test or Mann-Whitney U test. BMI, body mass index; FFM, fat-free mass; FFMI, fat-free mass index.

Table 3.

Comparison by FFMI among protein supplement users in Japanese young soccer players.

n=15	FFMI		p
	Upper	Lower
	n=9	n=6
BMI, kg/m²	22.6 ± 1.0	21.0 ± 1.0	0.008^*
FFM, kg	64.6 ± 6.7	56.9 ± 7.5	0.055
FFMI, kg/m²	20.4 ± 0.9	18.4 ± 0.4	<0.001^*
Exercise energy expenditure, kcal/day	1099 ± 273	647 ± 219	0.005^*
Energy availabity, kcal/kg FFM	33.9 ± 7.5	38.3 ± 8.2	0.301
Total energy expenditure, kcal/day	3526 ± 588	3034 ± 334	0.076
Energy intake, kcal/day	3286 ± 672	2812 ± 523	0.169
Energy intake/expenditure	0.93 ± 0.18	0.92 ± 0.08	0.872
Dietary protein, g/day	149.1 ± 43.3	128.2 ± 23.1	0.302
Dietary protein, E%	18.4 ± 5.0	18.4 ± 2.6	0.995
Dietary protein, g/kg bodyweight	2.1 ± 0.5	2.0 ± 0.5	0.847
Dietary fat, g/day	107.4 ± 43.9	104.0 ± 61.6	0.902
Dietary fat, E%	29.4 ± 9.0	31.6 ± 12.4	0.690
Dietary carbohydrate, g/day	455.9 ± 146.7	343.9 ± 31.8	0.092
K, mg/day	3817 ± 1104	3414 ± 921	0.474
Ca, mg/day	720 ± 298	589 ± 297	0.419
Mg, mg/day	407 ± 111	374 ± 125	0.594
P, mg/day	1635 ± 521	1360 ± 373	0.288
Fe, mg/day	13.2 ± 4.7	12.3 ± 3.7	0.694
Zn, mg/day	18.0 ± 6.3	13.8 ± 3.8	0.165
Cu, mg/day	2.1 ± 0.5	1.8 ± 0.4	0.157
Retinol activity equivalent, µg/day	650 ± 358	578 ± 262	0.683
Vitamin D, µg/day	20.4 ± 19.2	15.4 ± 8.4	0.559
Alpha-tocopherol, mg/day	12.7 ± 8.3	9.3 ± 3.4	0.365
Vitamin K, µg/day	451 ± 178	507 ± 126	0.474
Vitamin B₁, mg/day	2.9 ± 1.1	2.1 ± 0.7	0.148
Vitamin B₂, mg/day	2.9 ± 1.4	2.0 ± 0.7	0.148
Vitamin B₆, mg/day	3.6 ± 1.4	3.1 ± 1.6	0.580
Vitamin B₁₂, µg/day	14.3 ± 23.3	6.3 ± 3.4	0.423
Folic acid, µg/day	519 ± 167	447 ± 129	0.388
Vitamin C, mg/day	230 ± 120	189 ± 101	0.609
Salt equivalent, g/day	11.6 ± 2.3	10.3 ± 3.5	0.415
Energy-adjusted intake (/1,000 kcal)
Dietary protein, g/day	50.0 ± 12.4	46.0 ± 6.5	0.995
Dietary fat, g/day	32.6 ± 10.1	35.1 ± 13.8	0.690
Dietary carbohydrate, g/day	115.6 ± 37.2	87.2 ± 8.1	0.092
K, mg/day	968 ± 280	866 ± 234	0.474
Ca, mg/day	183 ± 75	149 ± 75	0.419
Mg, mg/day	103 ± 28	95 ± 32	0.594
P, mg/day	414 ± 132	345 ± 94	0.288
Fe, mg/day	3.3 ± 1.2	3.1 ± 0.9	0.694
Zn, mg/day	4.6 ± 1.6	3.5 ± 1.0	0.165
Cu, mg/day	0.5 ± 0.1	0.4 ± 0.1	0.157
Retinol activity equivalent, µg/day	165 ± 91	147 ± 66	0.683
Vitamin D, µg/day	5.2 ± 4.9	3.9 ± 2.1	0.559
Alpha-tocopherol, mg/day	3.2 ± 2.1	2.3 ± 0.9	0.365
Vitamin K, µg/day	114 ± 45	129 ± 32	0.513
Vitamin B₁, mg/day	0.7 ± 0.3	0.5 ± 0.2	0.125
Vitamin B₂, mg/day	0.7 ± 0.4	0.5 ± 0.2	0.148
Vitamin B₆, mg/day	0.9 ± 0.3	0.8 ± 0.4	0.580
Vitamin B₁₂, µg/day	3.6 ± 5.9	1.6 ± 0.9	0.423
Folic acid, µg/day	132 ± 42	113 ± 33	0.388
Vitamin C, mg/day	56 ± 30	48 ± 26	0.609
Salt equivalent, g/day	2.9 ± 0.6	2.6 ± 0.9	0.415

Value are mean ± SD.

^* p<0.05. Non-paired t-test or Mann-Whitney U test. BMI, body mass index; FFM, fat-free mass; FFMI, fat-free mass index.

Table 4.

Single correlations between dietary protein and carbohydrate intake and FFMI in Japanese young soccer players.

Variable	r	p
Protein	0.448	0.013
Carbohydrate	0.590	0.001

Spearman's rank sum correlation coefficient.

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