Can the supplementation of vitamin D, sun exposure, and isolation during the COVID-19 pandemic affect the seasonal concentration of 25(OH)D and selected blood parameters among young soccer players in a one-year training season?

Figures & data

Figure 1. The study timeline and flowchart of the participants.

Table 1. Training load (min) taking into account type (overall and special drills) and intensity (aerobic, or anaerobic) of exercises of typical weekly training during competition and preparation period of one1-year training season of young soccer players.

	Aerobic performance (min)	Aerobic-–anaerobic performance (min)	Anaerobic lactate performance (min)	Anaerobic nonlactate performance (min)	Total (min)
Competition period
Overall drills	225	30	5	20	280
Special drills	160	220	30	30	440
Total (min)	385	250	35	50	720
Preparation period
Overall drills	250	80	25	35	390
Special drills	190	185	18	27	420
Total (min)	440	265	43	62	810

Figure 2. The scope of the UV index for the city of Gdynia (Poland) during the project [33].

Table 2. Changes in selected blood parameters in young soccer players (n = 24) during a one-year training cycle.

Time point	T1	T2	T3	T4
25(OH)D (ng/ml)	35.0 ± 6.26	24.5 ± 4.89*	26.4 ± 5.32	40.5 ± 6.86*^#†
WBC (10^3 [3]/µl)	5.84 ± 1.10	6.16 ± 1.09	5.76 ± 1.15	6.00 ± 1.11
RBC (10^4 [4]/µl)	5.14 ± 0.29	5.29 ± 0.35	5.07 ± 0.30	5.25 ± 0.37
HGB (g/dL)	14.9±0.71	15.5 ± 0.73*	14.9 ± 0.61^#	15.4 ± 0.73
HCT (%)	44.6 ± 1.92	45.8 ± 1.97	44.3 ± 1.84^#	45.5 ± 2.24
PLT (10^3 [3]/µl)	228.3 ± 39.84	229.5 ± 34.08	213.4 ± 34.28	221.0 ± 42.87
TC (mg/dL)	142.5 ± 21.83	145.9 ± 22.28	147.6 ± 22.03	146.4 ± 23/27
HDL-C (mg/dL)	55.6 ± 9.19	56.1 ± 8.47	56.8 ± 7.97	53.00 ± 8.41
LDL-C (mg/dL)	74.3 ± 18.26	69.5 ± 15.98	77.2 ± 20.67	76.8 ± 17.74
TG (mg/dL)	63.4 ± 17.37	95.6 ± 48.56	64.3 ± 18.56	84.6 ± 30.63
ALT (U/L)	18.5 ± 4.77	19.3 ± 8.83	21.5 ± 6.62	17.5 ± 4.99
AST (U/L)	27.5 ± 7.93	22.8 ± 5.96*	33.5 ± 12.04^#	25.7 ± 6.57†
CK (U/L)	341.7 ± 172.9	180.8 ± 91.9*	564.3 ± 335.7^#	271.6 ± 156.1†

*Significantly different from T1; ^#Significantly different from T2; †Significantly different from T3; WBC-: white blood cells; RBC-: red blood cells; HGB-: hemoglobin; HCT-: hematocrit; PLT-: thrombocytes; TC-: total cholesterol; HDL-C-: high-density cholesterol; LDL-C-: low-density cholesterol; TG-: triglycerides; ALT-: alanine aminotransferase; AST-: asparagine aminotransferase; CK-: creatine kinase.

Figure 3. Correlations between 25(OH)D concentration and white blood cells in young soccer players (n = 24) during a one1-year training cycle.

Table 3. Changes in selected blood parameters in young soccer players in the supplemented (GS) and placebo group (GP) in the T2, T3T3, and T4.

Group	GS			GP			Inter-actions	p	pη2	OP
Time- point	T2	T3	T4	T2	T3	T4
25(OH)D [ng/ml]	24.1 ± 5.23	26.6 ± 6.35	41.7 ± 6.95*†	24.8 ± 4.77	26.2 ± 4.52	39.5 ± 6.91*†	Time	<0.001	0.82	1.00
WBC (10^3 [3[3]/µl)	6.63 ± 0.93	6.23 ± 1.23	6.17 ± 0.90	5.77 ± 1.05	5.37 ± 0.95	5.86 ± 1.28
RBC (10^4 [4]/µl)	5.34 ± 0.26	5.11 ± 0.27*	5.34 ± 0.34†	5.24 ± 0.42	5.03 ± 0.33*	5.18 ± 0.38	Time	<0.001	0.38	0.99
HGB (g/dL)	15.5 ± 0.71	15.1 ± 0.64	15.5 ± 0.73	15.4 ± 0.77	14.8 ± 0.59*	15.2 ± 0.72	Time	<0.001	0.37	0.99
HCT (%)	46.0 ± 1.89	44.9±1.92	45.9 ± 2.11	45.7 ± 2.10	43.8 ± 1.70*	45.1 ± 2.36	time	<0.001	0.28	0.96
PLT (10^3 [3]/µl)	238.2 ± 33.87	216.3 ± 36.41*	223.5 ± 38.40	222.2 ± 33.79	210.9 ± 33.47	219.0 ± 47.79	Time	0.001	0.27	0.94
TC (mg/dL)	151.5 ± 27.94	148.9 ± 22.78	152.1 ± 28.81	141.2 ± 15.74	146.5 ± 22.25	141.6 ± 17.08
HDL-C (mg/dL)	54.8 ± 10.54	58.5 ± 8.64	53.5 ± 9.73	57.2 ± 6.50	55.3 ± 7.36	52.5 ± 7.50	Time	0.011	0.19	0.79
LDL-C (mg/dL)	74.7 ± 18.10	76.8 ± 19.96	79.7 ± 20.63	64.7 ± 12.65	77.5 ± 22.06	74.2 ± 15.29
TG (mg/dL)	110.1±57.74	65.7 ± 19.13*	96.5 ± 26.64	83.3 ± 37.21	63.0 ± 18.75	74.5 ± 31.08	Time	0.002	0.25	0.93
ALT (U/L)	22.4 ± 11.57	22.5 ± 8.61	18.0 ± 4.69	16.6 ± 4.63	20.5 ± 4.50	17.1 ± 5.38	Time	0.027	0.15	0.68
AST (U/L)	24.1 ± 5.17	35.8 ± 15.87	25.5 ± 6.04	21.7 ± 6.55	31.5 ± 7.64	25.8 ± 7.22	Time	<0.001	0.41	0.99
CK (U/L)	176.3 ± 70.21	598.8 ± 432.29	224.8 ± 108.50	184.6 ± 109.71	535.1 ± 241.28	311.2 ± 182.26	Time	<0.001	0.50	1.00

*Significantly different from T2; †Significantly different from T3; WBC-: white blood cells; RBC-: red blood cells; HGB-: hemoglobin; HCT-: hematocrit; PLT-: thrombocytes; TC-: total cholesterol; HDL-: C-high-density cholesterol; LDL-C-: low-density cholesterol; TG-: triglycerides; ALT-: alanine aminotransferase; AST-: asparagine aminotransferase; CK-: creatine kinase; pη2-: partial eta square; OP-: observed power.

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