Table 1.

Summary of study sample characteristics

Variable		Boys			Girls
		N a	Mean or proportionb	SE	N a	Mean or proportionb	SE
	Age (years)	8327	13.0	0.18	9100	13.0	0.18
	Height (cm)	8327	157.8	0.97	9100	152.6	0.68
	Weight (kg)	8327	50.3	0.84	9100	45.7	0.60
	BMI (kg/m2)	8327	19.4	0.11	9100	19.1	0.10
	Height z-score	8327	0.0	0.01	9100	0.0	0.01
	BMI z-score	8327	0.0	0.01	9100	0.0	0.01
	Birth weight (g)	7387	3433.0	7.41	8206	3281.0	6.66
Low birth weight (<2500 g)	Yes	358	4.7	0.27	554	6.7	0.27
	No	7029	95.3	0.27	7652	93.3	0.27
Body weight category	Normal weight	5932	71.5	0.57	6550	72.1	0.55
	Overweight incl. obesity	1562	18.6	0.53	1304	14.3	0.46
	Thinness all grades	833	9.9	0.36	1246	13.6	0.42
	Thinness grade 1	713	8.4	0.34	996	10.8	0.37
	Thinness grade 2	91	1.1	0.11	193	2.1	0.17
	Thinness grade 3	29	0.4	0.07	57	0.6	0.09
Maternal education	University	1577	19.1	0.94	1611	18.1	0.92
	Secondary	3247	40.1	0.64	3431	38.4	0.72
	Primary or vocational	3280	40.7	1.12	3822	43.6	1.17
Type of school	Primary	4372	50.1	2.99	4337	49.7	2.97
	Middle	2114	27.0	2.68	2258	26.6	2.65
	Secondary	1841	22.8	2.32	2505	23.7	2.36
School locationc	Big city	591	8.9	1.94	636	9.5	2.06
	Town or small city	3309	51.4	3.29	3353	50.5	3.29
	Rural	2586	39.7	3.10	2606	40.0	3.11
Income per capita	≤500 PLN	2819	48.4	1.14	3095	49.4	1.14
	501–1000 PLN	1850	31.9	0.72	1968	31.5	0.69
	>1000 PLN	1139	19.7	0.91	1212	19.1	0.87
Gross Domestic Product	Region 1	1124	13.0	1.03	1276	13.0	0.92
	Region 2	5253	64.0	1.51	5642	64.0	1.45
	Region 3	1950	23.1	1.33	2182	23.1	1.29
Participation in physical education	Yes	7935	96.8	0.23	8617	95.8	0.32
	No	249	3.2	0.23	400	4.2	0.32

Thinness grade 1, 2 and 3 corresponds to the BMI 17 to <18.5, 16 to <17 and <16 kg/m² at the age of 18 years, respectively.

SE standard error, PLN Polish zloty

^aUnweighted

^bWeighted

^cOnly pupils from primary and middle schools