Novel association between chronic obstructive pulmonary disease and osteoporosis: A prospective cross-sectional study

Table 1 Baseline characteristics of participants (n = 85), (mean ± standard deviation)

Characteristic	Low	Middle	High	P value
No. of participants	28	29	28
Age (years)	71.0 ± 10.2	65.1± 8.7	57.1 ± 11.1	< 0.001
Height (cm)	1.6 ± 0.1	1.7 ± 0.1	1.7 ± 0.1	0.007
Weight (kg)	60.6 ± 9.2	64.6 ± 11.0	67.4 ± 10.4	< 0.001
BMI (kg/m2)	22.3 ± 3.2	23.1 ± 3.5	24.0 ± 3.4	< 0.001
FEV1 (L)	1.8 ± 0.7	1.9 ± 0.9	2.4 ± 0.1	< 0.001
FEV1/FVC% (L)	62.2 ± 11.9	64.2 ± 16.3	71.2 ± 15.9	< 0.001
BMD (mg/cm3)	53.9 ± 17.1	98.2 ± 9.0	139.8 ± 24.7	< 0.001
Sex, n (%)				< 0.001
Male	55.7	75.5	77.8
Female	44.3	24.5	22.2
Smoke, n (%)				< 0.001
Yes	52.2	39.1	54.3
No	5.9	20.3	24.3
Yes, give up	41.9	33.8	33.0
Physical activity, n (%)				0.014

Grouping of bone mineral density (BMD): Low group: ≤ 80.5 mg/cm³; middle group: 80.6-115.7 mg/cm³; high group: > 115.7 mg/cm³. BMI: Body mass index; FEV1: Forced expiratory volume in 1 second; FVC%: Forced vital capacity percentage.

Table 2 Univariate analysis of forced expiratory volume in 1 second/forced vital capacity percentage, n (%)

Covariate	Statistics	β (95%CI)	P value
BMD, mg/cm3	99.1± 37.6	0.1 (0.1, 0.1)	< 0.0001
Low	140 (33.4)	Reference
Middle	139 (33.2)	2.0 (-1.8, 5.7)	0.3094
High	140 (33.4)	9.0 (5.3, 12.8)	< 0.0001
Sex
Male	290 (69)	Reference
Female	130 (31.0)	3.4 (0.0, 16.8)	0.0488
Age, years
Low	140 (29.4)	Reference
Middle	125 (29.4)	-8.9 (-12.8, -5.0)	< 0.0001
High	175 (41.2)	-19.5 (-40.2, 1.2)	0.0002
Height, cm
Low	140 (32.9)	Reference
Middle	140 (32.9)	0.3 (0.1, 0.3)	< 0.0001
High	145 (34.1)	0.5 (0.3, 0.8)	0.0028
Weight, kg	64.2± 10.6	0.9 (0.9, 1.0)	< 0.0001
Low	140 (32.9)	Reference
Middle	135 (31.8)	-0.9 (-4.8, 3.0)	0.6579
High	150 (35.3)	2.8 (-0.9, 6.5)	0.1383
BMI, kg/m2	23.2± 3.4	0.8 (0.3,1.3)	0.0006
Low	140 (32.9)	Reference
Middle	135 (31.8)	3.7 (-0.2, 7.6)	0.0669
High	150 (35.3)	3.8 (-0.1, 7.5)	0.0439
FEV1, L	2.0± 0.9	10.9 (9.7,12.1)	< 0.0001
Low	120 (32.4)	Reference
Middle	125 (33.8)	16.9 (14.2, 19.6)	< 0.0001
High	125 (33.8)	25.9 (23.2, 28.7)	< 0.0001
1Smoke
0	205 (48.8)	Reference
1	70 (16.7)	2.7 (-1.4, 6.8)	0.1950
2	145 (34.5)	-10.7 (-14.0, -7.4)	< 0.0001
Tea
Yes	270 (63.5)	Reference
No	155 (36.5)	4.1 (0.9, 7.4)	0.0135
2Physical activity
0	115 (27.1)	Reference
1	225 (52.9)	5.9 (2.4, 9.5)	0.0012
2	25 (5.9)	5.4 (-1.8, 12.6)	0.1395
3	60 (14.1)	-2.2 (-7.3, 2.8)	0.3856

¹Smoke: Smoking status: 0 = None, 1 = Current smoker, 2 = Former smoker.

²Physical activity: 0 = Sedentary; 1 = Normal activity; 2 = Regular exercise; 3 = Occasional exercise. BMD: Bone mineral density; BMI: Body mass index; CI: Confidence interval; FEV1: Forced expiratory volume in 1 second.

Table 3 Relationship between bone mineral density and forced expiratory volume in 1 second/forced vital capacity percentage

Outcome	Crude model		1Model I		2Model II
	β (95%CI)	P value	β (95%CI)	P value	β (95%CI)	P value
BMD (mg/cm3)	0.1 (0.1,0.1)	< 0.0001	0.0 (-0.0, 0.1)	0.0387	-0.0 (-0.0, -0.0)	0.896
BMD (quartile)
Low	Reference		Reference		Reference
Middle	2.0 (-1.8, 5.7)	0.309	3.4 (0.7, 6.1)	0.0143	-0.0 (-2.1, 2.0)	0.9694
High	9.0 (5.3, 12.8)	< 0.0001	2.9 (-0.1, 6.0)	0.0579	0.8 (-1.6, 3.3)	0.5052

¹Adjust I adjusts for: Sex (male, female); age; height; weight; body mass index (BMI); Smoke (0, 1, 2); tea (0, 1); physical activity (0, 1, 2, 3); forced expiratory volume in 1 second (FEV1).

²Adjust II adjusts for: sex (male, female); age (smooth); height (smooth); weight (smooth); BMI (smooth); smoke (0, 1, 2); tea (0, 1); physical activity (0, 1, 2, 3); FEV1 (smooth). BMD: Bone mineral density; CI: Confidence interval.

Table 4 Threshold effect analysis of bone mineral density and forced expiratory volume in 1 second/forced vital capacity percentage using piece-wise linear regression

Inflection point of BMD (mg/cm3)	Effect size (β)	95%CI	P value
≤ 128.08	0.245	0.094, 0.396	0.0019
> 128.08	-0.136	-0.284, 0.013	0.0753

Outcome variable: Forced expiratory volume in 1 second/forced vital capacity percentage (FEV1/FVC%). Exposure variable: Bone mineral density (BMD). Adjusted variables: Sex, body mass index, age, smoke. When BMD was ≤ 128.08 mg/cm³, the effect size for FEV1/FVC% was 0.245 (P = 0.0019), suggesting a significant positive correlation between lower BMD and FEV1/FVC%. However, when BMD exceeded 128.08 mg/cm³, the effect size changed to -0.136 (P = 0.0753), indicating a negative shift in the effect direction, suggesting that at higher levels of BMD, FEV1/FVC% may decrease. CI: Confidence interval.