Vitamin D status is associated with muscle strength and quality of life in patients with COPD: a seasonal prospective observation study

Figures & data

Table 1 Characteristics of COPD patients at the end of winter and end of summer

	End of winter	End of summer	P-valuea
n (% male)	51 (55)
Age (years)	68.7±7.2
Height (cm)	162.4±9.6
Weight (kg)	73.6±21.8	73.0±21.2	0.195
BMI (kg/m^2)	27.3±7.8	27.2±7.5	0.399
FFM (kg)	50.2±9.6	50.1±9.5	0.883
FFM (%)	69.3±11.8	70.4±12.6	0.506
FFMI (kg/m^2)	18.57±2.64	18.54±2.69	0.849
Muscle strength (kg)	23.2±10.9	23.9±11.3	0.963
25(OH)D (nmol/L)	33.7±28.4	52.5±30.5	⩽0.001
Adjusted Ca^+ (mmol/L)b	2.17±0.17	2.32±0.07	⩽0.001
PTH (pg/mL)	70.1±38.1	64.4±40.3	0.059
Creatinine (μmol/L)	84.9±21.0	93.6±20.7	⩽0.001
eGFR (mL/min/1.73 m^2)	76.1±19.0	67.0±11.9	⩽0.001
CRP (mg/L)	8.35±10.29	8.25±9.06	0.792
Smoking status
Current n (%)	16 (31)	21 (41)
Previous n (%)	34 (67)	29 (57)	⩽0.001
Never n (%)	1 (2)	1 (2)
Smoking pack years	44.6±39.3	44.8±39.4	0.030
SGRQ score (%)	60.8±19.8	61.8±21.2	0.948

Notes: Data presented as mean±SD, unless otherwise stated.

a Paired samples t-test (conducted on log transformed data) or Chi-squared test (for categorical data) to assess significant differences between the end of winter and end of summer.

b Calcium adjusted for albumin. Values in bold indicate a significant difference, P⩽0.05.

Abbreviations: BMI, body mass index; CRP, C-reactive protein; FFM, fat free mass; FFMI, fat free mass index; 25(OH)D, 25-hydroxyvitamin D; PTH, parathyroid hormone; eGFR, estimated glomerular filtration rate; SGRQ, St George’s Respiratory Questionnaire.

Figure 1 Mean (SE bars) 25(OH)D concentration for housebound (n=16) and nonhousebound (n=35) COPD patients at the end of winter and at the end of summer. Notes: Differences in 25(OH)D concentration between housebound and nonhousebound patients, within each season tested using independent samples t-test and differences in 25(OH)D concentration for each group between each season tested using paired samples t-test (conducted on log transformed data. Dashed lines represent the level of vitamin D sufficiency (25(OH)D ≥50 nmol/L) and deficiency (25(OH)D ⩽25 nmol/L). *P⩽0.05, **P⩽0.001.

Abbreviations: 25(OH)D, 25-hydroxyvitamin D; SE, standard error.

Table 2 Correlation analysis between serum 25(OH)D concentration and muscle outcomes, quality of life and CRP concentration of COPD patients at the end of winter and end of summer

	End of winter		End of summer
	R	P-value	R	P-value
FFM (kg)	0.089	0.588	0.270	0.092
FFMI (kg/m^2)	−0.028	0.866	0.171	0.297
Muscle strength (kg)	0.191	0.243	0.344	0.024
SGRQ score (%)	−0.058	0.712	−0.323	0.028
CRP (mg/L)	−0.223	0.141	−0.149	0.327

Notes: R, Spearman’s rho correlation coefficient. Values in bold indicate a significant difference, P⩽0.05.

Abbreviations: 25(OH)D, 25-hydroxyvitamin D; FFM, fat-free mass; FFMI, fat-free mass index; SGRQ, St George’s Respiratory Questionnaire; CRP, C-reactive protein.

Table 3 Stepwise linear regression models to investigate 25(OH)D concentration as a predictor of FFM, muscle strength and quality of life in COPD patients at the end of winter and at the end of summer

	End of winter				End of summer
	R^2	β	SE	P-value	R^2	β	SE	P-value
FFM (kg)
1	0.008	0.089	0.041	0.588	0.073	0.270	0.036	0.092
2	0.554	0.034	0.028	0.760	0.485	0.169	0.028	0.166
3	0.554	0.009	0.035	0.761	0.486	0.169	0.028	0.171
4	0.652	0.120	0.026	0.261	0.504	0.179	0.028	0.147
FFMI (kg/m^2)
1	0.001	−0.028	0.028	0.866	0.029	0.171	0.026	0.297
2	0.168	−0.066	0.026	0.662	0.116	0.123	0.026	0.444
3	0.204	−0.068	0.026	0.653	0.119	0.123	0.026	0.448
4	0.351	0.035	0.025	0.804	0.158	0.140	0.026	0.388
Muscle strength (kg)
1	0.037	0.191	0.109	0.243	0.118	0.344	0.110	0.024
2	0.325	0.170	0.092	0.222	0.501	0.243	0.085	0.038
3	0.434	0.179	0.086	0.168	0.530	0.258	0.081	0.021
4	0.490	0.227	0.084	0.079	0.582	0.254	0.080	0.022
5	0.553	0.285	0.082	0.041	0.610	0.271	0.082	0.022
SGRQ (%)
1	0.003	−0.058	0.072	0.712	0.105	−0.323	0.079	0.028
2	0.014	−0.069	0.073	0.663	0.112	−0.306	0.081	0.042
3	0.045	−0.054	0.073	0.737	0.123	−0.303	0.082	0.046
4	0.039	−0.064	0.076	0.698	0.132	−0.316	0.083	0.042

Abbreviations: R², coefficient of determination; β, standardized beta regression coefficient; SE, standard error; FFM, fat-free mass; FFMI, fat-free mass index; SGRQ, St George’s Respiratory Questionnaire; 25(OH)D, 25-hydroxyvitamin D.

Notes: Statistical models (1) unadjusted; (2) adjusted for sex; (3) adjusted for sex and age; (4) adjusted for sex, age and smoking status and (5) adjusted for age, sex, smoking status and FFMI. Values in bold indicate a significant difference, P⩽0.05.

Table 4 Muscle function outcomes according to vitamin D insufficiency (25(OH)D ⩽50 nmol/L) and sufficiency (≥50 nmol/L) in COPD patients at the end of winter and at the end of summer

	End of winter			End of summer
	Insufficient (<50 nmol/L)	Sufficient (≥50 nmol/L)	P-value	Insufficient (<50 nmol/L)	Sufficient (≥50 nmol/L)	P-value
Housebound (%)	36.8	18.2	0.425	52.2	15.4	0.015
Muscle strength (kg)	24.4±11.0	24.3±11.5	0.559	19.0±9.5	27.6±11.6	0.016
SGRQ (%)	62.7±18.4	51.8±24.2	0.189	71.3±9.4	54.5±5.5	0.014

Notes: Chi-squared crosstabs test to assess associations between patients who are housebound and level of vitamin D sufficiency. Independent samples t-test to assess significant differences in muscle strength and SGRQ between vitamin D insufficient and vitamin D sufficient individuals. Values in bold indicate a significant difference, P⩽0.05.

Abbreviations: SGRQ, St George’s Respiratory Questionnaire; 25(OH)D, 25-hydroxyvitamin D.