Joint associations of sauna bathing and cardiorespiratory fitness on cardiovascular and all-cause mortality risk: a long-term prospective cohort study

Figures & data

Table 1. Baseline characteristics of study participants.

	Mean (SD) or median (IQR) or n (%)
Cardiorespiratory fitness (ml/kg/min)	30.3 (8.0)
Frequency of sauna bathing (times/week)	2 (1–2)
Questionnaire/prevalent conditions
Baseline age (years)	53 (5)
Alcohol consumption (g/week)	74.2 (121.8)
History of type 2 diabetes	79 (3.5)
Current smoking	714 (31.4)
History of CHD	539 (23.7)
Physical measurements
BMI (kg/m^2)	26.9 (3.5)
SBP (mmHg)	134 (17)
DBP (mmHg)	89 (10)
Physical activity (KJ/day)	1539 (1402)
Socio-economic status	8.41 (4.23)
Lipid markers
Total cholesterol (mmol/l)	5.91 (1.07)
HDL-C (mmol/l)	1.29 (0.30)
Triglycerides (mmol/l)	1.09 (0.79–1.54)
Metabolic, inflammatory and renal markers
Fasting plasma glucose (mmol/l)	5.33 (1.20)
C-reactive protein (mg/l)	1.24 (0.69–2.37)
Serum creatinine (µmol/1)	89.7 (21.5)
Estimated GFR (ml/min/1.73 m^2)	87.1 (17.2)

BMI: body mass index; CHD: coronary heart disease; DBP: diastolic blood pressure; GFR: glomerular filtration rate; HDL-C: high-density lipoprotein cholesterol; SD: standard deviation; SBP: systolic blood pressure.

Figure 1. Cumulative Kaplan–Meier curves for cardiovascular and all-cause mortality during follow-up according to CRF and sauna bathing frequency groups. CRF: cardiorespiratory fitness; CVD: cardiovascular disease; FSB: frequency of sauna bathing.

Table 2. Associations of cardiorespiratory fitness with risk of cardiovascular and all-cause mortality.

		Model 1		Model 2		Model 3
CRF (ml/kg/min)	Events/total	HR (95% CI)	p Value	HR (95% CI)	p Value	HR (95% CI)	p Value
CVD mortality
Low CRF (6.36–30.03)	378/1139	Ref		Ref		Ref
High CRF (30.04–65.40)	142/1138	0.34 (0.28–0.42)	<.001	0.49 (0.39–0.60)	<.001	0.51 (0.41–0.63)	<.001
All-cause mortality
Low CRF (6.36–30.03)	722/1139	Ref		Ref		Ref
High CRF (30.04–65.40)	402/1138	0.51 (0.45–0.58)	<.001	0.63 (0.55–0.72)	<.001	0.65 (0.57–0.75)	<.001

CI: confidence interval; CRF: cardiorespiratory fitness; HR: hazard ratio; ref: reference; Q: quartile; Cut-off for CRF was based on the median.

Model 1: Adjusted for age.

Model 2: Model 1 plus body mass index, smoking status, systolic blood pressure, total cholesterol, high-density lipoprotein cholesterol, history of type 2 diabetes, prevalent coronary heart disease and alcohol consumption.

Model 3: Model 2 plus socioeconomic status and C-reactive protein.

Table 3. Association of frequency of sauna bathing with risk of cardiovascular and all-cause mortality.

		Model 1		Model 2		Model 3
Frequency of sauna bathing (sessions/week)	Events/total	HR (95% CI)	p Value	HR (95% CI)	p Value	HR (95% CI)	p Value
CVD mortality
Low FSB (≤2)	437/1788	Ref		Ref		Ref
High FSB (3–7)	83/489	0.67 (0.53–0.85)	.001	0.73 (0.57–0.92)	.009	0.74 (0.59–0.94)	.014
All-cause mortality
Low FSB (≤2)	921/1788	Ref		Ref		Ref
High FSB (3–7)	203/489	0.77 (0.66–0.90)	.001	0.82 (0.70–0.96)	.011	0.84 (0.72–0.97)	.021

CI: confidence interval; CRF: cardiorespiratory fitness; FSB: frequency of sauna bathing; HR: hazard ratio; ref: reference; Q: quartile

Model 1: Adjusted for age.

Model 2: Model 1 plus body mass index, smoking status, systolic blood pressure, total cholesterol, high-density lipoprotein cholesterol, history of type 2 diabetes, prevalent coronary heart disease and alcohol consumption.

Model 3: Model 2 plus socioeconomic status and C-reactive protein.

Figure 2. Interactions of the associations of CRF and sauna bathing frequency with cardiovascular and all-cause mortality. CI: confidence interval; CRF: cardiorespiratory fitness; CVD: cardiovascular disease; FSB: frequency of sauna bathing; HR: hazard ratio; HRs are adjusted for age, body mass index, smoking status, systolic blood pressure, total cholesterol, high-density lipoprotein cholesterol, history of type 2 diabetes, prevalent coronary heart disease and alcohol consumption; *, p values for interaction.

Table 4. Joint associations of cardiorespiratory fitness and frequency of sauna bathing with risk of cardiovascular and all-cause mortality.

		Model 1		Model 2		Model 3
CRF (ml/kg/min) and frequency of sauna bathing (times/week)	Events/total	HR (95% CI)	p Value	HR (95% CI)	p Value	HR (95% CI)	p Value
CVD mortality
Low CRF & Low FSB	324/931	Ref		Ref		Ref
High CRF & Low FSB	113/857	0.33 (0.27–0.41)	<.001	0.48 (0.38–0.60)	<.001	0.50 (0.39–0.63)	<.001
Low CRF & High FSB	54/208	0.65 (0.49–0.86)	.003	0.71 (0.53–0.95)	.020	0.72 (0.54–0.97)	.029
High CRF & High FSB	29/281	0.27 (0.19–0.40)	<.001	0.40 (0.27–0.59)	<.001	0.42 (0.28–0.62)	<.001
All-cause mortality
Low CRF & Low FSB	609/931	Ref		Ref		Ref
High CRF & Low FSB	312/857	0.49 (0.42–0.56)	<.001	0.61 (0.52–0.71)	<.001	0.63 (0.54–0.74)	<.001
Low CRF & High FSB	113/208	0.72 (0.59–0.88)	.001	0.77 (0.63–0.95)	.013	0.78 (0.64–0.96)	.019
High CRF & High FSB	90/281	0.45 (0.36–0.56)	<.001	0.57 (0.45–0.72)	<.001	0.60 (0.48–0.76)	<.001

CI: confidence interval; CRF: cardiorespiratory fitness; FSB: frequency of sauna bathing; HR: hazard ratio; ref: reference; Q: quartile.

Model 1: Adjusted for age.

Model 2: Model 1 plus body mass index, smoking status, systolic blood pressure, total cholesterol, high-density lipoprotein cholesterol, history of type 2 diabetes, prevalent coronary heart disease and alcohol consumption.

Model 3: Model 2 plus socioeconomic status and C-reactive protein.