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Sauna bathing is inversely associated with dementia and Alzheimer’s disease in middle-aged Finnish men

TANJANIINA LAUKKANEN 1,SETOR KUNUTSOR 2,JUSSI KAUHANEN 1,JARI ANTERO LAUKKANEN 1

1 Institute of Public Health and Clinical Nutrition, University of Eastern Finland, Kuopio, Finland

2 School of Clinical Sciences, University of Bristol, Bristol, UK

Address correspondence to: L. Tanjaniina. Tel: +358505053013. Email: tanjaniina.laukkanen@uef.fi

Abstract

Background: there are no previous studies linking repeated heat exposure of sauna and the risk of memory diseases. We aimed to investigate whether frequency of sauna bathing is associated with risk of dementia and Alzheimer’s disease. Setting: prospective population-based study.

Methods: the frequency of sauna bathing was assessed at baseline in the Kuopio Ischaemic Heart Disease population-based prospective cohort study of 2,315 apparently healthy men aged 42–60 years at baseline, with baseline examinations conducted between 1984 and 1989. Hazard ratios (HRs) with 95% confidence intervals (CIs) for dementia and Alzheimer’s disease were ascertained using Cox-regression modelling with adjustment for potential confounders.

Results: during a median follow-up of 20.7 (interquartile range 18.1–22.6) years, a total of 204 and 123 diagnosed cases of dementia and Alzheimer’s disease were respectively recorded. In analysis adjusted for age, alcohol consumption, body mass index, systolic blood pressure, smoking status, Type 2 diabetes, previous myocardial infarction, resting heart rate and serum low-density lipoprotein cholesterol, compared with men with only 1 sauna bathing session per week, the HR for dementia was 0.78 (95% CI: 0.57–1.06) for 2–3 sauna bathing sessions per week and 0.34 (95% CI: 0.16–0.71) for 4–7 sauna bathing sessions per week. The corresponding HRs for Alzheimer’s disease were 0.80 (95% CI: 0.53–1.20) and 0.35 (95% CI: 0.14–0.90).

Conclusion: in this male population, moderate to high frequency of sauna bathing was associated with lowered risks of dementia and Alzheimer’s disease. Further studies are warranted to establish the potential mechanisms linking sauna bathing and memory diseases.

Keywords: Older people, dementia, Alzheimer’s disease, prospective study, epidemiology

Introduction

Although there is a direct correlation between ageing and the incidence of dementia, its risk factors and pathogenesis are not fully understood and various possible preventive strategies are still under investigation. A multifactorial aeti-ology has been proposed and pathways notably implicated include inflammation, haemodynamic and vascular function [1, 2]. Evidence from observational studies suggests that poor vascular function (as indicated by arterial stiffening) and vascular diseases maybe associated with worse cognitive per-formance and dementia [3–5]. Given this, there remains a pos-sibility that improved circulatory function may be linked to better cognitive outcomes. Sauna bathing, an activity linked to relaxation and well-being, is associated with better haemo-dynamic function, leads to skin sweating-induced fluid loss and increase in heart rate [5, 6]; andhas beensuggestedto be associated with a better cardiovascular and circulatory func-tion, especially among patients with chronic heart failure [7–11]. Previous studies have also shown that heat exposure of sauna bathing is associated with lower blood pressure [12, 13]; however, some of these reports were based on limited study designs. We have recently shown that regular sauna bathing is associated with reduced risks of fatal cardiovascular diseases and all-cause mortality [14]. The relationship between sauna bathing and dementia is however not known. Thus, the aim of this prospective study was to investigate the association between frequency of sauna bathing and the risk of dementia and Alzheimer’s disease.

Methods

Study population

This study was designed to investigate risk predictors for atherosclerotic cardiovascular outcomes in a population-based sample of men from eastern Finland. Subjects com-prised a randomly selected sample of 3,433 men aged 42–60 years who resided in Kuopio or its surrounding rural communities [15]. Of those invited, 2,682 (83%) partici-pated in the study, and those with complete information on sauna bathing were included (N = 2,327). Twelve men who did not use sauna were excluded, leaving 2,315 men for the analyses [14]. Baseline examinations were conducted between March 1984 and December 1989. The study was approved by the research ethics committee of the University of Eastern Finland, Kuopio, Finland. Each participant gave written informed consent.

Assessment of sauna bathing habits

A traditional Finnish sauna has dry air (10–20%) with rela-tively high temperature. The recommended temperature for sauna is usually from 80°C to 100°C at the level of the bather’s face [16]. Humidity is temporarily increased by throwing water on the hot rocks of sauna heater. The use of sauna bathing was assessed by a self-administered ques-tionnaire based on weekly sauna sessions, duration and temperature. The assessment represents a typical sauna use during the week. The temperature in sauna room was mea-sured using a thermometer in and was also self-reported. The questionnaires were checked by an experienced nurse at the time of baseline examination [14].

Assessment of baseline characteristics

A subject was defined as a smoker if he had ever smoked on a regular basis and pack-years were defined. Resting blood pressure was measured between 8:00 and 10:00 AM with a random-zero sphygmomanometer. Alcohol consumption was assessed using the Nordic Alcohol Consumption Inventory [17]. Body mass index (BMI) was calculated as the ratio of weight in kilograms to the square of height in metres. The collection of blood specimens and the measurement of serum lipids, lipoproteins, creatinine and glucose have been described previously [17]. Diabetes was defined as a fasting blood glucose level ≥7.0 mmol/l or clinical diagnosis of dia-betes with dietary, oral or insulin treatment. The use of medi-cations, medical history and the level of physical activity were assessed by self-administered questionnaires. Physical activity (kcal/day) was computed by multiplying the duration and intensity of each physical activity by body weight and was assessed using a 12-month physical activity questionnaire described in detail previously [17]. Briefly, this detailed quantita-tive questionnaire deals with the most common leisure-time physical activities of middle-aged Finnish men (conditioning physical activity, e.g. walking, skiing, bicycling, swimming, row-ing and ball games; and non-conditioning physical activity, e.g. crafts, repairs, building, gardening, hunting and fishing) and enables the assessment of all components of physical activity [18]. Prevalent coronary heart disease was defined as either a previous myocardial infarction, angina pectoris or the use of nitroglycerin for chest pain once a week or more frequently. Socio-economic status is a summary index that combines mea-sures of income, education, occupation, occupational prestige, material standard of living and housing conditions, all of which were assessed with self-reported questionnaires [14].

Ascertainment of outcomes

All dementia and Alzheimer’s disease cases that occurred from study enrolment through 2013 were included. There were no losses to follow-up given the complete follow-up of the Finnish population using personal identification codes. In the Kuopio Ischemic Heart Disease risk factor study, par-ticipants are under annual monitoring for incident dementia cases and cardio-metabolic outcomes [14, 17]. Data on out-come events were obtained by record linkage to the national computerised hospitalisation registry, which covers every hos-pitalisation in Finland. The diagnoses of memory diseases were coded according to the International Classification of Diseases Codes. In addition, sources of information on dementia outcomes were also based on a comprehensive review of hospital records, inpatient physician claims data and medico-legal reports. An independent committee of researchers reviewed all potential cases of dementia to obtain a consensus on the diagnosis and aetiology.

Statistical analysis

For all analyses, natural logarithm (loge) transformed values of non-normal distributed variables were used. Descriptive data are presented as means (SDs) for continuous variables and numbers (percentages) for categorical ones. Analyses of the associations between sauna bathing, dementia and Alzheimer’s disease involved Cox-regression modelling. Hazard ratios (HRs) with confidence intervals (CIs) were estimated per frequency of sauna bathing with adjustment for potential confounders selected on the basis of their previ-ously established role as risk factors [14]. Two-sided analyses were performed using SPSS version 18.0 (IBM Corp., Armonk, NY, USA) and CIs are presented at the 95% level.

Results

Baseline characteristics of the study population are shown in Table 1. During a median follow-up of 20.7 (interquartile range 18.1–22.6) years, a total of 204 newly diagnosed dementia and 123 Alzheimer’s disease cases were recorded; only 4 dementia cases and 6 Alzheimer disease cases were defined at time of death. Of all men with diagnosed out-come events, 124 men with dementia and 67 men with Alzheimer’s disease died during the follow-up period, respectively. A total of 601, 1,513 and 200 participants reported having a sauna bath once a week, 2–3 times and 4–7 times per week, respectively. The numbers (proportions

Table 1. Baseline characteristics of the study participants (N = 2,315)

in each group) of dementia cases were 59 (10%), 137 (9%) and 8 (4%) respectively in the three frequency groups of sauna bathing. The corresponding numbers (proportions) of Alzheimer’s disease cases were 34 (6%), 84 (6%) and 5 (3%), respectively.

After adjustment for age, compared with men with only 1 sauna bathing session per week, the HR of dementia was 0.77 (95% CI: 0.57–1.04) for 2–3 sauna bathing sessions per week and 0.38 (95% CI: 0.18–0.80) for 4–7 sauna bath-ing sessions per week. The corresponding HRs for Alzheimer’s disease were 0.80 (95% CI: 0.54–1.2) for 2–3 sauna bathing sessions per week and 0.41 (95% CI: 0.16–1.07) for 4–7 sauna bathing sessions per week.

In a multivariable analysis adjusted for baseline age, alco-hol consumption, BMI, systolic blood pressure, smoking status, Type 2 diabetes, previous myocardial infarction, rest-ing heart rate and serum low-density lipoprotein cholesterol, there was a 66% risk reduction (HR: 0.34) of dementia and 65% (HR: 0.35) risk reduction of Alzheimer’s disease when sauna bathing 4–7 times a week was compared with subjects having 1 sauna session per week (Table 2). After additional adjustment for physical activity and socio-economic status, the respective associations between sauna bathing and the risk of memory diseases (HR 0.34, 95% CI: 0.16–0.71, P = 0.004 for dementia and HR 0.35, 95% CI 0.16–0.90, P = 0.029 for Alzheimer’s disease) remained similar for menwith4–7 sauna bathing sessions per week.

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