Study design

The CoLaus Study (www.colaus-psycolaus.ch) is a prospective study designed to assess the prevalence of cardiovascular risk factors and to identify new molecular determinants of cardiovascular disease in the population from Lausanne (Switzerland). The baseline and the follow-up methodologies of the CoLaus study have been reported previously [16, 17]. Briefly, recruitment began in June 2003 and ended in May 2006. The follow-up visit was performed between April 2009 and September 2012 and was similar to the baseline evaluation. As body temperature was collected only at the follow-up visit, only data from this visit were used.

Anthropometric data

Participants were asked to attend the outpatient clinic at the Lausanne university hospital in the morning after an overnight fast. The data were collected by trained field interviewers in a single visit lasting about 60 min. Participants had to be fasting, take their medication as usual, avoid strenuous physical activity during the previous 12 h and abstain from consuming caffeine or alcohol-containing beverages during 24 h before the analysis.

Body weight and height were measured with participants standing without shoes in light indoor clothing. Body weight was measured in kilograms to the nearest 0.1 kg using a Seca™ scale (Seca, Hamburg, Germany). Height was measured to the nearest 5 mm using a Seca™ height gauge (Seca, Hamburg, Germany). BMI was defined as weight (kg)/height2 (m2). Underweight was defined as BMI <18.5 kg/m2; normal weight as BMI ≥18.5 and <25 kg/m2; overweight as BMI ≥25 and <30 kg/m2 and obesity as BMI ≥30 kg/m2.

Waist circumference was measured twice with a non-stretchable tape over the unclothed abdomen at the mid-point between the lowest rib and the iliac crest. Hip circumference was also measured twice at the greater trochanters. For waist and hip, the mean of the two measurements was used and waist-to-hip ratio (WHR) was calculated.

Fat mass was assessed by electrical bioimpedance in the lying position after a 5-min rest using the Bodystat® 1500 body mass analyzer (Bodystat Ltd, Isle of Man, England). This device has been shown to correlate well (r = 0.968) with measurements from dual energy X-ray absorptiometry (DEXA) [18]. In a subset of 794 CoLaus women who had also their body composition assessed by DEXA, the correlation between fat mass estimated by bioimpedance and DEXA was 0.852 (p < 0.001). All metallic adornments were removed, and measurement was performed after a 5-min rest in the lying position. The electrodes were positioned in the right side of the body according to the manufacturer’s instructions. Care was taken to ensure that the participants did not touch any metallic component of the bed and that the inner part of the thighs did not touch each other. Results were obtained as percentage (%BF); body fat mass was calculated as weight × %BF and expressed in kg. Non-fat mass was obtained by subtracting body fat mass from body weight. (Non) fat mass indexes were calculated as (non) body fat mass (kg)/height2 (m2). Body area was assessed using the method of Mosteller [19].

Temperature measurement

Body temperature was measured in degrees Celsius (°C) to the nearest 0.1 °C using a tympanic thermometer (Genius™ 2, Covidien, Dublin, Ireland) according to the manufacturer’s instructions. The measurement was performed in a temperature-controlled room ~20 min after the participant’s admittance.

Other data

Smoking status was categorized into never, former, and current smoker. Menopause was defined as the absence of menstruations for >1 year. All drugs (prescribed or over the counter) were systematically screened for acetaminophen, NSAIDs or corticosteroids. Resting heart rate was measured thrice on the right arm, after at least 10 min rest in the seated position, using an Omron® HEM-907 automated oscillometric sphygmomanometer (Matsusaka, Japan). Values averaged between the last two readings were used.

Most biological assays were performed by the clinical laboratory of the Lausanne university hospital on fresh blood samples within 2 h of blood collection. The following analytical procedures (with maximum inter and intra-batch CVs) were used on cobas® 8000, Roche Diagnostics, Basel, Switzerland: glucose by hexokinase (1.6%; 0.8%); high sensitive CRP by immunoturbidimetry HS (8.0%; 7.4%); insulin by ECLIA (electrochemiluminescence method) (3.7%; 1.5%). Care was taken that no hemolysis was present so not to bias the results. The assay has been validated and is used for diagnostic procedures, and the technical documentation can be obtained from the authors upon request.

Inclusion and exclusion criteria

Participants were excluded if they (1) missed data for temperature; (2) missed data for BMI, waist and hip; (3) reported regular or occasional use of acetaminophen, NSAIDs or corticosteroids; (4) presented with an inflammatory syndrome, defined as a high-sensitivity C-reactive protein (hs-CRP) level ≥20 mg/l, and (5) missed data regarding menopausal status (women only). For sensitivity analyses, participants were further excluded if they missed data for bioimpedance.

Statistical analysis

Statistical analyses were performed with Stata® version 14.1 (Stata Corporation, College Station, TX, USA). As body composition and adiposity markers differ considerably by gender, analyses were stratified by gender. As menstrual cycle influences body temperature in women, a further stratification on menopausal status was performed. Due to their distribution, hs-CRP and insulin were log transformed prior to analyses. Results were expressed as mean ± standard deviation for continuous data or number of participants (percentage) for categorical data. Bivariate analyses were performed using Student’s t-test or analysis of variance for continuous data and χ2-test for categorical data. Bivariate associations between temperature and adiposity and metabolic markers were assessed by Spearman correlation. Multivariable associations between body temperature and continuous markers were assessed using linear regression and the results were expressed as standardized coefficients, which can be interpreted as multivariable-adjusted correlation coefficients. Multivariable associations between body temperature and BMI categories were assessed using analysis of variance and results were expressed as multivariable-adjusted mean ± standard error; test for a linear trend was performed using command contrast p. of Stata®. Statistical significance was considered for a two-sided test with p < 0.05.

Ethical statement

The institutional Ethics Committee of the University of Lausanne, which afterwards became the Ethics Commission of Canton Vaud (www.cer-vd.ch) approved the baseline CoLaus study (reference 16/03, decisions of 13th January and 10th February 2003); the approval was renewed for the first follow-up (reference 33/09, decision of 23rd February 2009). The full decisions of the CER-VD can be obtained from the authors upon request. The study was performed in agreement with the Helsinki declaration and its former amendments, and in accordance with the applicable Swiss legislation. All participants gave their signed informed consent before entering the study.