Serum selenium level and risk of lung cancer mortality: a 16-year follow-up of the Copenhagen Male Study

Patient characteristics

The CMS was set up in 1970–1971 as a prospective cardiovascular cohort study of 5,249 Caucasian males with a mean age of 48 yrs (range 40–59 yrs). The examination comprised a short interview based on a previously completed questionnaire from which information was obtained on lifestyle, including smoking habits [11, 12]. Based on information about education and job profile, the males were subdivided into five social classes as previously described [13].

In 1985–1986, a new baseline was established giving more comprehensive information about study participants. This baseline was used for the present study. All males from the 1970–1971 study were traced by means of the Danish Central Population Register. Between June 1985 and June 1986, all survivors (except 34 emigrants) from the original cohort were invited to take part in this study; 3,387 (75%) males agreed and gave informed consent; their mean age was 63 yrs (range 53–74 yrs). The 1985–1986 study took place as part of the Glostrup Population Studies at Glostrup Hospital (University of Copenhagen, Copenhagen, Denmark). Each subject was interviewed about a previously completed questionnaire and subjected to a clinical examination; a venous blood sample was taken for determination of serum selenium concentration. With respect to serum selenium and lung cancer mortality diagnoses, useful information was available for 3,333 males. The proportion of missing values did not exceed 3% for any other single variable.

Serum selenium determination

Serum selenium level was determined at Medisinsk Laboratorium (Oslo, Norway) using a graphite furnace atomic absorption spectrophotometric method after a simple dilution with a solution containing nickel in nitric acid [9]. The solutions were analysed for selenium at 196 nm with a deuterium background corrector after single injections into the graphite furnace and a standard curve of reference serum was used for the calibration. All samples were analysed in duplicate. The coefficient of variation was 5.5%.

As described in our previous report on serum selenium and ischaemic heart disease risk [9], it has been shown that at levels of selenium in whole blood >100 μg·L⁻¹ and of plasma selenium >1.25 μmol·L⁻¹, there is no noticeable increase in the glutathione peroxidase enzyme activity, indicating saturation. The indicated levels both correspond to ∼1 μmol·L⁻¹ of serum selenium. Almost one-third of the males in the present study had serum levels ≤1 μmol·L⁻¹. Accordingly, for analysis, we subdivided the population into three groups, i.e. into males with the lowest, potentially insufficient level of selenium (≤1 μmol·L⁻¹) and those with higher levels were subdivided into two almost equally large groups.

Tobacco smoking habits

Subjects classified themselves as never-smokers, previous smokers or current smokers. Current tobacco consumption was calculated from information about the number of cigarettes, cheroots or cigars, or the weight of pipe tobacco smoked daily. One cigarette was taken as equivalent to 1 g tobacco, one cheroot as 3 g tobacco and one cigar as 4 g tobacco. As previously estimated by means of serum cotinine, the validity of tobacco reporting in the CMS was high [14]. Number of pack-years was calculated based on information about average total daily use of tobacco in grams multiplied by number of years as a smoker divided by 20.

Dietary factors

Salt and fat intake was crudely measured based on two simple questions: “Do you use more salt in your food than others?” and “Do you try to avoid fatty foods?” We have previously shown that both these dietary markers were associated with risk of lung cancer mortality, independently of tobacco history and other factors [10].

Alcohol consumption

Total weekly consumption of alcohol was calculated from questionnaire items about average alcohol consumption on weekdays and at weekends. Intakes of beer, wine and spirits were reported separately. One drink corresponded to 10–12 g ethanol.

Occupational exposures

Information on potentially relevant physical and chemical respirable occupational exposures was obtained from the questionnaire. Long-term exposure was defined as frequent occupational exposure for ≥5 yrs. The question was phrased as follows: “Have you at your present or previous work places often, i.e. several times per week or more, been exposed to dust? If yes, for how many years?”

End-points

Notification of death and causes of death within the period from 1985–1986 to the end of 2001 was obtained from the Danish Register of Causes of Death. The International Classification of Diseases (ICD) lung cancer mortality diagnoses used were codes ICD-8 162 and, from 1994, ICD-10 C34. Information on histological type of lung cancer was available for 128 out of the 170 lung cancer deaths.

Statistical analysis

All basic analyses, including unpaired t-test, Chi-squared analyses, Mann–Whitney rank-sum tests and regression analyses, were performed with the SPSS statistical software for Windows (IBM, Somers, NY, USA). Relative risks were estimated by exp(β), where β is the hazard coefficient for the variable of interest in Cox proportional hazards regression models following different adjustments. Assumptions for use of Cox proportional hazards were met. A two-sided probability value of p≤0.05 was a priori taken as significant.

Ethics

Each participant was informed that all personal data were confidential and gave written consent to participate. The study was approved by the Committee for Ethics in Medical Research of the County of Copenhagen (Copenhagen, Denmark).