The regulations of the Swiss federal authority specify that all young Swiss men be called up for conscription during the year in which they turn 19. Conscription either slightly earlier or later is possible upon request. The medical assessments that are part of the conscription process include not only the measurement of height and weight but also the recording of the socioeconomic status (indicated by current occupation) and the place of residence of every conscript, including those who subsequently receive either a deferral or an exemption. The medical examinations are made under professional medical supervision at six dedicated conscription centers with identical qualitative standards for technical equipment and organizational structures. Previous studies have shown that Swiss conscription data are representative for more than 90 % of a given male birth cohort . However, the medical causes of the up to 10 % classified in absentia as unfit for service include the full range of severe diseases and severe physical and psychiatric disabilities. Because of the size of this group and the fact that the list of all possible reasons is not limited to diseases linked to lipid profiles, absenteeism was not considered to have an appreciable effect on the cholesterol distribution.
The multiday recruitment concept also includes a sports performance test (Test Fitness Rekrutierung, or TFR) consisting of five components: speed-strength of the legs, speed-strength of the arms, muscular strength of the global trunk, coordination and endurance (maximum number of points: 125). Approximately 80 % of the present conscripts are physically evaluated by the sports performance test/TFR [18, 19]. The medical examination of the conscripts includes a voluntary laboratory test, for which approximately 65 % of the young men consent at conscription. The blood samples are taken by medical personnel at the conscription centers (fasting status is not recorded) and shipped to a laboratory center in Allschwil (Viollier AG) to be tested by state-of-the-art equipment and assays, usually within 12 h by laboratory personnel. Until 2013, the laboratory test included a small chemistry profile including TCL (no cholesterol subfractions were determined). TCL was measured by enzymatic assay (‘CHOL_2’) redundantly on two Siemens Advia 1650 (Siemens Healthcare Diagnostics AG, Zürich, Switzerland). In August 2012, two Siemens Advia 1800 analyzers replaced them; however, the new devices are identical to their predecessors in terms of measurement techniques and reagents. The laboratory ensured application of identical measurement standards (evaluated by regular internal and external quality control) during the entire time period under observation.
Data and study population
Fully anonymized individual conscription records for the period 01/01/2004-31/12/2012 (N = 345,684) were provided by the Swiss Army (Logistikbasis der Armee - Sanität) in February 2013 under contractual agreement with the study authors. The data included date of birth, date of conscription, height (cm), weight (kg), current occupation (recorded as free-text entry), postcode of place of residence, stage of conscription (first, regular visit versus reassessment, NIAX-code), sports test/TFR results (1–125 points), and TCL (mmol/l). To ensure temporal comparability, we restricted our analyses to the time period during which TCL was measured with identical laboratory test devices and standards (11/07/2006-31/12/2012, N = 269,636). In addition, we restricted our dataset to male conscripts appearing for the first, regular assessment in the recruitment centers (NIAX conscription status code = S, N = 265,569). The dataset was checked for implausible height, weight, sports test results and cholesterol level values; none were found. We calculated BMI (weight [kg]/height [m]2) and age at conscription based on date of birth and date of conscription. BMI was categorized according to the official WHO subgroups for underweight (BMI < 18.5 kg/m2), normal weight (BMI 18.5–24.9 kg/m2), overweight (BMI 25.0–29.9 kg/m2) and obesity (BMI ≥ 30.0 kg/m2, including the official subcategories) . Furthermore, each conscript was assigned to one of seven regions (Grossregionen: Région Lémanique, Mittelland, Nordwestschweiz, Zürich, Ostschweiz, Zentralschweiz, Ticino) on the basis of the ZIP-code of his residential community . We also converted free-text entries of the current occupation to the International Socio-Economic Index of occupational status (ISEI) code as described by Ganzeboom et al. . The ISEI major groups (“low,” “medium,” and “high”) were then built by dividing the ISEI scores into tertiles according to Panczak et al. . We created separate categories for individuals who were still in school (“pupils”) and for those with insufficient or missing data for the occupational status (“imprecise”). The individual sports test/TFR scores (1-125 points, available for N = 225,388 or 84.9 %) were then categorized into tertiles, (“low”, “medium”, “high”), in order of increasing physical performance. Further, elevated TCL was categorized in “borderline high” (5.17-6.18 mmol/l) and “high” (≥6.18 mmol/l) .
Excluding individuals not participating in the voluntary laboratory test, the sample for which TCL was available consisted of 174,872 conscripts (65.8 %, Appendix Table 4). Overall, voluntary blood-test participants and non-participants were similar in terms of mean age – 19.75 years (95 % CI 19.75–19.76) versus 19.89 years (95 % CI 19.88–19.89) – and mean BMI – 23.39 kg/m2 (95 % CI 23.37–23.40) versus 23.40 kg/m2 (95 % CI 23.37–23.42). The yearly comparison between participants and non-participants is reported in Appendix Table 4: The blood-analyses participation rate was steady, lying between 60.4 % and 68.5 % for each of the conscription years between 2006 and 2012 (overall 65.8 %). Age distribution was very similar among participants and non-participants, with the 19-year-old conscripts contributing 45.4 % and 43.6 %, respectively, to the whole dataset and both the 18-year-old and the 20-year-old conscripts contributing 20.3 % to 22.7 %. Regarding socioeconomic occupational status (ISEI), the participant and the non-participant populations were comparable as well. However, non-participants were – in a low single-digit percentage – slightly less overweight, more obese and had slightly lower sports test results than the participants. The greatest differences between participants and non-participants were found in their regional origins: Non-participants were more likely to be residents of Région Lémanique and Nordwestschweiz and less likely to be from Zürich and Ostschweiz, whereas the percentages of participants from Mittelland, Zentralschweiz, and Ticino were again comparable to those of non-participants. All of the percentages reported in Appendix Table 4 are surprisingly stable over time when single conscription years are compared. The total number of conscripts in the first analyzed conscription year, 2006 (N = 18,502), is lower than the total number in the other years (N between 39,115 and 43,201) because only laboratory tests after 11/07/2006 were included.
To test the association between TCL and draft year, age, socioeconomic status, BMI, sports test performance, and regional origin, a multivariable linear regression analysis with TCL as dependent variable was performed. We further controlled TCL among BMI subgroups by sports test performance/TFR. Stata version 13 (Stata Corporation, College Station, TX, USA) was used for all analyses.
The data and the permission to use them were available from the Swiss Armed Forces (Logistikbasis der Armee) upon submission and approval of a study protocol. According to the bilaterally signed data contract, the Swiss Armed Forces fully anonymized the records by removing all names, social security numbers, and exact residential addresses prior to delivery to the study authors. The anthropometric and laboratory data used in this study are considered nonclinical, governmental data. According to Swiss federal law (Bundesgesetz über die militärischen Informationssysteme MIG, BG 510.91, Art. 2, 9, 24–29), the Swiss Army is authorized to make these data accessible in an anonymous form for academic research. Furthermore, the conscripts sign a detailed informed consent form for the voluntary laboratory test (available from the Swiss Army upon request). As for anonymized and nonclinical governmental data, no additional ethical approval is needed (Swiss data privacy act, SR 235.1; 19.6.1992 and Federal Act on Research involving Human Beings HRA, 810.30; 1.1.2014).