The Whitehall II study was established in 1985 as a longitudinal population-based study to examine the socioeconomic gradient in health and disease among 10,308 civil servants (6,895 men and 3,413 women).(18
) All civil servants aged 35–55 years in 20 London based departments were invited to participate by letter. In total, 73 percent of those invited agreed to take part in Phase 1. Baseline examination (Phase 1) took place during 1985–1988, and involved a clinical examination and a self-administered questionnaire containing sections on demographic characteristics, health, lifestyle factors, work characteristics, social support and life events. Clinical examination included measures of blood pressure, anthropometry, biochemical measurements, neuroendocrine function, and sub-clinical markers of cardiovascular disease. Subsequent phases of data collection have alternated between postal questionnaire alone (phases 2, 4, 6) and postal questionnaire accompanied by a clinical examination (phases 1, 3, 5 and 7). The median length of follow up from Phase 1 to Phase 7 was 17 years, with 535 individuals dying during this period. The clinical examination where physical functioning and cognitive tests were administered, phase 7, was attended by 6944 participants. The University College London Ethics Committee approved the study.
Measures of socioeconomic position (SEP) at phase 1: Based on salary and work role, the civil service defines a hierarchy of employment grades ranging from senior executive officers to clerical and support staff. This is a three level variable representing high (administrative grades), intermediate (professional or executive grades) and low (clerical or support grades) SEP. People in the three SEP groups differ with respect to salary, social status and level of responsibility. Although mostly white collar, respondents covered a wide range of SEP, with annual salaries in 1995 ranging from £4,995 to £150,000.
Early life factors (father’s social class, age when left education and adult height) were ascertained from self-completed questionnaire and clinic data. Adult height was used in this way as it has been shown to be associated with prenatal and childhood exposures. (19
Measures of health behaviors at Phase 1: Smoking, exercise, diet, and alcohol were assessed by questionnaire. Smoking was grouped as current, ex-smoker and never-smokers. Exercise was derived from questions on frequency and number of hours per week taken in activities that were mildly energetic, moderately energetic or vigorous. These were grouped as vigorous or moderate if they did one or more hours per week of these, or as none/mild. A summary index of poor diet was defined if 2 or all of the following applied: most frequently used bread was white, usually used milk was whole, and fruit or vegetables were eaten less often than daily. Alcohol consumption in the last week was expressed in units of alcohol where 1 unit = 8 grams ethanol.
Measures of psychosocial factors at Phase 1: The first set of factors were work based and consisted of the central components of the job strain model, that is, psychological job demands, decision latitude, and social support at work (20
). Four items dealt with psychological job demands, and 15 items dealt with decision authority and skill discretion, which were combined into an index of decision latitude (or job control). Social support at work is the sum of two subscales: support from co-workers and support from supervisor. Social network was assessed using an adapted version of the Berkman & Syme scale (1979). This scale assesses the frequency and the number of friends, relatives and work colleagues seen; and participation in social and religious groups; high scores indicating a bigger network (21
). Sample selection: Participants were classed as aging successfully if they were free from major disease up to Phase 7 and had good physical and mental functioning at Phase 7. For the analysis of successful aging we included the 5,823 men and women with no prevalent disease at phase 1, who had measures of functioning at phase 7 and who had attended five or more phases of follow-up. The latter exclusion was to reduce potential reporting bias since those who attend most phases have the greatest opportunity for declaring the presence of major diseases. Any residual confounding due to this was removed by adjusting for the number of phases attended in all analyses. Prevalent disease at Phase 1 was defined as a self-report in the questionnaire on coronary heart disease (CHD), cancer, diabetes or depression. The incidence of disease (CHD, stroke, cancer, diabetes, depression and the ATPIII metabolic syndrome) was determined from all relevant data collected between Phase 1 and Phase 7 from self-reports in questionnaires, medication use and from clinical examinations (metabolic syndrome) together with supporting evidence from general practitioners and hospitals (CHD). Good functioning at Phase 7 was defined as being in the best third of the sex-specific distribution for three or four of the four measures: Walking speed, lung function, Alice Heim 4-I (AH4-I) cognitive test and Physical Component Score of Short Form36 General Health Survey (22
) (or two or three in the top third for the 26 percent of subjects who only had three of the four measures). Walking speed was measured by a trained study nurse over a clearly marked eight foot walking course. Participants were asked to “walk to the other end of the course at your usual walking pace, just as if you were walking down the street to go the shops. Walk all the way past the other end of the tape before you stop”. Times were recorded in seconds to two decimal places. Three tests were conducted and the mean walk time was used in the analysis. Lung function was assessed using forced expiratory volume (FEV) which is the volume of air expelled in the first second of a forced expiration starting from full inspiration (23
) The AH4-I is composed of a series of 65 verbal and mathematical reasoning items of increasing difficulty. This is a test of inductive reasoning that measures the ability to identify patterns and infer principles and rules (24
Logistic regression analyses were performed, separately in men and women, to determine the association between phase 1 factors and successful aging at phase 7. All analyses were adjusted for age at Phase 1 and the number of phases attended (5, 6 or 7) to remove potential reporting bias since participants who came to more phases had more opportunities to report adverse health outcomes. The overall effect of early life factors, midlife health behaviors and psychosocial factors were summarized by creating a score for each from the individual factors (see ). A high score on each summary measure indicates a more favourable level. The effect of each of these on successful aging was expressed per 1 standard deviation change in each score.
Scores allocated to each measure to create socio-economic position, early life factor, health behaviors and psychosocial factor.
We undertook additional sets of sensitivity analyses using slight variations on the definition of successful aging in order to assess whether it influenced the role of risk factors for successful aging. First, successful aging was defined using only the disease criteria and then only the functioning criteria. Second, the occurrence of metabolic syndrome was excluded from the definition of disease in recognition that it measures a constellation of risk factors not a clinical disease. Finally, cognitive functioning was excluded from the definition of successful aging.