Data source 1: The Thai Cohort Study (TCS)

The TCS is a prospective observational study of a large nation-wide cohort of distance learning Open University students. It is a study of health risks and outcomes in transitional Thailand. The overall aims are to investigate how health-risks are changing over time, how they are distributed among Thai adults, and how they affect health status. Also under investigation is the effect of rapid socio-economic development. Appropriate interventions to control and prevent emerging disease epidemics are also considered by the TCS (Sleigh et al., 2008).

To start the TCS in 2005, a 20-page baseline questionnaire was mailed to all 200,000 students enrolled at Sukothai Thammithirat Open University (STOU). A total of 87,151 (44%) students returned their questionnaire and formed the baseline cohort. In 2009, a follow-up questionnaire was sent to all participants of the baseline cohort. Of these a total of 60,569 (70%) were successfully followed-up. In 2013, a further follow-up questionnaire was sent to all participants who responded in 2009; 42,785 (70%) returned a completed questionnaire again in 2013 Table 2.1 shows selected characteristics of the cohort across the three surveys.

Table 2-1 Selected characteristics of TCS participants responding to each survey Characteristics Participants in 2005 n (%) Participants in 2009 n (%) Participants in 2013 n (%) Sex 87,151 60,569 42,785 Male 39,484 (45.3) 27,406 (45.3) 19,330 (45.2) Female 47,659 (54.7) 33,163 (54.8) 23,455 (54.8) Age 15-29 46,714 (53.6) 28,681 (47.4) 18,361 (42.9) 30-39 27,320 (31.4) 20,907 (34.5) 15,576 (36.4) 40-49 10,953 (12.6) 9,173 (15.1) 7,399 (17.3) 50 and over 2,145 (2.5) 1,808 (3.0) 1,449 (3.4)

Body Mass Index

Normal (<23.0) 59,471 (69.2) 40,327 (67.5) 27,937 (66.2) At risk (23.0-24.9) 12,969 (15.1) 9,484 (15.9) 7,018 (16.6) Obese (>25) 13,560 (15.8) 9,953 (16.6) 7,278 (17.2) Income <10,000 55,434 (65.2) 36,500 (61.6) 24,733 (58.9) 10,001-20,000 20,570 (24.2) 15,729 (26.5) 11,857 (28.3) >20,001 8,956 (10.5) 7,042 (11.9) 5,370 (12.8) Education level

Junior high school 3,039 (3.5) 2,023 (3.4) 1,325 (3.1)

High school 39,415 (45.4) 25,974 (43.0) 17,594 (41.2) Diploma/certificate 23,468 (27.0) 16,265 (26.9) 11,384 (26.7) university 20,985 (24.2) 16,165 (26.8) 12,380 (29.0) Current residence Rural 41,748 (48.3) 29,339 (48.8) 21,201 (49.9) Urban 44,764 (51.7) 30,819 (51.2) 21,319 (50.1) Fruit/vegetable Serves per day

<5 serves 52,486 (62.8) 37,046 (63.5) 26,350 (63.7) >5 serves 31,114 (37.2) 21,324 (36.5) 15,026 (36.3) Smoking Never smoked 61,124 (73.6) 42,959 (74.1) 30,646 (74.8) Ex-smoker 13,409 (16.1) 9,596 (16.6) 6,844 (16.7) Current smoker 8,539 (10.3) 5,388 (9.3) 3,508 (8.6) Alcohol intake Never drinks 22,712 (26.4) 16,106 (26.9) 11,620 (27.5) Quit 7,724 (9.0) 5,321 (8.9) 3,633 (8.6) Occasional drinker 51,371 (59.8) 35,433 (59.2) 24,923 (58.9) Regular drinker 4,171 (4.8) 2,981 (5.0) 2,116 (5.0)

2.1.2.1 Eligibility for the TCS

STOU students are an informative group of Thai adults that represent the national population well in terms of demography, geography, religion, and socio-economy (Sleigh et al., 2008). All 200,000 distance-learning students enrolled at STOU in 2005 were eligible to take part in the TCS. At baseline, cohort members were adults ranging in age from 15 to 87, of modest means, who were distance-learning students embedded in their communities all over Thailand. Just over half of this cohort was female and living in an urban area of residence. At baseline, cohort members were younger and had a higher level of education than the national Thai population. Accordingly, they were expected to undergo the health-risk transition ahead of their fellow Thais. Thus this cohort can provide information on health and its determinants that will inform trends in the national Thai population in future years.

2.1.2.2 TCS protocols

All TCS participants were asked to self-complete a questionnaire in 2005 and in 2009. Those who were followed-up in 2009 were also asked to complete an additional survey in 2013. In all three surveys, participants were asked questions about their socioeconomic and demographic characteristics (e.g. age, marital status, income, occupation, education level, area of residence), cultural and lifestyle characteristics (e.g. ethnic/religious practices and their dietary intake), health-risk behaviours (e.g. smoking and alcohol intake, transport, seat belt and helmet use), family characteristics (number of children, cause of parental death), and self-reported health outcomes (e.g. dental health, anxiety and depression, weight and health conditions including diabetes). Details on diabetes, the focus of the thesis, are given below.

In 2005, participants were asked ‘Have you ever been told by a doctor that you have

diabetes (needing or not needing insulin)’; in 2009 participants were asked ‘As a result

of a diagnosis by a doctor, have you ever been told that you have diabetes (needing or

not needing insulin)’; and in 2013 participants were asked ‘Have you ever received a

The three surveys (2005, 2009, 2013) were developed by TCS investigators including experts from the fields of public health, medicine, anthropology, nutrition, demography, and epidemiology. Where possible, the surveys utilized validated and standardized questions (e.g. the Medical Outcomes (SF8) instrument). Surveys were translated and then back translated to ensure that the meanings of the questions remained stable. All returned surveys were scanned using Scan Devet intelligent character recognition software that creates a linked image and digital data for each questionnaire. Alongside the 2005, 2009 and 2013 surveys, additional validation studies have been conducted to assess the validity and reliability of self-reported data (e.g. for height and weight data).

2.1.2.3 Data processing and variable development

All scanned data were verified by a team of experts at Khon Kaen University who compared the resulting digital data to the scanned data. In addition to this, a group of TCS investigators cleaned and prepared variables for analyses at each wave of follow-up. All actions taken to edit or revise the data and the name and structure of master flies have been described in detail with the project metadata. Categorical and continuous variables were checked for impossible, improbable outlying and missing data points using tabulations and frequency tables, respectively. Data inconsistencies were checked against the original questionnaires for errors and remaining inconsistencies were resolved by discussion among the TCS investigators. Where possible, errors were corrected and where not possible an ‘error in data’ code was given.

For the purpose of this thesis, additional variables have been prepared. Variables for the main outcome of interest in this study were derived as follows: for diabetes in 2005 and 2009 ‘yes’ responses to having diabetes (needing insulin or not) were combined and considered as diabetes; those responding ‘no’ on both 2005 and 2009 surveys were considered as not having diabetes. In 2013, there was no specific question on insulin. A response of ‘yes’ meant having diabetes and ‘no’ meant not having diabetes. If the participants said ‘I am at risk’ this was considered as not yet having diabetes. Other decisions regarding how variables were used for the different research questions in this research are outlined specifically in each chapter.

2.1.2.4 Ethics

The chief TCS investigators obtained ethical approval for TCS data from Sukhothai Thammathirat Open University Research and Development Institute (protocol 0522/10) and the Australian National University Human Research Ethics Committee (protocols 2004/344, 2009/570). All participants gave informed written consent. TCS participants were advised that they were free to withdraw from the research at any time.

2.1.2.5 Use of TCS data in the thesis

In this thesis, the first research component to use TCS data was the validation study. This involved generation of additional data as described for data source two (section below) and in Chapter 3.

Other components of the thesis to use TCS data were the incidence study, the SSB study, and the BMI study (Chapters 4, 5 and 6). Study participants for these three components included TCS members who were followed from 2005 to 2013 and did not have a missing response to the TCS diabetes survey question in 2013. Participants who reported diabetes at baseline (N=902) and those who reported having diabetes in 2009 and not in 2013 (N=180), mostly women with gestational diabetes, were excluded from analyses reported in Chapters 4, 5 and 6. Details are given in each chapter regarding criteria for inclusion.