Obesity Clinical Trial
To assess genetic effects on the variation of cardiovascular and pulmonary risk factors in a cohort of 514 pairs of white male veteran twins.
BACKGROUND:
In 1966, the National Academy of Sciences-National Research Council (NAS-NRC) began the
development of a twin registry. Through state birth records, a group of twins born in
1917-1927 was ascertained and matched to veteran records to obtain a roster of 16,000 twins
where both members of the pair were in military service. Veterans formed the basis of the
registry because Veterans Administration files provided access to current addresses as well
as potential for mortality follow-up. From this roster, a group of twins living within 200
miles of the examining centers and agreeing to cooperate was examined. This procedure
yielded a set of 514 pairs of twins where both members of the pair were examined. At the
time of initial examination, the World War II veteran twins were 42-56 years old. Since the
initial examination, the twins have been followed by the NAS-NRC to maintain current
addresses and mortality information.
The World War II veteran cohort was originally examined during the 1969-1973 period at five
examination centers located in Framingham, Massachusetts; Davis, San Francisco, Los Angeles,
California; and Indianapolis, Indiana. The objective of the initial examination was to
estimate the heritability of cardiovascular risk factors. Results of the first examination
showed significant heritability for height, weight, relative weight, systolic and diastolic
blood pressure, glucose tolerance, uric acid, hematocrit, plasma triglycerides, and forced
expiratory volume but not for total plasma cholesterol or any of the lipoprotein cholesterol
fractions
.A second exam of the cohort was completed during 1981-1982. Of the original cohort of 1028
subjects, 792 or 77 percent were reexamined and approximately 5.5% had died. The objective
of the second examination was to assess the change in cardiovascular risk factors over the
ten year interval, assess the disease experience of the cohort and gather additional data on
new cardiovascular risk factors as well as pulmonary function measurements and covariates.
Examination two differed from examination one in that there was greater opportunity to
standardize data collection for methods, laboratories, equipment, and time of examination.
Greater emphasis was placed on pulmonary function testing. One objective of examination two
that was not achieved was the determination of a sufficient number of cardiovascular events
to do analyses of concordance of disease and analyses of disease occurrence in relation to
risk factors. Analysis of data on 167 monozygotic pairs of twins and 176 dizygotic pairs of
twins showed results similar to those of the first examination for systolic and diastolic
blood pressure but, unlike the first examination, also showed significant genetic variance
for total cholesterol and LDL cholesterol. HDL cholesterol continued to show no genetic
variance.
The third examination of the original cohort was conducted in 1986-1987. The primary
objective of the third examination was to assess the incidence of cardiovascular and
pulmonary diseases for analyses relative to risk factor data and determine if the
heritability of cardiovascular disease exceeded what was expected based on the known
cardiovascular risk factors. Extensive pulmonary function testing was part of examination
three as it was for examination two and provided longitudinal data for analyses of change in
pulmonary function in relation to genetic and environmental factors. The exercise
electrocardiogram was expanded to the entire cohort for examination three to provide
additional data on sub-clinical disease.
DESIGN NARRATIVE:
The study was longitudinal in design and consisted of three examinations of the cohort over
a period of eighteen years. Data collected in examination one in 1969-1973 included: a twin
history questionnaire; personal, family and medical histories; a diet history; a physical
examination; numerical data on anthropometry, hematology, and pulmonary function;
supplemental lab data; an X-ray; clinical diagnostic impression; electrocardiogram; and
blood analysis-fasting lipids. Data collected in examination two in 1981-1982 included: a
personal and family history; deceased twin information; residence and occupational history;
Thurstone Activity Scale; leisure time physical activity; social support; medical history;
physical examination; clinical diagnostic impression; job mobility and stress; retirement
information; Jenkins Activity Survey; blood lipid measurements. Data collected in
examination three in 1986-1987 included the same data as collected in examination two plus
an assessment of the prevalence for all twins and the incidence for a subgroup of twins of
preclinical cardiovascular disease detected through noninvasive techniques.
The follow-up exam in 1986-87 collected baseline data on cognitive performance in 622
individuals, including 132 monozygotic and 134 dizygotic twin pairs. Beginning in December,
1994, a research grant was used to conduct a fourth exam in the surviving subjects of this
cohort that used this rich resource of available data to determine the relationship between
cardiovascular risk factors collected over 23 years of follow-up, changes in cognitive
function since the last exam, and brain morphology assessed by magnetic resonance imaging
(MRI). Given the mean age of this cohort (72 years) in 1995, the next several years
represented a critical time to monitor age related changes in brain morphology and
cognition. The addition of a fourth exam made the NHLBI Twin Study the longest longitudinal
study of cardiovascular disease epidemiology in twin subjects (23 to 25 years of follow-up)
and the first to investigate the contribution of genes and the environment to associations
between CVD risk factors, cognitive performance, and brain morphology. There were five
specific objectives of this study. The first was to evaluate the prospective and
cross-sectional relationships of cardiovascular risk factors collected over 23 years of
follow-up (e.g., blood pressure, lipids, obesity, smoking) and indices of brain morphology
assessed at the fourth exam. The second objective was to determine the extent that twins
discordant for essential hypertension or non-insulin-dependent diabetes mellitus (NIDDM) or
cardiovascular disease (CHD) showed different MRI profiles. The third was to characterize
the neuropsychological changes in performance from Exam 3 to Exam 4 and determine the
contribution of genes and the environment to stability or change. The fourth objective was
to use the data from exams 1 to 3 to investigate prospective relationships between CVD risk
factors and decline in cognitive performance from Exam 3 to Exam 4. The fifth was to
determine the heritability of the new measurements obtained for the first time at the fourth
examination cycle (e.g., MRI measures of infarct area and localization, white matter
hyperintensities (WMHI), and ventricular volumes).
The study completion date listed in this record was obtained from the "End Date" entered in
the Protocol Registration and Results System (PRS) record.
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