Scientific Concept of the National Cohort (status ... - Nationale Kohorte

A.2
A.2. Scientific background and rationale for study elements
that regular physical activity should be promoted as a major approach to reduce the chronic
disease burden. The American College of Sports Medicine, the American Heart Association,
and the CDC, for example, formulated recommendations for a combined regimen of moderate-
and vigorous-intensity exercise for the prevention of heart disease and the promotion
of overall health. With regard to cancer, international expert panels judged that increasing
physical activity levels, together with tobacco control and body weight control, could be the
most promising approach to alleviate the burden of cancer in industrialized countries 236 , 237
although precise estimates of the magnitude of this preventive potential are still lacking.
Expert panels, however, also recognized several important limitations of the epidemiologic
evidence so far. Most studies to date have included measures of the total amount of physical
activity (used to characterize participants as ‘‘active’’, ‘‘moderately active’’, or ‘‘inactive’’,
for example), and only few studies addressed the effects of different physical activity profiles
in terms of intensity, duration, or frequency, independently of the overall energy expenditure
due to physical activity. Thus, up to now, it is still relatively unclear which type, intensity, and
temporal pattern of activity is most beneficial for health.
A second limitation of most studies on physical activity and health has been that activity
measurements were based exclusively on questionnaire assessments, which can contain
substantial measurement error 238, 239 . Finally, in most of the large-scale prospective cohort
studies so far, physical activity assessments were made only at a single point in time, and
intraindividual variations over time in activity levels were not sufficiently accounted for in
RR estimates. Taken together, we anticipate that substantially stronger and more valid estimates
of both relative and attributable risks for chronic disease will be obtained if more
accurate methods are used to assess activity and assessments are repeated over time 240 .
Assessment of physical activity:
Examinations and questionnaires (at baseline and during reassessment)
Level 1: Questionnaire
7-day triaxial accelerometry
Level 2: 24-h physical activity recall
Excess body weight and specific measures of body composition: Excess body weight
is one of the best established risk factors for type 2 diabetes and heart disease and is increasingly
being recognized as a major risk factor for many forms of cancer 85 . However, a
major limitation of most prospective cohort studies is that these have relied primarily on the
measurement of BMI to assess the impact of excess weight on risk of chronic disease. BMI
does not take body fat distribution into account, nor does it distinguish between fat mass
and lean body mass (e.g., skeletal muscle). BMI provides only a very approximate measure
of visceral fat mass – the metabolically more active body fat compartment that secretes,
for example, a large part of cytokines and hormones implicated in the development of type
2 diabetes 241 . Waist circumference measurements, although correlating more strongly with
the amount of visceral fat, still also provide only an approximate measure of intra-abdominal
body fat, in comparison to more accurate imaging measurements 242, 243 . A number of observations
indicate that a substantial part of “hidden” obesity is not properly accounted for by
these indices and that even among subjects with comparatively low BMI intra-abdominal fat
may considerably increase risks of chronic diseases and overall mortality. In the USA and
Europe the “metabolic” syndrome has a non-negligible prevalence among subjects classified
as normal weight according to their BMI. Furthermore, waist circumference has been
shown to be more strongly associated with mortality risk among subjects with a normal BMI
(