Epidemiology cohort studies

During the last 15 years, various additional aspects of the diethylstilbestrol problem have given rise to concern. One emerged in 1988 from a large multicenter epidemiological cohort study established by the US National Cancer Institute (DESAD Project), in which it was found that women exposed in utero to diethylstilbestrol had a 50% increased incidence of autoimmune disease (46). 

Thus, epidemiological cohort studies can provide associations between vitamin intake either from food or from fortified food or supplements and a specific disease, and the RCTs can provide a proof whether this association is causal or not. The major differences are that the cohort studies usually include healthy people at baseline, while the RCTs usually include patients who suffer from the disease (secondary prevention). Observational studies usually have longer follow-up periods and they assess food intake, from which vitamin intake is calculated from. It may also be mentioned that an observational study may either find an increased disease risk at low intake or low plasma levels of a nutrient (usually in the lowest quartile or quintile of the cohort), which is opposite to the finding of a reduced disease risk at high intake or high plasma levels. RCTs, however, aim to find a reduced disease risk at high intake levels that is achieved through the nutrient supplement used. This difference is discussed in more detail below. 

Conclusions from Prospective Epidemiology Cohort Studies... 

Cohort Study—A type of epidemiological study of a specific group or groups of people who have had a common insult (e.g., exposure to an agent suspected of causing disease or a common disease) and are followed forward from exposure to outcome. At least one exposed group is compared to one unexposed group. 

Baghurst PA, McMichael AJ, Tong S, et al. 1995. Exposure to environmental lead and visual-motor integration at age 7 years The Port Pirie cohort study. Epidemiology 6(2) 104-109. 

Epidemiological studies can be grouped according to how exposure is measured (acute exposure studies and chronic exposure studies) and how health effects are measured (individual-based panel or cohort studies and population-based or ecological studies). Most studies in the scientific literature have examined acute, not chronic, health consequences. 

Epidemiological studies have different strengths and weaknesses associated with their design (Table 4) shows some of the strengths and weaknesses of the two main methods of prospective cohort studies and retrospective case-control studies. 

Three types of epidemiological studies of cancer contribute to the assessment of carcinogenicity in humans—cohort studies, case-control studies and correlation (or ecological) studies. Rarely, results from randomized trials may be available. Case series and case reports of cancer in humans may also be reviewed. 

Case-control and convenience-cohort studies are susceptible to unobserved differences between experimental- and control-group members. This leads to selection and detection bias, which accounts for the alternative explanations of the observed statistical relationship between exposure and disease incidence. The best individual-level epidemiological studies go to great lengths to avoid bias, use large numbers of people, and study large exposures. 

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