Analytical Studies

Cohort studies Cohort studies involve the collection of information from healthy participants who are followed over time and observed for the occurrence of new cases of disease (incident cases). During or at the end of follow-up, the disease frequency within a cohort may be measured as either a cumulative incidence rate (the number of cases divided by the entire base population) or an incidence density rate (the number of cases divided by the total follow-up time accumulated by all members of the population, or 'person-time' follow-up). The relative risk is the rate of disease (cumulative incidence rate or incidence density rate) in the exposed (e.g., those with a high intake of dietary fat) divided by the rate of disease in the unexposed (e.g., those on a low-fat diet). A relative risk of 2 implies that the exposed group has twice the rate of disease compared with the unexposed group.

For illustration, in a study of 121700 women, a group of participants who completed dietary questionnaires and had no previous diagnosis of cancer in 1980, were followed through 1988 to address the hypothesis that dietary fat increases and fiber intake decreases the risk of breast cancer. This outcome was defined by histologically confirmed cases of breast cancer. In one analysis, the primary exposure of interest was energy-adjusted intake of total dietary fiber. Among the women in the highest quintile of energy-adjusted dietary fiber intake there were 299 cases of breast cancer compared with 305 cases among the women in the lowest quintile. This gave a relative risk (with adjustment for established breast cancer risk factors as well as alcohol intake) of 1.02 for those in the highest quintile of energy-adjusted dietary fiber intake compared with those in the lowest quintile.

There is also a growing body of evidence available from cohort studies for the assessment of dietary fat intake and breast cancer in developed countries. The average relative risk was 1.03. This observation was based on the results from nine prospective studies with at least 50 incident breast cancer cases each (n = 2742) and a large comparison series (i.e., non-cases). At the same time, as these results suggest no overall association for total fat intake, emerging evidence suggests that monounsaturated fat may be protective against breast cancer.

The use of cohort studies can be advantageous in many ways when studying the relationship between diet and cancer. A cohort study allows the assessment of multiple effects of a given dietary exposure. Dietary data can be updated during follow-up and the temporal relation between diet and cancer can be addressed. For example, the beneficial effects of alcohol in reducing the risk of gallstone formation and coronary heart disease, and the potentially deleterious effects of alcohol on cancer and hemorrhagic stroke, can be weighed against each other in a cohort study. It is also possible to measure the absolute rates of disease according to the level of food or nutrient intake.

Among the limitations of cohort studies is the concern that current practice, usage, or exposure may change over the duration of the follow-up, limiting the ability to come to any relevant conclusions in studies of diet and cancer that have measured exposure just once at the beginning of the study. Controlling for extraneous variables such as smoking, which are related both to risk of cancer and to dietary intake, and separating the effects of specific dietary factors from those that exist together, also limit the range of knowledge that can be extracted from cohort studies.

Some investigators believe that the large number of subjects required to study rare disease and the high expense of management and maintenance also limit the usefulness of cohort studies. Others believe that the larger overall monetary investment most cohort studies require can be advantageous: More variables can be studied and in the long run further hypotheses can be generated and more conclusions produced than in a single case-control study that relies on recall of past habits.

Case-control studies In case-control studies information is obtained from diseased participants and compared with information provided by disease-free controls with respect to a possible risk factor (e.g., level of a dietary factor). Data collected from these studies can be used to evaluate the hypothesis that the risk factor is a cause of the disease. The cases are selected from a defined population, such as a country population. The population represents those at risk of developing the disease under study. Each time someone in the defined population is diagnosed with the disease during the duration of the study, this individual joins the case series. As each case arises from the population, one or more controls should be sampled to estimate the prevalence of the exposures among those remaining free from disease. The controls may be chosen from any population of individuals that provides valid information about those at risk for the disease. It is important to choose controls so that their probability of selection is unrelated to the exposure being studied.

In the study of the relationship of diet and cancer, case-control studies may be used to evaluate the hypotheses that individual or multiple dietary factors are the cause of the cancer under investigation. For example, a study in 1977 identified all cases of lung cancer diagnosed during an 18-month period from 1972 in three Singapore hospitals. Controls were chosen from other hospital patients free of any smoking-related diseases. There were a total of 233 cases and 300 controls interviewed regarding their frequency of consumption of dark-green leafy vegetables and food preparation habits. The investigation found a substantially increased risk of lung cancer among those reporting a low consumption of dark-green leafy vegetables.

Case-control studies are better suited to the study of rare diseases because in cohort studies tens of thousands of individuals must be followed in order to study the most common cancers. It is also thought that case-control studies may be quicker and less expensive to conduct because they require fewer subjects, and they are therefore often employed as an alternate mode of investigation to cohort studies.

Among the limitations of case-control studies is the comparability of information between the cases and the controls. While in a cohort study the exposure of interest is measured before the onset of disease, in case-control studies the exposure is assessed in individuals who (in most cases) already know their own disease status. Often the person collecting the data will also know the disease status of the patient. This may influence the accuracy of the data collected, either through differential recall by cases and controls, or by an interviewer being more persistent in questioning cases than controls. In cohort studies neither the participant nor the investigator knows whether or not the subject will be a case or noncase by the end of the follow-up period.

Intervention studies In principle, the most powerful means of determining the effects of dietary factors on cancer risk is an intervention study (i.e., a randomized trial). In randomized trials bias is removed because of the equal distribution of risk factors in each group. For example, it has been proposed that a randomized trial of fat reduction could help uncover the mystery of the relationship between dietary fat intake and breast cancer. The Women's Health Initiative was started by the US National Institutes of Health with the goal of enrolling and randomizing several tens of thousands of women, half of whom will be trained to follow a diet deriving less than 20% of energy from fat. Unfortunately, such a trial would not be able to address the most promising modification of the dietary fat hypothesis—that dietary fat reduction at an early age may reduce breast cancer risk several decades later. Other problems with such a randomized trial include the difficulty of maintaining compliance with a diet incompatible with prevailing food consumption habits, and the gradual secular decline in total fat consumption already under way which may reduce the size of the comparison of fat intake between the intervention group and the control groups. The Women's Health Initiative Trial will also counsel the women in the intervention group to adopt a diet that is high in fruits, vegetables, and grain products as well as low in total and saturated fat, therefore making it more difficult to distinguish between the effect of the fat reduction and that of increasing intake of fruits, vegetables, and grain products. All in all, intervention studies may in principle have a great chance of determining effects of dietary factors on cancer risk, but trials of sufficient duration and size may not be feasible because of long-term compliance and cost.

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