Resolved and Unresolved Issues

Some of the issues that researchers have encountered in their attempt to uncover the mystery of the dietary factors linked to cancer include the difficulty of distinguishing the importance of parts of dietary factors from the overall effect of each dietary factor (e.g., total dietary fat intake compared with type of dietary fat intake). In a meta-analysis in 1990 of 12 case-control studies of dietary fat intake and cancer, 4 studies observed a significant positive association, 6 uncovered nonsignificant positive associations, and 2 saw inverse associations. When the data were analyzed together there was a positive association observed for both total fat intake and saturated fat intake. Investigators must ask themselves which factor has larger implications in the study of diet and cancer, as not all studies have included analyses of the individual types of fats along with their data on overall fat consumption. In the study of the influence of dietary fiber intake (which includes crude fiber and many soluble fiber fractions) on cancer rates, there is debate about the most appropriate method of biochemical analysis for determining fiber content of individual foods. This same issue arises with the study of most dietary factors and could affect any important advances in the study of diet and cancer.

Biochemical indicators of food and nutrient intake have two fundamental uses in epidemiological studies. Most often they serve as a 'surrogate' for actual dietary intake in studies of disease occurrence. For nutrients that vary widely in concentration within foods and for which food composition tables are inaccurate, biochemical indicators may be the most feasible way of measuring intake. Within-food variation may occur owing to differences in food storage, processing, or preparation, or may be owing to geographical differences in soil nutrient content. For example, it has been found that selenium content in US soil can vary by as much as 100-fold, which in turn causes the selenium content of swine muscle to vary more than 15-fold. Another example is that of fat. When the composition of fats in commercial food products is not known to study participants, it is possible to assess the fat components of the diet by subcutaneous fat aspirates which reflect long-term dietary patterns.

Like most exposures in chronic disease, nutrient exposures relevant to disease are usually long-term. As the promotion period for cancers may be years or decades, it is usually desirable that a biomarker indicates the cumulative effect of diet over an extended period of time. There are a couple of methods to surpass the barrier of an indicator that is only sensitive to short-term intake, and to overcome the day-to-day intake fluctuations that occur with most nutrients: (1) experimental studies, in which nutrient levels are manipulated; and (2) sampling levels in individuals longitudinally. Biomarkers of nutrient levels in blood or other tissues can provide a useful assessment of intake of certain nutrients, although the above considerations must be acknowledged, and careful attention must be given to specimen collection, storage, and analysis in order to avoid misclassification or bias. With an expanding array of biochemical indicators that have been validated as measures of dietary intake, their use in nutritional epidemiology will continue to grow.

The limited range of diet within most populations adds its own set of complexities to the epidemiolog-ical study of nutrition and cancer. For example, in the majority of populations where foods high in fat are readily available, very few individuals consume less than 30% of their energy from fat. This makes it difficult to study the impact of reducing fat intake to less than 30% of total energy intake. At the same time, some individuals of a relatively homogencous population may have very different dietary patterns: For example, a range of dietary fat intake from 25% to 40% of total energy was seen within a cohort of 52 000 male health professionals in the USA.

Given that most neoplasms have a long induction period (the time from an exposure to a carcinogen to the development of cancer), often spanning several decades, accurate measure of long-term dietary intake is of utmost importance in the study of the implications of diet on cancer. Therefore, short-term methods of dietary assessment such as 24-h recalls are usually insufficient. In the context of case-control studies these short-term methods are inappropriate because they measure current diet, and it has been found that individuals alter their diet after the diagnosis of cancer. The most feasible method of measuring long-term intakes in large numbers of individuals is the food frequency questionnaire: These questionnaires measure the usual frequency of a selected list of foods.

Food frequency questionnaires to assess dietary intake need to be carefully designed. First of all, the food items on the questionnaire must represent the major source of nutrients of interest within the study population. Depending on the consistency of the concentration of a nutrient in a given food, the precision of dietary questionnaires varies. Food frequency questionnaires may provide rankings by level of intake but they do not quantify actual intake. A dietary questionnaire may efficiently distinguish between participants with low-fiber and high-fiber intakes in a given population, but it will not necessarily provide a precise assessment of the absolute fiber intake. In the case of larger studies, it is possible for a random sample of participants to provide a more comprehensive assessment of intake by keeping several weeks of dietary records. This additional information will provide a more precise quantification of dietary intake by helping estimate true dose-response relationships between a nutrient and diet expressed in absolute intake.

Gaining Weight 101

Gaining Weight 101

Find out why long exhausting workouts may do more harm than good. Most of the body-building workout and diet regimens out there are designed for the guys that gain muscle and fat easily. They focus on eating less and working out more in order to cut the excess fat from their bodies while adding needed muscle tone.

Get My Free Ebook

Post a comment