Diet and Health Implications for Reducing Chronic Disease Risk (1989) / Chapter Skim
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2 Methodological Considerations in Evaluating the Evidence
2 Methodological Considerations in Evaluating the Evidence
Pages 23-40
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From page 23... ...
Additional considerations include the costs, burden on respondents, and availability of critical resources such as trained interviewers and accurate food composition tables. The chosen dietary intake assessment method must be tested to ensure its accuracy and reliability in the study population, and adequate training of personnel involved in collecting and analyzing data is essential.
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From page 24... ...
In most case-control studies tO determine the etiology of chronic diseases, investigators have recognized the difficulty of determining past dietary intake and have assumed that the current diet (or the diet prior to the onset of symptoms of the disease) reflects past intake sufficiently well to identify associations of dietary factors with disease (Morgan et al., 1978~.
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From page 25... ...
These include sampling errors, reporting errors, and errors due tO wide day-to-day variation in dietary intake. · Sampizng Errors: Because of interindividual variation in usual diet, small samples may provide highly unrepresentative estimates of food intakes by populations even when individual data items are accurate.
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From page 26... ...
Strengths and Weaknesses of Dietary Intake Assessment Methods Each method of ascertaining dietary intake has its strengths and weaknesses. None of them is suitable for every purpose.
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From page 27... ...
Several 24-hour recalls obtained periodically from the same people over several months or more can increase the accuracy of estimates of their usual intakes; this may be especially important for nutrients that are highly variable in the diet (Bazzarre and Myers, 1980; Liu et al., 1978; Rush and Kristal, 1982~. A common misuse of data from a single 24-hour recall is the designation of cutoff points below which dietary intakes of individuals are considered to be inade' quate (see Chapter 3~.
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From page 28... ...
Biologic Markers Because of difficulties in validating dietary intake assessment methods, biologic markers are receiving more and more attention as independent validity checks. Such markers can indicate dietary intake, but the complexities of nutrient metabolism and genetic and environmental factors may affect their usefulness.
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From page 29... ...
Problems Common to Observational Studies Assessment of Dietary Intakes Most knowledge about the relationship of diet to human health and disease, particularly chronic disease, has been derived from observational studies of people who selected their own food and drink over a lifetime. Me strengths and weaknesses of different methods of measuring dietary intakes of groups and individuals are discussed earlier in this chapter.
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From page 30... ...
Dietary intake after a disease is established may not be an index of the relevant exposure since the disease may have affected the diet, rather than vice versa. Misclassification of this type is especially serious in studies of chronic diseases, most of which develop over long periods during which they do not produce readily detect' able signs or symptoms.
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From page 31... ...
In ecological correlations, averaging across individuals to determine the mean dietary intake or disease occurrence of a population greatly reduces the effects of variation among individuals and of random errors in classification. Further' more, there is often an opportunity to select populations representing high and low extremes of exposure and disease rates.
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From page 32... ...
PurtherTnore, as in case-control studies, misclassifications of dietary exposure can occur. Nevertheless, cohort studies have been very successful in the investigation of suspected risk factors for chronic disease, e.g., associations of serum cholesterol concentration and dietary components with cardiovascular diseases (Shek' elle et al., 1981~.
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From page 33... ...
In studies involving noninstitutionalized people, larger study populations and longer exposures are possible, but control of the diet is limited, blinding is rarely possible, and assessment of actual diet is difficult, as discussed above for observational studies. Strengths and Weaknesses of Laboratory Experiments Studies ~ Animals Historically, Experiments in animals have played an important role in research on nutrition, even though there are considerable differences among species in their needs for various nutrients and in their responses to specific dietary manipulations.
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From page 34... ...
The most valuable role of animal models in research on nutrition and chronic diseases in humans is in the study of mechanisms and pathogenesis. Studies of the effects of diet on serum lipids and lipoproteins, calcium absorption and metabolism, tumor initiation and growth, blood pressure, and other physiological processes in selected animals have been useful in testing and refining hypotheses regarding mechanisms.
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From page 35... ...
Animal models are most valuable in studying the physiological and molecular mechanisms involved in nutritional effects; modem molecular biology has made them even more useful. Results from studies in animals cannot be used alone either to affimn or negate relationships between human diet and chronic diseases, nor can they be used to estimate accurately the size of the effects in hu mans.
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From page 36... ...
, for inferring causality in associations between diet and chronic diseases: strength of association, dose-response relationship, temporally correct association, consistency of association, specificity of association, and biologic plausibility. All criteria were accorded roughly equal weight, with the exception of biologic plausibility, which in practice was given a little less weight since it is more dependent on subjective interpretation.
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From page 37... ...
Experiments on dietary exposure of different animal strains can take genetic variability into account and permit more intensive observation. However, extrapolation of data from animal studies to humans is limited by the ability of animal models to simulate human diseases and the comparability of absorption and metabolic phenomena among species.
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From page 38... ...
1983. Dietary Intake Source Data: United States, 197~1980.
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From page 39... ...
1980. Relationship of dietary intake to subsequent coronary heart disease incidence: the Puerto Rico Heart Health Program.
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From page 40... ...
1983. An assessment of the relative validity of retrospective interviewing for measuring dietary intake.
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