The women's health initiative dietary modification trial: overview and baseline characteristics of participants
Introduction
The Dietary Modification (DM) component of the Women's Health Initiative (WHI) is a randomized controlled evaluation of a low-fat diet that is high in fruits, vegetables, and grains. This low-fat dietary pattern is hypothesized to reduce the risk of breast and colorectal cancer and secondarily, coronary heart disease, in postmenopausal women. To test these hypotheses, 48,836 postmenopausal women were randomly assigned to either the low-fat eating pattern (40%) or self-selected dietary behavior (60%). The nutrition goals for women in the intervention arm are to reduce energy from fat to 20% and energy from saturated fat to 7%, and to increase fruit and vegetable intake to at least five servings per day and grains to at least six servings per day. Participants will be followed for an average of 8.5 years.
The DM was motivated by animal studies 1., 2., international ecologic studies of diet and disease 3., 4., migrant studies 5., 6., 7., and epidemiologic studies (8) indicating that the diet, particularly lower levels of fat intake, has the potential to reduce risk of breast cancer, colon cancer, and heart disease. Within-country analytic epidemiologic studies of fat and breast and colorectal cancers have yielded inconsistent or null results 9., 10., 11.. However there are substantial obstacles to finding clear and interpretable relationships in these studies (12):
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Current or recent fat intakes may differ from intakes during the years pertinent to the development of chronic diseases, likely attenuating associations.
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Fat intakes in Western populations may not be highly variable, in spite of the variety of foods available.
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It is difficult to estimate the relationship between fat intake and disease because diet is a complex mixture of foods, nutrients, and other bioactive compounds.
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Dietary patterns often relate to other disease risk factors, offering the potential for confounding (or over-control) in these studies.
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Considerable random, systematic, and person-specific errors exist in all available dietary assessment methods and the key measurement properties of these instruments are not well understood.
The purpose of this report is to describe the baseline characteristics of participants in the DM trial, with emphasis on sociodemographics, health behavior, medical history, dietary intake, and other factors that could relate to the clinical outcomes.
Section snippets
Screening and eligibility for the dietary modification trial
The WHI included postmenopausal women aged 50 to 79 years. Women with previous or existing breast cancer or invasive cancer of any type within the past 10 years were excluded. General WHI trial eligibility criteria are provided in Hays' article in this issue. The DM component also excluded women who were: 1) on a low-fat diet (<32% energy from fat); 2) had dietary needs incompatible with the intervention program (e.g. celiac sprue); 3) ate 10 or more meals per week outside the home; 4) could
Results
Similar to the presentation of results for the other components, the baseline description of participants in the Dietary Modification trial is stratified by age. Because there was a study-wide emphasis on inclusion of minorities, all demographic, medical history, dietary intake and blood analytes are given by race/ethnicity group in the Appendix to Hays' article. Differences by age and race/ethnicity are generally statistically significant because of the large sample sizes. Therefore, we simply
Comparisons of key variables to national data
Since DM participants were not recruited as a representative sample from the US population, it is instructive to compare this sample to US women aged 50 to 79 years. Compared with women from the NHANES III, DM participants are more obese. Specifically, in the three age decades, 73%, 76%, and 73% of DM participants have BMI greater than 25 as compared with 64%, 64%, and 58% of NHANES III women (19). In contrast, DM participants have lower rates of hypertension: 28%, 38%, and 45% compared with
References (42)
- et al.
Epidemiologic studies of coronary heart disease and stroke in Japanese men living in Japan, Hawaii, and California: incidence of myocardial infarction and death from coronary heart disease
Am J Cardiol
(1977) - et al.
Feasibility of a randomized trial of a low-fat diet for the prevention of breast cancer: Dietary compliance in the Women's Health Trial vanguard study
Prev Med
(1990) - et al.
Correlates of over- and under-reporting of energy intake in healthy older men and women
Am J Clin Nutr
(1994) Assessment of the US diet in national nutrition surveys: national collaborative efforts and NHANES
Am J Clin Nutr
(1994)- et al.
Literacy and body fatness are associated with underreporting of energy intake in US low-income women using the multiple-pass 24-hour recall: a doubly labeled water study
J Am Diet Assoc
(1998) - et al.
Measurement of total energy expenditure provide insights into the validity of dietary measurements of energy intake
J Am Diet Assoc
(1993) - et al.
Dietary methods research in the third National health and Nutrition Examination Survey: underreporting of energy intake
Am J Clin Nutr
(1997) - et al.
Dietary assessment instruments are susceptible to intervention-associated response set bias
J Am Diet Assoc
(1998) - et al.
Dietary Approaches to Stop Hypertension: Rationale, design, and methods
J Am Diet Assoc
(1999) - et al.
Rationale and design of the Dietary Approaches to Stop Hypertension (DASH) trial. A multicenter controlled-feeding study of dietary patterns to lower blood pressure
Ann Epidemiol
(1995)