RATIONALE

General Issues

Cardiovascular disease, breast cancer, and osteoporotic fractures are among the leading causes of morbidity and mortality in postmenopausal women. As such, they are reasonable and defensible targets for a large prevention study. Coronary heart disease is the leading cause of death in U.S. women. The mortality and incidence rates of breast cancer are high; over an average 85-year lifespan, one in nine women develop breast cancer and approximately one in thirty die of it. Osteoporotic fractures, which are associated with aging, affect many more women than men; complications are life threatening and reduce both longevity and quality of life.

These diseases are not alone among the severe disablers of women, however. The CT does not directly address arthritis, dysmobility, poverty and isolation, depression, dementia, hearing, vision, and dental losses, and institutionalization. Neither does it address other compelling outcomes, such as dysfunction or pain, that are not linked to solitary etiologies. This should not imply that these issues are not troubling sources of morbidity, nor that they would be inappropriate targets for future prevention and treatment research. Similarly, that the focus of the CT is on postmenopausal women should not be mistaken for a disregard of the myriad unanswered questions about younger women, or about the effects of behavior and disease in earlier stages of life on morbidity and mortality in later stages. One study cannot answer all questions.

The primary hypotheses of the CT are as follows:

  • A low fat dietary pattern reduces the risk of breast cancer.

  • A low fat dietary pattern reduces the risk of colorectal cancer.

  • Hormone replacement therapy reduces the risk of coronary heart disease.

  • Calcium and vitamin D (combined) supplementation reduces the risk of hip fracture.

The numerous secondary hypotheses include: DM reduces risk of CHD; HRT increases risks of breast and endometrial cancers; HRT reduces risk of fractures; and CaD reduces risk of colorectal cancer. The CT outcomes are presented in Figure 2-1; the CT hypotheses are listed in Appendix F.

There are reasonably good rationales for some aspects of each of the three branches of the CT, although evidence for the central hypothesis for the DM branch–that a change to a low fat dietary pattern by women over the age of 50 will reduce the incidence of breast cancer over the following nine years—is the weakest and least consistent of the three. There are stronger rationales for expecting that there are effects of DM on colorectal cancer and various cardiovascular disease endpoints. Similarly, there is a strong rationale for the HRT branch, which will test not only the relationship between HRT and coronary heart disease,



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