WHI trial finds no benefit of low-fat diet on cancer, CVD

February 07, 2006

Washington, DC - Results of the Women's Health Initiative (WHI) Dietary Modification Trial following almost 50 000 women for eight years has found that a low-fat diet does not significantly reduce the risk of breast cancer or colorectal cancer, nor does it reduce cardiovascular disease, a prespecified secondary outcome.

Three papers reporting the findings in breast cancer[1], colorectal cancer[2], and cardiovascular disease[3] are published in the February 8, 2006 issue of the Journal of the American Medical Association, accompanied by two editorials[4,5].

Lead author of the cardiovascular paper, Dr Barbara V Howard (Howard University, Washington, DC), told heart wire : "This study tells us a lot. This is the first time there has ever been a long-term test of a lower-fat diet, increasing carbohydrates, grains, fruit, and vegetables. We have established the safety of this strategy. It didn't hurt, and although there was no significant effect on cardiovascular disease, the tendencies were in the right direction."

No significant effects on any CHD outcomes or in any subgroup

In the trial, 48 835 postmenopausal women aged 50 to 79 years were randomly assigned to an intervention (19 541, 40%) or comparison group (29 294, 60%). Enrollment occurred between 1993 and 1998 in 40 US clinical centers, and average follow-up was 8.1 years. In the intervention group, behavior modification in group and individual sessions was designed to reduce the total fat intake to 20% of calories and increase intake of vegetables and fruits to five servings per day and that of grains to at least six servings per day.

For cardiovascular disease (which was a prespecified secondary outcome of the overall study), the main outcome measures were fatal and nonfatal coronary heart disease, fatal and nonfatal stroke, and CVD (composite of CHD and stroke).

"The WHI Dietary Modification Trial is the largest long-term randomized trial of a dietary intervention ever conducted to our knowledge, and it achieved an 8.2% reduction at year six in total fat intake and a mean daily increase of 1.1 servings of vegetables and fruits and 0.5 servings of grains," Howard and colleagues note.

However, no significant effects on incidence of CHD or stroke were observed during the 8.1 years of follow-up, they add, nor were there any beneficial or adverse effects of the intervention on any subcomponents of CVD.

Clinical outcome of the Dietary Modification Trial participants by random assignment

Outcome Intervention group (n=19 541) Comparison group (n=29 294) HR
Nonfatal MI* 435 671 0.98
CHD death 158 234 1.02
CABG/PCI 717 1113 0.96
Composite CHD (nonfatal MI, CHD death, or CABG/PCI) 1000 1549 0.97
Fatal stroke 55 86 0.97
Nonfatal stroke 379 556 1.03
Stroke class      
Ischemic 256 383 1.01
Hemorrhagic 67 113 0.90
Total cardiovascular disease 1357 2088 0.98
*Nonfatal MI is defined as no definite CHD death within 28 days of MI
†Total cardiovascular disease includes clinical MI, silent MI, death due to CHD, CABG/PCI, and stroke
Not just reducing fat intake but targeting the right fats to eat

In one of the accompanying editorials, Drs Cheryl AM Anderson and Lawrence J Appel (Johns Hopkins University Bloomberg School of Public Health, Baltimore, MD) discuss the reasons there was a null finding in this study.

"The intervention reduced intakes of fats that increase risk of CVD but also of those that might be protective," they note, "but the intervention did not address weight, the primary risk factor for diabetes. Over the course of the follow-up, the intervention group lost only 0.4 kg more than the comparison group."

 
The reason the effect was so small was because we didn't target saturated fats enough.
 

Howard agrees: "The reason the effect was so small was because we didn't target saturated fats enough. For heart disease, there needs to be more focus on reducing saturated fats and trans fats and increasing polyunsaturated fats, eating more fish and more fruits and vegetables," she told heart wire .

Since this trial began, Anderson and Lawrence explain, "there has been a major evolution in the thinking about the role of fat intake in reducing risk of CV, specifically a shift in focus from reducing total fat intake to modifying specific types of fats. The results from WHI have implications for the design of future lifestyle-intervention trials in which CVD is an outcome."

Howard told heart wire her group is following the women in the trial for another five years.

Some effect on breast cancer, none on colorectal

Using the same study, Dr Ross L Prentice (Fred Hutchinson Cancer Research Center, Seattle) and colleagues found no effect of the intervention on breast cancer, with 3.35% of those in the low-fat group developing the disease compared with 3.66% of those in the comparison group.

But Howard told heart wire there was a nonsignificant trend toward a reduction in breast cancer in the intervention group, with the strongest association seen in women with estrogen-positive tumors.

For colorectal cancer, there was no benefit of a low-fat diet, with 201 cases of invasive colorectal cancer in the intervention group (0.13% per year) compared with 279 in the comparison group (0.12% per year), Dr Shirley AA Beresford (University of Washington, Seattle) and colleagues report.

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