Yes, Diet Matters!

One week they recommend one diet—the next week they recommend another!”

“I guess I can just eat whatever I want!”

These were the comments of the day when the low-fat dietary pattern and risk of cardiovascular disease (CVD) results of the Women's Health Initiative (WHI) Dietary Modification Trial were announced in February (DOC News, March 2006), with the following conclusions:

Over a mean of 8.1 years, a dietary intervention that reduced total fat intake and increased intakes of vegetables, fruits, and grains did not significantly reduce the risk of CHD [coronary heart disease], stroke, or CVD in postmenopausal women and achieved only modest effects on CVD risk factors, suggesting that more focused diet and lifestyle interventions may be needed to improve risk factors and reduce CVD risk.¹

The challenge of establishing clinical practice guidelines for nutrition recommendations is that each patient will eat, every day, regardless of the state of his or her knowledge at any point in time. It is inherent in the scientific endeavor that knowledge evolves. Nutrition recommendations must evolve as well. The challenge is to establish appropriate recommendations based on an incomplete knowledge base—and when a major study provides incomplete answers, the challenge is even greater. My view is that for several reasons the WHI results are not entirely negative or positive; they are simply incomplete.

STUDY LIMITATIONS

The Women's Health Initiative is a major, long-term study that incorporated a randomized clinical trial of the effect of a dietary intervention on the risk of breast and colorectal cancer as primary outcomes and on the risk of CVD as a secondary outcome. The dietary intervention was designed to reduce dietary fat to 20% of calories, to increase intake of vegetables and fruits to at least five servings per day, and to increase intake of grains to at least six servings daily. The study included nearly 50,000 women from 40 clinical centers across the U.S. Specially trained nutritionists delivered an intensive behavioral modification program with 18 sessions in the first year and quarterly sessions thereafter. Why did this intervention not yield the expected statistical results?

The published paper itself provides most of the answer to this question. First, the dietary intervention group did not reach the study goal of 20% calories from fat. At baseline, intake of total fat was 37.8%; after 1 year, fat intake was down to 24.3%, but by year 6, intake had risen to 28.8%. Second, the CHD incidence rate in the comparison group was only about two-thirds of what was expected in the original design. Together, from a statistical perspective, these differences between the original design and reality led to a problem of having only a 40% chance of finding a true difference (if one existed) in CHD incidence between the dietary intervention group and the comparison group.

Now consider two additional aspects of this work: adherence and the role of obesity. There are potential problems with post hoc analyses of subgroup data, and the WHI authors themselves have noted. This is offset to some degree, however, by the need to advise patients about what to eat. There were beneficial effects of the dietary intervention on CHD incidence in the subgroups of individuals who reached the lowest levels of saturated fat intake, the lowest levels of trans fat intake, or the highest intakes of fruits and vegetables. And for the subgroups of individuals with the lowest intake of saturated and trans fat, LDL cholesterol was reduced significantly.

Women in the WHI dietary intervention lost a very modest amount of weight (DOC News, March 2006).² Given the impact of weight loss to improve CFD risk profile,³ the WHI dietary intervention might have hd greater impace if weight loss had been an intervention goal.

Where does that leave us? My best advice is to continue to follow the American Diabetes Association (ADA) position statements on nutrition principles that relate to CVD risk reduction.⁴ The lifestyle recommendations focus on glycemic control and cardiovascular risk reduction, including weight management by reduced caloric intake and increased physical activity, limiting intake of saturated fat and trans fatty acids, and increasing intake of dietary fiber.

The results of the WHI are not nearly as compelling as one would have hoped, but this should not discourage us. The risk of CVD for patients with diabetes is high; so is the risk of inaction. Stay the course. Encourage patients to follow current ADA lifestyle recommendations to manage weight and improve CVD risk factors. ▪