点击显示 收起
1 From the Department of Epidemiology and Surveillance Research, American Cancer Society, Atlanta (MLM), and the Departments of Epidemiology and Nutrition, Harvard School of Public Health, and the Channing Laboratory, Department of Medicine, Brigham and Women’s Hospital and Harvard Medical School, Boston (MS).
See corresponding article on page 889.
In this issue of the Journal, Harnack et al (1) evaluate whether compliance with the Dietary Guidelines for Americans (2) was associated with a reduced risk of cancer among postmenopausal women in the Iowa Women’s Health Study. We applaud such evaluations of dietary guidelines with the use of health outcomes. Although the Dietary Guidelines for Americans and the companion pyramid have been assessed for public familiarity, few studies have evaluated whether people who follow the Dietary Guidelines actually remain healthier.
Harnack et al developed their own dietary guidelines index to quantify conformance to the Dietary Guidelines for Americans. Presumably, they did not use the US Department of Agriculture’s own index of adherence to the guidelines—the Healthy Eating Index (HEI; 3)—because it has not yet been updated to reflect some changes to the Dietary Guidelines in 2000 and because it focuses specifically on dietary intake. De novo development of such indexes necessarily involves arbitrary cutoffs and judgment. The Harnack et al score aptly distinguished between grain types and sugar-based foods and included new recommendations to account for variety in fruit and vegetable intakes. However, consistent with the US Department of Agriculture guidelines and the older HEI, they gave higher credit for more meat (any type, no limits) and more dairy products and across-the-board less credit for > 30% of energy as fat (treating all types of fat as bad). Their range of possible scores for each component was narrow, 0–2, even though they had a sufficient sample size to examine finer levels of concordance. Some components ended up with awkward double counting (eg, grains), probably because of nuances and repetition in the new guidelines (including "follow the pyramid").
At first glance, the results seem encouraging, with a 15% overall reduction in cancer incidence among women in the top quintile of degree of adherence to the guidelines. However, closer inspection reveals that much of the apparent benefit was driven by the specific guidelines relating to body mass index and physical activity. In Table 4, the authors showed that diet alone had only a modest and nonsignificant effect on cancer incidence in their study. These findings are similar to our previous reports using the HEI (4). Although another study reported lower risk of cancer mortality with a food-based Recommended Food Score (RFS; 5), mortality studies may combine the effect of diet on disease incidence with the effect of disease on diet and on survival. The potential for confounding by early diagnosis, access to medical care, and other behaviors that are difficult to measure and control can also complicate the interpretation of mortality studies. This might explain why the RFS more strongly predicted total mortality in that cohort of women (5) than it predicted incident cancer in the Nurses’ Health Study and Health Professionals Follow-up Study (6). Also, the RFS was heavily weighted toward fruit and vegetables and did not specifically reflect the Dietary Guidelines.
Why is it that scores for dietary recommendations do not work better to predict lower cancer risk? One possible reason is that the Dietary Guidelines for Americans promote dairy and meat intake and overemphasize carbohydrate intake relative to fat intake. In contrast, dietary guidelines established through major cancer organizations recommend limiting red and processed meat intake and acknowledge the uncertainties about the role of carbohydrate and fat quality and of dairy products in carcinogenesis (7,8). That said, we recently evaluated the performance of an alternate HEI in reducing major chronic disease risk, which addressed many of these concerns. Although it predicted a substantially better risk reduction for cardiovascular disease compared with the HEI, it unfortunately did not perform better than the original HEI at predicting cancer risk (6). Finally, few dietary factors in adult life have been associated with breast cancer, one of the most common cancers in female cohorts.
Harnack et al appropriately recognized the importance of obesity and physical activity in carcinogenesis. These were included as part of their score, reflecting the "cluster of nutrition-related behaviors included in the Dietary Guidelines." However, this complicates the interpretation of their findings. Because body mass index and physical activity are included in their score, the main results of Harnack et al’s article are not comparable to other assessments of adherence to diet recommendations (4,5). They are more comparable to studies that combined several lifestyle changes in relation to disease risk (9,10). However, the present article still does not present the full picture of the potential role of lifestyle modification; for example, smoking has a large effect.
There is good news. Next to smoking cessation, weight control and increased physical activity appear to be crucial lifestyle behaviors that can reduce the risk not only of cancer but of several other chronic diseases. Their consistent relation with cancer risk even later in life, as supported by this study, suggests that these behaviors should have a more prominent role in cancer prevention. In this study, women who followed the guidelines for physical activity, weight control, and diet concurrently (compared with those complying only minimally), appeared to reduce their risk of cancer by 15%. Taken together with guidelines for smoking cessation, this article underscores the potential for reduction of cancer incidence in middle-aged and older women by lifestyle modification.
What should we suggest that people eat to reduce cancer risk throughout life? Recent cancer guidelines on nutrition and physical activity emphasize diets that avoid overweight and a diet pattern that is low in red and processed meats and high in a variety of vegetables, fruit, and whole grains (7). This is a good place to start. As suggested by our data (6,10), additional changes in dietary guidelines may further reduce the risk of cardiovascular disease without adversely affecting cancer risk. The study of nutrition and cancer is less developed than that of nutrition and cardiovascular disease, and the role of nutrition is likely to vary by cancer type, stage of cancer development, and genotype, not to mention interactions between biological and genetic factors. Dietary guidelines for cancer risk reduction will become more focused as new and improved data on dietary factors and overall dietary patterns accumulate. The future of this area will be even more promising if these questions are approached with an open mind and a willingness to let go of preconceived ideas. Importantly, guidelines for diet should continue to be assessed for predictive validity as Harnack et al have done.
REFERENCES
Related articles in AJCN: