Reply to JE Lincoln

¹ Epidemiology and Surveillance Research American Cancer Society 1599 Clifton Road, NE Atlanta, GA 30329 E-mail: marji.mccullough{at}cancer.org
² Department of Nutrition Harvard School of Public Health 665 Huntington Avenue Boston, MA 02115

Dear Sir:

In his letter, Lincoln suggests that the Alternate Healthy Eating Index (AHEI) (1) would have more strongly predicted chronic disease risk had more specific attention been paid to fatty acids, namely, monounsaturated fatty acids and n-3 polyunsaturated fatty acids. In designing the index, we included components with well-established relations to disease, including trans fatty acids and the ratio of polyunsaturated to saturated fatty acids, and gave credit for the consumption of fish and nuts (both rich sources of n-3 fatty acids). The intakes of both n-3 and n-6 polyunsaturated fatty acids were substantially (P < 0.001) higher in the fifth quintile of the AHEI than in the first quintile (Tables 1 and 2).

View this table:
TABLE 1 . Unsaturated fatty acid intakes by quintile (Q) of Alternate Healthy Eating Index score in 38615 men who participated in the Health Professionals’ Follow-up Study¹  

View this table:
TABLE 2 . Unsaturated fatty acid intakes by quintile (Q) of Alternate Healthy Eating Index score in 67271 women who participated in the Nurses’ Health Study¹  
Ecologic studies, such as the study by Keys et al (2) that is cited by Lincoln, are useful in generating hypotheses but are not confirmatory, because of the great potential for uncontrolled confounding (3). Nevertheless, monounsaturated fatty acids have been found to have beneficial effects on both blood lipids (4) and disease risk (5). Currently, monounsaturated fat intake in the United States is mainly from beef, margarine, and baked goods (6) rather than from olive oil, which is still not commonly used in the United States. We avoided overemphasizing these foods because they contain other potentially adverse components.

Specific polyunsaturated fatty acids have different metabolic effects (7), but both n-3 and n-6 fatty acids have protective associations with blood lipids and cardiovascular disease risk (5, 8). In the Lyon Diet Heart Study (9), subjects were given defined diets to test specific hypotheses. Therefore, the concentrations of fatty acids in their blood after the intervention reflected the diet and the hypotheses being tested. The experimental subjects had significantly higher oleic acid concentrations and significantly lower linoleic acid concentrations than did the control subjects. The experimental subjects also had significantly higher concentrations of linolenic and eicosapentaenoic acids than did the control subjects. Because the study was not designed to test the n-6 hypothesis, linoleic acid concentrations (and thus the ratio of polyunsaturated to saturated fatty acids) were not expected to differ between the experimental and control groups. In the free-living populations that we studied, the ratio of polyunsaturated to saturated fatty acids captured healthy trends in ingested lipids and was strongly associated with the risk of ischemic heart disease (10). Inclusion of -linolenic acid in the AHEI score might have improved our prediction of cardiovascular disease, but again, we chose to include only those factors with longer, more established relations.

As we learn more about the relation between diet and health, scores such as the AHEI can continue to be refined and improved, and diet patterns being recommended to the US public can become more precise. For now, advice that emphasizes the intake of unsaturated fats and the restriction of saturated and trans fats and that encourages the consumption of fish, nuts, and whole grains clearly represents an improvement in recommendations to reduce chronic disease risk.

REFERENCES

1. McCullough ML, Feskanich D, Stampfer MJ, et al. Diet quality and major chronic disease risk in men and women: moving toward improved dietary guidance. Am J Clin Nutr 2002;76:1261–71.
2. Keys A, Menotti A, Karvonen MJ, et al. The diet and 15-year death rate in the Seven Countries Study. Am J Epidemiol 1986;124:903–15.
3. Willett WC. Nutritional epidemiology. 2nd ed. New York: Oxford University Press, 1998.
4. Mensink RP, Katan MB. Effect of monounsaturated fatty acids versus complex carbohydrates on high-density lipoprotein in healthy men and women. Lancet 1987;1:122–5.
5. Hu FB, Stampfer MJ, Manson JE, et al. Dietary fat intake and the risk of coronary heart disease in women. N Engl J Med 1997;337:1491–9.
6. Subar AF, Krebs-Smith SM, Cook A, Kahle LL. Dietary sources of nutrients among US adults, 1989 to 1991. J Am Diet Assoc 1998;98:537–47.
7. Ulbricht TLV, Southgate DAT. Coronary heart disease: seven dietary factors. Lancet 1991;338:985–92.
8. Mensink RP, Katan MB. Effect of dietary fatty acids on serum lipids and lipoproteins: a meta-analysis of 27 trials. Arterioscler Thromb 1992;12:911–9.
9. de Lorgeril M, Renaud S, Mamelle N, et al. Mediterranean alpha-linolenic acid-rich diet in secondary prevention of coronary heart disease. Lancet 1994;343:1454–9.
10. Hu FB, Stampfer MJ, Manson JE, et al. Dietary saturated fats and their food sources in relation to the risk of coronary heart disease in women. Am J Clin Nutr 1999;70:1001–8.