The preferred macronutrient dietary composition and its potential impact on health has been debated for decades. The importance of this question is obvious given the increasing prevalence of diet-related health issues such as certain cancers, type 2 diabetes, and coronary heart disease.

This has prompted numerous dietary intervention studies to determine if various dietary regimens can reduce disease risk and to better understand the optimal macronutrient balance. The Women’s Health Initiative Dietary Modification Trial, which began in 1993, recruited almost 50,000 women between the ages of 50 and 79 years. This randomized controlled clinical trial was designed to test the hypothesis that a low-fat dietary pattern compared to a usual dietary pattern would reduce the risk of breast and colorectal cancers and coronary heart disease in postmenopausal US women.

During the intervention, approximately 40% of subjects were randomly assigned to follow a low-fat diet, whereas the remaining 60% were assigned to a usual-diet comparison group.  Subjects assigned to a low-fat dietary pattern intervention group reduced fat intake from ~35% to 20% of total energy, in conjunction with increasing vegetables and fruit to 5 serving/d and grains to 6 servings/d. Over an 8.5 y intervention period, intervention and comparison group differences included lower fat and higher carbohydrate consumption, in conjunction with higher consumption of vegetables, fruit, and grains.

A follow-up study, published in the September 2019 issue of The Journal of Nutrition, provides updated trial results over the longest period for which complete outcome data are available. At the end of the 19.6 year follow-up period, researcher Ross L Prentice (Fred Hutchinson Cancer Research Center) and colleagues, noted that the intervention and comparison groups did not differ significantly in terms of important health outcomes – breast cancer, colorectal cancer, and coronary heart disease.

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Although overall mortality rates did not differ significantly between groups, a significant reduction in breast cancer followed by death occurred among intervention group women. A reduction in diabetes requiring insulin was also observed in the intervention group, as was a reduction in coronary heart disease incidence among healthy women without hypertension at study enrollment.

These results suggest that a reduction in dietary fat with a commensurate increase in carbohydrate, with vegetable, fruit, and grain increases, did not show significant benefits for breast or colorectal cancer incidence or for coronary heart disease incidence overall, but evidently led to some important health benefits during the intervention period and over the longer term cumulative follow-up, without observed adverse health consequences.

Reference Prentice RL, Aragaki AK, Howard BV, Chlebowski RT, Thomson CA, Van Horn, L, Tinker LF, Manson JE, Anderson GL, Kuller LE, Neuhouser ML, Johnson KC, Snetselaar L, Rossouw JE. Low-Fat Dietary Pattern among Postmenopausal Women Influences Long-Term Cancer, Cardiovascular Disease, and Diabetes Outcomes. The Journal of Nutrition, Volume 149, Issue 9, September 2019, Pages 1565–1574,

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