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Etiology

Monounsaturated fat was associated with a decreased risk for breast cancer, and polyunsaturated fat showed an increased risk

ACP J Club. 1998 Jul-Aug;129:19. doi:10.7326/ACPJC-1998-129-1-019


Source Citation

Wolk A, Bergström R, Hunter D, et al. A prospective study of association of monounsaturated fat and other types of fat with risk of breast cancer. Arch Intern Med. 1998 Jan 12;158:41-5.


Abstract

Question

In women, are dietary saturated, monounsaturated, and polyunsaturated fats associated with breast cancer?

Design

Population-based cohort study with mean follow-up of 4.2 years.

Setting

2 counties in central Sweden.

Participants

61 471 women between 40 and 76 years of age at baseline (1987 to 1990). Mean age at baseline was 57.3 years. Exclusion criteria were missing questionnaire data, extreme energy intake estimates on dietary questionnaires, or previous diagnosis of cancer other than nonmelanoma skin cancer.

Assessment of risk factors

A 67-item self-administered food-frequency questionnaire was used to determine dietary fat intake and other nutritional information. The questionnaire was validated for a subsample of 129 women. Saturated, monounsaturated, polyunsaturated, and total fat intakes were calculated in grams per day and divided into quartiles; the lowest quartile was the reference standard (relative risk [RR] 1.0). Data were adjusted for age; parity; age at first child's birth; family history of breast cancer; body mass index; education level; and daily intake of cholesterol, fiber, alcohol, and kilojoules.

Main outcome measures

Women with histologically confirmed invasive breast cancer were identified in cancer registries in the 2 counties. Death and relocation were ascertained by consulting national databases.

Main results

674 women developed invasive breast cancer during follow-up. All 3 types of fat were analyzed simultaneously in the same model, using incremental increases for daily intake. This analysis showed that monounsaturated fat was associated with a decreased risk for breast cancer (RR 0.45 for each 10-g increment, 95% CI 0.22 to 0.95), polyunsaturated fat was associated with an increased risk for breast cancer (RR 1.69 for each 5-g increment, CI 1.02 to 2.78), and saturated fat had no association with breast cancer. After each type of fat was analyzed separately and compared across quartiles of fat intake, no association was shown between breast cancer and total fat (P = 0.82 for trend), saturated fat (P = 0.83 for trend), monounsaturated fat (P = 0.38 for trend), or polyunsaturated fat (P = 0.92 for trend).

Conclusions

Monounsaturated dietary fat was associated with a decreased risk for breast cancer, and polyunsaturated dietary fat was associated with an increased risk for breast cancer. Total and saturated dietary fat intake were not associated with breast cancer.

Sources of funding: Swedish Cancer Society; Uppsala County Council; Västmanland County Council.

For correspondence: Dr. A. Wolk, Department of Medical Epidemiology, Karolinska Institute, Box 281, S-171 77 Stockholm, Sweden. FAX 46-8-31-49-57.


Commentary

The study by Wolk and colleagues represents another attempt to understand the vexing relation between dietary fat intake and breast cancer. The study has methodologic strengths, including the prospective collection of data and the concurrent validation of self-reported diet with weighted-food records in a subset of women. Further, it seems that the investigators pursued a limited number of specific a priori hypotheses about the association of types of fat with the risk for breast cancer.

The study's failure to identify an association of total fat intake with the risk for breast cancer is consistent with most of the published literature. The identification of an apparent protective effect of monounsaturated fat intake and an apparent detrimental effect of high levels of polyunsaturated fat intake is interesting and consistent with animal data. However, the results must be interpreted with caution. A linear association does not seem to exist between intake of monounsaturated fat or polyunsaturated fat and risk for breast cancer. Indeed, the apparent protective effect of monounsaturated fat is present in the upper 3 quartiles of polyunsaturated fat intake but is significant only in the highest quartile of intake, and there is a suggestion of an opposite effect in the lowest quartile of polyunsaturated fat intake. These data may reflect the small numbers of women in some categories of intake, particularly because intake of monounsaturated fat and intake of polyunsaturated fat are strongly correlated. However, they may also reflect a more complex relation among intake of different types of fat than was anticipated.

Although these results provide an interesting perspective on the contributions of different types of fat to the risk for breast cancer, they should not result in changes in population-based dietary recommendations. As the authors acknowledge, further research is necessary to confirm their observations and to explore the potential protective effects of monounsaturated fat when intake of polyunsaturated fat is low.

Pamela J. Goodwin, MD, MSc
Marvelle Koffler Breast Centre Mount Sinai HospitalToronto, Ontario, Canada