ARCHIVED - Health Canada Scientific Summary on the U. S. Health Claim Regarding Dietary Fat and Cancer
Bureau of Nutritional Sciences
Food Directorate, Health Products and Food Branch
Since the U. S. health claim pertaining to dietary fat and cancer was accepted in the U. S. in 1993, new evidence has weakened the support for this claim.
The searches to update the evidence included Medline from 1993 to December 2003 inclusive, Food Science and Technology Abstracts for the period 1997 to 2002 and Current Contents from 1998 to 2002. Key words fat, dietary fat, fat intake, saturated fat, saturated fatty acids and meta analysis were intersected with the words cancer, breast cancer, prostate cancer, gastric cancer, colon cancer, colorectal cancer, rectal cancer, lung cancer, oesophagus cancer, endometrial cancer, testicular cancer and carcinogenesis. The search was limited to papers involving human subjects and published in English. Manual searches were also made of primary studies and reviews for additional papers not identified in the electronic search. Experimental studies were excluded if the dietary intervention was conducted in subjects with diagnosed cancer. Each study was classified by design type and evaluated for quality with reference to the Standards of Evidence for Evaluating Foods with Health Claims. Four controlled trials, 18 cohort studies and 60 case-control studies of acceptable quality of 63 identified were included in the evaluation.
Several large prospective cohort studies have examined the relationship between dietary fat and risk of breast, prostate and colon cancer in particular. For the nine cohort studies focussing on breast cancer risk, only one found a relationship with both total fat and saturated fat intake, one identified an association with saturated fat only, two identified an inverse association with monounsaturated fatty acids (MUFA) while one found a positive association with MUFA. Of the three studies that assessed trans fatty acid intake, one identified an increased risk of breast cancer with trans fatty acid intake, one found an decreased risk with trans fatty acid intake, and one found no effect. For the three prospective cohort studies focussing on prostate cancer, energy-adjusted total fat intake was not associated with total prostate cancer risk in any of these studies. Eight of nine cohort studies focussing on colon cancer found no relationship with total fat intake.
In addition six expert committee reports (Food and Agriculture Organisation of the United Nations / the World Health Organisation (FAO/WHO), 1994; The World Cancer Research Fund / the American Institute for Cancer Research (WCR/AICR), 1997; The Committee on Medical Aspects of Food and Nutrition Policy (COMA), UK, 1998 (Gurr, 1998); WHO, 1999 (Scheppach et al.,1999); the Institute of Medicine(IOM), 2002 and FAO/WHO, 2003) have been published during this period as well as several systematic reviews. Among the expert reports, the first, that of the FAO/WHO on Fats and Oils in Human Nutrition (1994), concluded that cohort and case-control studies cast some doubt on animal studies and inter-country correlations linking dietary fat with cancer of the breast, colon, pancreas and prostate, but commented that there were serious limitations in case-control and cohort studies.
The WCR/AICR (1997) concluded that "there is a consistent pattern suggesting that diets high in total fat possibly increase the risk of lung, colorectal, breast and prostate cancers. Diets high in fat increase the risk of obesity; therefore high-fat diets are an indirect risk factor for cancers, the risk of which is increased by obesity." This tentative conclusion was based on early case-control studies which showed a relationship between total fat intake and these cancers.
The COMA, UK stated that "in a rigorous review of the literature, the authors found little evidence for a strong association between the intake of total fat or of individual fatty acids and any type of cancer" (Gurr, 1998).
A WHO concensus statement, 1999, on fat intake and promotion of cancer was that "fat intake is probably associated with colorectal cancer risk. The data available are not sufficient to identify clearly the role of total fat, compared with specific types of fat" (Scheppach et al.,1999).
The IOM (2002) recommended an acceptable macronutrient distribution range (AMDR) for fat to be 20 to 35 percent of energy for adults. Reduction in risk of cancer was not a factor in setting the AMDR for fat due to insufficiency of the data to determine a defined level of fat intake for prevention of chronic disease.
The most recent joint FAO/WHO report Diet, Nutrition and the Prevention of Chronic Diseases (2003) concluded that the evidence does not support a relationship between dietary fat intake and the increased risk of breast, prostate, colorectal and other cancers.
Overall the scientific evidence of an association between dietary fat intake and incidence of breast, prostate and colon cancer has weakened over the past ten years. Although cross sectional population studies continue to provide some support for the relationships between breast cancer and fat components and between colorectal cancer and animal fat intake, prospective cohort and case-control studies generally do not. The primary change in the literature has been the softening of support for the relationship on the basis of case-control studies especially where relative risks and odd ratios have been adjusted for energy intake. Further, several recent large cohort studies did not support a relationship between dietary fat intake and breast, prostate or colon cancers. There is also increased inconsistency in the relationship between specific foods and cancer. Evidence for an effect of meat, in particular red meats, on cancer risk has not been consistently supported by case control and cohort studies reported during the past 10 years.
The evidence indicating no relationship between total fat intake and different types of cancers greatly outweighs the evidence for the claim and thus it is recommended that Canada does not accept a health claim for an association between dietary fat and cancer.
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