The Controversial Association Between Dietary Fat Intake and Breast Cancer

If you eat a high-fat diet, will you have a greater chance of getting breast cancer? If you eat certain types of fat, such as olive oil or fish oil, will your breast cancer risk be decreased? Although it seems that these questions might be easy to answer (and that the answer to both might be "yes"), there is a surprising lack of consensus among scientists on the relationship between fat intake and the likelihood of developing breast cancer.


Animal Studies

The controversy stems from a lack of agreement between several types of studies used to evaluate the link between dietary fat and breast cancer. Animal feeding experiments have provided strong evidence that there is a link between the consumption of high-fat diets and development of breast cancer (breast cancer is usually called "mammary cancer" in experiments involving animals). A number of laboratories have reported that diets high in fat stimulate mammary tumor formation in mice and rats. Diets high in polyunsaturated fatty acids, such as the fats found in corn oil and safflower oil, have been shown repeatedly to promote mammary cancer in these animal breast cancer models. Animal experiments with olive oil have been inconsistent, while fish oil has been shown to not increase mammary tumors, and in many cases even to prevent tumor growth and metastasis.

The major difficulty with mammary tumorigenesis studies is that effects due to fat intake and energy intake have been difficult to separate. Fat is a high-energy food, and it may be that a high-energy intake from any source in the diet (carbohydrates, fats or proteins), rather than fat intake itself, may be the culprit in mammary cancer. Some research has suggested that energy intake is much more important than fat intake in the development of mammary tumors.

Human Studies of Fat and Breast Cancer
Population-based international studies have also provided strong evidence that a high-fat diet is associated with an increased risk for breast cancer. In these analyses, fat intake in women in a certain country is matched with the incidence of breast cancer in that country. Regions such as East and Southeast Asia that have a low fat intake also have a low incidence of breast cancer. In contrast, North America, which has the highest incidence of breast cancer in the world, also has one of the highest intakes of total dietary fat.

So you might ask "what's the problem - there's a link, right?" The problem is that there are differences other than fat intake between geographic regions. For example, in Japan the rates of both fat consumption and breast cancer have risen noticeably in this century. However, other factors that contribute to breast cancer risk have also changed in Japan during this period. It turns out that rapid growth and greater adult height shows the strongest relationship to breast cancer risk. Thus, girls who grow rapidly during puberty and are taller as adults will be more likely to develop breast cancer. Other hormone-related factors including a late age at first birth, no children (or a small number of children), and late natural menopause also contribute to the risk for breast cancer. Changes in many breast cancer risk factors other than dietary fat intake have most likely played a role in the increased incidence of breast cancer in Japan in the last century. These non-dietary factors are probably more important than fat intake in determining whether a woman will develop breast cancer.

To try to get a more accurate picture of the importance of fat in the development of breast cancer, scientists use two other types of studies to evaluate risk. In case-control studies, patients with breast cancer (the cases) are compared to a group of women without breast cancer (the controls). Both groups are from a closely matched population (that is, the groups are not from different countries, different races, etc.). An interviewer then asks each group about their dietary history and eating habits, and their answers are used to determine whether there is a pattern of intake of fat in the breast cancer patients that was not seen in the controls. The flaw in these types of studies is that the interviewer or the subject may subconsciously be biased in their questioning or reporting. For example, a breast cancer patient may have heard or read that a high-fat diet is associated with breast cancer, and may then report that her fat intake was higher than it actually really was. These studies are referred to as "retrospective" studies, because the subjects are looking back to recall their past behavior.

Another type of study, called a prospective cohort study, is believed to be the best way to assess how risk factors such as dietary fat influence the likelihood of developing breast cancer. In cohort studies, the diet of a large group of healthy subjects is assessed before the development of breast cancer. This group is followed for many years, and some members of the group will develop breast cancer over this time (sometimes more than a decade). Cohort studies avoid the bias mentioned above for the case-control studies because neither the interviewer nor the subject knows whether cancer will develop in the subject. In addition, the diet can be assessed at regular intervals during the study, and blood and other samples can be taken from the volunteers during the period of observation. The main disadvantage of this study design, as might be imagined, is the cost of following a group of subjects over many years, and waiting until a significant number of subjects develop breast cancer. However, a number of cohort studies have been carried out and the results of the studies have been analyzed for possible associations between dietary fat intake and risk for breast cancer.

Results of Human Case-Control and Cohort Studies

There have been over 20 case-control studies of the association between total dietary fat intake and the risk of breast cancer to date. These studies have produced inconsistent results. Two of the studies have shown a significant increase in breast cancer risk as fat intake increased, whereas the remainder of the studies showed no association. In 1990, all of the case-control studies done to that time were summarized by a technique called "meta-analysis." The results of this analysis of 4,312 cases and 5,978 controls showed that total fat intake was associated with increased risk of breast cancer, and the association was stronger for postmenopausal than premenopausal breast cancer. Another meta-analysis in 1993, which was based on analysis of about 3,000 cases, also showed a link between increased total fat intake and breast cancer risk, but the association was weaker than in the earlier study.

In contrast to case-control studies, which indicate that total fat intake may increase the odds of developing breast cancer, the more accurate cohort studies have never shown a link between dietary fat and breast cancer. A meta-analysis of these studies in 1996 revealed that there is no increase in risk of breast cancer with greater intake of total fat. Results of a large prospective cohort study of 88,795 women who were followed for 14 years between 1980 and 1994 were reported last year in the Journal of the American Medical Association. This study found no evidence that intake of total fat or specific types of fat (such as fat in corn oil, fish oil or olive oil) were associated with an increased risk of breast cancer in humans.

Conclusion

Although many types of studies in humans and animals have suggested an association between fat intake and breast cancer risk, the cohort studies, which are the most powerful ones for detecting associations between diet and breast cancer risk, have consistently failed to show a link between dietary fat and breast cancer. So is it OK to eat a high fat diet without worrying about developing breast cancer? Of course no one knows the answer to this question with certainty. It may be that a high fat diet indirectly increases risk by promoting a more rapid growth in early life, or by contributing to obesity, which itself is a probable risk factor for breast cancer. The best advice at present, from the standpoint of preventing breast cancer, would likely be to avoid a high fat diet until the discrepancies between the animal and human study results are resolved.

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