Review
Nutrition and primary prevention of breast cancer: foods, nutrients and breast cancer risk

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Abstract

Worldwide, each year approximately one million women are newly diagnosed with breast cancer (BC), in Germany 65 new cases per 100,000 inhabitants are registered, yearly. The fact that incidence has been rising in parallel with economic development indicates that environmental factors might play a role in the causation of BC. Migrational data have pointed to nutrition as one of the more relevant external factors involved. Preventive dietary advice often includes a reduction of alcohol, red meat and animal fat and increasing the intake of vegetables, fruit and fibre and lately, phytoestrogens from various sources. Clearly, the scientific basis for these recommendations appears sparse. The available prospective data from epidemiological studies and interventional trials do not support the overall hypothesis that higher fat-intakes are a relevant risk factor for BC development, more important seems the relative distribution of various fatty acids. A non-vegetarian eating habit (consumption of animal products) per se does not elevate BC risk, while consumption of broiled or deep fried meats cannot be ruled out as a risk factor in genetically susceptible individuals.

It appears prudent to abstain from regular and increased alcohol consumption. This should be particularly true for pubescent girls, in whom glandular breast tissue is particularly vulnerable. In general, if alcohol is consumed on a regular basis, a sufficient supply of fresh vegetables and fruit is essential. While there is no overall protective effect of a high fruit and vegetable consumption speculation remains over possible beneficial effects of certain subcategories, especially brassica vegetables like broccoli, cauliflower and cabbage.

In essence, regional differences in BC incidence are probably partially attributable to life long dietary habits. There is no need to adopt a foreign dietary plan in order to protect oneself against BC. Traditional western diets also have their beneficial ingredients that should be regular constituents in our meals. Lignans from traditionally made sourdough rye bread, linseed/flaxseed and berries are local sources of potentially canceroprotective phyto-estrogens. Furthermore, indole-3-carbinol rich cabbage species might contribute to BC protection by diet. Nevertheless, clear cut recommendations for or against single nutrients or secondary plant metabolites are not yet possible, lacking sufficient data on individual bioavailability, safety and long term outcome. BC prevention by dietary means therefore relies on an individually tailored mixed diet, rich in basic foods and traditional manufacturing and cooking methods.

Section snippets

Epidemiological data on BC and nutrition

It is the aim of this review to present relevant human data on the relationship between nutrition and BC risk. In this context results from epidemiological and if possible interventional studies (Table 2) have been evaluated. There are as many publications on cancer and its relation to nutrition as there are different foods to savour. Even with respect to BC, the number of publications is still much too high to be reviewed in extension, not to mention the many methodological problems inherent

Fat intake and BC risk

For many years high energy intake, most of all high fat intake was considered the decisive explanation for the regional differences in BC incidence. Mainly the consumption of animal fat and saturated fatty acids was accused of a causal relationship to BC. However, the results from 13 prospective cohort studies (Table 3) clearly account against a general association between fat consumption and BC risk.

Prospective cohort studies start with a survey on the observed population, recording individual

Fatty acids versus oils

In an attempt to precisely define the food-ingredient responsible for an alleged risk elevation or reduction, researchers are frequently misled to focus on single nutrients rather than on foods. The discussion about the influence of different fatty acids may serve as an example to explain why this is the case.

BC is clearly less prevalent in the Mediterranean as compared to Europe north of the Alps. Case control-studies [13], [14], retrospectively analysing the eating habits of BC patients or

Interventional studies

It is believed that the majority of BCs are hormone-responsive, at least in a very early stage of disease development. Later on this hormone-responsiveness might be lost, especially in younger patients [17]. Since the hormonal equilibrium can be influenced by the fat content of the diet it has been attempted in several interventional studies to reduce circulating levels of bioavailable estrogenic sex steroids by the administration of low-fat, vegetable-based, fibre-rich diets. Some of these

Conclusion

The available prospective data from epidemiological studies and interventional trials do not support the overall hypothesis that higher fat-intakes are a relevant risk factor for BC development. Neither do they support a protective effect of a low-fat diet. These days a general recommendation is often issued to reduce the fat intake to 30% of daily calorie consumption. This threshold could not be recognized in any of the prospective studies. Willett and Hunter concluded that “the evidence from

Meat consumption and BC

Four out of 10 prospective cohort studies found a positive correlation between a high level of meat consumption or the consumption of broiled meat and the risk for later BC. One of these four papers however had only limited power due to a low number of subjects included. The remaining six studies did not find a significant correlation with meat intake [9], [25], [26], [27], [28], [29], [30], [31], [32], [33].

The most recent meta-analysis including more than 25,000 BC cases used a slightly

Vegetarian diet and BC

A vegetarian diet per se does not necessarily protect against BC. This is the conclusion from the meta-analysis from five mortality studies comparing vegetarians with health-conscious meat-eaters [39]. One study in Californian Seventh-day Adventists revealed a significantly reduced risk, while neither the four other studies nor the pooled analysis showed any significant risk difference (Table 7). There is even some suggestion that BC risk may increase with duration of being a vegetarian [24].

Fish consumption and BC

On one hand many valuable nutrients are delivered by fish. On the other hand, fish can be heavily contaminated with persistent organic residues and heavy metals.

Seven prospective cohort studies are only of limited value in the clarification of the nutritional impact of fish consumption on BC risk [41]. This is partly due to limited comparability as a consequence of methodological differences and short follow-up. Four of these studies have been conducted in the US, with vast regional differences

Alcohol consumption and BC

Ethanol as a chemical itself is not a carcinogen. It has however profound metabolic effects by which it interferes with the metabolism of other, potentially cancer-promoting agents. Of particular relevance is the ability of ethanol to interfere with endogenous hormone-metabolism: after the ingestion of 65 ml pure ethanol postmenopausal women under estrogen replacement therapy exhibited prolonged prolactin levels, daily ingestion of 30 g ethanol raised bioavailable total estrogens as compared to a

Conclusion

In an attempt to lower the individual BC risk it appears prudent to abstain from regular and increased alcohol consumption. This should be particularly true for pubescent girls, in whom glandular breast tissue is particularly vulnerable. In this context the authors welcome recent legislative initiatives to clamp down on “alco-pops”, alcoholic beverages designed to incite under-age drinking. In later adulthood, moderate drinking has been shown to exert a cardioprotective effect and to extend

Fruit and vegetable consumption and BC

A high consumption level of fruit and vegetables is generally considered a healthy diet. Prophylactic effects have been shown against hypertension, obesity and coronary heart disease. However, a significant overall protective effect could not be shown against BC in prospective cohort studies. A meta-analysis of the pooled data from eight prospective studies incorporating 350,000 women and more than 7300 cases (Table 6) did not find a single significant correlation. Neither was any association

Fibre consumption and BC

The term fibre encompasses a great variety of chemically diverse plant constituents which are non-digestible to humans. They are believed to protect from BC by binding enteral carcinogens and endogenous sex hormones on enterohepatic cycling and facilitating their enteral elimination. They are known to improve insulin resistance and counteract obesity. However, the sparse prospective data from cohort studies account against a general protective effect of fibre against BC (Table 9). A possible

Phyto-estrogens and BC

The discovery that chemicals in our daily surroundings can exert potent hormone-like activity has fuelled substantial public interest and research activities [58], [59], [60], [61]. Since BC is a hormone dependent entity is was logical to ask, whether the exposure to environmental estrogens was related to the rising incidence of BC. The finding that potent estrogenic substances are part of daily diets sparked both further public concern and questions about the pathogenetic relevance of

Summary conclusion

Regional differences in BC incidence are probably partially attributable to life long dietary habits. There does not seem to be a need to adopt a foreign dietary plan in order to protect oneself against BC. Foreign diets without the century long co-evolution of antigen tolerance carry the risk of inducing food allergies and could trade in the unwanted side effects of these diets (e.g. early stomach cancer with Japanese diets). Clear-cut risk-foods could not be identified in prospective studies

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