Rosa ribbon day (15.10.2010) made me curious about what new information has accumulated in regards to breast cancer and diet. After several PubMed literature reviews I could conclude that majority of the scientific human data is based on cohort trials. Few randomized trials have been published, we need to rely on epidemiological data. Anyway, here is what I found.

Overweight increases the risk of ER+PR+ -subtype tumors

My impression is that the strongest dietary predictor of breast cancer is overweight.  Obesity increases the estrogen levels which leads to the increased of breast cancer. According to a study published in 2009, women with BMI over 30 had 82 % higher risk for ER+ PR+ -breast cancer than those with normal weight.  Each increment of 5 units in BMI tended to increase the risk of breast cancer by 10 % among pre-menopausal and  by 33 % among post-menopausal women with ER+PR+ -subtype. In other variants of breast cancer (more rare ones) the observed risk was weaker or non-existing. (Suzuki et al. 2009)

Alcohol once again

Alcohol is well-established risk factor for breast cancer. In a meta-analysis published in 2008, each 10 grams of alcohol increased the risk of breast cancer by 10 % in all sybtypes excluding ER-PR-subtype (this is rather rare type of breast cancer)  (Suzuki et al 2008)

Dietary fat

Diets high in fat are also linked to breast cancer. The association has been thought to mediate via estradiol, high fat diet increases estradiol levels in a body (Wu AH et al. 1999). Evidence from meta-analyses is not always unanimous  (Hunter et al. 1996). However, some recent randomized interventions have supported the deleterious effect of high fat diet on the risk of breast cancer  (Prentice RL et al. 2006 ja Chlebowski RT et al. 2006). Saturated fat per se is not likely to be associated with breast cancer (Alexander DD et al. 2010 ).

Omega-3 and omega-6 fatty acids

How about spesific fatty acids? Most of the data above were based on meta-analyses. It takes often decades before sufficient data are accumulated for meta-analyes. Therefore, we cannot rely on them only. According to the some epidemiological studies omega-3 to omega-6 ratio has an effect on breast cancer risk. The smaller the ratio, the bigger the risk (Simonsen et al. 1998). 

However, the story on omegas is not straight-forward. According to a French cohort trial omega-6 fatty acids from nuts increased the risk of breast cancer whereas omeag-6 faty acids from oils and vegetables or fruit decreased it. In addition, in this trial biggest risk reduction was achieved among those had highest overall omega-3 and omega-6 intake  (Thiebaut AC et al 2009). Thus the role of omega- 6 fatty acids still requires confirmation.

Vitamin D

Last July a Chinese meta-anaysis consluded that high intake of vitamin D may decrease the risk of breast cancer. In this analysis, thise belonging to the highest quintile of vitamin D intake had 45 % smaller risk for breast cancer than those with the lowest intake (Chen P et al 2010).  In another meta-analysis the results were less clear  (Gissel D et al. 2008)

Olive oil and Mediterranean diet

Even if the cancer preventive effect of olive oil is known long, the accumulated data is not vast. There is no meta-analysis available on olive oil’s effect on breast cancer. Italian, Greek and Spanish cohort studies have supported the preventive effect of olive oil. The possible mechanism might be inhibition of mitogenis (due to suppression of FASN enzyme)  (Menendez JA  et al 2008). Olive oil is the most abundant fat in Mediterranean diet. Mediterranean diet as whole has also been associated with decreased risk in some studies (Murtaugh MA et al. 2008 , Trichopulou A et al 2010)

Green tea and soya

Prevalence of breast cancer is lower in Asian countries. Consumption of green tea and soya are speculated contributing factors. In a meta-analysis from 2005, 5 cups of green tea daily was associated with decreased risk of breast cancer  (Seely D et al. 2005), even if the results from background trials were heterogenous. Polyphenols, and spesifically catechines are thought mediator of the protective effects. Intake of soya protects only the Asian women from breast cancer according to the latest data (Nagata C 2010).

Calcium

In some cohort trials, high intake of calcium has decreased the risk of breast cancer. There is one meta-analysis that has  shown significant risk reduction among those with highest calcium intake (Chen P et al 2010).

Carbohydrates and fiber

In some American, Swedish and Chinese cohort studies the high intake of fiber has protected from breast cancer, whereas the high intake of refined carbohydrates has increased the risk. In one American randomized trial high intake of fiber and vegetables did not decrease the risk of breast cancer  (Pierce JP et al 2007). High intake of refined carbohydrates have been thought to stimulate  insulin receptors and cause mitogenesis via IGF (Insulin like Growth Factor).

Recap

Most important dietary means to prevent breast cancer are avoidance of obesity, moderate alcohol consumption and sufficient vitamin D intake, and potentially avoidance of very high fat diet (Figure) . The potential other factors, mentioned towards the end of this article, have been documented less conclusively. However, as the same factors are also promoting cardiovascular health, the application of them to daily routines is warranted without further evidence. Perhaps, with calcium one should be cautious not to overdo (see my previous post).

Figure.  Recap: diet and breast cancer