Rates of breast cancer have been increasing among Asian American women, particularly women of Japanese and Filipina background. Asian American women have different patterns of breast cancer types than the general U.S. population. While Asian Americans are subject to breast cancer risk factors common to all women, certain foods, dietary patterns, and food preparation methods that are more prevalent in Asia also influence breast cancer risk. Childhood diet can also help determine breast cancer risk in adulthood. Increases in breast cancer rates among Asian Americans highlight the need for breast cancer screening in groups that may not perceive themselves as being at significant risk for breast cancer.
Chinese, Korean and Japanese studies make up most of the breast cancer research concerning Asian populations. Obviously, there are large differences between South Asians, Pacific Islanders and Eastern Asians. Studies of Asian Americans tend to be inclusive of more Asian groups. We present the information below by specific ethnic group, where possible.
Rates of breast cancer have been increasing among Asian Americans
Breast cancer rates are substantially higher in the U.S. than in Asia. When Asian women migrate to the U.S., their breast cancer rates rise over several generations. However, rates of breast cancer have been increasing in some Asian American groups beyond the level of the general population. A 2011 study designed to compare breast cancer rates among young women in California reported that Asian American and Pacific Islander women had a 1.6-fold higher risk of breast cancer than white women. The risk was highest for women of Filipina and Japanese ancestry. All of the women in the study were under age 45 and born in California during the 1960s. Note that the breast cancer incidence in the Philippines is the highest in Asia.
Another 2011 study examined breast cancer trends in Los Angeles County from 1972 to 2007 among various racial groups (non-Hispanic white, Latina, African-American, Chinese, Filipina, Japanese, and Korean women). Increasing breast cancer rates were found in all of the groups during the 1980s and 1990s. However, the breast cancer risk increased more in Japanese and Filipinas compared to women of Chinese and Korean background. After the turn of the century (2000-2007), breast cancer rates fell significantly among white women and slightly in African Americans, stayed level for Latinas, and continued to increase significantly among all Asian groups. By the 2000-2007 period, younger Latina women had the lowest breast cancer risk, whereas Chinese and Korean women formerly had had the lowest risk.
Breast cancer rates among Asian Americans are higher than that of women living in Asia. A 2010 study that compared the incidence of breast cancer in Asian countries with their Asian-American counterparts for the period 1993-2002 reported that the breast cancer incidence among Asian women living in the U.S. was 1.5 to 4 times higher than the corresponding incidence in the women’s respective countries of origin.
Asian American women have different mix of breast cancer types
Asian American women have different proportions of breast cancer subtypes than Caucasian women, reflecting the rates of women in their countries of origin. Compared to white women, Chinese American women have a lower rate of hormone receptor positive (ER+/PR+) disease and higher rates of hormone receptor negative (ER-/PR-) and mixed receptor (ER+/PR- or ER-/PR+) tumors (all of which have more aggressive characteristics than ER+/PR+ disease).
Some Asian American women are more likely to have HER2 overexpressing (HER2+) disease than white women. A study that examined that the distributions of breast cancer subtypes among six Asian groups (Chinese, Filipina, Japanese, Korean, South Asian, and Vietnamese) in California found that compared to white women, Chinese, Filipina, Korean, and Vietnamese women had significantly increased risk of HER2+ breast cancer. Another study that examined the prevalence of various breast cancer subtype combinations in California breast cancer patients reported that Pacific Islanders had increased likelihood of having the ER-/PR-/HER2+ subtype.
Another study examined the distributions of ER and PR status across five ethnic groups in Hawaii and Los Angeles. ER/PR status was found to vary significantly across racial/ethnic groups. The highest fraction of ER-/PR- tumors was seen in African Americans (31%), followed by Latinas (25%), whites (18%), Japanese (14%), and Native Hawaiians (14%).
A study of U.S. women from India and Pakistan found that South Asian breast cancer patients had significantly younger median age at diagnosis, larger tumor size, higher tumor stage and grade, more positive lymph-nodes, and more ER-/PR- disease than white women. While the age-adjusted incidence of breast cancer was lower in South Asian women, their five-year survival rate (84%) was lower than that of white women (89%). In other words, U.S. women of South Asian background have lower rates of breast cancer than white women, but when they do develop the disease, it is more aggressive and more deadly.
Another study comparing South Asian with white breast cancer patients in the U.S. found that 16.2% of the Indian/Pakistani breast cancer patients were under 40 at diagnosis compared to 6.23% of white women. Indian women had more ductal breast cancer (69.1% compared to 65.7%), more inflammatory breast cancer (1.4% compared to 0.8%) and less lobular breast cancer (4.2% compared to 8.1%) than white women. Indian/Pakistani women also had more ER-/PR- disease (30.6% compared to 21.8%).
Perhaps surprisingly, given the information above, women in most Asian-American subgroups have better breast cancer-specific survival rates than U.S. white women with breast cancer according to one 2015 study.
Most breast cancer risk factors occur across races
While there are some genetic differences between races that influence breast cancer susceptibility, for the most part, Asian and Asian American women are influenced by the same factors as other racial groups. Breast cancer risk factors over which women have some control and that have been confirmed in Asian populations include high breast density, high body mass index (BMI), use of birth control pills, use of hormone replacement therapy (HRT), vitamin D deficiency, high omega-6 to omega-3 fat ratio in the diet, moderate to high alcohol intake, and high red meat consumption.
Soybeans reduce premenopausal breast cancer risk
Numerous studies have reported that soybean consumption (including foods such as tofu, edamame and tempeh) reduces the risk of premenopausal breast cancer in Eastern Asians. Soybean consumption has also been associated with enhanced survival after a diagnosis of breast cancer. The impact of soy consumption among Asians living in countries with a long history of soy consumption is considerably higher than among Caucasians.
The reason for the difference is that only 25% to 35% of the U.S. Caucasian population is capable of converting the soy isoflavone daidzein to equol in the gut, whereas people in high soy consumption areas of Asia have rates closer to 40% to 60%. Equol has been shown to inhibit proliferation in human breast cancer cells. Seaweed consumption has also been found to enhance intestinal production of equol, which could partially explain some of the breast cancer protective effects of Asian diets that are high in both seaweed and soy. It makes sense that Asian populations, which have had many thousands of years to develop their ability to digest and extract nutrients from soybeans (which are less digestible than most other commonly-consumed foods), would have a slightly different genetic profile than populations that are relatively new to soy consumption.
Soy consumption appears to be most beneficial during childhood. One study examined the association between childhood soy intake and breast cancer risk in U.S. women of Asian descent. The study included 597 breast cancer cases and 966 cancer-free controls of Chinese, Japanese, and Filipino descent, living in San Francisco-Oakland, Los Angeles or Oahu (Hawaii). Study participants were interviewed about adolescent and adult diet and cultural practices. For those with mothers living in the U.S. (39% of the participants), the mothers were interviewed about their daughter's childhood diets and exposures. Inverse associations between breast cancer and childhood soy intake were found in all the groups and in women born both in Asia and the U.S. Note that soybean oil, soy protein isolate and soybean paste are not healthy foods and there is some evidence that they increase breast cancer.
Other foods common in Asia also reduce breast cancer risk
High green tea, mushroom, turmeric and seaweed consumption levels are also associated with reduced risk of breast cancer in both Caucasian and Asian populations. One Chinese study found a joint effect between green tea and mushroom consumption; high dietary intake of mushrooms decreased breast cancer risk and green tea consumption further decreased the risk.
Some Asian dietary patterns are associated with breast cancer risk
A number of Asian and Asian American studies have taken into account dietary patterns in evaluating the influence of food on breast cancer risk:
- One study investigated the association between dietary patterns and breast cancer risk in Asian Americans. The authors identified three dietary patterns (Western-meat/starch, ethnic-meat/starch, and vegetables/soy). Women who were both high consumers of Western or ethnic meat/starch and low consumers of vegetables/soy had the highest risk of breast cancer.
- A Hong Kong study identified two dietary patterns: (1) vegetable-fruit-soy-milk-poultry-fish; and (2) refined grain-meat-pickle. Women in the highest fourth of the vegetable-fruit-soy-milk-poultry-fish dietary pattern were found to have a 74% lower risk of breast cancer than those in the lowest quartile. On the other hand, the refined grain-meat-pickle dietary pattern was linked to 2.6-fold higher risk of breast cancer.
- A Shanghai study reported that women in the highest fourth of fruit and vegetable intake (3.8 servings/day) had approximately half the risk of breast cancer of women in the lowest quartile of intake (2.3 servings/day).
- A Guangdong, China study reported that total overall fruit and vegetable consumption was inversely associated with risk of breast cancer. Consumption of bananas, watermelon/papaya/cantaloupe, dark green leafy vegetables, cruciferous vegetables, carrots, and tomatoes were each significantly associated with lower breast cancer risk.
- A Korean study reported that breast cancer patients consumed a significantly lower quantity of eggs, tofu, onions, garlic, green pepper, sweet pepper, kale, cucumber, seasoned bean sprouts, sesame leaf, zucchini, radish, mushroom, crown daisy, red pepper paste, bean paste, spicy bean paste, orange juice, grape juice, and tomato juice than cancer-free women. On the other hand, the breast cancer patients consumed significantly greater quantities of cooked rice, noodles, deep fried chicken, satsuma mandarin, Korean melon, kimchi and coffee than the cancer-free controls.
- Another Korean study designed to examine the relationship between fruit, vegetable, and soy intake and the risk of breast cancer reported that high grape, green pepper, soybean intake were each associated with reduced breast cancer risk.
- A study designed to investigate the association between dietary patterns and sexual maturation among children in Seoul identified four distinct dietary patterns among the children: (1) rice and Kimchi; (2) shellfish and processed meat; (3) pizza and drinks; and (4) milk and cereal. Early breast development in girls was found to be significantly positively associated with the shellfish and processed meat dietary pattern.
- A Philippine study examined the possible contribution of method of cooking to the remarkably high rates of breast cancer among Filipinas. Detailed information was collected regarding the study participants' current method of cooking, as well as the usual method of cooking in the household of the women when they were 12 years of age. The cooking method of boiling food in coconut milk was found to be associated with more than double the risk of breast cancer.
Regular breast cancer screening is important
The finding that Filipina and Japanese American women have comparatively high rates of breast cancer at relatively young ages points to the need for them to be screened for breast cancer starting at age 40 (younger if there is a family history of breast cancer). Many older Asian American and Asian immigrant women are also not undergoing the regular breast cancer screening that they should receive.