Our study hypothesis was confirmed where we showed for the first time that dietary intake of beans is associated with reduced odds of overall, hormone receptor-positive and triple-negative breast cancer in African women after adjusting for well-established covariates including obesity, leisure time physical activity, menopausal status, marital status, number of pregnancies, breastfeeding experience, demographic variables, and type of occupation.
Our findings are similar to studies conducted in other populations [12-15]. For example, in prospective study of dietary beans intake, Adebamowo et. al. studied 90,630 nurses from the United States who participate in the Nurses’ Health Study II, aged 26-46 years at baseline and followed for 8 years to examine their dietary intake and the association with breast cancer risk. The authors found that high cumulative average intake of beans or lentils, two or more times a week, compared with intake of less than once a month is associated with reduced risk of breast cancer (Relative Risk (RR)=0.76, 95% CI: 0.57-1.00; p-trend=0.03) . Also, in experimental animal studies suggest inverse associations between consumption of a 60% bean diet with chemically induced breast cancer in rat models .
Other studies examined intake of beans while using factor analyses to derive dietary patterns. In a US-wide prospective cohort study of 40,559 women by Velie et. al., three major dietary patterns were derived after using principal components factor analysis: vegetable-fish/poultry-fruit, beef/pork-starch, and traditional southern dietary patterns. The traditional southern pattern which is characterized by high intake of traditional rural southern US foods, including cooked greens, cooked beans and legumes, sweet potatoes, cornbread, cabbage, fried fish, chicken, and rice but low intake of cheese, mayonnaise–salad dressing, wine, liquor, and salty snacks was inversely associated with invasive breast cancer (Relative Hazard (RH)=0.78, 95% CI: 0.65-0.95; p-trend = 0.003) .
In a case–control study of 2,135 breast cancer cases and 2,571 controls, the researchers showed that high intake of total beans was associated with reduced risk for breast cancer (OR=0.81, 95%CI: 0.66–1.01, p-trend= 0.03). There was an inverse association between estrogen and progesterone breast cancer and high intake of beans (OR=0.72, 95% CI: 0.50–1.05, p-trend= 0.04). In another case-control study of breast cancer in the United States, the Four-Corners Breast Cancer Study, Murtagh et. al. found an inverse association between breast cancer risk and factor-derived “Native Mexican” dietary pattern that heavily loaded with Mexican cheeses, soups, meat dishes, legumes and tomato-based sauces in Hispanic and non-Hispanic white women (OR=0.68, 95%CI: 0.55-0.85; p-trend < 0.01) .
Beans is one of the commonest food items eaten in SSA and Latin America even though the amount consumed per capita is falling. In Nigeria, it is most commonly consumed alone or as porridge beans, stewed, soups, pudding or cakes. In this study, we examined intake of beans only because approximately half of our study participants consumed it in this form daily whereas consumption in other forms was less common. Other survey showed that approximately two third of Nigerians consume bean in different types of dishes daily . Beans is an excellent source of plant proteins (23%), complex carbohydrates, dietary fiber, flavonoids, some vitamins and minerals .
There are several mechanisms that might explain our study findings. The first one is the high fiber content in beans. Animal studies have shown that soluble fiber reduced mammary tumor growth, angiogenesis and metastasis in mice . Fiber has been associated with slowing glucose absorption, reducing insulin secretion, and regulation of the bioavailability of insulin-like growth factors, which are important pathways in breast carcinogenesis [25,26]. Epidemiological studies have shown inconsistent association between dietary fiber intake and serum estrogen. Conjugated estrogens in the liver are excreted into the bile and reabsorbed in the intestine and fiber may bind estrogens in the colon during the enterohepatic circulation and increase the fecal excretion of estrogens [25-26]. Fiber may also act by reducing β-glucuronidase activity in the feces leading to a decrease in the reabsortion of estrogen in the colon . Furthermore, high intake of dietary fiber may reduce breast cancer risk by reducing the risk of overweight and obesity.
The second explanation is that beans contain a wide range of flavonoids and is the source of the greatest amount of flavonoids intake in the Nigerian diet. The Mexican diet is also rich in flavonoids, including flavones and lignans from beans, corn and green vegetables . A hospital-based case–control study conducted in Mexico City found that in postmenopausal women, high dietary intake of flavonols and flavones was associated with a significant reduction of breast cancer risk . Flavonoids may reduce carcinogenesis by preventing oxidative stress, inhibiting cell proliferation, inducing cell-cycle arrest, and maintaining DNA methylation [12, 29-31].
Lastly, women who have high beans’ intake may also have retained other components of the traditional African diets, which are rich in anti-cancer agents. Whereas, women who reported never eating beans may have replaced it with other food items that are associated with increased risk for breast cancer such as charred red/organ meats, fat and calories. However, more studies are needed to test these hypotheses because we did not examine them in the current study.
The limitations of our study include recall bias, and potential impact of breast cancer on dietary intake in general and specifically of bean intake. We did not adjust for family history of breast cancer, but previous studies showed low prevalence of this risk factor in Nigerian breast cancer patients. We also did not adjust for intake of other foods and nutrients. However, we adjusted for alcohol intake, the dietary factor that is most consistently associated with breast cancer risk as well as obesity measured by BMI and WHR. We used a modified FFQ, which has not been extensively validated in African populations. Despite these limitations, the strengths of our study include histological and immunohistochemical confirmation of breast cancer and its molecular subtypes, a large sample size with sufficient power to detect significant results, inclusion of a broad range of well-established covariates and confounders such as BMI, menopausal status, demographic variables, types of occupation, as well as relative homogeneity of the study population.