Our findings indicate that a high consumption of legumes is associated with a decreased risk of several cancers including those of the upper aerodigestive tract, colorectal, ovary and kidney cancers. When stratified for sex our results showed that the association with cancers of the upper aerodigestive tract, colorectum and kidney cancer was greater in males.
The strongest inverse associations between legume consumption and cancer were observed for esophagus and larynx cancer with OR of 0.50 and 0.55, respectively, for consumption of 2 or more portions per week. These results are consistent with previous case-control studies which have reported OR of 0.54–0.62 for esophagus and larynx cancer with the highest intake of legumes (32, 33). A case-control study of 11 cancer sites conducted in Uruguay between 1996 and 2004 and including 3,539 cancer cases and 2,032 hospital controls reported an OR of 0.54 for esophagus and 0.55 for laryngeal cancer among the highest as compared to the lowest tertile of consumption (34). Other studies from the United States (Connecticut and Los Angeles) looking at associations between legumes and esophageal cancer reported significant inverse associations between legume intake and risk of esophageal cancer (particularly a decreased risk of esophageal squamous cell carcinoma), although the legume group within these studies included beans and nuts (35, 36).
We found that even a moderate consumption of legumes (i.e. 1 portion a week) is associated with a significant reduction of colorectal cancer risk (OR: 0.70). In line with our findings a recent meta-analysis of observational studies (n = 14: 3 cohort studies, 11 case‐control studies) found a decreased risk of colorectal adenoma for the highest versus lowest intake of legumes (OR = 0.83) (11). However, other studies provided mixed results with some indicating an inverse association or no association (10). In the Polyp Prevention Trial, an increased consumption of legumes was associated with a reduced risk of advanced adenoma recurrence. The OR in individuals in the highest quartile of change in dry bean intake from baseline (median change: +41.5 g/day) versus the lowest quartile (-5.7 g/day) was 0.35 (9).
In our study, legume intake was also linked to a reduced risk of kidney and ovarian cancer. Diet has been related to kidney cancer although the role of specific foods or nutrients is still controversial (12). A limited number of studies have specifically examined the association between legume consumption and kidney cancer (12) (37) (38). Consistent with our findings, a case-control study in Uruguay reported a significant inverse association between legume intake and kidney cancer (OR = 0.41) (34). In addition, a large US cohort (N = 1816) showed a dose-response relationship with a 12% reduced risk of kidney cancer per 2.5g/day increment of dietary legume fibre(12, 38).
To date, a few studies have considered dietary patterns in relation to ovarian cancer risk and none, to our knowledge, has looked specifically at legumes (14, 39). Some studies have examined the intake of plant-based foods and fibre and showed inconsistent results (14, 40).
We found inverse but not significant associations between legume consumption and risk of stomach, breast, endometrium, and prostate cancer. Previous studies for these cancer sites reported mixed results, some reporting weak/moderate associations (OR ranging from 0.42–0.84) or null associations (39,41−44).
As to the mechanisms that could explain a possible protective effect of legume intake on cancer risk there are several possible explanations (45–48). Legumes are recognised as a protein source but are often overlooked as a source of fibre, with 100g of cooked legumes containing, at a minimum, 5g of dietary fibre (5). Our results showed that those who consumed at least 1 portion of legumes per week had 6–8 more grams of fibre per week than individuals who had less than 1 portion per week. Consumers of 2 or more portions per week had up to 7.5 more grams of fibre per day. This represents half of the recommended 14g/1000Kcal/day to reduce chronic disease risk(7).Thus, the beneficial effects related to legume consumption are likely related to their fibre content and this is particularly true for colorectal cancer. When entering the large bowel, fibre increases stool weight, dilutes colonic contents and stimulates bacterial anaerobic fermentation. This process reduces contact between the intestinal contents and mucosa and leads to the production of short chain fatty acids (SCFA) through the fermentation of fibre by gut bacteria. SCFAs reduce cell proliferation, the first biological mechanism promoting carcinogenesis. SCFA reduce colonic pH thereby inhibiting the histone deacetylase enzyme and decreasing the conversion of primary to secondary bile acids (deoxycholic acid and lithocholic acid) which are cytotoxic to colonocytes (6).Furthermore, dietary fiber is a substrate for the gut microbiota affecting amount and composition favouring anti-inflammatory strains which have local and systemic health benefits via modulation of the immune system, production of microbial metabolites, conversion of polyphenols into biologically active forms, and modifying also distant organ tissue-specific strains (6, 45). Beyond fibre, other bioactive compounds in legumes, such as phenolics, may also play a role in inhibition of colorectal cancer (45).
Dietary fibre and proteins from legumes also contribute to lower the glycaemic load of the diet (6, 21) thus preventing hyperglycaemia and hyperinsulinemia(27, 48). Hyperglycaemia and hyperinsulinemia are both sustained by excess body fat and consequential changes in hormonal status, growth factors, inflammatory markers, and oxidative stress – all contributing factors in the development of chronic diseases, including cancers (6–8). Pulses have been linked to improvements in these markers (45).
In addition to fibre, legumes are also rich in vitamins (i.e. B vitamins), minerals (i.e. iron, folate, calcium and zinc) and a series of biological active compounds, known as phytochemicals which also have antitumor effects (47). These compounds include tannins, flavonols, isoflavones, phenolic acids and phytic acids (45). For example, phytate are excreted in the urine where they inhibit the formation of kidney stones (37), which have been related to kidney cancer (49). Legumes are also a good source of folate, which may protect against cancers of the esophagus and colon (13, 33).
In addition to the direct cancer preventative effects of legume intake, indirect effects may also be at work as well. Higher intake of legumes may replace other sources of protein such as meat or high glycaemic index carbohydrates, both of which have been shown to be linked to several cancers(48).
The sex-specific pattern of association for oral cavity, larynx, esophagus, colorectum and kidney cancers is difficult to explain, but could be related to the differences in dietary habits between men and women. In the studies included in this work, men tended to eat less vegetables and fruit than women (Online Supplement Table S2), thus legumes in men can be an important source of dietary fibre that compensate for the low fibre intake from other sources (29, 39, 42). The observed sex differences may also be linked to a greater effect in men than in women of dietary changes on microbial composition reported in some studies (50). However, whether the microbial composition is involved in the development of cancer remains to be determined.
Strength And Weakness
In this work, we quantified the association between legume consumption and several cancer sites using a network of large case-control studies. In these studies, the same validated and reproducible questionnaires have been used to collect information on legume consumption and to measure potential confounders. Several confounders have been considered including age, education, overweight/obesity, smoking, alcohol, consumption of fruit, vegetables and processed meat, energy intake and for female hormone-related cancers also age at menarche, menopausal status and number of children.
The study has also some limitations. The first lies in the potential inaccurate measure of legume consumption in a case control design. In addition, the inverse association between legume consumption and various cancers can at least be partially attributable to a generally healthier diet of legume consumers who also had high intake of fibre from other dietary sources. Finally, although the majority of studies included more than 1000 cases, for some cancer sites only a few cases were in the highest category of consumption (i.e. ≥2 portions). This should be considered when interpreting the significance of the estimates.