- Legumes are defined as the immature, germinated, or mature edible seeds of legume plants (Fabaceae). The primarily consumed legumes are dry beans, dry peas, lentils, chickpeas, cowpeas, fava beans, and pigeon peas, which are grouped as mature legumes, green beans and peas, which are considered green legumes, and sprouted or germinated beans and peas, which are considered sprouted legumes. Soybeans and peanuts are usually excluded because of their higher lipid content. Legumes are an important part of a daily diet because of their nutritional and health benefits. They are a good dietary source of protein, fiber and folate, and are very low in saturated fatty acids, cholesterol, and sodium, and contain many non-nutritive bioactive components. Based on their nutritional benefits, the current U.S. dietary guidelines consider mature legumes as a food group with a daily recommendation of 37.5 g for non-vegetarians and 75 g for vegetarians. In addition to their nutritional benefits, daily mature legume
consumption may have disease prevention benefits, specifically obesity, metabolic syndrome, and cardiovascular diseases, as concluded by several meta-analyses.
The overall objective of my thesis is to provide evidence that regular legume consumption is an effective strategy for chronic disease prevention in U.S. adults. Using the PRECEDE/PROCEED planning model for health promotion and change, I assessed the public health problem by evaluating current legume consumption patterns in Aim 1 and then identified reasons for the current low legume consumption patterns in U.S. adults using the Health Belief Model in Aim 2. My other research focus was to provide scientific evidence of the benefits of regular legume consumption for chronic disease prevention. I evaluated the scientific evidence of the chemo-preventive role of regular legume consumption against colorectal neoplasia in human and animal model studies through a meta-analysis in Aim 3 and identified biomarkers of dry bean consumption in human and animal model studies for future legume human intervention studies in Aim 4.
Given that the most recently published legume consumption data for U.S. adults are from the NHANES 1999-2002 two-year cycles, in which 7.9% of U.S. adults consumed mature legumes and 11.1% of adults consumed dry beans on a given day, there is a need to evaluate current legume consumption patterns in U.S. adults. Therefore, the Aim 1 was to evaluate current legume consumption patterns in U.S. adults by using cross-sectional data from the 2011-12 and 2013-14-year cycles of NHANES (n=11,855) and a cross-sectional, on-line survey conducted in 2017 in adults around Corvallis, Oregon named “Beans, Lentils, Peas (BLP) Survey” (n=43). Participants were grouped into legume non-consumers and consumers, which were
further grouped into inadequate legume consumers (<37.5 g/d, which are the dietary recommendations for mature legume consumption), marginal legume consumers (37.5-87.49 g/d, the latter being the cut-off point demonstrating nutritional and disease prevention benefits), and disease-prevention legume consumers (≥87.5 g/d legume consumption). My results indicate that legume consumption remained low in U.S. adults with a declining trend from 2011 to 2014 (mature legumes: 12.8 to 8.3%; dry beans: 10.0 to 6.5%). Less than 5% of the population consumed legumes on a daily basis; approximately one third of the population did not consume legumes during the last month. Inadequate consumers ate a limited variety of legumes (dry beans and green legumes) on a weekly to monthly basis. Disease-prevention consumers (16% of the population) ate legumes daily or every other day and included chickpeas, lentils and dry peas added to their legume mix. My data suggest that legume consumption declined, rather than increased, in U.S. adults, warranting improved communication about the disease prevention benefits of regular legume consumption.
Although legumes are recognized as one of the food groups in the U.S. dietary recommendations, less than 15% of U.S. adults consume legumes on a given day. The Aim 2 was to identify reasons for the disconnection between dietary recommendations and legume consumption patterns in U.S. adults. In an online survey, I identified benefits, barriers, and preferences for legume consumption. The selected response group were parents or guardians in the Corvallis, Oregon, area who were interested in healthy nutrition. To better understand their food choices, I subdivided respondents based on their current legume consumption pattern: none, below current dietary recommendation (inadequate), at current dietary recommendation (marginal), at levels
showing efficacy for disease prevention (disease prevention). Based on their perceptions, I identified three legume-consumer groups corresponding to their current legume consumption: ‘skeptics’ (non-legume consumers), ‘starters’ (Inadequate or marginal legume consumers), and ‘experienced’ (disease prevention legume consumers). Regardless of their legume consumption levels, participants were not aware of the disease prevention benefits of legumes. The differences between those who consume sufficient amounts of legumes and those who do not centered around differences in perceptions of taste and texture and of gastro-intestinal health. Those who consume sufficient amounts of legumes enjoy their taste and their digestive benefits, whereas, non-consumers dislike their taste and texture and experienced gastro-intestinal discomfort after consuming them. Reasons for these differences are that regular legume consumers are experienced in using a variety of legume types, dishes, and preparation techniques, whereas non-consumers have limited interest or knowledge about legumes’ disease prevention benefits, type and dish variety, and preparation techniques, which should be the focus of future legume information materials. Mature legume consumption has been linked to a decreased risk of metabolic syndrome, obesity, and cardiovascular diseases in meta-analysis studies; however, the evidence for a chemo-protective effect of mature legume consumption against colorectal tumorigenesis has been considered inconclusive. Therefore, the Aim 3 was to conduct a meta-analysis of human and animal studies to evaluate the effect of mature legume consumption on colorectal cancer (CRC) and its precursors. Twelve case-control studies (42,473 controls and 12,408 cases) and 11 prospective cohorts
(1,533,527 participants including 12,274 cases) were included in the meta-analysis; the pooled risk ratio (95% confidence interval) for the highest versus the lowest legume intake group based on a random-effects model was 0.72 (0.60-0.89) for incident adenoma, 0.91 (0.84-0.99) for prevalent adenoma, and 0.82 (0.74-0.91) for incident CRC. Fourteen animal studies (355 on grain legume diets and 253 on control diets) were included in the meta-analysis and showed a chemo-preventive effect against colorectal tumorigenesis in all but one study. Mature legumes contain various compounds (e.g., resistant starch, soluble fiber, insoluble fiber, phytosterols, saponins, phytates, flavonoids, proanthocyanidins, and phenolic acids) that have been shown to inhibit colorectal tumorigenesis in animal studies at concentrations that are relevant for human diets. Mature legume consumption alters several molecular pathways (e.g., p53, mTOR, NF-κB, Akt, and AMPK), that are critical for tumor induction, promotion, and progression. Based on the meta-analysis, daily mature legume consumption may confer chemo-preventive effects against CRC.
To demonstrate a disease protective effect in humans, biomarkers of dry bean intake are needed. Currently, there are a very few established dietary biomarkers and none exists for dry beans. Metabolomics provides a global analysis of metabolites in biological samples and is a novel and promising tool for identifying exposure biomarkers in humans. Therefore, Aim 4 was to identify and monitor metabolite markers of dry bean consumption in parallel human and mouse studies. Using liquid chromatography/mass spectroscopy +/- electrospray ionization and gas chromatography/mass spectroscopy, serum metabolites of dry beans were measured in 46 men before and after a four-week dry bean-enriched diet (250 g/d) in Legume
Inflammation Feeding Experiment (LIFE) and 12 mice that received a standardized diet containing either 0 or 10% navy bean ethanol extract for 6 weeks; I also investigated fecal metabolites in the mice. The serum metabolites identified in these controlled feeding studies were then investigated in 212 polyp-free participants from the Polyp Prevention Trial (PPT) who self-reported either increased (≥+31 g/d from baseline) dry bean intake of ≥42 g/d in year 3, or unchanged dry bean consumption of <8 g/d; serum was analyzed from baseline and year 3. I observed that serum pipecolic acid and S-methyl-cysteine were elevated after dry bean consumption in the LIFE study and in the mouse study and reflected dry bean consumption in the Polyp Prevention Trial. In conclusion, serum pipecolic acid and S-methyl-cysteine are specific to dry beans; therefore, serum pipecolic acid and S-methyl-cysteine can be used as metabolite markers for dry bean consumption.
Collectively, the findings from these completed studies provide evidence that regular legume consumption can prevent colorectal neoplasia. However, current legume consumption in U.S. adults is low and insufficient to confer any nutritional and chemo-preventive benefits. There is a disconnect between peer-reviewed nutritional and health benefits of legumes, current dietary recommendations for legumes, and current legume consumption patterns. This disconnect questions the effectiveness of current efforts in providing consumers information about the nutritional and health benefits of legumes and how to prepare a variety of legume types and dish types that include legumes.