Dry beans (Phaseolus vulgaris L.) or common beans, have been characterized as a nearly perfect food because of their high protein, fiber, prebiotic, vitamin B, and chemically diverse micronutrient composition [1,2]. Dry beans can also be grown in a variety of eco-agricultural regions and distributed in multiple forms, such as whole unprocessed seeds, as part of mixes, canned products, or as a gluten free wheat flour substitute. As a result, dry beans are used throughout the world representing 50% of the grain legumes consumed as a human food source. Alternatively, overall US intake of dry beans is low despite links to reduced disease risks or states prevalent in western cultures, such as oxidative stress, inflammation, cancer, heart disease, and metabolic syndrome [3]. Of the 14 different market classes grown in the US, the pinto bean account for vast majority in terms of production (Figure 1) and consumption [4]. Although all the market classes contain similar major components (protein, fat, carbohydrates, minerals), each have unique minor chemical profiles that can affect their functional food outcomes. Yet, dry beans are understudied with research programs remaining critically underfunded compared to other commodities. Therefore, the objective of this review is to provide information on dry beans as an important crop not only for the US agriculture sector (past and present) but as a potential functional food for western societies with an emphasis on pinto beans.

The dry bean is truly a “new world crop” originating 7000 years ago in two different parts of the North and South American continents [5,6]. The clear separation of the two domestication centers, i.e., Mesoamerica (southern Mexico and Guatemala) and regions along the Andes mountain range (principally Peru and Columbia), resulted in the small seeded Mesoamerican beans and the large seeded Andean gene pools. The Mesoamerican beans spread northward during the next several millennia to what is now the US, Canada, and the Caribbean Islands whereas the Andes migrated throughout South America to the eastern coast.

In the US, dry beans can be traced to 1300 AD, specifically in the northeast where they played an important role in the “three sisters” farming practices of the Native American Indians. As the middle sister to corn and squash, dry beans fixed nitrogen in the soil supplying fertilizer to its two sisters. Corn provided a trellis for the beans to grow while squash served as a cover crop preventing moisture, evaporation and weed pressure. The three sisters in combination provided most of the daily nutrients needed for a balanced diet.

It was not until the 15–16th century that dry beans crossed the Atlantic Ocean. Spanish and Portuguese explorers first transported dry beans to Europe and Africa, respectively, where beans then spread rapidly to the rest of the world [2]. Dry beans have since evolved into multiple market classes and are grown/consumed in most parts of the world. Currently, Brazil is the No. 1 producer growing on average 3.2 million metric tons (mmt) followed by India, Myanmar, and China growing ~3.0 mmt, 1.7 mmt, and 1.2 mmt, respectively, on a per annual basis [7]. The US and Mexico fluctuate between the 5th and 6th position producing ~1.0 mmt of dry beans per year [7].

New York was the first US state on record to grow dry beans and was the leading producer from the 1800s until the early 1900s when Michigan assumed this position. Commercial production of dry beans is now scattered across 19 states with North Dakota, Michigan, Nebraska, Minnesota and Idaho being the leaders relative to total yields (Figure 2). These states have remained in the top five positions for the last decade with Nebraska and Minnesota switching in the 3rd and 4th ranking depending on the year. Typically, Nebraska exceeds the other top ranking states in yield per acre. According to the USDA 2010 statistics [8], Nebraska produced on average 910 kg/acre (2010 lbs/acre) of dry beans followed by Idaho (860 kg/acre, 1900 lbs/acre), Michigan (820 kg/acre, 1800 lbs/acre), and North Dakota (657 kg/acre, 1490 lb/acre). Pinto beans account for the largest market class produced in the US and are grown in most of the dry bean growing states (Figure 2) with the navy and black beans following a distance 2nd and 3rd [4,9,10] (Figure 1).

The dry bean is a staple food in Latin America, Africa, and India [11], but intake in the US and other western societies is substantially low in comparison. US consumption of dry beans is on average ~6.5 lbs (2.95 kg) per person per year with pinto beans again being the top choice. Whole pinto beans are usually purchased as canned pre-cooked or plastic packaged dry beans and are then taken to private homes where they are prepared as ingredients for other foods, such as soups, casseroles, Latin American dishes, or salads. Approximately 75%–80% of bean consumption takes place in the home, while the remaining 20%–25% occurs at restaurants, as fast foods, or in cafeterias [12]. Mexican Americans or other Hispanics are the highest consumers of pinto beans with equal intake occurring by men and women (31–50) regardless of ethnicity [3]. The rising Hispanic population has been credited with the mainstream consumption of all types of cooked beans in the US [12].

The compositional profile of dry beans in general, and pinto beans specifically, are included in this section to provide possible links to their disease protecting properties, as discussed in Section 4. Although a critical gap in knowledge exists on these mechanisms, dry beans currently hold two positions on the newly developed US MyPlate nutrition guide [13]. Firstly, dry beans are grouped with meat, fish, eggs, nuts as a high source of protein. Dry beans supply approximately 8%–10% of the daily recommended allowance of protein per 100 g serving of cooked beans with methionine being the only absent essential amino acid. Secondly, dry beans are categorized with the vegetable group due in part to their high fiber content. Dry beans are also composed of several types of prebiotics, including resistant starch (RS) and the fructooligosaccharides, stachyose and raffinose [14]. These compounds serve as substrates for bacterial fermentation in the human intestine, thereby influencing the microbial ecology of the gastrointestinal (GI) tract and gut metabolism [15,16]. Additionally, dry beans contain iron and calcium at levels that respectively fulfill ~11 and 2%–6% (100 g serving) of the daily reference intake (DRI) for a 2000 Calorie (kcal) diet. Of the vitamins, the B vitamins are present in particularly high levels. One cup of beans (225 g cooked) supplies ~74% of folic acid (vitamin B9) [17]. This nutrient supports multiple biological functions, including synthesis, repair and methylation of DNA, and acts as a cofactor in many reactions [18]. Folic acid is needed for healthy blood cell production to prevent anemia in children and adults [19]. In addition, one cup of beans provides ~30% of the DRI for vitamin B6 and at least 25% of the DRI for thiamin [17]. Lastly, dry beans (100 g cooked) contain vitamins A and C that account for 3%–8% of the DRI. Table 1 shows the basic compositional profiles of the cited compounds relative to pinto beans.

Considerable less fat-soluble nutrients are present in dry beans as total lipid levels are low (1%–2%). Still, the lipid fraction contains the essential vitamins, E and K. One cup of beans provides ~11% and ~5% DRI of Vitamin E and K, respectively [17]. Vitamin E is a potent antioxidant and anti-inflammatory agent with the isomers α-tocopherol and γ-tocopherol being the most abundant forms in dry beans [20,21,22]. Vitamin K intake is widely associated with bone health, but has been linked to anti-cancer properties [23,24,25,26]. Polyunsaturated fatty acids (PUFAs) represent another important lipid class of compounds present in dry beans, specifically the omega fatty acids (linoleic acid (n-6) (LA) and alpha linoleic acid (n-3) (ALA)) [27]. Epidemiologic studies have shown overall improvement in the health of individuals with coronary heart disease (CHD) when fed the alpha linolenic acid (n-3) (ALA) [28]. After absorption into a cell, LA is elongated and desaturated to arachidonic acid while ALA is elongated and desaturated into eicosapentaenoic acid (EPA) and then into docosahexaenoic acid (DHA). EPA and DHA are omega-3 fatty acids prevalent in fish that have received intense interest due to their cardioprotective and anti-inflammatory properties [29,30]. Leukotrienes, prostaglandins, and thromboxanes, metabolic products derived from AA are generally pro-inflammatory and proaggregatory agonists, while those derived from the ALA are able to inhibit platelet aggregation and inflammation. The latter mechanisms are involved in the prevention of cardiovascular disease (CVD), hypertension, type 2 diabetes, chronic obstructive pulmonary disease, among others [31]. In particular, pinto beans contain ~0.3 grams of ALA per 100 g of raw portion [32]. The fatty acid levels present in pinto beans are shown in Table 1 based on their degree of saturation. (The reader is referred to Winham et al. [33] for compositional information relative to other types of dry beans.)

In addition to the nutrients described previously, other phytochemicals are present in dry beans but at low levels (parts per million or parts per billion). Nonetheless, these phytochemicals (phenolic compounds, lignins, lectins, and trypsin inhibitors) exert potent protective properties against multiple chronic conditions with the phenols playing a major role [35,36,37,38,39,40]. Figure 3 illustrates the different classes phenols based on their chemical structures. As over 8000 structurally distinct phenols have been identified [41], numerous quantities and combinations can arise affecting their physiochemical characteristics and thus their bioavailability and biological targets [42,43]. Phenolic acids are a subclass of the phenolic compounds that contain at least one aromatic ring and one hydroxyl group (Figure 3). Flavonoids are the largest subclass of polyphenols in the human diet and are characterized by two or more aromatic rings containing at least one hydroxyl group in each and connected with a heterocyclic pyran (Figure 3). Flavonoids can be sub-divided further into two main groups, anthocyanins (glycosylated derivative of anthocyanidin) and anthoxanthins (Figure 3). Anthoxanthins are composed of several categories, such as flavones, flavanones, flavonols, flavanols, isoflavones and their glycosides [44]. It was generally believed that the phenols exerted their health benefit by directly scavenging free radicals or reactive oxygen species (ROS) or chelating of redox metals, but these properties are now largely attributed to the phenols’ ability to act as signaling agents of cellular endogenous responses [36,45].

All plant based systems contain phenolic compounds, but these compounds are typically more diverse and abundant in fruits, vegetables, leaves, and seeds [48]. However, dry beans contain various types and quantities of phenolic compounds depending on the market class [49,50,51,52,53]. As such, different bean market classes can possess different functional food properties. Thompson et al. [52] showed that cooked bean powders isolated from different market classes were able to reduce cancer tumors in rats, but the response differed based on the type of bean. The response did not correlate to the amount of total phenols present, but rather to the domestication origin of the bean suggesting that the type or combination of bioactive agents were as important if not more so than the quantity. Moreover, as secondary metabolites, phenols can be impacted by the level of stress (environment, farming practices, production location) potentially altering their phenolic compositional profile on a lot to lot basis [54]. The pinto bean is especially high in the phenols shown in Figure 4 and listed in Table 2 [55,56]. Additionally, dry beans contain lignin, an indigestible component that cross-links with phenolic compounds changing their structure and solubility [57]. Lignins have been shown to promote gastrointestinal health [58]. It also must be noted that dry beans are composed of anti-nutrients (phytates, tannins, lectins, protease, a-amylase inhibitors, saponins, among others), although many have been associated with human health benefits [59]. For example, phytate has been linked to anticancer activity [60], but it also binds cations (i.e., iron and zinc) negatively impacting mineral bioavailability in the intestinal lumen [61].

Chronic conditions such as diabetes, cancer, and heart disease are becoming more prevalent among all ages and ethnic groups throughout the US and other western societies resulting in global economic burdens reaching the $USD trillions [64]. A possible contributor to these debilitating diseases may lie in the consumption of overly processed foods and the high cereal:legume consumption ratio that has emerged in the US over the last 60 years. Instead of the recommended 2 cereal:1 legume ratio for optimal consumption, the ratio has reached 8 or 9:1. Moreover, epidemiology studies have shown positive links between legume (bean) rich diets, such as increased longevity [65], heart health [66,67], and reduced risks for many types of cancers [68,69,70,71,72]. As the pinto bean is the most commonly consumed dry bean in the Americas, this section provides a brief review of the limited but seminal studies on the functional food properties of this market class. A summary of the discussed studies are also shown in Table 3.

Dry beans are an important US agricultural crop with our country being a top world-wide producer and distributor, particularly of pinto beans. Moreover, dry beans are an important source of numerous nutrients and phytochemicals that protect against multiple diseases currently afflicting western cultures. Despite their importance, bean consumption is low in the US. This review therefore provides information on dry beans, with an emphasis on the pinto bean, from a historical context, current production and consumption patterns in the United States, their chemical composition, and links to human health benefits. Although our understanding of the latter attributes are increasing, critical gaps in knowledge still remain on the role that pinto beans as any dry beans play in protecting against multiple disease risks or states. As dry beans contain multiple nutrients and other phytochemicals they most likely exert these effects as synergists or additives within the complex bean system, but again research remains limited in this area. Such studies are particularly important considering that dry beans are consumed primarily as whole products or parts of the product while cooking practices can affect their compositional profiles, both chemically and physically, as can the market class, lines, production location, etc. However, the existing studies show the potential of pinto beans and other dry bean market classes as a highly effective functional food capable of providing multiple health benefits.