Legumes

Summary

Introduction

Legumes are plants of the Leguminosae family with seed pods that split into two halves. Edible seeds from plants of this family include beans, peas, lentils, soybeans, and peanuts. The most recent Dietary Guidelines for Americans do not include green peas or green (string) beans as legumes due to a dissimilar nutritional profile when compared to the other legume family members (1). Peanuts are nutritionally similar to tree nuts; therefore, information on their health effects is presented in a separate article on Nuts and not discussed below. Many studies have examined how the consumption of pulses is related to health; pulses are legumes harvested as dry grain and thus exclude green peas, green beans, as well as crops used for oil extraction (soybeans, peanuts) (2).

Although legumes are an important part of traditional diets around the world, they are often neglected in typical Western diets. Legumes are inexpensive, nutrient-dense sources of protein that can be substituted for dietary animal protein (3). While sources of animal protein are often rich in saturated fats, the small quantities of fats in legumes are mostly unsaturated fats. Legumes are also good sources of several essential minerals, rich in dietary fiber (both soluble fiber and resistant starch), and considered to be low-glycemic index foods (4, 5). Moreover, legumes contain numerous phytochemicals that may affect health. Soybeans have attracted the most scientific interest, mainly because they are a unique source of phytoestrogens known as isoflavones (6). Overall, legumes represent unique packages of nutrients and phytochemicals that may work synergistically to reduce risk of chronic disease, as other edible seeds do (7).

Note: Research on the health effects of diets rich in legumes, including peas and soy foods, is summarized below. For a discussion of the potential health benefits and risks of soy isoflavones, see the separate article on Soy Isoflavones.

Disease Prevention

Type 2 diabetes mellitus

Consumption of legumes, which are rich in dietary fiber, other nutrients, and bioactive compounds, might help improve insulin sensitivity and regulate blood glucose concentrations, thereby influencing risk of type 2 diabetes mellitus.

Beans, peas, and lentils

The glycemic index is a measure of the potential for carbohydrates in different foods to raise blood glucose concentrations. In general, consuming foods with high-glycemic index values causes blood glucose concentrations to rise more rapidly, which results in greater insulin secretion by the pancreas, than after consuming foods with low-glycemic index values. Chronically elevated blood glucose concentrations and excessive insulin secretion are thought to play important roles in the development of type 2 diabetes mellitus (8). Low-glycemic load diets have been associated with reduced risk of developing type 2 diabetes in some, but not all, prospective cohort studies (see the article on Glycemic Index and Glycemic Load). High-fiber diets have also been associated with a decreased risk of type 2 diabetes (see the article on Fiber).

Together, the high-fiber content and low-glycemic index nature of legumes may contribute to help lower risk of type 2 diabetes in those who incorporate them into their daily diets. However, a 2017 meta-analysis of 12 prospective cohort studies found that higher legume intakes were not associated with risk of developing type 2 diabetes compared to lower intakes (range, 0-190 g/day; ~0-2 servings/day) (9), although a more recent prospective study not included in this pooled analysis found an inverse association (10).

Results of clinical trials of legume or pulse consumption have been more promising. A meta-analysis of 41 randomized controlled trials (some in individuals with normoglycemia and some in individuals with type 1 or type 2 diabetes) found that pulse consumption, either alone or in combination with a low-glycemic index diet or a high-fiber diet, improved some measures of glucose control, including fasting blood glucose and insulin concentrations, as well as markers of long-term glycemic control (11). The effect was stronger in patients with diagnosed diabetes compared to individuals with normoglycemia (11). In a three-month trial in 114 people with type 2 diabetes, those randomized to a low-glycemic diet that emphasized 1 cup/day of legumes had significant improvements in their hemoglobin A1C value — the main fraction of glycated hemoglobin that reflects glycemic control over the past four months — compared to those randomized to a high-wheat fiber diet (12).

Small studies in healthy individuals have found that consumption of legumes improved postprandial glucose response at the subsequent meal (called the 'subsequent meal effect' or 'second meal effect') (13-15), although this has not been extensively studied. In addition to improving glycemic and insulin control, consumption of legumes may increase satiety and decrease both food intake and body weight, indirectly improving glycemic control (see Obesity below).

Soy

Observational studies on the association of soy intake and type 2 diabetes mellitus have reported mixed results. A 2018 meta-analysis of eight observational studies (six prospective cohort and two cross sectional) found an inverse association between soy intake and risk of type 2 diabetes; however, there was a high degree of heterogeneity in the analysis (16). Moreover, when a subgroup analysis was done and only prospective cohort studies were examined, the association was no longer significant (16).

A meta-analysis of 19 randomized controlled trials, including 1,518 participants, found that soy consumption — as soy foods/whole soy, isolated soy protein, or soy isoflavone supplements — had no effect on fasting glucose or insulin concentrations (17). Yet, subgroup analyses reported the effect on hyperglycemia differed by type of soy intervention: adherence to a whole-soy diet lowered fasting glucose concentrations, while soy protein isolate or isoflavone supplementation had no effect on fasting glucose concentrations (17).

Cardiovascular disease

Several characteristics of legumes may contribute to protection against cardiovascular disease. They are rich in soluble fiber and phytosterols, which are known to have cholesterol-lowering effects, and their high folate content may help lower homocysteine concentrations. Additionally, legumes are good sources of magnesium and potassium, which may decrease cardiovascular disease risk by helping to lower blood pressure (18). The low-glycemic index values of beans mean that they are less likely to raise blood glucose and insulin concentrations (see Type 2 diabetes mellitus above), and this may also decrease cardiovascular disease risk. Further, the substitution of legumes for foods high in saturated fat or refined carbohydrates may offer some cardioprotection.  

Beans, peas, and lentils

Some, but not all, observational studies have found regular legume intake to be associated with a lower risk of cardiovascular events, and overall, most clinical trials have found legume consumption decreases cardiometabolic risk factors, especially circulating LDL-cholesterol concentrations. In a meta-analysis of eight prospective cohort studies, legume intake as part of a Mediterranean diet was associated with a 9% lower risk of cardiovascular disease, although one of the included studies grouped legume consumption with nuts, which are known to be cardioprotective (see the article on Nuts) (19). In a systematic review and meta-analysis of five prospective cohort studies, consuming four 100-g servings of legumes (beans, peas, lentils, tofu) weekly was associated with a 14% lower risk of coronary heart disease (CHD) (20). This association was driven mainly by a prospective cohort study that followed participants for 19 years, finding those who ate dry beans, peas, or peanuts at least four times weekly had a 21% lower CHD risk compared to those who ate them less than once weekly (21). Most recently, in a meta-analysis that included both CHD incidence and CHD-related mortality together as outcomes (eight prospective cohort studies in total), the highest versus lowest legume intakes were associated with a 10% decreased risk (22). In contrast, legume intake has not been linked to risk of stroke in four separate meta-analyses (20, 22-24).

Serum lipid profiles: Some, but not all, clinical trials have found that regular legume consumption results in favorable changes in the lipid profile. A 2014 systematic review and meta-analysis of 26 randomized controlled trials reported that pulse intake (beans, lentils, peas; median intake of 130 g/day or ~1.3 servings/day) reduced LDL-cholesterol concentrations by 0.17 mmol/L (6.6 mg/dL) compared to an isocaloric diet without pulses; however, there was no effect of pulse intake on non-HDL cholesterol or apolipoprotein B concentrations (25). It is important to note that there was significant heterogeneity among included trials of this meta-analysis, and the baseline lipid status of participants varied among the trials: eight trials included hyperlipidemic subjects, three trials were done in participants with normal lipid profiles, and 15 trials had a combination of normal and high lipid levels (25). A previous meta-analysis of randomized controlled trials found that non-soy legume consumption decreased total and LDL-cholesterol concentrations by 11.76 mg/dL (10 studies) and 7.98 mg/dL (9 studies), respectively, when compared to a matched control (matched for total calories or macronutrients; baseline total cholesterol concentrations of participants ranged from 199 to 295 mg/dL among studies) (26). A trend for a reduction in serum triglycerides (-18.94 mg/dL, p=0.05) was observed in this meta-analysis, and no effect on HDL cholesterol was found (26).

Hypertension: Adherence to the Dietary Approaches to Stop Hypertension (DASH) eating plan, which includes dry beans and peas among many other components (27), has been shown to reduce blood pressure (28), but it is not known whether legumes contribute to this effect. A 2014 meta-analysis of eight isocaloric, controlled feeding trials (median of 10 weeks’ duration) — including both normotensive and hypertensive participants (n=554; median age, 49 years) — found that pulse consumption (median of 162 g/day or ~1.6 daily servings) decreased systolic blood pressure by 2.25 mm Hg but had no effect on diastolic blood pressure (29). It is important to note that there was significant heterogeneity among trials included in the meta-analysis (29).

Soy

Serum lipid profiles: In 1999, the US Food and Drug Administration (FDA) approved the following health claim: "Diets low in saturated fat and cholesterol that include 25 grams of soy protein a day may reduce the risk of heart disease" (30). Most of the evidence to support this health claim was included in the Anderson et al. meta-analysis of 38 controlled clinical trials that was published in 1995. This meta-analysis found that an average intake of 47 g/day of soy protein decreased serum total cholesterol concentrations by an average of 9% and LDL-cholesterol concentrations by an average of 13% (31). Hypocholesterolemic effects were primarily noted in individuals with high baseline cholesterol concentrations (31). A meta-analysis of 33 studies published since 1995 confirmed the hypocholesterolemic effect of soy protein reported in the Anderson et al. publication (32). Another meta-analysis of 30 studies in individuals with normal or mildly elevated cholesterol levels concluded that about 25 g/day of soy protein significantly lowers LDL-cholesterol concentrations by about 6% (33). Yet, a 2006 science advisory from the Nutrition Committee of the American Heart Association concluded that earlier research indicating that soy protein consumption results in clinically important reductions in LDL-cholesterol compared to other proteins has not been confirmed (34). A 2011 meta-analysis of randomized controlled trials, mostly in participants with hypercholesterolemia, found consumption of soy protein decreased LDL-cholesterol by 5.5% in parallel-design studies (20 trials; median soy protein intake of 31.5 g/day for a mean of 9.2 weeks) and 4.2% in cross-over studies (23 trials; median soy protein intake of 26.0 g/day for a mean of 6.0 weeks) — such decreases may translate to 6%-10% decreases in risk for coronary heart disease (35).

Consumption of isolated soy isoflavones (as supplements or extracts), however, does not appear to have favorable effects on serum lipid profiles (35-42).

In addition to possibly lowering cholesterol, incorporating soy foods into the diet may benefit overall cardiovascular health due to their relatively high content of polyunsaturated fat, fiber, and phytosterols compared to animal products (43).

Cancer

Legumes

Colorectal cancer: Legumes, including soybeans, contain fermentable dietary fiber and certain micronutrients and phytochemicals that may have anticancer effects. Some legumes contain serine protease inhibitors of the "Bowman-Birk family" (called Bowman-Birk inhibitors) that may have potential chemopreventive effects in the large intestine (44). Moreover, simply replacing red and processed meats, which are linked to a higher risk of colorectal cancer (45), with legumes in the diet might lower risk.

To date, observational studies of legume intake and colorectal cancer have reported conflicting findings. A meta-analysis of 14 prospective cohort studies found that higher legume intake was linked to a 9% lower risk of colorectal cancer, although there was significant heterogeneity among studies (46). Subgroup analyses revealed an inverse association between soybeans and colorectal cancer (RR, 0.85; 95% CI, 0.73-0.99) but not with dry beans and colorectal cancer (RR, 1.00; 95% CI, 0.89-1.13) (46). An earlier meta-analysis of cohort and case-control studies found that higher versus lower legume intake was associated with a 17% lower risk of colorectal adenoma (47).

Beans, peas, and lentils

Although beans, peas, and lentils are rich in a number of compounds that could potentially reduce the risk of certain cancers, the results of observational studies are too inconsistent to draw any firm conclusions regarding bean intake and cancer risk in general (48, 49).

Prostate cancer: Several prospective cohort studies have found non-soy legume intake to be inversely related to incidence of prostate cancer. In a six-year prospective study of more than 14,000 Seventh Day Adventist men living in the United States, those with the highest intakes of legumes (beans, lentils, or split peas) had a 47% lower risk of prostate cancer (50). In a prospective study of more than 58,000 men in the Netherlands, those with the highest intakes (median intake, 62 g/day) of legumes (including pulses and dried seeds) had a risk of prostate cancer that was 29% lower than those with the lowest intakes (median intake, 11 g/day) (51). Moreover, a prospective study in a multiethnic cohort of 82,483 men examined the risk of prostate cancer in men who consumed legumes excluding soy products. In this study, men who consumed the highest amount of non-soy legumes had a 10% lower risk of total prostate cancer and a 28% lower risk of nonlocalized or high-grade prostate cancer compared to those who consumed the least amount of non-soy legumes (52). However, another study found no association between dry bean intake and prostate cancer (53). Most recently, in a prospective study in 3,313 French men, non-soy legume consumption was linked to a 44% reduced risk of prostate cancer (54). A meta-analysis of these five prospective cohort studies found legume consumption was associated with a 15% lower risk of prostate cancer development (95% CI, 0.72-0.99) (55). Moreover, a pooled analysis of 13 prospective cohort studies found a similar association: a 14% lower risk of prostate cancer in men who consumed ≥100 g/day of mature beans (all beans excluding green beans and soybeans) compared to those who had intakes lower than 15 g/day (56).

Soy

Prostate cancer: Incidence rates of prostate cancer are much higher in North America, Northern and Western Europe, Australia, and New Zealand compared to Asian countries, such as Japan and China, where soy is common in the diet (57). Soy food consumption has been associated with a reduced risk of prostate cancer in pooled analyses of observational studies (58-62). In the most recent meta-analysis, total soy intake (16 studies) and unfermented soy intake (11 studies) were associated with a 29% and 35% lower risk of prostate cancer, respectively; no association was found between fermented soy intake and prostate cancer (eight studies) (62). Subgroup analyses revealed that the protective association of total soy intake was much stronger in case-control (39% lower risk) versus prospective cohort and nested case-control studies (10% lower risk) — unfermented soy intake was only associated with prostate cancer risk in case-control studies (45% lower risk) (62). Results of case-control studies are more likely to be distorted by bias (i.e., the selection bias with the selection of cases and controls, as well as dietary recall bias) than results of prospective cohort studies.

For a review of studies on soy isoflavone intake and prostate cancer, see the separate article on Soy Isoflavones.

Breast cancer: More than 25 observational studies have assessed the relationship between soy food intake and the risk of breast cancer. A 2008 meta-analysis of prospective cohort studies and case-control studies reported differential effects based on the typical level of soy consumption (63). In Asian populations, where soy intake is high, the authors found an inverse association between soy food intake and breast cancer; however, no association was observed in studies completed in Western populations, where soy food intake is much lower (63). A 2016 meta-analysis of 10 prospective cohort studies — four conducted in Japan, three in the US, two in China, and one in France — found that higher intake of soy food was associated with an 8% lower incidence of breast cancer compared to lower intake (64).

Age at exposure to soy might affect subsequent risk of developing breast cancer. For instance, a few case-control studies have reported that higher soy intake during childhood or adolescence was associated with a lower risk of developing breast cancer later in life (65-68). Soy intake later in life may not have as strong as an effect on breast cancer compared to exposure during childhood and adolescence (63), and lifelong exposure to soy food may be needed to lower risk of breast cancer (69).

Isoflavones are likely responsible for any protection of soy against the development of breast cancer; studies of soy isoflavone intake and breast cancer are reviewed in the separate article on Soy Isoflavones.

Endometrial cancer: Since soy isoflavones have estrogenic activity, studies have investigated whether isoflavone intake from soy food or supplements might be related to development of endometrial cancer. There is only limited evidence of an inverse association, primarily from case-control studies (70); see the separate article on Soy Isoflavones.

Gastrointestinal cancers: Results of observational studies examining soy intake and gastrointestinal cancers, including gastric cancer and colorectal cancer, have been mixed. A 2016 meta-analysis that included 18 case-control and 16 prospective cohort studies found soy consumption was associated with a 7% lower risk of gastrointestinal cancer; however, soy intake was not significantly linked to gastrointestinal cancer when the data from only cohort studies were considered (OR, 0.97; 95% CI, 0.90-1.03) (71). A separate meta-analysis reported soybean intake was associated with a 15% lower risk of colorectal cancer (95% CI, 0.73-0.99), but only three prospective cohort studies were included in this analysis (46). High-quality studies that control for potential confounders are needed to determine whether soy food intake — or legume intake in general — is linked to a lower risk of gastric or colorectal cancer.

Obesity

Numerous clinical trials have shown that consuming low-glycemic index food delays the return of hunger, decreases subsequent food intake, and increases the sensation of fullness compared to consuming high-glycemic index food (72, 73). Some trials have specifically examined the effect of legume or pulse consumption on food intake amount, satiety, or body weight. A 2016 meta-analysis that included 21 randomized controlled trials, mostly in overweight or obese subjects, reported an overall weight loss of 0.34 kg (0.75 lb) with diets that contained pulses (median intake 132 g/day for a median of six weeks; one serving ~100 g/day) (74). This analysis found a greater weight loss in negative-energy balance trials (i.e., energy-restricted diets; weight loss averaging 1.74 kg or 3.8 lb) than in neutral energy-balance trials (i.e., weight-maintenance diets; weight loss averaging 0.29 kg or 0.64 lb), although both effects were statistically significant (74). Six trials included in this meta-analysis examined the effect of pulse consumption (161 g/day; trial average duration of 10 weeks) on other measures of obesity: no effect was seen on waist circumference, but a trend for reduced body fat percentage was found with pulse consumption (p=0.07) (74). A meta-analysis of 21 controlled clinical trials (duration ranging from 4 weeks to 2 years) found that soy consumption — as whole soy or soy protein — had no effect on body weight, and a subgroup analysis showed that soy consumption actually increased body weight in obese subjects (75). Additionally, this meta-analysis found no effect of soy intake on fat mass (10 trials) or waist circumference (16 trials) (75).

A meta-analysis of nine acute, feeding trials in mostly normal-weight subjects reported pulse consumption, administered as a single bolus, increased measures of satiety by 31% compared to an isocaloric control; however, no effect on food intake at the subsequent/second meal was seen (seven trials) (76). Increases in acute satiety, likely due to the high protein and fiber content and the low-glycemic load of legumes, could lead to reduced food intake and the weight loss observed in some studies. Although currently available studies are limited, the effect on satiety appears to be following consumption of pulses and not soy food (reviewed in 77). High-quality trials are needed to determine whether regular pulse or legume consumption results in weight loss in the long term.

Cognitive health

Some studies have linked adherence to either the Dietary Approaches to Stop Hypertension eating pattern (78, 79) or a Mediterranean-style diet (78, 80-82) with better overall cognitive function in healthy older adults. Legumes are one of many components included in these dietary patterns that might contribute to this purported association; studies reporting specifically on legume intake and cognitive endpoints are limited and have reported mixed results. In a prospective cohort study of 2,613 middle-age and older adults (ages 43-70 years) residing in the Netherlands, followed for five years, legume intake was not associated with changes in global cognitive function (memory, speed of information processing, and cognitive flexibility) (83). In contrast, a prospective, nested case-control study of the Chinese Longitudinal Health Longevity Study found that older adults (≥65 years) who included legumes in their diets nearly every day had a 22% lower risk of cognitive decline over a three-year period compared to those who had lower intakes — this study included 5,691 illiterate older adults who had no evidence of cognitive decline at baseline (84). Most recently, a small study in 214 older, Italian adults (≥65 years) with normal cognitive function used the method of principal component analysis and identified legumes as one of the food categories linked to better cognitive scores at the end of a one-year period (85). Large-scale prospective cohort studies are needed to examine whether legume intake is associated with improvements in cognitive health or risk of dementia in older adults.

Mortality

A 2017 meta-analysis of six prospective cohort studies found a weak, inverse association between legume intake and risk of all-cause mortality (RR: 0.96; 95% CI: 0.94, 1.00) (86). In a large, international prospective cohort study (N=135,335) not included in the 2017 meta-analysis, stronger inverse associations were found with respect to all-cause mortality and non-cardiovascular mortality; however, legume consumption was not associated with cardiovascular-related mortality (87). In this study, consuming more than one serving of legumes daily was associated with a 42% lower risk of non-cardiovascular mortality and a 41% lower risk of all-cause mortality compared to consuming fewer than one legume serving per month (87). In a smaller prospective study (N=7,212) in Spain, conducted among older individuals at high risk for cardiovascular disease, non-soy legume intake was associated with a lower risk of cancer-related mortality but a higher risk of cardiovascular-related mortality (88). It is important to note that average intakes of legumes in this study were low, with those in the highest tertile consuming a mean 28 g/day of legumes — less than one-third of a serving (88). Moreover, a meta-analysis of prospective cohort studies did not find soy intake to be associated with all-cause (six studies), cardiovascular-related (six studies), or cancer-related (eight studies) mortality (89)

Intake Recommendations

Substituting beans, peas, and lentils for foods that are high in saturated fat or refined carbohydrates is likely to help lower the risk of type 2 diabetes mellitus and cardiovascular disease. Soybeans and foods made from soybeans (soy foods) are excellent sources of protein. In fact, soy protein is complete protein, meaning it provides all of the essential amino acids in adequate amounts for human health (6). Like beans, peas, and lentils, soy foods are also excellent substitutes for protein sources that are high in saturated fat like red meat or cheese. Legumes are not only good sources of protein and dietary fiber but also of vitamins, minerals, as well as phytochemicals that may benefit health (see Table 1).

The 2015-2020 Dietary Guidelines for Americans — healthy eating recommendations issued jointly by the US Department of Health and Human Services and the US Department of Agriculture — include legumes (beans, lentils, dried peas, and edamame [green soybeans]) in two different food groups: the protein group and the vegetable group (1). Recommended intake of legumes at the 2,000-calorie level translates to 1½ cup-equivalents weekly for those who follow a Healthy US-style or a Healthy Mediterranean-style eating pattern and 3 cup-equivalents weekly for those following a Healthy Vegetarian eating pattern (1). Current intakes of legumes among Americans are well below these recommendations (see Figure 2-4 in the Dietary Guidelines). The Dietary Guidelines also recommend 8 oz-equivalents/week of soy products for vegetarians consuming 2,000 calories/day and 5 oz-equivalents per week for those following other recommended eating patterns, although nuts and seeds can contribute to meet this recommendation. Moreover, fortified soy beverages are emphasized as alternative to dairy in the latest Dietary Guidelines (1).

Table 1. Some Potentially Beneficial Compounds in Legumes
Macronutrients Vitamins Minerals Phytochemicals
Essential Fatty Acids Folate Iron Fiber
  Niacin Magnesium Flavonoids
  Riboflavin Potassium Lignans
  Vitamin B6 Zinc Phytosterols
      Soy Isoflavones

Authors and Reviewers

Originally written in 2004 by:
Jane Higdon, Ph.D.
Linus Pauling Institute
Oregon State University

Updated in December 2005 by:
Jane Higdon, Ph.D.
Linus Pauling Institute
Oregon State University

Updated in April 2009 by:
Victoria J. Drake, Ph.D.
Linus Pauling Institute
Oregon State University

Updated in September 2019 by:
Victoria J. Drake, Ph.D.
Linus Pauling Institute
Oregon State University

Reviewed in December 2019 by:
Emilio Ros, M.D., Ph.D.
Endocrinology and Nutrition Service
Hospital Clinic
Barcelona, Spain

Copyright 2004-2024  Linus Pauling Institute


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