An Evidence-based Approach to Phytochemicals and Other Dietary Factors

3 Legumes

Legumes are plants with seedpods that split into two halves. Edible seeds from plants in the legume family include beans, peas, lentils, soybeans, and peanuts. Since peanuts are nutritionally similar to tree nuts, information on the health benefits of peanuts is presented in Chapter 4 on Nuts. 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.¹ While sources of animal protein are often rich in saturated fats, the small quantities of fats in legumes are mostly unsaturated fats. Not only are legumes excellent sources of essential minerals, but they are also rich in dietary fiber and other phytochemicals that may affect health. Soybeans have attracted the most scientific interest, mainly because they are a unique source of phytoestrogens known as isoflavones.² Although most other legumes lack isoflavones, they also represent unique packages of nutrients and phytochemicals that may work synergistically to reduce the risk of chronic diseases. In this chapter, the health effects of diets rich in legumes and soy foods are summarized. Research on the health effects of soy isoflavones is discussed in Chapter 12.

Disease Prevention

Type 2 Diabetes Mellitus

The glycemic index is a measure of the potential for carbohydrates in different foods to raise blood glucose levels. In general, consuming foods with high-glycemic-index values causes blood glucose levels 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 levels and excessive insulin secretion are thought to play important roles in the development of type 2 diabetes mellitus (DM).3 Because legumes generally have low-glycemic-index values, substituting legumes for high-glycemic-index foods like white rice or potatoes lowers the glycemic load of one's diet. In several large prospective studies, low-glycemic-load diets have been associated with reduced risk of developing type 2 DM.^4–7 Obesity is another important risk factor for type 2 DM. Numerous clinical trials have shown that the consumption of low-glycemic-index foods delays the return of hunger, decreases subsequent food intake, and increases the sensation of fullness compared with high-glycemic-index foods.^8,9 The results of several small, short-term trials (1–6 months) suggest that low-glycemic-load diets result in significantly more weight or fat loss than high-glycemic-load diets.^10–13 Thus, diets rich in legumes may decrease the risk of type 2 diabetes by improving blood glucose control, decreasing insulin secretion, and delaying the return of hunger after a meal. For more information on glycemic index values and glycemic load, see Appendix 1 on Glycemic Index and Glycemic Load.

One study in elderly men and women reported that consumption of legumes was protective against the development of glucose intolerance.¹⁴ More recently, a prospective cohort study in 64 227 middle-aged Chinese women found that their total legume consumption, which included soybeans, peanuts, and other legumes, was associated with a 38% lower risk of developing type 2 DM.¹⁵ Moreover, a prospective study that followed 10 449 individuals with diabetes for 9 years found that legume intake was inversely associated with cardiovascular-related mortality and all-cause mortality, but not with cancer-related mortality.¹⁶

Cardiovascular Disease

Beans, Peas, and Lentils

One prospective cohort study that examined the effect of legume intake on cardiovascular disease risk followed men and women for 19 years and found those who ate dry beans, peas, or peanuts at least four times weekly had a risk of coronary heart disease (CHD) that was 21% lower than for those who ate them less than once weekly.17 When compared with a typical Western diet, legume intake as part of a healthy dietary pattern that included higher intakes of vegetables, fruits, whole grains, fish, and poultry was associated with a risk of CHD that was 30% lower in men 18 and 24% lower in women.¹⁹ The results of controlled clinical trials suggest that increasing bean consumption improves serum lipid and lipoprotein profiles. A meta-analysis that combined the results of 11 clinical trials found that increasing the consumption of dry beans resulted in modest (6%–7%) decreases in total cholesterol and low-density lipoprotein (LDL) cholesterol.²⁰ Several characteristics of beans may contribute to their cardioprotective effects. Beans are rich in soluble fiber, which is known to have a cholesterol-lowering effect. Elevated plasma homocysteine levels are associated with increased cardiovascular disease risk, and beans are good sources of folate, which helps to lower homocysteine levels. Beans are also good sources of magnesium and potassium, which may decrease cardiovascular disease risk by helping to lower blood pressure.²⁰ The low-glycemic-index values of beans means that they are less likely to raise blood glucose and insulin levels, which may also decrease cardiovascular disease risk. For more information on glycemic index, see Appendix 1.

Soy

In 1999, the US Food and Drug Administration 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.”²¹ 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 levels by an average of 9% and LDL cholesterol levels by an average of 13%.²² Hypocholesterolemic effects were primarily noted in individuals with high baseline cholesterol levels.²² A more recent meta-analysis of 33 studies published since 1995 confirmed the hypocholesterolemic effect of soy protein reported in the Anderson et al. publication.²³ Another meta-analysis of 30 studies in individuals with normal or mildly elevated cholesterol levels concluded that approximately 25 g/day of soy protein significantly lowers LDL cholesterol concentrations by about 6%.²⁴ Yet, a recent science advisory from the Nutrition Committee of the American Heart Association concluded that earlier research indicating that soy protein consumption resulted in clinically important reductions in LDL cholesterol compared with other proteins has not been confirmed.²⁵ The consumption of isolated soy isoflavones (as supplements or extracts) does not appear to have favorable effects on serum lipid profiles.^26–29 In addition to possibly lowering cholesterol, many soy products may be beneficial for overall cardiovascular health due to their relatively high content of polyunsaturated fat, fiber, and phytosterols compared with animal products.³⁰

Cancer

Beans, Peas, and Lentils

Although beans are rich in several compounds that could potentially reduce the risk of certain cancers, the results of epidemiological studies are too inconsistent to draw any firm conclusions regarding bean intake and cancer risk in general.31,32

Prostate cancer. There is limited evidence from observational studies that legume intake is inversely related to the risk of prostate cancer. In a 6-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 significantly lower risk of prostate cancer.³³ More recently, a prospective study of more than 58 000 men in the Netherlands found that those with the highest intakes of legumes had a risk of prostate cancer that was 29% lower than those with the lowest intakes.³⁴ Similarly, in a case–control study of 1619 North American men diagnosed with prostate cancer and 1618 healthy men matched for age and ethnicity, those with the highest legume intakes had a risk of prostate cancer that was 38% lower than in those with the lowest intakes.³⁵ Excluding the intake of soy foods from the analysis did not weaken the inverse association between legume intake and prostate cancer, suggesting that soy was not the only legume that conferred protection against prostate cancer. A recent prospective study in a multi-ethnic 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 with those who consumed the least amount of non-soy legumes.36

Soy

Prostate cancer. Although there is considerable scientific interest in the potential for soy products to prevent prostate cancer, evidence that higher intakes of soy foods can reduce the risk of prostate cancer in humans is limited. Only two out of six case–control studies found that higher intakes of soy products were associated with a significantly lower risk of prostate cancer. In the largest case–control study, North American men who consumed an average of at least 1.4 oz of soy foods daily were 38% less likely to have prostate cancer than men who did not consume soy foods.³⁵ A much smaller case–control study of Chinese men found that men who consumed at least 4 oz of soy foods daily were only half as likely to have prostate cancer as those who consumed less than 1 oz daily.³⁷ However, case–control studies conducted among North American,^38,39 Japanese,⁴⁰ and Taiwanese men⁴¹ did not find that higher soy intakes were associated with significantly lower prostate cancer risk. A 6-year prospective cohort study of more than 12 000 Seventh Day Adventist men in the United States found that those who drank soy milk more than once daily had a risk of prostate cancer that was 70% lower than in those who never drank soy milk,⁴² but a 23-year study of more than 5000 Japanese American men found no association between tofu consumption and prostate cancer risk.⁴³ More recently, a prospective study in a cohort of 43 509 Japanese men found that consumption of soy foods was associated with a decreased risk of localized prostate cancer in men who were older than 60 years.⁴⁴ Clinical trials are needed to determine whether consumption of soy foods affects the risk of prostate cancer.

Breast cancer. More than 25 epidemiological studies have assessed the relationship between soy food intake and the risk of breast cancer. A recent meta-analysis of prospective cohort studies and case–control studies reported differential effects based on the typical level of soy consumption.⁴⁵ 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.⁴⁵ Age at exposure to soy foods may affect the subsequent risk of developing breast cancer. For instance, two case–control studies have found higher soy intake during adolescence may lower the risk of developing breast cancer later in life.^46,47 Soy intake later in life may not have as strong an effect on breast cancer as exposure during adolescence.⁴⁵

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 DM 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.2 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. Although several health-related organizations recommend daily consumption of five to nine servings (2.5–4.5 cups) of fruits and vegetables daily (see Chapter 1), few make specific recommendations for legumes. In the 2010 Dietary Guidelines for Americans, an intake of 1.5 cups (three servings) of legumes (beans and peas, excluding green beans and green peas) weekly is recommended for people who consume 2000 kcal/day. A serving of legumes is equal to 0.5 cup of cooked dried beans or peas.⁴⁸

Summary

• Foods from the legume family include beans, peas, lentils, peanuts, and soybeans.

• Legumes are excellent sources of protein, lowglycemic-index carbohydrates, essential micronutrients, and fiber.

• Substituting legumes for foods that are high in saturated fats or refined carbohydrates is likely to lower the risk of cardiovascular disease and type 2 diabetes mellitus.

• Although legumes are rich in several compounds that could potentially reduce the risk of certain cancers, the results of epidemiological studies are too inconsistent to draw any firm conclusions regarding legume intake and cancer risk in general.

• The 2010 Dietary Guidelines for Americans recommend a weekly intake of three servings (1.5 cups) of legumes, defined as beans and peas, not including green beans and green peas, for people who consume 2000 kcal/day.