Maybe we don’t know beans . . .
By Donna M. Winham
Greater media attention is being paid to the health benefits of beans and legumes, but how much do we really know about the topic?
News articles, cooking magazines, and science features are recommending beans for lowering cholesterol, improving glycemic control, increasing dietary fiber, and providing a gluten-free alternative to cereal grains. The trend is not surprising, given the rise in chronic disease incidence, consumer interest in functional foods, renewed interest in sustainable agriculture, and economic hard times. Beans can be part of the solution for many of these issues. Beans are a familiar and culturally important food, inexpensive, easy to prepare, and have great menu versatility. Yet, the scientific evidence to support many of these health claims is often generalized from a few original studies limited in scope and based on a few bean types. While the health benefits from beans are likely true, what remains unknown invites the question,"Just how much do we know about beans?"
What is a bean or legume?
The legume family, or Fabaceae (Leguminosae), encompasses a wide range of plants, including the oil seeds (peanuts, soybeans), animal foods (clover, alfalfa), and the dry grain pulses (lentils, peas, chickpeas, beans). In the United States, varieties of the common bean or Phaseolus vulgaris are the dominant crops produced for domestic consumption and export. These include pinto, black, red kidney, white kidney, and navy beans. Wax beans, string beans, or green beans are the young or vegetative stage of P. vulgaris varieties. They have a different nutrient profile and are bred to be eaten whole before they have advanced to dried seed pods. Research on soybeans (Glycine max) has been more extensive than with P. vulgaris species, but this evidence does not directly translate across bean species.
Cardiovascular benefits of legume/bean consumption
The health-promoting properties of diets, such as the traditional "Mediterranean" diet, that are high in legumes have been recognized since the early 1960s. After high intakes of fruits, vegetables, grains, and pulses, to name a few components, followers of this dietary pattern have exhibited decreased levels of chronic disease. Individuals who live in other cultures characterized by traditional diets rich in pulses have showed similar results. Darmadi-Blackberry et al. (2004) found that general pulse consumption was linked with greater longevity for elderly in several different countries (Japan, Sweden, Greece, Australia) even though dietary patterns varied. In contrast, deviations from the traditional Mediterranean eating pattern due to immigration or cultural change have been associated with increased cardiovascular risk. Large-scale epidemiological studies such as the National Health and Nutrition Examination Survey (NHANES) and cross-cultural surveys have observed that cardiovascular disease (CVD) risk is lower among adults who frequently consume beans. Results from the NHANES I Epidemiologic Follow-up Study indicated that men and women who reported consuming pulses four or more times per week had a 22% reduction in coronary heart disease risk compared with those who consumed pulses less than once a week. Higher intakes were associated with lower body mass index (BMI), blood pressure, serum total cholesterol value, and a lower incidence of diabetes mellitus, compared with lower intakes (Bazzano et al., 2001).
Case of cholesterol
At least 100 million Americans have high cholesterol and are at increased risk of CVD, the leading cause of death in the United States. The National Cholesterol Education Program and the American Heart Association recommend dietary and lifestyle changes as the first intervention step to reduce CVD risk by improving blood lipid profiles (i.e., cholesterol). Some of the behavioral interventions used to prevent or treat CVD include smoking cessation; weight loss; exercise; dietary reductions of saturated fat, trans fat, and sodium; and increased consumption of fiber, fruits, and vegetables. Dietary and lifestyle changes have the added benefit of reducing inflammation and risk factors for other chronic diseases such as cancer, diabetes, and hypertension.
If a person is unable to meet treatment goals with diet and lifestyle changes alone, prescription medication therapy is typically the next resort. Although statins are frequently dispensed and effective at lowering low-density lipoproteins (LDL), they are not without risk; side effects may include muscle pain or weakness (myopathy). Statins are also contraindicated in persons with conditions such as hypothyroidism and impaired liver function. Furthermore, statins or other medications do nothing to alter behavior or diet from a broader perspective.
On the other hand, dietary modifications such as the inclusion of beans can improve overall nutrition and reduce the risk of other nutrition-related chronic diseases such as type 2 diabetes mellitus, hypertension, and cancer. Bean consumption has been associated with improvements in dyslipidemia, triacylglycerides, metabolic syndrome, inflammatory markers such as high sensitivity C-reactive protein, and folate status, all risk factors or biomarkers of CVD risk.
Several studies have examined cholesterol reduction with mixed types of cholesterol-lowering foods. A portfolio diet containing a variety of functional foods, including legumes (beans), showed additive effects in reduction of CVD markers. The experimental diet was able to lower low-density lipoprotein cholesterol (LDL-C) as successfully as earlier versions of statins or in excess of 20%. Several single-food dietary interventions (e.g., almonds, garlic, oatmeal, walnuts, pomegranates) have been shown to reduce cholesterol, improve endothelial function, and reduce inflammation biomarkers. Beans, with their rich contents of fiber and micronutrients, are likely to have similar physiological effects. While cholesterol reduction will be the outcome best recognized by consumers, it is a surrogate marker of CVD risk. Endothelial function and systemic inflammation are increasingly recognized in the scientific community as significant predictors of CVD risk. Endothelial dysfunction is now recognized as the first step in the cascade of processes that lead to atherosclerosis. Endothelial function as measured by brachial artery flow-mediated vasodilatation (FMD) is a comprehensively validated, reproducible test that has been shown to predict cardiovascular risk in several clinical trials.
To fulfill its purpose of helping to prevent chronic disease, the 2005 Dietary Guidelines for Americans (DGA) recommended intake of 3 cups or six servings per week for all legumes. The new 2010 DGA calls for a greater emphasis on "a more plant-based diet that emphasizes vegetables, cooked dry beans and peas, fruits, whole grains, nuts, and seeds." However, the 2010 DGA has backed off on the amount of beans recommended, citing limited evidence to support the relationship of cooked dry beans and peas-including soy-in lowering lipids. The 2010 recommendation is 0.21 cups or ~50 grams of beans per day, or 360 grams per week. While this turn-around was unexpected to those of us who conduct bean research, it underscores the needs for further well-designed and well-funded studies to provide strong evidence or refute the epidemiological associations and research trends.
Research gaps and needs
Although consumers and health professionals tend to think all beans are alike, the available research to support health claims and beneficial effects varies across species. With the 2010 DGA concerns about the strength of the evidence, more scientific research on the health benefits of beans is needed for several reasons.
1. Not all beans are created equal. Different types of beans have variable amounts of fiber and nutrients. Although the message "eat more beans" is simple, it overlooks the diversity among beans, which affects the potential health benefits derived from consuming different types. For example, the amount of fiber in ½ cup of black-eyed peas is roughly half the amount of fiber in an equivalent amount of pinto beans. One cup of black beans contains four times the amount of folate as one cup of chickpeas, but the protein content of black beans is similar. Only a few types of beans have been examined for their effects on cholesterol reduction or postprandial glycemic response effects as part of meals. Most cholesterol studies have been done with navy or white beans.
2. Appropriate intake amounts or dose-response. Most retrospective or prospective epidemiological studies have relied on nonspecific frequencies of legume or bean consumption, rather than on controlled or known intakes. Although researchers have found associations with reduced CVD risk, they are often vague on details about the actual amount or type of bean consumed. With the 2010 DGA emphasis on a plant-based diet, and legumes in particular, there is greater urgency for prospective controlled efficacy trials to clarify which beans are effective and in what dosage. There have been no dose-response data for beans on CVD risk in humans.
3. Measurement of additional effects of beans beyond cholesterol. Also lacking are trials that have investigated endothelial function or inflammatory markers as a result of prolonged bean consumption.
4. Improving on the designs of previous bean and cholesterol studies. Additional research on the relationship of beans and CVD risk reduction is needed to overcome design flaws that are apparent in previous studies.
a. Use of multiple types of beans and inconsistent dosing in previous studies. Different market classes have wide-ranging nutrient contents. Most CVD reduction studies have used white bean varieties. Phytonutrient content, for example, varies in white beans from that of the colored bean varieties, including pinto beans. Previous studies vary in treatment amount, ranging from 80 grams or slightly less than ½ cup to over 300 grams or ~2 cups per dose; one can of drained beans is ~280 grams. Still needing confirmation are what dose sizes and bean varieties lead to optimal cholesterol reduction.
b. Study sample diversity. Few previous bean intervention studies have included sufficient numbers of women to determine possible gender differences. Since CVD is increasingly common among women, interventions that reduce risk in women will allow researchers to reach more solid conclusions and prevent the need to make assumptions that results will be similar to those with men. Furthermore, women often have a strong influence on household food choices and control most of the meal decisions and preparation. They are in a good position to enact dietary changes for CVD prevention through their intergenerational spheres of influence. Thus, if women realize the benefits of beans, they would be able more effectively to promote bean consumption. Over half of our participants have been women in our pilot pinto bean study (53%) and baked bean study (56%). Other bean intervention studies have not always reported the ethnic background of participants, making comparisons between trials difficult.
c. Needed study design improvements. Most previous studies have been conducted in metabolic wards or other controlled research environments. The ability to generalize results and augment external validity is enhanced by using free-living participants. Several previous studies have not noted the usual intakes of beans by participants prior to the intervention. Persons who routinely consume beans before an intervention study may show less of an effect than persons who do not eat beans at all.
The available evidence points to the value of beans in traditional diets and the benefits of adding them in our modern "grab-and-go" diets. More research on their nutritional value will only add to the benefit consumers perceive alongside the realization of how inexpensive they are when it comes to improving a major health problem-elevated cholesterol. Further studies using consistent and contemporary methodologies are critical to substantiate the benefits of beans. In addition, the mechanisms by which beans, such as pinto, black or kidney, reduce cholesterol and may modulate FMD and inflammation have not been fully identified. These beneficial changes likely occur via increased fiber and micronutrients, reduced glycemic load, and anti-inflammatory phytochemicals. It is essential that funding for bean research come from national health research organizations and federal research agencies, not just industry. While we still may not know a lot about beans, like Jack and his beanstalk, a treasure of nutritional benefits awaits discovery.
Donna M. Winham is an assistant professor, Nutrition Program in the College of Nursing and Health Innovation, Arizona State University Polytechnic (Mesa, USA). Contact her via email at Donna.Winham@asu.edu.
For further reading:
Albala, K. Beans: A History, Berg Publishers, New York, New York. USA, 2007, 256 pp.
Bazzano, L.A., M.T. Tees, and D.M. Winham, Non soy legume consumption lowers cholesterol levels: a meta analysis of ran domized controlled trials. Nutr. Metab. Cardiovasc. Dis. 21:94 103 (2011).
Darmadi Blackberry, I., M.L. Wahlqvist, A. Kouris Blazos, B. Steen, W. Lukito, Y. Horie, and K. Horie, Legumes: the most important dietary predictor of survival in older people of different ethnicities, Asia Pac. J. Clin. Nutr. 13:217 220 (2004).
Winham, D., D. Webb, L Marr, and A. Barr, Beans and good health: Compelling research earns beans expanded roles in dietary guidance, Nutr. Today 43:201 209 (2008).