Dietary guidance for pulses: the challenge and opportunity to be part of both the vegetable and protein food groups
Abstract
Pulses are a dry, edible variety of beans, peas, and lentils that have been consumed for 10,000 years. Pulses are rich in plant-based protein and fiber, as well as micronutrients such as iron and potassium. The satiating effect of both fiber and protein assists in managing weight and combating obesity. The high fiber content and low glycemic index of pulses aid people with diabetes in maintaining blood glucose and insulin levels. Pulse consumption may improve serum lipid levels to reduce the risk of cardiovascular disease. Pulses developed as a member of both the protein and vegetable food groups as a result of its high content of plant-based protein and dietary fiber. The last two revisions of the Dietary Guidelines saw the transformation from the MyPyramid “meat and beans group” to the MyPlate “protein foods group,” a nutrient name rather than a food source. Research suggests that consumers better identify with food source examples rather than nutrient names. The 2015 Dietary Guidelines also came with a new area: sustainable diets. Encouraging the consumption of sustainable food sources, like pulses, is imperative to ensuring a secure, healthy food supply for the U.S. population over time and for future generations.
Introduction
Pulses are annual crops whose life cycle, from germination to the production of seeds, is terminated after 1 year and members of the Leguminosae family.1 Pulses are estimated to have been consumed for at least 10,000 years, and they are one of the most extensively used foods in the world.2 Historically, the consumption of legumes and other plants was thought to prevent illness and disease ranging from headache to heart disease.3
Today, in the developed world, many consumers may find it challenging to include pulses in their daily diet owing to a lack of proper preparation and cooking skills.4 Pulses are part of more than one food group in the Dietary Guidelines for Americans 2015–2020 (DGAs): the protein and the vegetable groups. A food group is a collection of foods that share common nutritional and biological compositions. Under the dietary guidelines, recommended dietary serving sizes are provided for each food group.5 The vegetable food group comprises plant-based dark-green vegetables, red and orange vegetables, starchy vegetables, beans and peas, and other vegetables, and the serving size for each subgroup of vegetable varies depending on the diet pattern and caloric intake of the individual.5 The protein food group comprises protein-rich meat, poultry, seafood, beans and peas, eggs, processed soy products, nuts, and seeds, and the recommended serving size is also dependent upon the diet pattern and caloric intake of the consumer.5 With pulses being part of both the protein and vegetable groups because of their nutritional composition, confusion and misinterpretation of dietary guidelines may further discourage pulse consumption. The United Nations (UN) declared 2016 the International Year of Pulses, which helps facilitate discussion on the nutritional composition, health benefits, dietary guidance, and sustainability of pulse grains.6
Definition of foods referred to as pulses
Pulses are defined by the Food and Agricultural Organization (FAO) of the UN as, “limited to crops harvested solely for dry grain, thereby excluding crops harvested green for food (e.g., green peas, green beans), which are classified as vegetable crops.”1 Pulses also exclude crops, such as soybeans and groundnuts, which are used primarily for oil extractions, and leguminous crops used solely for sowing purposes, such as alfalfa. The FAO acknowledges 11 principal pulses: dry beans (e.g., kidney, lima, adzuki, butter, mungo, black gram, scarlet runner, and golden beans), dry broad beans (e.g., horse, broad, and field beans), dry peas, chickpeas (including garbanzos), dry cowpeas (including black-eyed peas/beans), pigeon peas, lentils, bambara beans, vetches, lupins, and minor pulses (jack, winged, sword, guar, velvet, and yam beans).1, 7
Nutritional composition
Pulses are moderately low in energy density, offering an average of 1.3 kilocalories per gram.8 Additionally, pulses are high in fiber (∼7 g/0.5 c serving), with the majority containing both insoluble and soluble fibers.9 Pulses are very low in fat (0.8–1.5%) when compared with oilseeds, such as soybean, canola, and flax; they are beneficial sources of mono- and polyunsaturated fat and contain plant sterols.10-14 Pulses are characterized as a good source of digestible protein, providing an average of 7.7 g/0.5 c serving and 17–30% of the dry weight of pulses.8, 15 Pulses are abundant in the essential amino acids lysine (∼64 mg/g of protein) and threonine (∼38 mg/g of protein), which are typically low in other plant-based protein sources; however, pulses are low in other amino acids, including methionine, tryptophan, and cysteine.14-16 Since pulses are lacking in these essential amino acids, they are considered to be lower quality sources of protein.7 Therefore, it is recommended that pulses be consumed in conjunction with other plant-based and/or animal protein sources that contain the limiting essential amino acids to form a meal or diet containing a high-quality protein mixture. Pulses also contain a high carbohydrate content (∼50–65%) and 22–45% of pulse grain weight from starch, depending on the source, cooking, and processing.17, 18 Slowly digestible starches, resistant starches, protein content, and the protein–starch matrix have been attributed to the low glycemic index (GI) of pulses, which ranges from approximately 29to 48.8, 19 Table 1 outlines the calorie, protein, fat, carbohydrate, and fiber content of various pulses per 100 g.
| Calories | Protein (g) | Fat (g) | Carbohydrate (g) | Fiber (g) | Iron (mg) | Potassium (mg) | |
|---|---|---|---|---|---|---|---|
| Adzuki beans, cooked | 128 | 7.52 | 0.10 | 24.77 | 7.3 | 2.00 | 532 |
| White beans, cooked | 139 | 9.73 | 0.35 | 25.09 | 6.3 | 3.70 | 561 |
| Small white beans, cooked | 142 | 8.97 | 0.64 | 25.81 | 10.4 | 2.84 | 463 |
| Lima beans, cooked | 115 | 7.80 | 0.38 | 20.88 | 7.0 | 2.39 | 508 |
| Great northern beans, canned | 114 | 7.37 | 0.39 | 21.02 | 4.9 | 1.57 | 351 |
| Pink beans, cooked | 149 | 9.06 | 0.49 | 27.91 | 5.3 | 2.30 | 508 |
| Black beans, cooked | 132 | 8.86 | 0.54 | 23.71 | 8.7 | 2.10 | 355 |
| Black turtle beans, cooked | 130 | 8.18 | 0.35 | 24.35 | 8.3 | 2.85 | 433 |
| Refried beans, canned, traditional | 89 | 4.98 | 2.01 | 13.55 | 3.7 | 1.44 | 319 |
| Pinto beans, cooked | 143 | 9.01 | 1.11 | 15.41 | 9.0 | 2.09 | 436 |
| Kidney beans, cooked | 127 | 8.67 | 0.50 | 22.80 | 6.4 | 2.22 | 403 |
| Navy beans, cooked | 140 | 8.23 | 0.62 | 26.05 | 10.5 | 2.36 | 389 |
| Yellow beans, cooked | 144 | 9.16 | 1.08 | 25.28 | 10.4 | 2.48 | 325 |
| Lentils, cooked | 116 | 9.02 | .38 | 20.13 | 7.9 | 3.33 | 369 |
| Chickpeas, cooked | 164 | 8.86 | 2.59 | 27.42 | 7.6 | 2.89 | 291 |
| Yardlong beans, cooked | 118 | 8.29 | 0.45 | 21.09 | 3.8 | 2.64 | 315 |
| Cranberry (romano) beans, cooked | 136 | 9.34 | 0.46 | 24.46 | 8.6 | 2.09 | 387 |
| Split peas | 118 | 8.34 | 0.39 | 21.10 | 8.3 | 1.29 | 362 |
- Note: Data derived from the USDA Food Composition Database.78
Pulses are rich sources of many micronutrients, including selenium, thiamin, niacin, folate, riboflavin, pyridoxine, potassium, zinc, vitamin E, and vitamin A.10, 11, 13 Pulses can also be a good source of iron, depending on the pulse variety. For example, cooked white beans (3.70 mg iron/100 g) contain almost three times as much iron as split peas (1.29 mg iron/100 g) (Table 1). Since pulses are a plant-based protein, bioavailability of nutrients may be a limitation. Iron is a nutrient of concern for both adolescent and premenopausal females and, unfortunately, the bioavailability of nutrients, such as iron, from plant-based diets is usually lower than from a diet containing animal-based food sources.20 Iron, as well as calcium and protein, from a vegetarian diet is less bioavailable owing to the presence of phytates. Iron found in pulses is bound to phytates, which reduces iron adsorption in the body.21-23 Pulses are also a good source of potassium, a mineral associated with a lower risk of metabolic syndrome and a regulator of blood pressure.24 Potassium has been recognized by the 2015 Dietary Guidelines Advisory Committee as a shortfall nutrient that is underconsumed in the U.S. population.20 Increased consumption of pulses could assist in increasing the overall U.S. population's potassium consumption. Eating a variety of foods is beneficial to maintaining a balanced diet, and the inclusion of pulses as a part of a healthy diet may improve overall micronutrient consumption.20
Health benefits
Consumption of pulses is encouraged in the diets of the general population, as their nutrient profile can have a positive health impact.6, 10, 20 Nearly half of all adult Americans have one or more preventable chronic diseases related to physical inactivity or poor diet quality, including cardiovascular disease (CVD), diabetes, and hypertension.25 Nonnutrient and nutrient components of pulse grains have been linked to a reduction in the risk of obesity, CVD, and diabetes, which may help reduce the incidence of preventable chronic diseases among groups such as the American population.10, 14, 26
The high fiber content of pulses has been shown to result in an increase in satiety.27-29 Satiety, brought on by hormonal and neural signals within the body, is the sensation of fullness an individual experiences after consuming a meal.30 The sensation of fullness is enhanced by increased gastric distention and a slowed rate of gastric emptying into the intestine, which are results of the gelling effect of viscous soluble fibers.31 In addition, fiber fermentation within the colon increases satiety-related hormone release, resulting in an increased perception of fullness.31 A meta-analysis by Howarth et al. analyzed the results of more than 50 intervention studies that evaluated the relationship between body weight, energy intake, and fiber intake. It was determined that increasing fiber intake by 14 g per day was associated with a weight loss of approximately 2 kg and a 10% decrease in energy intake over a 4-month period.32 Similar to fiber, protein is also a component of pulse grains that affects satiety. Protein is the most satiating macronutrient, and, when consumed, it elicits the secretion of satiety-related hormones in the small intestine, such as cholecystokinin, peptide YY, and glucagon-like peptide 1.33 Protein and fiber both contribute to satiety and may also contribute to decreasing the occurrence of obesity by interfering with caloric intake.31 With pulses being rich sources of both protein and fiber, they are ideal candidates for increasing satiety, reducing calorie intake, and managing body weight over time.34 Modest weight loss has been shown to decrease the risk of developing type 2 diabetes and CVD.35
Evidence suggests that the consumption of pulses may improve lipid profiles, thus reducing atherosclerosis and overall risk of CVD.35 Many growing bodies of research suggest that a pulse-rich diet can decrease two leading factors of CVD, total serum triglycerides and cholesterol.26, 36-39 Both plant sterols and mono- and polyunsaturated fats, which are present in pulses, lower low-density lipoprotein (LDL) and total cholesterol and increase high-density lipoprotein.12-14 Bazzano et al. conducted a meta-analysis on 9632 men and women who participated in the First National Health and Nutrition Examination Survey Epidemiologic Follow-up Study and evaluated the effect of pulse consumption on cholesterol levels.40 The study indicated a significant inverse relationship between legume intake and risk of coronary heart disease and suggests that a diet rich in pulses reduced both total and LDL cholesterol.40 Similarly, Finley et al. conducted a study on 40 adult men and women aged 18–55 years for a 16-week period and reported that consuming a 1/2 cup serving of cooked dry beans daily reduced total cholesterol by 8%.37 While much evidence supports an improved lipid profile with pulse consumption, other studies assessing blood pressure, platelet activity, and inflammation also support the relationship between consuming a diet rich in pulses and CVD risk reduction.39, 41-44
Pulses are an ideal food choice for individuals with diabetes because of their low GI and high fiber content.8, 9, 19 Sievenpiper et al. conducted a systematic review and meta-analysis of 41 randomized controlled experimental trials and found that low-GI and high-fiber (e.g., pulses) diets improved glycemic control by lowering fasting blood glucose and insulin levels, glycosylated blood proteins, and fasting blood glucose.45 Additionally, Jenkins et al. conducted a study on 121 participants with type 2 diabetes mellitus. Participants were asked to either consume a low-GI legume diet or a diet high in insoluble fiber, such as whole-wheat products, for 3 months. The study found the reduction in hemoglobin A1c (HbA1c) values was greater in low-GI legume diets than in whole-wheat fiber diets by –0.2%. Jenkins et al. concluded that consuming legumes as part of a low-GI diet improved glycemic control.46 Consumption of high-GI foods leads to an elevated postprandial blood glucose and insulin response, which is not ideal for individuals with diabetes.47 Therefore, consuming pulses as part of a low-GI diet may help lower the risk of diabetes-related complications.46 Additionally, insulin sensitivity and glucose tolerance are improved with the presence of resistant starches in pulses, which also helps reduce risks associated with diabetes.48
Evolution of dietary guidelines
As mentioned above, pulses are composed of a rich nutritional profile that may assist in the prevention of obesity, CVD, and diabetes. The nutritional composition of pulses has also afforded pulses the opportunity to be categorized in both the vegetable and protein groups in the DGAs. Historically, pulses have not always been explicitly considered to belong to both food groups. Pulses, usually considered as either beans and peas or legumes in the DGA, have shifted positions within food groups from the 1992 Food Guide Pyramid, to the 2005 MyPyramid, to the current 2011 MyPlate. The advancement of nutritional understanding, the reformation of the dietary guidelines, and the evolution of new food guidance tools mark the journey of the pulse through the food groups.
In 1992, the United States Department of Agriculture officially released the Food Guide Pyramid after having conducted studies to determine the ideal graphic for effective food guidance.49 On the Food Guide Pyramid graphic, pulses fell in the protein group, which was labeled as the “meat, poultry, fish, dry beans, eggs, and nuts” group.50 In the 1995 DGAs, it was first explicitly written that dry beans, peas, and lentils could be listed in either the “meat and bean group or vegetable group,” but not both.51 Though not listed on the Food Guide Pyramid diagram, legumes were also listed as food sources in the vegetable group in the supplementary reading materials that accompanied the Food Guide Pyramid graphic of 1992.52 Although additional reading materials concerning the Food Guide Pyramid were available, many people did not utilize these documents. Instead, the Food Guide Pyramid diagram was often used alone, leading to misinterpretation and confusion among users. The Pyramid was contested for combining red meat with foods low in saturated fat, fish, poultry, nuts, and legumes; not distinguishing whole grains from refined grains; and not defining unsaturated fats as separate from saturated fats.53
The drawbacks of the Food Guide Pyramid of 1992 led to the development of a more simplistic food guidance diagram: MyPyramid. The new illustration was introduced in 2005 along with the DGAs of 2005. In the 2005 DGAs, pulses were listed as a serving in either the “meat and beans” group or the “vegetable” group, but not both.54 The MyPyramid symbol did not include detailed diet guidance; users needed to access dietary guidance materials via MyPyramid.gov. The simplicity of MyPyramid was designed to engage users through their participation in the online programs provided on MyPyramid.gov.55 Online users could enter their anthropometric details to receive personalized dietary guidance. The personalized diet plan, MyPyramid Tracker (to track diet and physical activity over time), informational food materials, and promotion of physical activity were all online resources intended to help Americans live a healthy lifestyle.56 However, as with the Food Guide Pyramid, MyPyramid faced opposition. Experts were disappointed that the grain recommendation did not exclusively promote whole grains, the healthfulness of unsaturated fat was not addressed, MyPyramid.gov did not include information on healthy body weight (most individuals are not aware of their healthy weight range), and the underprivileged may not have Internet access to utilize the online guidance materials.57 Although some experts believed that MyPyramid succeeded in emphasizing physical activity and was less complicated than the Food Guide Pyramid of 1992, experts were critical, such as Walter Willett, professor of epidemiology and nutrition at the Harvard School of Public Health, Boston, who stated, “unfortunately, people will need to go elsewhere for better information.”57
Owing to the many limitations, the MyPyramid guidance tool was short-lived, with MyPlate replacing MyPyramid in 2011. The MyPlate diagram features the fruit, grain, vegetable, protein foods, and dairy groups depicted as a plate and cup. Interestingly, the previously named “meat and beans,” group was changed to the protein foods group. Unlike other groups of MyPlate, the protein foods group is named for a nutrient rather than a food type. Haven et al. suggested that the group was renamed to “teach consumers that protein is available in a variety of foods.”58 However, a consumer research study for educational messages revealed that consumers understand food-based names better than nutrient terminology.59 Though the food group name changed, the inclusion of pulses in both the protein foods and vegetable groups did not. ChooseMyPlate.gov recognizes that pulses are excellent sources of plant proteins and iron and zinc (similar to meats, poultry, and fish), as well excellent sources of dietary fiber, folate, and potassium (comparable to foods in the vegetable group).5 Papanikolaou found that consumers of beans had better overall nutrient intake of fiber, potassium, magnesium, and iron than did nonconsumers, suggesting that plant-based protein sources improve dietary quality.41 On the other hand, animal products provide higher quality and greater quantity of protein than do plant foods; therefore, individuals should consume a variety of both plant- and animal-based protein food sources to meet nutrient recommendations.20, 60 ChooseMyPlate.gov also explains that pulses should be considered protein food if the total amount consumed does not meet or exceed the suggested intake from the protein foods group. Alternately, pulses should be considered as part of the bean and peas subgroup in the vegetable group if the amount consumed exceeds the suggested intake level in the protein foods group.61 This way of considering pulses is applicable for individuals who do not consume adequate amounts of meat, poultry, and fish, such as vegetarians.
The importance of sustainability and increasing pulse consumption
The Dietary Guidelines Advisory Committee commented on the sustainability aspect of diets for the first time in 2015.20 Sustainable diets recognize the impact of food and beverages on environmental outcomes, such as farm to plate to waste disposal. Food sustainability is imperative to ensuring a secure, healthy food supply for the U.S. population over time and for future generations. Global food production is responsible for over 70% of fresh water use, 80% of deforestation, and up to 30% of greenhouse gas (GHG) emissions.62 Energy costs, population growth, climate change, and shifts in dietary patterns will put pressure on land use, soil quality, and sustainable water sources in the future.63
The scientific report of the 2015 Dietary Guidelines Advisory Committee concluded that a diet high in plant-based foods and lower in animal-based foods promotes health and is associated with less environmental impact than an average U.S. diet. Thus, a diet rich in pulse grains could help promote sustainable energy, water, and land use.20 Pulses, such as lentils and peas, support a lower carbon footprint because they are nitrogen-fixing crops requiring very little, if any, nitrogen fertilizer.64 Nitrogen-fixing pulse crops can be grown in rotation with other crops to lower GHG emissions in comparison with nitrogen-fertilizer systems.65 Padhan et al. further explored GHG emissions by analyzing global dietary patterns that varied by food and energy content. The dietary patterns were measured per capita and sorted into four categories: intake of low, moderate, high, and very high kcal diets. The dietary patterns were then compared to corresponding GHG emissions. Low-energy diets comprised more than 50% cereals or more than 70% cereals, starchy roots, and pulses and contained less than 2100 kcals/cap/day. Very high-calorie diets, which were composed of high amounts of meat and more than 2800 kcal/cap/day, exhibited high total per capita CO2 emissions. The high CO2 emissions are associated with the developed world and can be attributed to high carbon intensity and high intake of animal products. Conversely, low-energy diets had the lowest total CO2 emissions per capita.66 Since pulses are a low-energy food, it can be deducted that the pulse carbon footprint would be much lower compared with a meat food source.
Pulses have the potential to significantly reduce GHG emissions, yet their consumption remains low. The 2015 Dietary Guidelines suggest a legume serving size of 1/2 cup (∼90 g) for a Healthy U.S.-Style Eating Pattern.5 The Data from the National Health and Nutrition Examination Survey 1999–2000 revealed that adults are consuming an average of 0.1–0.3 servings of legumes per day, which is one third or less than the recommendation.67 Mitchell et al. also found that only 7.9% of the U.S. population from 1999 to 2002 consumed beans, peas, or lentils on any given day, with refried beans, pinto beans, baked beans, chili and Latin dishes being the main sources.68 Though pulse consumption within the United States has been low, recent literature suggests that culinary education may assist in increasing pulse consumption. Confidence in cooking, health outcomes, an increase in time devoted to cooking, an enhanced approach toward cooking, and consumption of healthy foods have all been attributed to culinary education.69-71 Published reports have established a strong correlation between time passed in the kitchen and mortality rate and between improved dietary quality and healthy food preparation skills.72, 73
As a result of the influence of cooking on health, it has been recommended that nutrition education focuses on developing culinary skills, such as food storage, food preparation, grocery shopping, and meal planning, rather than only nutrients.74-76 Establishing knowledge of cooking techniques may help the U.S. population become familiarized with skills that promote the preparation of legumes. Becoming engaged in cooking skills may help Americans learn to soak, rinse, boil, freeze, and puree legumes, along with incorporating legumes into their everyday lifestyle. Polak et al. offer not only legume recipes but also practical suggestions for increasing pulse consumption, such as to always have a kitchen pantry stocked with lentils since they are easily prepared; purchase legumes even if there is no foreseeable or intended use; save time by cooking more lentils than necessary and freezing the leftovers; soak a double portion of legumes at once and freeze the remaining soaked legumes; eat homemade legume salads at work for lunch; and use leftover legumes in at soup or stew.4 Incorporating legume consumption into everyday life may help Americans learn to enjoy the flavor, aroma, and texture of legumes while also inadvertently offering nutritional, health, and environmental benefits.
Conclusions
Pulses contain a nutrient-dense profile, which offers many health benefits for consumers. While research supports the beneficial effects of pulses on cardiovascular health, diabetes, and obesity, more research must be conducted to clarify results. In addition, the unique nutritional composition of pulses allows them to be considered in both the protein and vegetable food groups. Though the nutrient profile of pulses warrants the inclusion of pulses in both groups, users may misunderstand or misinterpret dietary recommendations. Further education regarding recommended pulse consumption is needed to improve consumer understanding and increase consumption. Dietary consumption of pulses in the United States is currently low, especially among nonvegetarians. Orlich et al. conducted a study on more than 89,000 men and women over a 5-year period and found that nonvegetarians consumed on average less legumes (52 g/day) than vegans (84 g/day) and pescovegetarians (75 g/day).77 The 2015 Dietary Guidelines suggest a legume serving size of 1/2 cup (∼90 g), and, for a Healthy U.S.-Style Eating Pattern and a Healthy Mediterranean-Style Eating Pattern, 1 1/2 cups/week is recommended for a 2000 calorie diet.5, 78 The dietary guidelines promote not only an increase in pulse consumption but also diets, such as the Healthy U.S.-Style Eating Pattern and Healthy Mediterranean-Style Eating Pattern, which are rich in a wide variety of foods.5 These diets promote the consumption of foods like legumes; dark green, red, and orange vegetables; fruits; whole grains; and seafood, all of which contribute to a balanced diet while limiting the consumption of high-calorie, low-nutrient-dense foods.5 Participating in culinary education may increase legume consumption, resulting not only in beneficial dietary impacts but also environmental impacts. Increased consumption of plant-based protein from pulses could help decrease the carbon footprint associated with animal-based proteins.
