Recent research highlights the importance of legumes. Legumes represent a vast family of plants including more than 500 genera and more than 10,000 species. “Legume” is the inclusive term for pulses (the dried non-oil seed of legumes) and all other forms of beans and peas from the Fabaceae (or Leguminosae) botanical family.
Legumes have been consumed for at least 10,000 years and are among the most extensively used staple foods worldwide, both for food and animal feed.
The Role of Legumes in Nutrition
Legumes are valued worldwide as a sustainable and inexpensive meat alternative and are considered the second most important food source after cereals. Legumes are nutritionally valuable, providing proteins (20–45%) with essential amino acids, complex carbohydrates (60%) and dietary fiber (5–37%). Legumes also have no cholesterol and are generally low in fat, with 5% energy from fat, with the exception of peanuts (45%), chickpeas (15%) and soybeans (47%) and provide essential minerals and vitamins.
In addition to their nutritional superiority, legumes have also been ascribed economical, cultural, physiological and medicinal roles owing to their possession of beneficial bioactive compounds. Research has shown that most of the bioactive compounds in legumes possess antioxidant properties, which play a role in the prevention of some cancers, heart diseases, osteoporosis, and other degenerative diseases.
Because of their composition, legumes are attractive to health-conscious consumers, celiac and diabetic patients as well as consumers concerned with weight management.
In prospective studies consumption of legumes has been associated with a lower risk of coronary heart disease, in particular, when compared with the consumption of animal proteins such as red meat.
Protein Content of Legumes
Legumes are an excellent source of good quality protein that is generally rich in the essential amino acid. Protein plays an important role in building cells, including those of your immune system. Peas and beans are on the lower side of the range with 17–20% proteins while lupins and soybeans are on the higher end of the range with 38–45% protein. Legumes have higher protein content than most plant foods with about twice the protein content of cereals. The high protein content of legumes can be attributed to their association with the activity of the nitrogen-fixing bacteria in their roots, which converts the unusable nitrogen gas into ammonium which the plant then incorporates into protein synthesis.
Leguminous protein may improve calcium retention in comparison with proteins of animal or cereal origin. Legume protein has also been reported to contribute to the reduction of low-density lipoproteins, a known factor in the development of coronary heart diseases.
Protein quality is significantly improved when legumes are eaten in combination with cereals. For nutritional balance, legumes and cereals are to be consumed in the ratio 35:65. Legumes are particularly important in vegetarian diets as they are the chief source protein and also provide vitamins and minerals. For vegetarians to get a good balance of amino acids, their diets need to combine legumes with cereals.
Micronutrients in Legumes
Legumes are a good source of B-group vitamins such as folate, thiamin, and riboflavin but are a poor source of fat-soluble vitamins and vitamin C. Folate is an essential nutrient and has also been reported to reduce the risk of neural tube defects like spina bifida in newly born babies. Legumes are also sources of the essential minerals zinc, iron, calcium, selenium, phosphorus, copper, potassium, magnesium, and chromium. These micronutrients play important physiological roles such as bone health (calcium), enzyme activity and iron metabolism (copper), carbohydrate and lipid metabolism (chromium, zinc), hemoglobin synthesis (iron) as well as antioxidative activity, protein synthesis and plasma membrane stabilization (zinc).
Generally, legumes are low in sodium and this is desirable considering the recent trends encouraging sodium reduction. Although, legumes have high iron contents, the bioavailability of the iron is poor hence diminishing the value of legumes as a source of iron. However, if legumes are consumed in combination with vitamin C rich foods, the absorption of iron is increased. In this manner, the high iron content would play a major role in the prevention of anemia especially in women of reproductive age.
Bioactive Compounds in Legumes
Legumes contain non-nutrient bioactive compounds such as phytochemicals and antioxidants. These include isoflavones, lignans, protease inhibitors, trypsin and chymotrypsin inhibitors, saponins, alkaloids, and phytoestrogens.
Most of these chemicals generate adverse physiological effects and interfere with protein digestibility and the bioavailability of some minerals. Most of these chemicals are heat labile and since legumes are consumed after cooking, they do not pose a health hazard. Legumes can also be detoxified by dehulling, soaking, boiling, steaming, sprouting, roasting and fermentation prior to processing.
Research has shown that most of these phytochemicals have antioxidant properties which play a role in the prevention of some cancers, heart diseases, osteoporosis, and other chronic degenerative diseases.
The antioxidant capacity of legumes allows them to inhibit or slow down oxidative processes which are largely responsible for degenerative diseases by interacting and scavenging free radicals and reactive oxygen species, chelating metal catalysts, activating antioxidant enzymes as well as inhibiting oxidases. As such, the incorporation of legumes into human diets all over the world could offer protection against chronic diseases.
Important bioactive compounds found in legumes include polyphenols and their derivatives such as flavanols, anthocyanins/anthocyanidins, and tocopherols. Legumes with colored seed coats such as black bean and red kidney bean, have long been associated with antioxidant and anticarcinogenic activity. It is believed that the denser the color of the seed coat, the higher the antioxidant activity.
Oligosaccharides
Most legumes contain up to 50 mg/g total oligosaccharides. Oligosaccharides are responsible for flatulence widely associated with the consumption of legumes. The absence of an α-galactosidase enzyme in the human gastrointestinal tract to cleave the α-1,6 galactose linkage in galactoside-containing oligosaccharides such as raffinose and stachyose means these oligosaccharides pass undigested to the colon where they are metabolized by bacteria forming large amounts of carbon dioxide, hydrogen, and methane. These gases may cause bloating and gastric discomfort and are expelled from the body as flatulence. However, although the oligosaccharides in legumes are viewed negatively, their beneficial attributes outweigh their negative properties.
Prebiotics are non-digestible oligosaccharides which have been shown to have properties that can modulate gastrointestinal problems and improve gut health and well-being.
Oligosaccharides are prebiotic in nature and therefore, promote the growth of the probiotics, Bifidobacteria spp, which play a major role in the maintenance of a healthy colon. Foods high in oligosaccharides are onions (including leeks and garlic), legumes and asparagus. In Japan, soybean oligosaccharides have been suggested as a substitute for table sugar.
Nutritional Benefit of Soaking Beans Prior to Cooking
Soaking increases digestibility. The outer coatings of many varieties of beans contain sugars called oligosaccharides. When beans aren't soaked, these sugars can bypass your stomach and small intestine without being fully digested. When these sugars enter your large intestine, bacteria break them down, producing intestinal gas in the process. Soaking dried legumes dissolves the membranes that cover beans and releases their oligosaccharides. After soaking, discard water and rinse beans.
Soaking removes contaminants. Soaking beans in water removes tiny particles of dirt, gravel and other debris. Beans go through threshing and cleaning processes before they're marketed to consumers.
However, they are not washed because moisture would encourage sprouting. Soaking beans removes their coating of field dust, which may contain residue from pesticides or other contaminants. Before soaking, remove visible particles and sprouted beans. After you've soaked dried beans, drain and rinse them to remove the remaining contaminants.
Soaking reduces phytic acid effects. Phytic acid, a compound in many legumes and grains, may reduce the bioavailability of zinc and other minerals, making it more difficult for your body to utilize these nutrients. Zinc is an essential element that supports cellular metabolism and promotes healthy neurological function, growth, immunity and wound healing. Soaking legumes may reduce the effects of phytic acid on mineral absorption.
Tips for Eating More Legumes
The incorporation of legumes in diets, especially in developing countries, could play a major role in eradicating protein-energy. Legumes could be a base for the development of many functional foods to promote human health. Add lentils to your own vegetable soup. Add chickpeas or soybeans to stir-fry dishes. Extend casserole dishes by adding beans and lentils. Use bean mixes as a salad base. Substitute around 10% of wheat flour with lapin flour when baking to prepare higher fiber and higher protein.
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