NUTRITIONAL AND HEALTH IMPLICATIONS OF LEGUMES

NUTRITIONAL AND HEALTH IMPLICATIONS OF LEGUMES

Mebrahtom Gebrelibanos*, Dinka Tesfaye, Y. Raghavendra and Biruk Sintayeyu

Course and Research Unit of Pharmacognosy, Department of Pharmacy, College of Health Sciences, Mekelle University, Mekelle, Ethiopia

ABSTRACT: Legumes are plants in the family Fabaceae characterized by seeds in pods that are often ediblethough sometimes poisonous. The nutrient content (protein, carbohydrate and micronutrients) of legumes contribute to address under-nutrition, especially protein-calorie malnutrition among children and nursing mothers in developing countries where supplementing cereal-based diets with legumes is suggested as one of the best solutions to protein calorie malnutrition. Anti-nutritional factors, in legumes, may limit their biological value and acceptance as a regular food item, yet they are readily removable and recent research has shown potential health benefits of some of these compounds; and hence, manipulation of processing conditions may be required to remove or reduce only those unwanted components.  Moreover, legumes play a role in prevention, improvement and/or treatment of disease conditions such as, diabetes mellitus, cardiovascular diseases, cancer diseases (e.g breast and prostate cancers) and lowers blood cholesterol level. Most of these disease conditions are associated with over-nutrition and obesity and are considered as diseases of the rich. It is, therefore, claimed that including legumes in a health-promoting diet is important in meeting the major dietary recommendations to improve the nutritional status of undernourished as well as over-nourished individuals, and to reduce risk of chronic diseases such as cardiovascular disease, diabetes mellitus and cancer. In this review, some of the scientific viewpoints that attempt to justify the nutritional contributions, anti-nutritional considerations and health implications of legumes are discussed.

Legumes, Nutrition, Anti-Nutritional Factors, Health Benefits

INTRODUCTION: A legume is a plant in the family Fabaceae (or Leguminosae), or a fruit of these specific plants,characterized by seeds borne in pods ¹ and often edible, though sometimes quite poisonous ².

Members of the family are impor­tant food sources; and food legumes are those species of the legume family that are consumed by human beings or domestic animals commonly as dry seeds, i.e. the grain legumes ³.They include peas, beans, lentils, peanuts, and other podded plants that are used as food ⁴.

Grain legumes are commonly subdivided into pulses and oilseeds ⁵.  Pulses are distinguished from oil seeds by their low fat content ³.  They are those grain legumes, which, in addition to protein, store high levels of carbohydrate and low amounts of lipid in their dry seeds, and leguminous oilseeds are those which boast higher lipid, but lower carbohydrate levels than pulses. Soybean (Glycine max) and peanut (groundnut, Arachis hypogaea) are typically categorized as oilseeds while common beans (Phaseolus vulgaris) and cowpea (Vigna unguiculata) are considered pulses ⁵.

The oil seeds are cultivated primarily for their protein and oil content, and the grain legumes as a protein source ⁶. The terms “legumes” and “pulses” are used interchangeably because all pulses are considered legumes but not all legumes are consid­ered pulses.The term “pulse”, as used by the Food and Agriculture Organization (FAO), is exclusively for crops harvested solely for the dry seed of leguminous plants. This excludes oil seeds and also green beans and green peas which are consumed and considered as vegetables ³.

History and ecological importance: Generally, legumes are well known crops with long history of utilization ⁷, which have been cultivated for thousands of years ⁴. Their consumption dated back to as far as 5500 BC and they are thought to be one of the first crops cultivated by man ⁸.  They have played an important role in the traditional diets of many regions throughout the world ⁴ and their value has long been appreciated in many countries ⁹.  In addition to their food value, legumes are important in cropping systems because of their ability to fix atmospheric nitrogen and increase the overall fertility of soil, reducing the need for expensive nitrogenous fer­tilizers.

The dominant food legumes of any region may vary from country to country or even from region to region, but most of them can be grown under a reasonably wide range of ecological conditions and many legumes can be grown reasonably well on poor soils even without the application of fertilizers ³.

Legumes are well adapted to adverse environmental conditions and highly resistant to disease and pests ¹⁰. They are widely grown in semi-arid regions while cereals require more water and intensive cultivation¹¹. Historically, legumes have complemented cereals in both human diets and production systems, especially in efficient use of land and water resources and sustainability of agriculture systems globally ¹².

Most legumes that are used for foods are multipurpose plants, serving for animal forage as well as soil improvement due to their ability to fix atmospheric nitrogen ¹³. Nowadays, in addition to their nutritional, agricultural and ecological significance, legumes are of great importance because of their numerous health benefits.

Generally, despite numerous advantages (improve soil fertility, serve as food and feed, provide cheaper protein source and health promoting components), legumeshave certain limitations such as their deficiency of sulfur amino acids, poor digestibility and the presence of antinutritional factors although these limitations are not far from control as there exist recommended manipulations for their optimization.

Objective: The main objective of this review is to discuss on some of the scientific arguments about the nutritional contributions, anti-nutritional considerations and their suggested options of minimization as well as health implications of legumes.

Nutritional role of Legumes: Legumes constitute a part of the diet of nearly all humans ¹³ andtheir nutrient content (protein, carbohydrate and micronutrients) contribute to address under-nutrition ¹⁴. They contain complex carbohydrates (oligo-saccha­rides, dietary fibers and resistant starch- i.e. starch that escapes digestion in the small intestine and may ferment in the large intestine), protein with a good amino acid profile (high lysine), im­portant vitamins (B vitamins, folates, ascorbic acid and tocopherols), minerals as well as antioxidants, polyphenols and numerous other phytochemicals endowed with useful biological activities ^{3, 14-17}.

They are good sources of water-soluble vitamins, especially thiamine (Vitamin B₁), riboflavin (Vitamin B₂), niacin (Vitamin B₃), pyridoxine(Vitamin B₆) and folate, but poor sources of fat-soluble vitamins and vitamin C.  They are low in sodium but are excellent sources of other minerals, including calcium, copper, iron, magnesium, phosphorus, potassium and zinc^{14, 18, 19}.   They have desirable characteristic such as low fat (except oilseeds), high concentration of polyunsaturated fatty acids and a long shelf life ^{20, 21}. The predominant fatty acid is linoleic acid, although they also contain α-linolenic acid¹⁴.

Thus, legumes contain many nutritionally important and health-promoting components responsible for addressing under-nutrition and promotion of health.

Malnutrition is currently widespread in many areas of the world; and the most serious one is protein-calorie malnutrition (PCM). PCM is a widespread problem throughout the world, especially among children in the developing countries; and has both health and economic consequences. It is a major nutritional syndrome affecting more than 170 million preschool children and nursing mothers in developing Afro-Asian countries where provision of adequate proteins of animal origin is difficult and expensive. Legumes are the major contributors of protein and calories in Afro-Asian diets for economic and cultural reasons ^{22, 23}.

The high cost and limited supply of animal proteins have necessitated contemporary research efforts geared towards the study of food properties and potential utilization of protein from locally available food crops, especially from under-utilized or relatively neglected high protein oilseeds and legumes ²³. As the lower income group of the population is particularly vulnerable, it is suggested that attention must be given to easily available, accessible, cheap but nutritious plant protein sources (especially legumes – because among plants, legumes are considered the major source of dietary proteins) to improve the nutritional status of the low-income groups of the population^22,23.

Legumes (poor man’s meat) play an important role in human nutrition since they are rich and economical sources of good quality protein, calories, certain minerals and vitamins, and fibre ^{14, 22, 24}.  They are inexpensive, nutrient dense sources of plant protein that can be substituted for dietary animal protein ²⁵. However, proteins in legumes are higher in nonessential amino acids compared with proteins from animal foods ²⁶; and their nutritive value has been known to be low because of their deficiency of sulfur amino acids, poor digestibility and the presence of antinutritional factors ^{23, 27}.

Yet, it is suggested that sulfur amino acidsdeficiency can be solved by complementing legume with cereals ⁵; and food processing can improve the nutritional quality and increases the bioavailability of nutrients, by inactivating antinutritional factors ^{11, 28}.

Thus, legumes, with the exception of soybeans, are not a complete protein like meat; however, when paired with grains or another complementary food, they provide a complete source of amino acids ¹⁹.

In developing countries, legumes complement the lack of proteins from cereals, roots and tubers ²¹. Their most vital role is that of supplying most of the protein in regions of high population density and in balancing the deficiencies of cereal protein ¹³.Legumes and cereals are considered ideal dietary partners because the amino acids making up their proteins are very good complements ⁵.

Cereal proteins are deficient in certain essential amino acids, particularly lysine. On the other hand, legumes have been reported to contain adequate amounts of lysine, but are deficient in sulfur-containing amino acids (methionine, cystine and cysteine). Therefore, intentional combination of cereals