**3. Plant foods utilization in Nigeria**

chronic diseases. Thomas [26] concluded that the contribution a food makes to nutrient intake of a person depends on the quantity consumed per 100g, the amount consumed per meal and

Obviously, plant foods have other values related to human nutrition and health apart from their nutritional importance. Plant foods are one of the elements in virtually all cultures, with symbolic and ritualistic values that link language and religion. In many cultures, especially the Asian countries, distinction between food and medicine as therapeutic agents is obscure as the traditional vegetable diets provide the people with disease fighting arsenals that serve both protective/preventive and curative functions. For this reason, foods are widely recognized

The bioactive compounds or secondary metabolites are the non-nutrient components in plant foods. They have some nutritional effects and health benefits. They are those substances contained in foods which supply no nutrients. They could contain some compounds that are beneficial to health or toxic to humans and/or act as antagonists to nutrients in foods [27]. They include tannins and other phenolic compounds (phenols, flavonoids, isoflavonoids), saponins, glucosinolates, alkaloids [28, 29], phytate and dietary fibre [30]. These chemical compounds are found in different classes and parts of plant foods in varying amounts. They are more concentrated in plant storage organs (leaves and seeds) than other parts of the plants [22]. These constituents have their individual health-promoting qualities that compel people to combine the different food sources to achieve healthy eating and maintain good health. Several authors [27, 31-35] studied therapeutic potentials and metabolic effects of foods rich in dietary fibre and phytochemical constituents. These include lower risk of colon cancer, promotion of early satiety and normal laxation, moderation of post-prandial blood glucose responses and improved insulin sensitivity, reduction in total and low density lipoprotein (LDL)-cholesterol, regulation of appetite and enhancement of sodium and fluid balance. They are also used to treat constipation and prevent development of diverticulosis and diverticulitis. These are features of a dietary pattern to treat and prevent obesity and its co-morbidities (especially type 2 diabetes mellitus). These two conditions are closely linked because hyperglycaemia and hypercholesterolaemia are well-known cardiovascular risk factors in type 2 diabetes mellitus. Invariably, diets adequate in dietary fibre are usually rich in micronutrients and phytochem‐ icals, and frequently less calorically dense and lower in fat and added sugars. However, environmental factors, cultural food habits and insufficient nutritional information about health benefits of traditional plant foods still pose a problem to healthy food choices. Drew‐ nowski and Gomez-Carneros [29] reported that most of the bioactive compounds are bitter, acrid or astringent and aversive to the consumer and may be wholly incompatible with consumer acceptance. These caused increasing epidemic of diet-related diseases across the regions. They suggest the need to take sensory properties and food preferences into account when advocating for increased consumption and diversification of rich sources of these secondary metabolites in plant foods. The challenge of achieving adequate supply of energy and nutrient intake as well as the health-promoting compounds from plant-based foods/diets without compromising the health of an individual, forms the basis for current dietary recom‐ mendations aimed at promoting consumption of plant foods to reduce diet-related NCDs.

the frequency of consumption.

62 Antioxidant-Antidiabetic Agents and Human Health

as the primary health provider.

Nigeria's diverse ethnic groups (over 250] are accommodated under three main ethnic regions - Igbo (in the East), Yoruba (in the West) and Hausa-Fulani (in the North). The country has a beautiful climate, which endows it with very rich biodiversity. A variety of plant foods (maize, rice, sorghum, millet, yam, cassava, cocoayam and legumes) are mainly produced as subsis‐ tence food crops in Africa. They are more commonly and widely consumed in Nigeria and other developing countries than in the developed world. They are relatively available, affordable and acceptable. They contribute appreciably to the nutrient intake (energy, protein, fat, vitamins and minerals) of the less developed world. In Nigeria, starchy staples (cereals, roots/tubers) and legumes constitute the major part of the traditional diets (up to 70% and 25%, respectively) [8]. However, cereals are the major staples in the north with much higher intake of animal protein sources (mutton, beef and milk), while starchy roots and tubers are the main staples in the south with relatively more consumption of legumes [10]. These foods contribute appreciably to nutrient intake of southerners [36]. The plant foods are cultivated, gathered wild or are semi-wild.

Roots and tubers are the thickened underground starch storage organs of some plants, propagated vegetatively from the underground stems and their stem cuttings. These edible roots and tubers belong to several families and are formed by both monocotyledons (yams and cocoyams) and dicotyledons (cassava and sweet potatoes) [37]. Cereals are seeds of grass family (Gramineae). Cereals are important crops which serve as industrial raw materials and staple foods for the world over [38]. Cereals are the most widely cultivated and consumed crops globally. World cultivated cereals include wheat, maize, rice, barley, oats, rye, sorghum, millet, wild rice and hungry rice (acha). In Nigeria, the starchy staples (maize, rice, sorghum, millet, yam, cassava, cocoayam and plantain) are utilized in many different forms for prepa‐ ration of various dishes. Their nutritional importance is that they provide most of the energy, contain high carbohydrate, low protein and appreciable amount of minerals and vitamins, especially the B-complex vitamins. They form the base and usually constitute the major ingredients in the traditional dishes. Some dishes are light and serve as breakfast. Others are solid and frequently made palatable in combination with a variety of legumes and by the addition of palm oil, vegetables and fruits, a range of spices, various sauces and fish/meats. They serve as main meals and eaten later in the day. Processing and preparation methods of these starchy staples and their recipes vary with ethic groups and geographical locations. Each of these staples irrespective of the preparation methods produces a thick paste known as "foofoo" eaten with soups/sauces. These dishes are highly cherished and consumed daily in all parts of Nigeria [39]. These starchy staples are also used to produce complementary foods and local snacks as roasted/fried/baked products or drinks. Some like yam, cocoyam, 'okoho' (*Cissus pulponea* root) are also used to thicken traditional soups.

Legumes are flowering plants in the family Fabaceae (or Leguminosae) that have pods, which contain beans or peas. According to Okafor [40], a list of edible woody leguminous plants, consisting of 150 species covering 103 genera in 48 families was recorded in Nigeria. Legumes rank second to the grasses as source of fodder (for animal) and food for man. The plants are most commonly known as pulses, peas or beans [38]. They contain more protein and less carbohydrate than cereals. There are two main types of legumes: those containing high protein [25-35%) and oil content [15-45%) and those containing moderate protein [18-24%) and less than 5% oil content. They are oil seeds and pulses, respectively. The most common forms in which legumes are used – they may be cooked alone as pottage, or eaten in combination with any starchy staple of choice. The more widely consumed legumes include cowpea, pigeon pea, soyabean, bambara-groundnut, african yam bean, groundbean and lima bean. It is due to their high protein content that legumes are widely used in combination with cereals for production of nutritious complementary foods for infants and young children both in Nigeria and other developing countries, in both industrial and household levels. This is because legumes are deficient in sulphur-containing amino acids (methionine and cystine) and contain high amount of lysine. Cereals are deficient in lysine and rich in the sulphur-containing amino acids. Legumes play very important culinary roles as soup thickeners. Many of the leguminous seeds are used for thickening soups, sauces and stews in Nigeria [41]. These soup thickeners are condiments/ingredients used in preparation of local soups to alter their consistency and give them special flavour [42]. Traditionally, roots/tubers (yam and cocoayam, 'okoho'), vegetables (okro, 'ogbamu' leaves), legumes (grain legumes/oil seeds and nuts) and fungi are used for thickening soups. Soups made from them are basically prepared with meat and/or fish (when available), oil, vegetables, crayfish, pepper and other condiments and water to obtain watery or thick consistencies depending on their composition. These soups are traditionally consumed in combination with cereals and starchy roots/tubers, processed into a form of paste known as "foofoo". Soups are very important accompaniment to main dishes in Nigeria. Ene-Obong et al. [39] recorded 110 soup recipes out of 322 recipes in all the 6 geo-political zones in Nigeria. The total number of recipes documented reflects the biodiversity of the Nigerian food system. The high viscous property possessed by these leguminous soup thickeners is associated with their dietary fibre content, implicated in attenuation of post-prandial blood glucose and control of lipid metabolism [32].

succinic may also contribute to flavour in vegetables. The colour of vegetables depends on the pigments they contain. Spices are known to exert several physiological effects including antidiabetic influence [47]. Anthocyanins impart the blue, purple and red colours to vegetables such as tomatoes, garden eggs and red peppers. Chlorophyll imparts green colour to green leafy vegetables and carotenoids are responsible for the yellow colour in carrots, sweet potatoes and maize. Green leafy vegetables also contain carotenoids but this is masked by the more intense green of chlorophyll. The indigestible dietary fibres – cellulose, hemicelluloses and lignin give the vegetables their characteristics structure. However, some vegetables also contain pectin as storage carbohydrate, which has been linked to reduction of post-prandial hyperglycaemia. Data collection and assessment of commonly consumed foods in all the 6 geopolitical zones in Nigeria published by Ene-Obong et al. [39] showed that fruit consumption was conspicuously absent in all the areas. Fruits are the fleshy or dry ripened ovary of a plant, enclosing the seeds. They are commonly eaten raw and used for desserts. Green and yellow vegetables are rich sources of beta-carotene and essential minerals. Most fruits, particularly the citrus species are abundant in vitamin C. These vitamins play antioxidant roles in the body

Anti-Diabetic Effects of Nigerian Indigenous Plant Foods/Diets

http://dx.doi.org/10.5772/57240

65

The term "Diabetes" has been defined by many authors from different perspectives. However, the similarity in these definitions is that diabetes is a chronic metabolic disorder, characterized by high blood glucose (hyperglycemia), associated with impaired carbohydrate, fat and protein metabolism, resulting from either insufficient or no release of insulin by pancreas in the body [48, 49]. Diabetes mellitus is a chronic life-long disease, which has been known to mankind for over 2000 years. It requires careful monitoring and control. Currently, diabetes ranks fourth worldwide among the NCDs (cardiovascular diseases, cancers, chronic respira‐ tory diseases and diabetes) with prevalence rates of 30%, 13%, 7% and 2%, respectively WHO [50]. It has been projected that by 2020 -2025, the number of people in the developing world with diabetes will increase by more than 2.5 fold; from 84 million in 1995 to 228 million in 2025 [7] and that 70% of deaths due to type 2 diabetes will occur in developing countries. Diabetes mellitus is increasingly being recognized as a major public health problem in developing

Aetiological classification of diabetes mellitus includes type I diabetes (immune-mediated and idiopathic), type 2 diabetes, gestational diabetes and other specific types [48, 49]. However, types 1 and 2 diabetes mellitus appeared to have gained much more popularity among researchers and have generally been considered as the two major categories. In Africa, type 2 diabetes is the predominant form of diabetes in sub-Saharan Africa, accounting for over 90% of cases. The IDF Atlas [51] estimated that 10.8 million people have type 2 diabetes in sub-Saharan Africa in 2006 and this would rise to 18.7 million by 2025, an increase of 80%, as such exceeding the predicted worldwide increase of 55% [52]. In the past, the estimates on mortality of diabetes as the leading cause of death for sub-Saharan Africa were amongst the lowest for all regions globally [53]. This was attributed to Africa's rich biodiversity and high consumption

of natural foods, which are rich in complex carbohydrates, proteins and dietary fibre.

and needs to be supplied in the diet.

countries.

**4. Diabetes and its public health importance**

Vegetables are succulent herbaceous plants that are harvested and eaten whole or in part, raw or cooked as part of a main dish or salad [43]. They can be classified into leaves (green, fluted pumpkin), stems/whole shoot (asparagus, elephant grass, 'achara'), roots (carrot), flowers, immature fruit (fresh corn, okra), mature green fruits (peppers), ripe fruit (tomato), fresh pod (vegetable cowpea), bulbs (onions), tubers (irish potatoes, yams) and fungi (mushrooms, puffballs). There are over 500 known edible vegetables in Africa. Some of these are valued for their bulking effect/thickening power and others may be used as garnishes or spices. Vegeta‐ bles are generally low in calories and protein. They are valued most for high vitamins and minerals contents. They are low in sodium and rich in potassium, which helps to lower blood pressure [44, 45]. They also contain dietary fibre and phytochemicals (pigments and other compounds that impart flavour to foods), which have heart-disease fighting properties. However, fruits and vegetables consumption is generally low in Nigeria despite their recog‐ nition as very important food items to reduce nutrient deficiencies from inadequate intake of minerals and vitamins [46].

Composition of vegetables depends on their species, part of plant and stage of maturity. Spices are vegetables used as ingredients, which contribute to the taste and flavour of foods. The flavour of vegetables is due to sugars, organic acids, minerals salts, volatile sulphur com‐ pounds and tannins. The strong flavour of some vegetables (onions and cabbage) is due to their sulphur-containing compounds. Non-volatile acids such as malic, citric, oxalic and succinic may also contribute to flavour in vegetables. The colour of vegetables depends on the pigments they contain. Spices are known to exert several physiological effects including antidiabetic influence [47]. Anthocyanins impart the blue, purple and red colours to vegetables such as tomatoes, garden eggs and red peppers. Chlorophyll imparts green colour to green leafy vegetables and carotenoids are responsible for the yellow colour in carrots, sweet potatoes and maize. Green leafy vegetables also contain carotenoids but this is masked by the more intense green of chlorophyll. The indigestible dietary fibres – cellulose, hemicelluloses and lignin give the vegetables their characteristics structure. However, some vegetables also contain pectin as storage carbohydrate, which has been linked to reduction of post-prandial hyperglycaemia. Data collection and assessment of commonly consumed foods in all the 6 geopolitical zones in Nigeria published by Ene-Obong et al. [39] showed that fruit consumption was conspicuously absent in all the areas. Fruits are the fleshy or dry ripened ovary of a plant, enclosing the seeds. They are commonly eaten raw and used for desserts. Green and yellow vegetables are rich sources of beta-carotene and essential minerals. Most fruits, particularly the citrus species are abundant in vitamin C. These vitamins play antioxidant roles in the body and needs to be supplied in the diet.
