An Overview of Nutritional and Antinutritional Factors in Green Leafy Vegetables

Generally, vegetables are widely designated as “protective foods” in human diet due to their varied health benefits attributable to the richness in vitamins, essential fatty acids, minerals, amino acids and dietary fiber [1] and various essential bioactive compounds [2]. These include health-promoting plant secondary metabolites composed of antioxidants and phenolic compounds. It is well acknowledged that to meet recommended daily allowance of nutrition, the World Health Organization (WHO) recommendation at least 400g of fruit and non-starchy vegetables (WHO, 2013) is used. The dietary guidelines for Americans recommend five servings of vegetables per day based on an intake of 2000 calories (HHS/USDA, 2015). It is also recommended that one of the five servings of vegetables should be green leafy vegetables. Nutritionists and dieticians are of the opinion that people should diversify their diets as no single vegetable meets all the nutritional requirements necessary for good health and wellbeing [3]. Hence in today’s agri-food systems, more emphasize is given to crop diversification with the intention of fulfilling human nutritional requirements, and to reduce the pressure on cereal production [4]. Globally, crop diversity and nutritional value of vegetable crops are of special significance for improving food and nutrition security [5]. Plants are major sources of numerous bioactive compounds collectively termed as phytochemicals, which are reported to be key to good health [2]. Researchers have shown that the composition of phytochemicals is very distinctive and varies widely amongst plants [6]. For instance, vegetables that belong to the Alliaceae family (e.g., onions, garlic, shallots, leek, Welsh onion and chives) are characterized by thiosulfides and flavonoids. Cruciferous vegetables that belong to the Brassicaceae family (e.g., cabbage, cauliflower, kale and broccoli) contain high sources of glucosinolates, and those that belong to Cucurbitaceae family (e.g., squash, pumpkin, cucumber, melon and bitter gourd) are rich in carotenoids and tocopherols [2]. However, the compositions of these phytochemicals have been documented to be affected by factors such as genotypic characteristics, climatic conditions, edaphic factors and management practices [7-10]. For instance, it was shown that for the same plant species, the mineral nutrients content and total phenolic compounds and antioxidants can be altered by the application of different sources of nutrients [7-11].