Approximately 40 micronutrients, minerals, fatty acids and vitamins cannot be produced by the human body and must be supplied via foods. Substances in food have recently been discovered that are not among those micronutrients but still exhibit beneficial effect on health (Harborne et al. 1999) when ingested in optimal amounts (Brandt et al. 2004). Many of these substances belong to the large group of secondary metabolites that are produced within the plants (Luckner 1990).
Evidence indicates that secondary plant metabolites play critical roles in human health and may be nutritionally important (Brandt and M0lgaard 2001). Some scientists believe that plant-based phenolic metabolites are very important due to their antioxidant activity (Rein et al. 2000; Asami et al. 2003), while others have found that other bioactive secondary metabolites, such as glucosinolates from broccoli, are more likely to account for the health-promoting effects of vegetables and fruits (Brandt et al. 2004). Secondary metabolites are common constituents of fruits and vegetables that function in the defence against disease, insect and animal herbivory (Stevenson et al. 1993). There is a growing concern that the levels of some secondary metabolites may be lower than optimal for human health in foods grown in conventional agricultural practices (Brandt and M0lgaard 2001). This concern arises because conventional agricultural practices utilize levels of plant nutrients that can result in a reduction of the natural production of secondary metabolites in the plant (Brandt and M0lgaard 2001; N0rbsk et al. 2003). Differences between the content of secondary metabolites in organically and conventionally produced fruits and vegetables is in line with the observation that organically grown plants are more resistant to diseases and pests than corresponding conventional plants (Evers 1989; Van-Bruggen 1995) and allows for the possibility that organically grown produce may be more beneficial for human health than corresponding conventionally grown produce (Brandt and M0lgaard 2001; Carbonaro et al. 2002; Asami et al. 2003).
In terms of levels of compounds indicated as positive for health, the composition of plants that obtain much of their nutrients from slowly released sources such as plant residues or compost, tend to differ from those provided large amounts of easily available mineral fertilizers. These include higher levels of ascorbic acid (vitamin C); lower levels of nitrate; lower levels of total N (often expressed as 'protein'); higher proportion of essential amino acids in protein; higher zinc (Zn) to phytate ratios (on tropical soils); lower levels of b-carotene; and higher levels of dry matter and plant secondary metabolites (Brandt and Kidmose 2002). Organic foods generally have a lower content of nitrate than conventional foods (Woese et al. 1997; Soil Association Organic Standards 2001). Several reports show higher levels of flavonoids in organic vegetables than in conventional products (Ren et al. 2001; Carbonaro et al. 2002; Asami et al. 2003; Grinder-Pedersen et al. 2003), and correspondingly greater oxidation of proteins in humans, which may be related to the pro-oxidative effect of these compounds and is probably a positive biomarker, even though it contradicts the 'traditional' model for effects of phenolic antioxidants (Grinder-Pedersen et al. 2003). Products from animals fed a large proportion of grass or other relevant roughage have higher levels of b-carotene and other carotenoids, such as lutein; higher proportions of conjugated linoleic acids in the fat; proportions of polyunsatu-rated fatty acids in the fat; and higher levels of vitamin E, in particular the active, natural isomer (e.g. Dhiman et al. 1999; Nielsen et al. 2004).
Cobalt is an essential element for animals but not for plants, found in vitamin B12 and is utilized by micro-organisms. Vitamin B12, in common with a range of other organic substances can be taken up passively by plants. Plants products can therefore, contain considerable quantities of vitamin B12 although it is not essential for normal plant development (Mozafar 1994; Lundegardh and Martensson 2003).
Application of organic fertilizer in soil increases the supply of vitamin B12 in plants and Mozafar (1994) found that barley and spinach fertilized with organic materials had higher concentrations of vitamin B12, whereas sheep grazing cobalt-deficient forage were deficient in vitamin B12 (Ulvund and Pestalozzi 1990). However, human diets normally provide more than sufficient intake of B12, so deficiency is normally caused by impaired absorption due to medical conditions such as gastric atrophy (Cuskelly et al. 2007), which are common among the elderly. It is essential that patients with vitamin B12 deficiency are identified and treated with injection of the vitamin, since the condition cannot be alleviated through the diet, and B12 deficiency causes irreversible damage to the nervous system if not treated. The symptoms of vitamin B12 deficiency can be masked if the diet contains large amounts of folate, so folate fortification of foods can be a serious health risk for older people (Cuskelly et al. 2007). However, organic foods are exempt from fortification in countries where this takes place, providing an additional nutritional benefit for a large group of consumers.
Therefore, the type of farming system has implications on nutritional quality and safety of food and there is growing evidence that certain foods from organic production may have a number of advantages for consumers over food from conventional systems.
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