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Friday, April 15, 2016

Why fortified cereals are actually the cause of many nutritional deficiencies?


Mineral deficiencies, especially of iron, zinc, and calcium, respectively, negatively affect human health and may lead to conditions such as iron deficiency anemia, rickets, osteoporosis, and diseases of the immune system. Cereal grains and legumes are of global importance in nutrition of monogastrics (humans and the respective domestic animals) and provide high amounts of several minerals, e.g., iron, zinc, and calcium. Nevertheless, their bioavailability is low. Plants contain phytates, the salts of phytic acid, chemically known as inositol-hexakisphosphate, which interact with several minerals and proteins. However, phytate may be hydrolysed by phytase. This enzyme is naturally present in plants and also widely distributed in microorganisms. Several food processing methods have been reported to enhance phytate hydrolysis, due to the activation of endogenous phytase activity or via the enzyme produced by microbes. In recent years, fermentation for food and feed improvement and preservation, respectively, has gained increasing interest as a promising method to degrade phytate and enhance mineral utilization in monogastrics. Indeed, several in vitro as well as in vivo studies confirm a positive effect on the utilization of minerals, such as P, Ca, Fe and Zn, using sourdough fermentation for baking or fermentation of legumes, mainly soybeans. This review summarizes the current knowledge regarding the potential of fermentation to enhance macro and trace element bioavailability in monogastric species.


Iron and zinc utilization of plant-based diets

Bioavailability of trace elements is known to be influenced by various dietary components which may act either as absorption inhibitors or enhancers and by various food processing methods [2] and [4].

Although it has been shown that InsP6 decreases Ca absorption, its affinity is greater for certain other divalent cations, including Zn and Fe [38]. Zinc appears to be most affected by InsP6 due to formation of the most stable and insoluble complexes [39]. Nevertheless, only InsP6 and InsP5 seem to have depressing properties on Zn absorption. Inositol-tetra-phosphate and InsP3 should be relatively inert regarding Zn absorption [40] and [41]. Interestingly, studies of meals based on white beans and soybeans indicate that the effect of InsP6 on Zn absorption is less pronounced in soybeans than in cereals [42] and [43]. However, low Zn absorption from a soya-protein-based infant formula has been found [44] and a removal of InsP6 from such a formula significantly enhanced Zn absorption [45].

Moreover, phytic acid and Fe form insoluble complexes at intestinal pH, which cannot be absorbed. In general, using isotope techniques Fe absorption from soybeans and soya protein products was found to be low with a range from 0.5 to 9% [46], [47], [48], [49] and [50]. Also, Fe absorption from single meals based on black beans, lentils, mung beans and split beans has been reported to be very low, ranging from 0.8% to 1.9% [51]. In cereal grains, whole-wheat flour showed slightly higher Fe dialysis percentages (0.81%) compared to that of refined wheat flour (0.52%) [3]. For sufficient enhancement of Fe absorption, InsP6 must be completely degraded. According to Sandberg et al. [52], even InsP5, InsP4 and InsP3 must be degraded, to enhance Fe absorption. Accordingly, a complete removal of InsP6 through the addition of a microbial phytase preparation in soya infant formula has been reported to improve Fe absorption significantly [50]. Nevertheless, the same authors found that even after removal of InsP6, soya protein itself is still relatively inhibitory to Fe absorption, probably due to the presence of Fe-binding peptides [50]. Davidsson et al. [49] demonstrated that the Fe bioavailability can be similarly increased by either removing phytic acid or increasing the ascorbic acid content in infant formula based on soy isolate. The enhancement effect of native or added ascorbic acid on Fe absorption is largely due to its ability to reduce ferric to ferrous iron but is also due to its potential to chelate iron [53].

Taken together, due to the low mineral bioavailability typically found in plant-based foods, deficiencies of Fe, and Zn, are highly prevalent in developing countries, but also in vulnerable groups with high requirements in industrialized countries, such as women of fertile age, infants and adolescents. Up to 80% of pregnant women in developing countries are suffering from Fe deficiency [54]. Iron deficiency anemia (IDA), the most severe form of Fe deficiency, affects as many as four billion people worldwide [55]. In worst cases, the cause of Fe deficiency may be inadequate dietary intake, but inhibitors of absorption, mostly found in plant foods, contribute significantly to these deficiencies [56]. In most developing countries with a high prevalence of Fe deficiency and the respective anemia, the socio-economic status of those affected limits their consumption of red meat, which is one of the best sources of highly bioavailable Fe [57]. In these countries, cereals, legumes and vegetables often constitute the main food sources. Grains and legumes contain non-haem Fe, which has a very low bioavailability compared to that of haem Fe in meat [58]. While the bran of grains contains the most Fe [59], it also contains InsP6 and sometimes tannins, depending on species and cultivar, thus further decreasing the availability of non-haem Fe [57]. Besides developing countries, and vulnerable people with high requirements in industrialized countries, the increased number of vegetarians might lead to increased prevalence of Fe deficiency [60], [61] and [62]. Fe deficiency retards normal brain development in infants and affects the success of a pregnancy by increasing premature deliveries, as well as morbidity of mother and child at or around childbirth. Nutritional Fe deficiency reaches its greatest prevalence and severity in populations subsisting predominantly on cereal and legume diets [63].

Additionally, deficiency of Zn, although not completely assessed, is believed to be as widespread as that of Fe and is a cause for concern especially in the developing countries [64]. Zn deficiency prevents normal child growth and greatly weakens the immune system, leading to more infections. Although animal-derived foods are rich sources of Zn, this micronutrient has to be provided mainly through food grains for a majority of the population in developing countries. Cereals are the primary source of Zn in most vegetarian diets, secondary sources being legumes [4].

In conclusion, it is of utmost importance to decrease anti-nutritional compounds in plant-derived foods and to enhance mineral bioavailability to counteract severe health problems in people reliant on plant-based food.


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