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Figure 1 The effect of phytate content on non-heme iron absorption in a meal without meat or ascorbic acid,
The inhibiting effect of phytate on iron absorption is non-linear and the phytate content needs to be reduced to below a threshold phytate content of 100 mg [141], (Figure 1) or below a phytate:iron molar ratio of 1:1 [142] to have significant positive effect on iron absorption. The inhibiting effect of phytate on zinc absorption has recently been modelled based on data from human absorption studies [143], predicting that the inhibiting effect of dietary phytate on Zn absorption is linear with no upper threshold for the inhibitory effect [144]. Modelling absorbed Zn as a function of dietary phytate:Zn molar ratios emphasize the potential benefits of phytate reduction and Zn fortification in diets that are habitually high in phytate.
A plant-based diet has been categorised to have “moderate” zinc availability when the molar phytate:zinc ratio is between 5 and 15, and “low” zinc availability when the molar ratio exceeds 15 [145]. A reduction of the phytate:zinc molar ratio in a maize based diet from 36 to 17 was shown to significantly improve zinc absorption [146], indicating that – unlike iron – any reduction in phytate may potentially contribute to improving zinc absorption.
Various traditional processing methods can reduce the phytate content in cereals and legumes. The potential impact of traditional processing methods, thermal processing, mechanical processing, fermentation, soaking and germination (malting), on improving bioavailability of micronutrients in a plant-based diet was reviewed by Hotz & Gibson [147]. Soaking of cereals and legumes can promote diffusion of phytate into the soaking water. Soaking unrefined maize flour reduced phytate content by approximately 50% [147], while soaking of legume seeds (peas, peanuts and pigeon peas) has been reported to reduce phytic acid by about 20% [148] [149]. Soaking may also wash out soluble vitamins and minerals to the soaking water.
Fermentation can reduce phytate by up to as much as 90%, depending on species and pH [147] as a result of the enzymatic activity of microbial phytase originating from the microflora in the fermentation culture, either present on the surface of the fermented foods (spontaneous fermentation) or added as a starter culture. Fermentation can also contribute to activating endogenous phytase in the cereal or legume being processed, especially in fermentation processes producing low-molecular organic acids such as lactic acid, as many endogenous cereal phytases have optimum activity at pH 6 or below [135].
The stimulation of germination of cereals and legumes will activate endogenous phytase as a step in the process of releasing the phosphorus that the plant has stored as phytate. The endogenous phytase activity is relatively higher in wheat and in other non-tropical cereals such as rye and barley, compared to tropical cereals such as maize, millet and sorghum [150]. Therefore, mixing for example wheat and maize in a germination process may stimulate a higher initial phytase activity and lead to higher reduction of phytate, compared to germination of pure maize.
The nutritional benefits of reducing phytate content through processing depend on the efficiency to reach a sufficiently low level of phytate to improve iron absorption. Dephytinisation will also benefit zinc absorption [142] and possibly protein utilisation.
In a community-based trial with 6-12 month old children, a complementary food based on millet, beans and peanuts was phytate reduced by soaking and germination. The phytate content was reduced by 34%, but the molar phytate:iron ratio remained high, being > 11 in the processed food. No impact on iron status in the intervention group was found [151].
In addition to reducing phytate in a plant-based diet through traditional processing technologies, the potential of reducing phytate content in cereal based foods by adding commercially produced phytase needs to be explored. Commercial phytases have been developed and are commercially available for mono-gastric animal feed. In particular in pig production it is a widely used additive to enhance protein and phosphorus absorption and thereby growth. The application of commercial phytases to foods for MM children to reduce phytate and thereby increase nutritional value, particularly mineral bioavailability, is unexplored and research is needed to clarify the potential for applications as also pointed out in the reviews by Golden [4] and by de Pee and Bloem [152]. In a recent study among women iron absorption from a whole maize porridge was increased if phytase was added to the porridge [153].
In conclusion it is important that foods used for rehabilitation of MM children should have a low content of phytate, especially if there are few or no animal source foods in the diet. This can be achieved by avoiding unrefined cereals and legumes with high phytate content and by using food processing methods that reduce the phytate content.
Table 8 Phytate distribution in morphological components of cereals and legumes a) Cereal or Legume Morphological
component Distribution (%) Peas Cotyledon Germ Hull 88.7 2.5 0.1 Wheat Endosperm Germ Bran 2.2 12.9 87.1 Maize Endosperm Germ Hull 3.0 88.9 1.5 Brown rice Endosperm
Germ Pericarp 1.2 7.6 80.0 a) Source: [154;155]