Improvement of iron density in rice grains

Compared to other cereal, iron density in rice grains is considered low.  The recommended daily allowance (RDA) of iron for Thais is approximately 15 mg.  At the 200-250 gm of polished rice consumed daily, it is estimated that the optimum iron density in the whole rice grains should be close to 5-mg/100 g in order to supply one third of the RDA. 

This number seems to be unreachable as compared to 1.3-1.5 mg/100 gm among popular rice cultivars.  To step up the breeding activities, the extensive survey of iron density among landraces identified potential donors of high iron density.  The majority of these high iron rice lines belong to the black purple rice. The major limitation to using these rice germplasm is high polyphenol and phytic acid contents in the rice bran. Because white color rice is low in polyphenol and phytic acid, attempts were made to hybridize white x purple rice parents to transmit high iron density to normal white rice.

 

The first successful white rice contains 2.1 mg / 100 g as the whole grain.  This advanced line coded as 313-19-1-1 is aromatic with intermediate amylose content. From crosses between upland rice, Kum Doi Chang (KDC) and Hom Pamah, we successfully identified white color rice with 2.8-mg/100 g of grain iron.  The highest iron density lines were identified as progenies from the hybridization between a wild rice, O. nivara and a high iron rice Jao Hom Nin (JHN).  One of the highest iron dense rice contained up to 5-mg/100 g.  Our emphasis is now directed to improving iron bioavailability.  


 

Iron Bioavailability

The most difficult task in improving iron density in cereal grains is how to enhance its bioavailability.  Iron bioavailability is well connected to the molecular forms of iron, inhibitors to iron absorption, and booster to iron absorption.  When iron is best absorbed as ferric ion or chelated to ferritin or frataxin.  Therefore, the density of ferritin or frataxin certainly affected iron density and absorption.  For the natural inhibitors to iron absorption, polyphenol, condensed tannin and phytic acid are the most potent inhibitors to iron absorption.  Reduction of these natural inhibitors was the primary objective in cross-breed new strains of while color rice.  Understanding on enhancers to iron absorption is not totally clear.  Well known enhancers to iron absorption are vitamin C, amino acid methionine, cysteine, short oligosaccharide such as inulin, etc. Strong attention has now been paid to polysaccharide fraction as potential enhancers to iron absorption in polished rice.   

First Laboratory for Iron Bioavailability in Thailand

In vitro digestion Caco-2 cell is the preliminary test for Fe bioavailability in rice. This technique can screen many rice varieties before the clinical test in human subjects. Results from Caco-2 cell found that the highest Fe bioavailability rice from cross-breeding line containing 9 ng feritin/mg cell protein. Furthermore, low phytic acid mutant line and Fe transporter mutant from TILLING project showed higher Fe               bioavailability than wide type.

Fe Absorption in Human

The first Fe absorption test in human subjects was conducted by using 50 male volunteers. Among three rice varieties in this experiment, new cross-breeding line ‘313-19-1-1’ was showed the highest Fe absorption (9.82%) by human. Interestingly, polished rice contain low iron density, but the Fe absorption was higher than brown rice at 3 times.


 

 

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