Micronutrients. Macro effects!

Rice: high energy, low nutritive value

Rice is a major food staple for over 3 billion people, representing the major carbohydrate and even protein source in SE Asia, but also in Africa. Unfortunately, rice is a poor source of many essential micronutrients (Table 1). Thus, a rice-based diet is the primary cause of micronutrient malnutrition throughout much of the developing world. Iron, zinc, and vitamin A deficiencies are common in rice-consuming regions. These deficiencies account for decreased work productivity, reduced mental capacity, stunting, blindness, increased child mortality, and elevated morbidity and mortality in general.

Rice is the major energy source of more than half the world population, yet it lacks many life-supporting nutrients. People who cannot afford a balanced diet suffer from multiple micronutrient deficiencies. Some important nutrients are lost during polishing, but unpolished rice cannot be stored, because the outer layer containing those nutrients is rich in lipids (fat) and is thus susceptible to oxidation processes that make the rice rancid and hence untasty.

Amelioration of micronutrient deficiencies can be achieved either by supplementation programs or by food-based approaches. Supplementation programs, while effective, are not sustainable without continuous funding, and do not always reach the neediest individuals. Food-based approaches include the fortification of common foodstuffs (during processing) or the consumption of micronutrient-dense foods (which may necessitate plant breeding, improved crop management, and social marketing). Micronutrient-dense cultivars can be selected from within existing germplasm, or can be generated de novo through genetic modification. In either case, scientists have coined the term "biofortified" for genotypes that deliver increased levels of essential minerals or vitamins. Biofortification, when applied to staple crops, such as rice, is a sustainable approach, provided that access to the technology in the form of seeds is unrestricted.

A shining example of biofortification was the creation of Golden Rice (GR), which earnes its name from its ability to produce the provitamin A carotenoid β-carotene in rice endosperm. This was achieved by expressing two foreign genes (encoding phytoene synthase and carotene desaturase) in rice (Ye et al. 2000). The first generation of GR produced a realtively low amount of provitamin A (Beyer et al. 2002). Improved versions of GR, with significantly higher β-carotene levels are now available.

A recent ex-ante economic impact study carried out in the Philippines, predicts that GR adoption would significantly improve the vitamin A nutritional status of a large proportion of the population (Zimmermann and Qaim, 2004). Ongoing efforts are aiming at adding other micronutrients to the rice grain, specifically iron, zinc, high-quality protein, and vitamin E.

What other strategies have been tried? Biofortification can be considered a complement to the classic intervention approaches of supplementation or fortification. These different strategies are all important, as no single intervention can solve micronutrient malnutrition, and there are limitations to each approach. For instance, although infrequent megadoses of vitamin A can be used to replenish liver stores, this is not feasible for iron due to its limited bodily stores. Furthermore, the frequency required for an effective program for iron and zinc—preferably daily— increases the cost of the intervention and may be limited to opportunities for frequent contact (eg, schoolchildren). For practical reasons, some programs have gone with weekly iron doses, but this is less effective than daily doses and not advisable during pregnancy. Unlike other interventions, biofortified staple crops could provide a daily, sustainable supply of dietary nutrients.

Plant scientists have taken other transgenic approaches, besides that of GR, to improve crop micronutrient status. Overexpression of the iron storage protein ferritin in rice grains has been reported to result in a three-fold (Goto et al. 1999) and a two-fold (Lucca et al. 2001) increase in seed iron content. Similarly, overexpression of an Arabidopsis zinc transporter to manipulate zinc status in barley resulted in a two-fold increase in seed zinc levels (Ramesh et al. 2004). These minor changes are not surprising, because they do not directly address the primary determinant of seed mineral content, ie, the loading of metals into the phloem transport pathway (Grusak, 2002a). No genes are yet available for manipulating phloem metal loading, but plant breeders have begun screening germplasm collections for mineral content variation. Thus far, a four-fold variation for both seed iron and zinc has been identified across 1600 rice accessions (Gregorio et al. 2000). Hence, part of our future strategy will be to use high-iron and high-zinc conventional rice germplasm, and cross it with plants containig other micronutrient traits (eg, β-carotene and vitamin E).

Another approach used has been to alter the levels of compounds that inhibit mineral bioavailability, such as phytate (the major phosphorus storage compound in seeds) and tannins (condensed phenolic polymers), both of which can complex minerals and prevent their absorption during digestion (Welch and Graham, 2004). Mutational breeding has produced maize and barley mutants with low phytate levels (up to 95% reduction; Raboy, 2000), but only one mutant is currently known in rice (45% phytate reduction; Raboy, 2000), and it is questionable whether this is sufficient to improve mineral bioavailability (Glahn et al. 2002). An alternative approach has been the transformation of rice with a heat-stable phytase enzyme (Lucca et al. 2001), aiming at an enzyme that can still degrade phytate in the food matrix after cooking. Unfortunately, this effort was not successful at the time, due to poor thermal stability of the fungal enzyme when expressed in the plant (Lucca et al. 2001), but thermally stable phytases have been developed for industrial purposes and may become available in the future.

Cited bibliography

• Beyer P, Al-Babili S, Ye X, Lucca P, Schaub P, Welsch R, Potrykus I (2002) Golden Rice:introducing the beta-carotene biosynthesis pathway into rice endosperm by genetic engineering to defeat vitamin A deficiency. J Nutr 132:506S-510S.
• Glahn RP, Chen Z, Welch RM Gregorio GB (2002) Comparison of iron bioavailability from 15 rice genotypes: Studies using an in vitro digestion/Caco-2 cell culture model. J Agr Food Chem 50: 3586-3591.
• Goto F, Yoshihara T, Shigemoto N, Toki S, Takaiwa F (1999) Iron fortification of rice seed by the soybean ferritin gene. Nat Biotechnol 17:282-286.
• Gregorio GB, Senadhira D, Htut H, Graham RD (2000) Breeding for trace mineral density in rice. Food Nutr Bull 21:382-386.
• Grusak MA (2002a) Enhancing mineral content in plant food products. J Amer Coll Nutr 21:178S-183S.
• Lucca P, Hurrell R, Potrykus I (2001) Genetic engineering approaches to improve the bioavailability and the level of iron in rice grains. Theor Appl Genet 102:392-397.
• Ramesh SA, Choimes S, Schachtman DP (2004) Over-expression of an Arabidopsis zinc transporter in Hordeum vulgare increases short-term zinc uptake after zinc deprivation and seed zinc content. Plant Mol Biol (in press).
• Welch RM, Graham R (2004) Breeding for micronutrients in staple food crops from a human nutrition perspective. J Exp Bot 55:353-364.
• Ye X, Al-Babili S, Klöti A, Zhang J, Lucca P, Beyer P, Potrykus I (2000) Engineering the provitamin A (beta-carotene) biosynthetic pathway into (carotenoid-free) rice endosperm. Science 287:303-305.
• Zimmermann R, Qaim, M (2004) Potential health benefits of Golden Rice: A Philippine case study. Food Policy 29:147-168.