Biofortification of crops with nutrients: factors affecting utilization and storage

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Highlights

  • Biofortification involves the synthesis or accumulation of nutrients at source.

  • Nutrient bioavailability is an intrinsic property that depends on the food matrix.

  • Biofortification may promote bioavailability compared to standard fortification.

  • Processing enhances bioavailability by releasing nutrients or eliminating inhibitors.

  • Nutrient levels are affected by genotype, cooking method and storage.

Biofortification is an effective and economical method to improve the micronutrient content of crops, particularly staples that sustain human populations in developing countries. Whereas conventional fortification requires artificial additives, biofortification involves the synthesis or accumulation of nutrients by plants at source. Little is known about the relative merits of biofortification and artificial fortification in terms of nutrient bioaccessibility and bioavailability, and much depends on the biochemical nature of the nutrient, which can promote or delay uptake, and determine how efficiently different nutrients are transported through the blood, stored, and utilized. Data from the first plants biofortified with minerals and vitamins provide evidence that the way in which nutrients are presented can affect how they are processed and utilized in the human body. The latest studies on the effects of the food matrix, processing and storage on nutrient transfer from biofortified crops are reviewed, as well as current knowledge about nutrient absorption and utilization.

Introduction

Nutrients in the human diet ultimately come from plants, but all our major food crops lack certain essential micronutrients (vitamins and minerals) [1]. The endosperm of cereal staples such as rice, wheat and maize are the most important source of calories for humans, providing ∼23%, ∼17% and ∼10% of total global calories, respectively [2]. However, endosperm tissue lacks sufficient amounts of vitamins (particularly vitamins A, E, C and folate) and minerals (particularly iron, zinc and selenium) [1, 3]. Iron and zinc deficiencies affect more than 50% of the human population, resulting in poor growth and development, an impaired immune system, fatigue, muscle wasting, sterility and even death [2, 3]. More than four million children worldwide suffer from severe vitamin A deficiency (VAD), including 250 000–500 000 per year who become partially or totally blind [4]. Women have a higher demand for vitamin A during pregnancy, and currently more than 20 million pregnant women in developing countries suffer from VAD [4].

Strategies to address micronutrient deficiency include dietary diversification, nutritional supplements, fortification and biofortification [1, 2, 3]. A combination of approaches is likely to provide the greatest overall benefit, but in some populations dietary diversification is impractical and supplements are only suitable as short-term interventions [2, 3]. Fortification requires the addition of nutrients to food products, for example, iodine is added to table salt, and iron, zinc and folate are added to flour to make bread [2, 3]. One major drawback of these approaches is the limited stability of the additives, for example, folate added to rice becomes more soluble at higher temperatures and is lost when the rice is boiled [2]. A second disadvantage is that additives can also affect the quality of food, for example, iron additives are oxidized over time and this has an impact on taste [3]. The third and major limitation of conventional fortification is that it is mainly suited to developed countries with the necessary technical infrastructure and distribution networks, but is less appropriate for developing countries with their extensive reliance on subsistence agriculture [2]. Biofortification can address all three issues by facilitating the development of nutrient-dense staple crops that can be grown and distributed using existing agricultural practices [3, 5].

Biofortification is well established in principle but there are few practical examples of deployment thus far. Zinc-enriched rice and wheat have recently been deployed in Bangladesh and China, respectively; an orange sweet potato rich in provitamin A carotenoids has been released in Mozambique and Uganda; and provitamin A rich maize has been released in Zambia and Nigeria [5]. Golden Rice II, the first transgenic biofortified crop engineered with provitamin A carotenoids in the endosperm, has incurred multiple delays in terms of deployment. It is currently being backcrossed into locally adapted varieties in the Philippines, Indonesia, India and Bangladesh [5]. Multivitamin corn (registered as the protected variety Carolight® in Spain) was developed by transforming an elite white-endosperm South African inbred line with four genes representing three different vitamin biosynthesis pathways, increasing the levels of β-carotene, other carotenoids, vitamin C and folate [6]. Carolight® also contains a Bacillus thuringiensis (Bt) gene making it pest resistant [7]. Biofortification is a sustainable approach which can bring nutritious staple crops to populations that are difficult to supply with supplements or fortified food products, and once the crop is developed there are no recurring costs other than those associated with normal agriculture. However, it is necessary to consider the efficiency of nutrient delivery by biofortified crops compared to other interventions in order to determine the long-term benefits of this approach. Data from the first biofortified crops are now available to allow such comparisons (Boxes 1 and 2).

Section snippets

Fate of nutrients produced in plants

The fate of organic nutrients in plant tissues is highly dependent on their solubility and their affinity for the constituents of the plant tissue matrix.

Food matrix

The major role of the food matrix in terms of nutrient bioaccessibility and bioavailability is to trap the nutrients within cells or subcellular compartments, and to provide constituents that interact chemically with specific nutrients to either encourage or delay their release, leading to their classification as absorption promoters and inhibitors (Table 1). Lipid food components increase the bioaccessibility of fat-soluble nutrients, so cooking methods that preserve fats (e.g., frying) tend

Downstream behavior of absorbed nutrients

Nutrient supplements and fortified foods are provided in well-controlled doses to avoid toxicity. One concern about biofortification is that dosing would be more difficult to control, but recent studies have shown that the uptake of nutrients from biofortified crops is regulated at the level of absorption from the gut, and also at the cellular level and by the modulation of storage reservoirs, based on the abundance of nutrients already in the body and the demand for certain nutrient molecules [

Conclusions

The biofortification of staple crops was envisaged as a sustainable strategy to deliver nutritious food to populations that are unsuitable for other intervention measures, but the bioavailability of nutrients in biofortified crops must be confirmed before they can be widely deployed. The bioavailability of nutrients is partly dependent on the intrinsic qualities of each nutrient molecule and partly dependent on their presentation in the context of the food matrix.

The major difference between

References and recommended reading

Papers of particular interest, published within the period of review, have been highlighted as:

  • • of special interest

  • •• of outstanding interest

Acknowledgements

Research at the University of Lleida is supported by MINECO, Spain (BIO2014-54426-P; BIO2014-54441-P, FEDER funds), the Catalan Government (2014 SGR 1296 Agricultural Biotechnology Research Group) and Recercaixa. J. Díaz-Gómez thanks the University of Lleida for a pre-doctoral grant.

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