The priorities for nutritional enhancement For the developed world

Although it is known that the distribution and processing of food can lead to a significant loss in nutritional quality, there are few instances where present evidence suggests there is a need to change current practices. There is very little evidence for nutritional deficiencies. In those cases where public health authorities have thought there is a potential problem, food supplementation with nutrients is a commonly adopted policy. The use of nutritional supplements is widespread. Whilst the focus of current interest is on the need to consider nutrients and other phytochemicals as protective against the development of disease in later life, the levels of intake that may be necessary to optimise protection are far from resolved at the present time.

The only plant-derived food product on the market where nutritional health benefits are claimed (as opposed to implied) is the enrichment of margarines with plant sterol and stanol esters for the reduction in plasma cholesterol levels (Fig.

sitostanyl ferulate

Fig. 8.1 Structure of plant sterol and stanol esters.

sitostanyl ferulate

  1. 8.1 Structure of plant sterol and stanol esters.
  2. 1) These products do not require the development of specifically bred plants since it is possible to extract stanols and sterols from existing plants (albeit in the case of the stanols from the bark of a tree) for use in their manufacture.

Experiments with plant stanol esters were shown to lower serum cholesterol consistently by about 10-15% and LDL-cholesterol by about 20% in patients with high serum cholesterol levels as well as in normal individuals.16, 17 Similar effects have been seen with plant sterol esters but at least 1 g/day of plant sterols need to be consumed.18 Consequently, they require extraction and addition to foods.

Plant sterols can be in the free form or predominantly esterified with long chain fatty acids or with phenolic acids as in rice-bran oil (ferulate) and shea butter (cinnamates). Sterol esters are better absorbed than the free sterols and most sterol esters are hydrolysed to the free sterols in the intestine.

As campesterol esters are better absorbed than sitosterol esters, serum levels of campesterol could rise to those levels that are found in the very few people who suffer toxic symptoms from phytosterolemia. Thus there may be a benefit in increasing the sitosterol to campesterol ratio in plants.

The ideal situation would be for sufficient sterols to be present in our diets to ensure that plasma cholesterol levels are kept reasonably low without the need to buy a specific functional food, and that they would be in a fat-soluble form for effective uptake. The evidence favours, in increasing order of preference, the use of:

  1. Plant sterol esters with low campesterol contents.
  2. Sterol esters from tall oil (derived from pine wood) which have a higher stanol content than edible oils.
  3. Plant stanol esters.

A vegetable oil rich in plant stanols, especially in sitostanol esterified with polyunsaturated fatty acids, would also have the benefit of being less susceptible to oxidation at frying temperatures than the sterols. The potential health benefits of this class of bioactive compounds are unlikely to be met by the use of classical plant breeding methods but genetic engineering could make these targets feasible.

8.4.2 For the developing world

The world-wide deficiency of vitamin A is being tackled both through conventional plant breeding and by genetic manipulation. However, the use of conventional plant breeding to deliver adequate intakes is dependent on the availability of carotenoid-rich staple foods. Often these are available for very restricted times of the year in some societies. In those countries where rice is a dietary staple the problem is particularly severe and the deficiency is likely to be corrected only by the introduction of rice that has been genetically manipulated to produce b-carotene. However, yellow rice is produced and this may give rise to problems of acceptability to consumers used to white rice.

Manipulation of the carotenoid pathway in rice

The nature of the challenges faced in manipulating plant secondary metabolites is well illustrated through the attempts that have been made to produce carotenoids in rice plants. A simplified version of the pathways leading to the synthesis of the carotenoids principally found in food plants is shown in Figure 8.2.

Immature rice endosperm is capable of synthesising the early pathway intermediate geranylgeranyl diphosphate (GP). Four plant genes corresponding to the enzymes phytoene synthase (psy) (1), phytoene desaturase (2), zeta carotene desaturase (3) and lycopene cyclase (crt) (4) are required. Enzyme (1) was obtained from the daffodil (narcissus pseudonarcissus), (2) from a bacterium Erwinia uredovora - which is capable of achieving steps (2) and (3) from the single enzyme, and (4) from the daffodil.

The genes need to be expressed in a tissue-specific manner through the insertion of specific promoters. This has been achieved in rice through the use of the daffodil psy gene.19 In rice the daffodil psy cDNA insertion is under the control

GGPP

Phytoene synthase * (psy)

Phytoene

Phytoene desaturase (crtl)

Lycopene

\ Lycopene P-cycIase (Icy)

a-carotene p-carotene

Fig. 8.2 The carotenoid biosynthetic pathway (simplified).

of an endosperm-specific promoter. The choice of promoter will very much affect the timing and tissue-specific expression of a gene.

Surprisingly, seeds that expressed psy and crt did not accumulate lycopene. Instead they contained b-carotene and other xanthophylls. Thus it would seem that the enzymes required to make these metabolites are either normally expressed in rice endosperm or are induced if lycopene is formed. The maximum level of carotenoids in the endosperm of plants that were heterozygous for the transgenes was 1.6mgkg-1 which is likely to help to meet the nutritional needs of people consuming rice as a staple. Interestingly, good progress is being made in adding a gene coding for ferritin - the iron storage protein found in mammals and plants - to rice.20 It is likely that this would, in addition, help improve the iron deficiency also seen in these communities if it is shown to be bioavailable.

The controversy over the use of advanced technologies for producing sustainable food in the developing world has been addressed by the developers of modified rice. They have in effect waived all intellectual property rights for exploitation of the technologies in the developing world and are actively involved in assisting the International Rice Research Institute to breed stable and agro-nomically successful lines for use in vitamin A-deficient areas.

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