198 The nutrition handbook for food processors Potential strategies for the enhancement of specific metabolites could target 1. Over-expression of enzymes that control the final steps in the biosynthesis f a metabolite 2. Over-expression of rate-limiting enzymes 3. Silencing of genes whose expression causes the metabolite to be degraded 4. Increased expression of genes that are not subject to metabolic feedback 5. Increasing the number of plastids in a plant. 6. Increasing metabolic flux into the pathway of interest. 7. Expression in storage organs using site-specific promoter e The strategy that has had the greatest success at present is the first one,espe- ally in conjunction with the last strategy. In practice, if a substantial Increas in the concentration of a metabolite is required, the use of specific promoters directing the synthesis to a particular organelle normally used for storage pur poses, or where the plant normally synthesises the metabolite, is essential. Failure to use these approaches could cause toxicity in the plant by interfering with the production or function of other essential metabolites. However, this strategy pre supposes the metabolite of interest is the final one in a particular pathway y. Few strategies have yet been applied where multiple gene insertions are nec essary to produce the metabolite, although these are progressing rapidly, and none where plastid numbers have been increased. However, the accumulation of sequence data of both chromosomal DNA and expressed sequence tags of plants and other species is providing rapid advances in knowledge of the genetic make up and functions of several plants, and it is expected that these other possibili ties will soon be feasible 8.4 The priorities for nutritional enhancement 8.4.1 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 evi- dence suggests there is a need to change current practices. There is very little evi- dence for nutritional deficiencies In those cases where public health authoritie have thought there is a potential problem, food supplementation with nutrients i 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(FigPotential strategies for the enhancement of specific metabolites could target on: 1. Over-expression of enzymes that control the final steps in the biosynthesis of a metabolite. 2. Over-expression of rate-limiting enzymes. 3. Silencing of genes whose expression causes the metabolite to be degraded. 4. Increased expression of genes that are not subject to metabolic feedback control. 5. Increasing the number of plastids in a plant. 6. Increasing metabolic flux into the pathway of interest. 7. Expression in storage organs using site-specific promoters. The strategy that has had the greatest success at present is the first one, espe￾cially in conjunction with the last strategy. In practice, if a substantial increase in the concentration of a metabolite is required, the use of specific promoters directing the synthesis to a particular organelle normally used for storage pur￾poses, or where the plant normally synthesises the metabolite, is essential. Failure to use these approaches could cause toxicity in the plant by interfering with the production or function of other essential metabolites. However, this strategy pre￾supposes the metabolite of interest is the final one in a particular pathway. Few strategies have yet been applied where multiple gene insertions are nec￾essary to produce the metabolite, although these are progressing rapidly, and none where plastid numbers have been increased. However, the accumulation of sequence data of both chromosomal DNA and expressed sequence tags of plants and other species is providing rapid advances in knowledge of the genetic make￾up and functions of several plants, and it is expected that these other possibili￾ties will soon be feasible. 8.4 The priorities for nutritional enhancement 8.4.1 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 evi￾dence suggests there is a need to change current practices. There is very little evi￾dence 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. 198 The nutrition handbook for food processors