I Phytoene desaturase * (crtl)
Figure 10 Biosynthesis of jS-carotene.
enable a segregation of the antibiotic resistance gene away from the phy-toene synthase and desaturase genes. This would eliminate one of the major issues that are raised when the safety of genetically manipulated foods is discussed. Fortunately, it has already been demonstrated that these plants will still produce /3-carotene because these authors have demonstrated that plants engineered with standard transformation procedures to express only the first two enzymes in the pathway do not accumulate lycopene as would be expected. It seems that the enzymes needed to convert lycopene into /3-carotene are constitutively expressed in normal rice endosperm or are induced when lycopene is formed.
VI. RELATIONSHIP OF STRUCTURE TO NUTRITIONAL QUALITY (BIOAVAILABILITY)
The overall content of a given nutrient in a food is not always a useful indicator of its nutritional value as not all of the nutrient present is absorbed. Nutritionists must concerm themselves with understanding the proportion of an available nutrient that is digested, absorbed, and ultimately utilized. In the case of nutrients or phytochemicals whose beneficial effects are directed toward inhibiting degenerative diseases, it is important to know whether or not the nutrient is reaching the particular target organ and in a form that is active. Otherwise, the claims concerning the health benefits of that chemical would not be justified, especially as it is difficult to demonstrate benefits from long-term human studies.
The diet plays an important role in the uptake of specific nutrients and phytochemicals. Those that are lipophilic are absorbed much more readily from a lipid-rich diet. Frying tomatoes in oil dramatically improves the uptake of lycopene compared with the consumption of fresh tomatoes (49). Raw carrots, which have high levels of provitamin A carotenoids, are poorer sources of /3-carotene than gently cooked carrots (50). The bioavailability of certain trace elements is increased by cooking or processing, such as the increased bioavailability of iron in canned spinach (51).
The chemical form of the phytochemical present in food is very important in determining the uptake through the gastrointestinal tract (Fig. 11). Quercetin-/3-glucoside is more easily absorbed than the aglycone quercetin. Isorhamnetin-/3-glucoside, which is chemically similar to quercetin, differing only by a single methoxyl group, is much more readily absorbed. Flavonoid rutinosides (rhamnosyl 1-6 glucosides) are not easily absorbed (52). Sterol esters are more bioavailable than the free sterols (53,54).
The chemical form of the phytochemical is of profound importance when considering the biological relevance of specific chemicals and their levels in the diet. Although some phenols might be better antioxidants than
lsorhamnetin-4'-0-ß -D-glucoside (OCH3)
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