| Summary: | 博士 === 國立臺灣大學 === 動物科學技術學研究所 === 94 === The role of folate in health maintenance has moved beyond the prevention of macrocytic anemia. Suboptimal folate status is now associated with neural tube defects, cardiovascular disease and certain cancers. In light of evidence supporting a need for humans to increase their dietary folate intakes, experiments were conducted to evaluate the extent to which egg folate levels could be increased, and to compare the bioavailability between reduced and oxidized forms of folate.
In experiment 1, a 5 weeks folate depletion protocol was employed, using 13-week old Hyline pullets assigned to receive a semi-purified folate-deficient diet or a commercial corn-soybean meal based diet supplemented with 1 mg/kg pteroylglutamic acid (PGA). Growth performances were greatly depressed, erythrocyte, hemoglobin and hematocrit were all markedly decreased in the folate deficient birds whereas mean cell volume was increased. A 4 weeks folate depletion period was judged to be optimal on the basis of the development of nadirs in both plasma and hepatic folate stores and elevated concentrations of plasma homocysteine.
In experiment 2, pullets previously maintained on a folate-devoid diet for 4 weeks, were used to compare the bioavailability of PGA with natural tetrahydrofolate (THF) from digested bacterial cell powder after a single forced feeding for 4 weeks repletion. Both forms of folate supported increases in plasma and hepatic folate stores, whereas the reduced folate lowered plasma homocysteine concentrations further than the oxidized form PGA. These data also support the use of both plasma folate and homocysteine as sensitive response criteria for folate status in pullets.
In experiment 3, Hyline W-36 laying hens received a corn-based diet, containing 0, 5 mg/kg of PGA or THF, to establish the potential for folate incorporation into table eggs. Results indicated that the level of folate in the yolk reached a maximum by week 3 after folate supplementation. Fortification of dietary folate led to an increase in egg folate content of approximately 2.5 fold, from 20 µg folate/egg to 50 µg folate/egg and the reduced form of folate increased more.
In experiment 4, 72 hens were divided into 12 treatment groups and received diets supplemented with 0.5, 1, 2, 4, 8, or16 mg folate/kg as either PGA or THF, in an attempt to establish a dose response relationship between dietary and egg folate levels. Supplementation of dietary folate increased both folate content of plasma and egg, with the most sensitive responses observed between 0.5 and 4 mg of folate/kg diet. There was no significant difference in egg folate concentrations between 4 and 16 mg folate/kg diet supplementation. Furthermore, THF treated hens increased the plasma and egg folate levels higher than the PGA.
In conclusion, laying hens have highly efficient conservation and delivery systems for folates. Enrichment of eggs with folate is possible when dietary folate levels are increased. The concentration of folate in the yolk is dependent on the concentration of dietary folate and that it is saturable. Maximal egg folate levels established at approximately 4 mg folate/kg diet. THF might be preferable for the supplementation of producing high folate eggs. Increasing the folate content of eggs may position the egg as an important source of human dietary folate and lead to an improvement in consumer acceptance of this commodity as a healthful product.
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