Similar results have appeared in other studies. Jiang et al. (1994) fed diets with vitamin E fortification ranging from 0 to 400 IU per kilogram. Yolk alpha-tocopherol levels rose from 144 µg per gram of yolk with the control diet to 477 mg with the 400 IU fortified diet. In an unpublished study on a commercial operation in the United States, hens received a diet fortified with either 6 or 45 IU of vitamin E per kilogram. Yolk vitamin E levels with the higher level of fortification were 54.02 mg per gram, or three times the 18.75 µg found with the lower vitamin E diet.
Meluzzi et al. (2000) noted that an egg with a vitamin E content of 300 mg per gram would provide roughly 50 to 100 percent of the recommended daily allowances published by various nations for this essential nutrient.
Surai (2000) reported that consumption of vitamin E-enriched eggs as part of normal diet for eight weeks effectively increased the blood levels of alpha-tocopherol in participants. In work with laboratory rats, Yoshizawa et al. (1991) reported higher serum and tissue tocopherol levels in animals receiving vitamin E-enriched egg yolk powder than in control animals. Lipid peroxide levels were inversely proportional to the tocopherol concentrations in the eggs.
Meluzzi et al. (2000) also noted that increased vitamin E content should help maintain egg quality by reducing peroxidation. Scheideler (1998) did indeed report improved egg quality (P < 0.01) following increased vitamin E fortification. In this study, eggs were assessed for oxidative damage by measuring yolk levels of TBARS (thiobarbituric acid- reactive substances), which indicate oxidized fats. Eggs from hens receiving 50 IU of vitamin E per kilogram had only one-eighth the TBARS levels of the hens receiving 10 IU per kilogram.
The egg-quality research by Scheideler was part of a study that also reported increased egg production in hens receiving the higher level of vitamin E fortification. Hen-day production was increased (P < 0.01) by an average of 3 percentage points, from 79 to 82 percent, in birds fed the higher level of supplemental vitamin E before and during exposure to heat stress.
Whitehead (1998) also reported two studies in which egg production increased as vitamin E intake levels rose. In the first, hens received 10, 125 or 500 IU of vitamin E per kilogram and underwent chronic heat stress of 89.6°F (32°C) for four weeks. Although heat stress depressed egg production in all the hens, there was 16.4 percent greater production (P < 0.05) with the highest levels of vitamin E supplementation than with the lowest. Egg production figures were 65.4 and 56.2 percent for 500 and 10 IU of vitamin E supplementation, respectively. The diet containing 125 IU of vitamin E gave intermediate results.
In follow-up work, the decrease in egg production during heat stress was again minimized with increased vitamin E supplementation (Table 2). Performance was optimized at 250 IU per kilogram (P < 0.02). Furthermore, significant differences in egg production by vitamin E treatment were still apparent four weeks after heat stress ended.