have been established for any of the carotenoids including those which do not have provitamin A activity. In conjunction with the review of vitamin A, efforts are under way to establish ratios for the provitamin A carotenoids—β-carotene, α-carotene, and β-cryptoxanthin—based on their ability to be converted to vitamin A. A subsequent report will provide this analysis of the potential contributions of the carotenoids to the requirement for vitamin A.
A risk assessment model is used to derive Tolerable Upper Intake Levels (ULs). The model consists of a systematic series of scientific considerations and judgments. The hallmark of the risk assessment model is the requirement to be explicit in all the evaluations and judgments made.
The ULs for adults for vitamin C (2,000 mg/day based on the adverse effect of osmotic diarrhea), vitamin E (1,000 mg/day of any form of supplemental α-tocopherol based on the adverse effect of increased tendency to hemorrhage), and selenium (400 µg/day based on the adverse effect of selenosis), shown in Table S-4, were set to protect the most sensitive individuals in the general population (e.g., those who might be below reference adult weight). Members of the general apparently healthy population should be advised not to exceed the UL routinely. However, intake above the UL may be appropriate for investigation within well-controlled clinical trials to ascertain if such intakes are of benefit to health. Clinical trials of doses above the UL should not be discouraged because it is expected that participation in these trials will require informed consent that will include discussion of the possibility of adverse effects and will employ appropriate safety monitoring of trial subjects.
The ULs for vitamin C and selenium are based on intake from diet and supplements. Vitamin E ULs are based on intake from supplements only.
A UL could not be established for β-carotene because of inconsistent data and could not be set for other carotenoids because of a lack of suitable data. In both cases, this signifies a need for additional information. It does not necessarily signify that people can tolerate chronic intakes of these substances at high levels. Like all chemical agents, nutrients and other food components can produce adverse effects if intakes are excessive. Therefore, when data are extremely limited, extra caution may be warranted. In particular, β-carotene supplementation is not advisable, other than for the