Maximum daily doses for vitamins C and E (in the form of a-tocopherol) and selenium have been recommended by the Institute of Medicine of the National Academies in the US. In a report published on April 10, which examined available scientific data on both the beneficial and harmful effects of these nutrients and of carotenoids, the institute calls for overall increases in the recommended daily doses "to exploit their role in maintaining good health". The report includes a recommendation that smokers take larger daily doses of vitamin C because, in the opinion of the institute, they are more likely to suffer from higher levels of oxidative stress and cellular damage. However, the report also recommends "tolerable upper intake levels", which, it says, represent the maximum intake that poses no risk of adverse effects to the majority of the population. As a general rule, healthy individuals should not exceed the upper intake levels, it says.
With regard to carotenoids, including b-carotene, the report recommends neither a daily intake nor an upper intake level because the authors felt that supplementation with these nutrients should only be given for the prevention and control of vitamin A deficiency. This was because it was unclear whether carotenoids were truly antioxidants in vivo and because data on the adverse effects of consuming too much b-carotene were contradictory, according to Professor Norman Krinsky, who chaired the group that put together the report. He added that most adults meet the recommended amounts of each nutrient through diet alone but recommended "that adults eat more fruits and vegetables that are rich in these nutrients".
Dr Pamela Mason, a pharmacist with a special interest in nutrition, told The Journal on April 17 that the Department of Health had set up an expert group on intakes of vitamins and minerals that would report its findings in 2001 but had made no recommendations, yet. In the meantime, the Council for Responsible Nutrition had suggested maximum doses about two years ago, which were the current UK standard. The Department of Health made no comment on the new US report.
The Food and Nutrition Board of the National Academies has set recommended dietary allowances (RDAs) since 1941. The system for determining these values was updated and expanded in recent years and RDAs are now called dietary reference intakes. More information can be found on the National Academies' website (www.nas.edu).
| The US Institute of Medicine’s recommended intakes | |||
| Substance | Recommended | Upper daily | Adverse effects |
| daily intake | intake level | at high doses | |
| Vitamin C | women, 75mg; men, 90mg; | 2,000mg | Diarrhoea |
| smokers, an additional 35mg | |||
| Vitamin E | 15mg | 1,000mg | Increased risk of |
| (22IU) | (1,500IU) | bleeding | |
| Selenium | 55µg | 400µg | Selenosis (hair loss, |
| nail damage) | |||
| Beta-carotene | n/a | n/a | |