work in the heat are increased beyond the levels recommended in the 1989 RDAs. The folate allowance was lowered in the tenth edition of the RDAs because it was recognized that diets containing approximately half the RDA listed in the ninth edition maintained both an adequate folate status and ample liver stores (NRC, 1989b). Similarly, the RDA for vitamin B12 in the tenth edition was reduced by one-third for the adult age groups. The committee that wrote the RDAs commented that this was a conservative approach, which left the recommendation at approximately twice the level deemed safe by the Food and Agricultural Organization of the United Nations (FAO, 1988). The MRDAs (see Table 1-1) directly reflect the values of the ninth edition and presumably can be revised downward in a similar fashion without undue concern about the levels needed for work in hot environments.
There is some evidence that increased intake of vitamin C may help to reduce heat stress during acclimatization, particularly in those individuals who may have low intakes that are nevertheless considered to be in the adequate range. There is some limited evidence that excess vitamin C may adversely affect the absorption of vitamin B12. The recommended dietary allowance for vitamin C remained at 60 mg per day in the tenth edition of the RDAs (NRC, 1989b). Vitamin C levels in the MRDAs directly reflect the RDAs for this nutrient (see Table 1-1). More research is needed before any conclusions can be drawn.
At present there is no evidence that requirements for fat-soluble vitamins increase for people working in hot environments. Vitamin D levels appear adequate for work in hot environments, and the exposure to sunlight in these climates would likely be adequate to meet any increased need that might exist.
Vitamins A and E, as well as vitamin C, function as antioxidants and may be useful in the reduction of lipid peroxidation induced by exercise stress. However, there are no studies that have adequately examined this issue. Intakes of vitamins A, D, and E, as recommended in the 1989 RDAs, appear adequate to meet the requirements for military personnel performing their duties in hot environments. The MRDAs are based directly on the 1980 RDAs; the 1989 revision of the RDAs for these vitamins did not change appreciably (see Table 1-1). Further studies on whether these vitamins will be important as antioxidants for those living and working in a hot environment are warranted.