RDA

As for other biological substances, states of dynamic equiUbrium exist for the various mineral nutrients as well as mechanisms whereby a system can adjust to varying amounts of these minerals in the diet. In forms usually found in foods, and under circumstances of normal human metaboHsm, most nutrient minerals are not toxic when ingested orally. Amounts considerably greater than the recommended dietary allowances (RDAs) can generally be eaten without concern for safety (Table 1) (3). 

Dietary Copper. Analytical data indicate that many diets contain less than the RDA for copper (109). Excessive copper has been reported to be fatal for oral dose levels of copper sulfate of 200 mg/kg body weight for a child and 50 mg/kg for adults. 

The Rheometric Scientific RDA II dynamic analy2er is designed for characteri2ation of polymer melts and soHds in the form of rectangular bars. It makes computer-controUed measurements of dynamic shear viscosity, elastic modulus, loss modulus, tan 5, and linear thermal expansion coefficient over a temperature range of ambient to 600°C (—150°C optional) at frequencies 10 -500 rad/s. It is particularly useful for the characteri2ation of materials that experience considerable changes in properties because of thermal transitions or chemical reactions. 

Along with increasing evidence of health benefits from consumption of vitamins at levels much higher than RE) A recommendations comes concern over potential toxicity. This topic has been reviewed (19). Like all chemical substances, a toxic level does exist for each vitarnin. Traditionally it has been assumed that all water-soluble vitamins are safe at any level of intake and all fat-soluble vitamins are toxic, especially at intakes more than 10 times the recommended allowances. These assumptions are now known to be incorrect. Very high doses of some water-soluble vitamins, especially niacin and vitamin B, are associated with adverse effects. In contrast, evidence indicates that some fat-soluble micronutrients, especially vitamin E, are safe at doses many times higher than recommended levels of intake. Chronic intakes above the RDA for vitamins A and D especially are to be avoided, however. 

It is generally assumed that adequate vitamin levels in humans can be obtained through a balanced diet. However, ongoing studies continue to indicate that the majority of the U.S. population is not receiving even the RDA through diet. Supplementary vitamins are thus provided for fortification of foods (20) and as oral or parenteral dosage forms. 

The most recent RDA has included a vitamin C recommendation of 100 mg/day for cigarette smokers. An increasing number of investigators have concluded that the current RDA for vitamin C may not be adequate for elderly individuals. Plasma vitamin C level is generally accepted as an indicator of vitamin C status. 

A recent study recommended that the current RDA be increased from 60 mg/d to 200 mg. The researchers indicated, however, that vitamin C daily doses above 400 mg have no value (163). 

The amount of foHc acid required for daily iatake is estimated based oa the minimum amouat required to maintain a certaia level of semm folate. The recommeaded dietary allowance (RDA) for foHc acid accounts for daily losses and makes allowances for variation ia iadividual aeeds and bioavailabiUty from food sources (85). The U.S. recommended daily allowance for adults is 400 p.g and for pregnant women is 800 ]1 (Table 4). 

The RDA for niacin is based on the concept that niacin coen2ymes participate in respiratory en2yme function and 6.6 niacin equivalents (NE) are needed per intake of 239 kj (1000 kcal). One NE is equivalent to 1 mg of niacin. Signs of niacin deficiency have been observed when less than 4.9 NE/239 kj or less than 8.8 NE per day were consumed. Dietary tryptophan is a rich source of niacin and the average diet in the United States contains 500—1000 mg of tryptophan. In addition, the average diet contains approximately 8—17 mg of niacin. In total, these two quantities total 16—34 NE daily. Table 5 Hsts the RDA and U.S. RDA for niacin (69). 

Pantothenic acid is widely distributed in food and because of the lack of conclusive evidence regarding quantitative needs, a recommended dietary allowance (RDA) for pantothenic acid has not been estabUshed. In 1989, the Food and Nutrition Board of the United States National Research Council suggested a safe intake of 4—7 mg/d for adults. The provisional allowance for infants is 2—3 mg daily (90). 

The typical U.S. daily diet contains 1.1—3.6 mg of vitamin B, most coming from meats and vegetables. Poor diets may provide less than half of these amounts and less than the RDA. Some populations require higher amounts persons with high protein intakes, pregnant and lactating women, users of oral contraceptives, alcohoHcs, users of dmgs which interfere with vitamin B function, and those afflicted with some diseases. Several reviews have examined the relationship of vitamin B and specific diseases in more detail (4,23). 

Thiamine requirements vary and, with a lack of significant storage capabiHty, a constant intake is needed or deficiency can occur relatively quickly. Human recommended daily allowances (RDAs) in the United States ate based on calorie intake at the level of 0.50 mg/4184 kj (1000 kcal) for healthy individuals (Table 2). As Httle as 0.15—0.20 mg/4184 kJ will prevent deficiency signs but 0.35—0.40 mg/4184 kJ are requited to maintain near normal urinary excretion levels and associated enzyme activities. Pregnant and lactating women requite higher levels of supplementation. Other countries have set different recommended levels (1,37,38). 

Animals cannot synthesize vitamin A-active compounds and necessary quantities are obtained by ingestion of vitamin A or by consumption of appropriate provitamin A compounds such as P-carotene. Carotenoids are manufactured exclusively by plants and photosynthetic bacteria. Until the discovery of vitamin A in the purple bacterium Halobacterium halobium in the 1970s, vitamin A was thought to be confined to only the animal kingdom (56). Table 4 Hsts RDA and U.S. RDA for vitamin A (67). 

Requirement. A daily intake of 1 pg should cover the daily loss of vitamin and maintain an adequate body pool. The RDA (34), however, has been estabHshed at 2 pg/day to cover metaboHc variation among individuals and to ensure normal semm concentrations and adequate pool sizes (Table 2). 

Table 2 Hsts the Recommended Dietary Allowances (RDA) for vitamin K. Although manufacture by intestinal bacteria represents a significant source of plasma menaquinone concentrations, rehance on this source alone is not sufficient to maintain healthy concentrations of menaquinone. Consequendy, dietary supplementation is necessary (76).

X) Conaideradon of the joist design is important withre rda to processing andservice issues relating to ... 

Threshold Limit Value Ceiling (TLV-C) The concentration that RDA should not be e, cceded during any part of the working e. posure. If conventional industrial hygiene instantaneous monitoring is not feasible, then the TLV-C can be assessed by sampling over a 15-minute period except for those substances that may cause immediate irritation when exposures arc short. 

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