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RDA is the recommended dietary allowance. Vitamins and minerals should not cause side effects when you exceed 100% RDA, but they can cause them when you exceed "the tolerable upper intake" (UI), which is usually considerably higher than RDA. According to Office of Dietary Supplements, the RDA for vitamin C for adults is 90 mg/day and the UI is 2,000 mg/day. ...


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In addition to Jan's answer it is important to recognize that the biological half-life of most vitamins is quite long, on the order of several days. This means that you do not really have a spike when you take a multivitamin: the amount of each vitamin already in your body is much higher than the daily intake amount. It is not actually necessary to take the ...


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Perhaps to emphasize the definition of vitamins: The vitamins are a disparate group of compounds; they have little in common either chemically or in their metabolic functions. Nutritionally, they form a cohesive group of organic compounds that are required in the diet in small amounts (micrograms or milligrams per day) for the maintenance of ...


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In addition to Narusan's comment on exercise, it's important to consider diet too. Sometimes a lack of concentration and mental alertness can result from lower-than-usual energy levels, which can in turn be caused by poor dietary choices. NHS Choices stresses that eating well (as in a healthy, balanced diet) is preferable to taking supplements when trying ...


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