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This study investigated whether high-dose vitamin D3 given once to healthy adults before winter will (1) prevent the wintertime decline in vitamin D status, (2) promote vitamin D sufficiency 1 year following the dose and (3) prevent the rise of parathyroid hormone (PTH) concentrations. Subjects/methods: In this double-blind, placebo-controlled trial, we assessed plasma 25(OH)D and PTH concentrations at baseline, 5, 90 and 365 days after drug administration in 28 healthy adults. In all, >80% of subjects returned at each time point.
Plasma 25(OH)D concentrations returned to baseline at 90 and 365 days in the vitamin D3 group and remained unchanged in the placebo group. Pth and calcium concentrations were unrelated to changes in 25(OH)D levels and similar between groups over time.
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Other benefits of vitamin D supplementation are implicated epidemiologically: prevention of some cancers, osteoarthritis progression, multiple sclerosis, and hypertension.
Total-body sun exposure easily provides the equivalent of 250 microg (10000 IU) vitamin D/d, suggesting that this is a physiologic limit. The assembled data from many vitamin D supplementation studies reveal a curve for vitamin D dose versus serum 25-hydroxyvitamin D [25(OH)D] response that is surprisingly flat up to 250 microg (10000 IU) vitamin D/d. To ensure that serum 25(OH)D concentrations exceed 100 nmol/L, a total vitamin D supply of 100 microg (4000 IU)/d is required.
Except in those with conditions causing hypersensitivity, there is no evidence of adverse effects with serum 25(OH)D concentrations <140 nmol/L, which require a total vitamin D supply of 250 microg (10000 IU)/d to attain. Because vitamin D is potentially toxic, intake of >25 microg (1000 IU)/d has been avoided even though the weight of evidence shows that the currently accepted, no observed adverse effect limit of 50 microg (2000 IU)/d is too low by at least 5-fol.
Abstract
The current recommended dietary allowance (RDA) for vitamin C for adult nonsmoking men and women is 60 mg/d, which is based on a mean requirement of 46 mg/d to prevent the deficiency disease scurvy.
However, recent scientific evidence indicates that an increased intake of vitamin C is associated with a reduced risk of chronic diseases such as cancer, cardiovascular disease, and cataract, probably through antioxidant mechanisms. Because the RDA is defined as “the average daily dietary intake level that is sufficient to meet the nutrient requirement of nearly all healthy individuals in a group,” it is appropriate to reevaluate the RDA for vitamin C. Therefore, we reviewed the biochemical, clinical, and epidemiologic evidence to date for a role of vitamin C in chronic disease prevention. This amount is about twice the amount on which the current RDA for vitamin C is based, suggesting a new RDA of 120 mg vitamin C/d.
Commentary On Methods
In the survey reported in 1984 by Jongen et al (22) of 19 laboratories, the mean CV for plasma 25(OH)D was 35%, much of which was attributable to the purification procedures used. Over the past 6 y, an international proficiency survey of >30 laboratories has been available through the North West Thames, External Quality Assurance Survey (Charing Cross Hospital, London). The current between-laboratory CV remains essentially the same when all methods are combined (personal file of survey reports).
Furthermore, the proficiency data available show that selection of any one method would result in only a marginal improvement in the CV of comparisons between laboratories. 25(OH)D values presented here are either published numerical values or they were redigitized from graphical data.