REVIEW
Vitamin D Insufficiency

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Vitamin D deficiency, which classically manifests as bone disease (either rickets or osteomalacia), is characterized by impaired bone mineralization. More recently, the term vitamin D insufficiency has been used to describe low levels of serum 25-hydroxyvitamin D that may be associated with other disease outcomes. Reliance on a single cutoff value to define vitamin D deficiency or insufficiency is problematic because of the wide individual variability of the functional effects of vitamin D and interaction with calcium intakes. In adults, vitamin D supplementation reduces the risk of fractures and falls. The evidence for other purported beneficial effects of vitamin D is primarily based on observational studies. We selected studies with the strongest level of evidence for clinical decision making related to vitamin D and health outcomes from our personal libraries of the vitamin D literature and from a search of the PubMed database using the term vitamin D in combination with the following terms related to the potential nonskeletal benefits of vitamin D: mortality, cardiovascular, diabetes mellitus, cancer, multiple sclerosis, allergy, asthma, infection, depression, psychiatric, and pain. Conclusive demonstration of these benefits awaits the outcome of controlled clinical trials.

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VITAMIN D METABOLISM

The terminology related to the biochemistry of vitamin D can be confusing. Vitamin D has 2 forms and several metabolites. The 2 forms are vitamin D2 and vitamin D3, also called ergocalciferol and cholecalciferol, respectively. Vitamin D3 is produced in the skin in response to ultraviolet B radiation from sunlight or can be obtained from the diet (ie, animal sources such as deep sea fatty fish, egg yolks, or liver) or from supplements. Few foods naturally have substantial vitamin D content, and

ASSESSING VITAMIN D STATUS

Determination of vitamin D status is not based on measurement of serum 1,25(OH)2D concentrations. Vitamin D status is assessed by measuring the prohormone 25(OH) D, which is an indicator of supply rather than function. The most stable and plentiful metabolite of vitamin D in human serum, 25(OH)D has a half-life of about 3 weeks, making it the most suitable indicator of vitamin D status. In the past, vitamin D deficiency was identified by the presence of bone disease, either rickets or

SOURCES OF VARIATION IN VITAMIN D STATUS

Factors known to influence 25(OH)D levels include race, vitamin D intake, sun exposure, adiposity, age, and physical activity. Even when all the factors known to influence 25(OH)D concentrations are taken into account, most of the individual variation of 25(OH)D values is difficult to explain. Consequently, it is difficult to assess the risk of clinical or biochemical consequences of vitamin D insufficiency in a patient on the basis of concentrations of 25(OH) D alone. The duration of vitamin D

MEASUREMENT OF 25(OH)D LEVELS

Some controversy exists regarding the best method for measuring 25(OH)D levels. Radioimmunoassay has been the most common method reported in the literature and was the method used in some of the large-scale population studies of vitamin D, such as the National Health and Nutrition Examination Survey (NHANES) and the Women's Health Initiative (WHI).

The accuracy of measurement varies widely between individual laboratories and between different assay methods. In one study, identical serum samples

Nutritional Rickets

The classical manifestation of vitamin D deficiency is nutritional rickets, which results from inadequate mineralization of growing bone. Consequently, rickets is a disease of children. Far from being eradicated, nutritional rickets continues to occur throughout the world, with reports from at least 60 countries in the past 20 years.29 In a review of published cases of rickets in the United States, most occurred in children younger than 30 months.30 The vast majority of cases in the United

POTENTIAL BENEFITS OF VITAMIN D

Apart from the deficiency diseases of rickets and osteomalacia, recent evidence suggests other skeletal and nonskeletal benefits of vitamin D. In evaluating the evidence, it is important to recognize the limitations inherent in the study design and methodology. Important issues that apply to vitamin D research include the following:

  • (1)

    Was the study design observational, which can only demonstrate associations and is subject to confounding, or was it an RCT that generally balances unmeasured

INDICATIONS FOR VITAMIN D TESTING

Measurement of serum 25(OH)D levels is indicated in select circumstances. If clinical symptoms of rickets in children or osteomalacia in adults are present, measurement of 25(OH)D levels will confirm vitamin D deficiency. Such testing would be appropriate in adults or children with bone pain, elevated serum alkaline phosphatase or PTH levels, and low serum calcium or phosphorus levels. Persons of advanced age, those with osteoporosis, or those at increased risk of falls or fractures may also

CONCLUSION

Critically evaluating the evidence regarding the purported benefit of vitamin D on a multitude of health outcomes is difficult. The bulk of current data is based on observational, epidemiological studies, which are useful for generating hypotheses but not for proving causality. It is particularly difficult to tease out the effects of confounding variables that relate both to health outcomes and to vitamin D status, such as physical activity, milk intake, and adiposity. Few of the observational

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