Vitamin D is a steroid hormone with a wide ranging nutritional role. Vitamin D can be synthesised from cholesterol in the skin in a reaction catalysed by ultraviolet light or can be derived from the diet. Vitamin D is not a true vitamin as it is only conditionally essential. In this regard, if adequate sunlight exposure is obtained there is no requirement for a dietary intake. However, many people living at high latitudes both north and south of the equator do not get adequate sunlight during the winter months and so without adequate dietary intake plasma concentrations begin to fall. Further many individuals do not obtain adequate sun exposure during the summer, and in combination with low winter synthesis rates, these individuals often become deficient in vitamin D. Although clinical deficiencies of vitamin D are rare, subclinical deficiencies (insufficiencies) are much more common, and can increase the risk of a number of Western lifestyle diseases such as cancer, cardiovascular disease, diabetes and obesity.
Vitamin D is converted to 25-hydroxyvitamin D in the liver and then enters the plasma. Plasma (or serum) levels of 25-hydroxyvitamin D are the biologically accepted marker for vitamin D. Recent research into the biological effects of vitamin D on immunity and insulin function has culminated in recommendations that plasma 25-hydroxyvitamin D levels remain above ~40 ng/mL (100 nmol/L), something that is only possible with access to strong sunlight. Without sunlight supplements must be taken as dietary sources of vitamin D are not commonly eaten, and those foods that do contain vitamin D might not be able to raise plasma levels to their optimal point. Many therefore choose to take supplements, and around 2000 IU is recommended for adults to obtain optimal plasma levels, however some may required greater intakes than this. Therefore supplementation is guesswork without a concomitant series of blood tests to assess the plasma levels. This is cost prohibitive in most cases and also inconvenient.
It might therefore be beneficial to be able to predict plasma 25-hydroxyvitamin D levels though the assessment of other more easily measured or observed criteria. To these ends, researchers1 have investigated the risk profile that could be used to identify older individuals at high risk of vitamin D insufficiency. Over 1000 elderly subject had their serum levels of 25-hydroxyvitamin D measured and then the researchers used backward logistic regression to create models that could be used to accurately predict serum 25-hydroxyvitamin D concentrations of less than 20 ng/mL (50 nmol/L) and less than 12 ng/mL (30 nmol/L). The results showed that 46.2 and 17.5 % of the subjects had serum 25-hydroxyvitamin D concentrations below 20 and 12 ng/mL, respectively. The model for predicting serum 25-hydroxyvitamin D concentrations below 20 ng/mL included 13 predictors, whereas the model for predicting serum 25-hydroxyvitamin D concentrations below 20 ng/mL included 10 predictors.
Predictors of a serum 25-hydroxyvitamin D concentration of less than 25 ng/mL included older age, being female, a body mass index of over 30 kg/m2, smoking, alcohol consumption of more than 13 drink per week, winter months, no vitamin D supplement use, no bicycling, no sport, no gardening, medication use, poor appetite and being single. For a 65 year old, the probability of a vitamin D deficiency ranged from 2 to 85 % when none or all of these predictors were present, respectively. However, for an 85 year old, these probabilities increased to 6 to 94 %. Prediction of a serum 25-hydroxyvitamin D concentration of less than 12 ng/mL included older age, smoking, alcohol consumption of more than 13 drinks per weeks, winter months, no vitamin supplement use, no bicycling, no gardening, medication use, public transport use, and the inability to remember the present year. For a 65 year old, the probability of a vitamin D deficiency ranged from 0.2 to 45 % when none or all of these predictors were present, respectively. However, for an 85 year old, these probabilities increased to 0.8 to 72 %.
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