Differences in dosing and type of vitamin D supplement
used are apparent and would be expected to have an effect on
reported efficacy outcomes. Cholecalciferol is thought to be
preferable to ergocalciferol because of its effects on 25(OH)D
concentrations and longer half-life.35 Nutritional supplement
forms require enzymatic activation of vitamin D, which is
tightly regulated.3 There exists a class of synthetically derived
vitamin D receptor activators that do not require the kidney
1a-hydroxylase reaction to potentiate an effect.36 Because
these molecules are ingested in active form, they can bypass
enzymatic activation and bind to the array of tissue vitamin D
receptors, eliciting responses in target tissues without regulation.
However, caution should be taken with these compounds
because they can have dangerous hypercalcemic and
hyperphosphatemic effects in the human body. Dosage,
including frequency of delivery, is another highly inconsistent
area of vitamin D therapy; particularly in relationship to
quality-of-life outcomes. In the articles we reviewed, dosages
ranged from 400 IU/day to a single large dose of 300,000 IU.
In addition, route of delivery differs (oral vs intramuscular
injection), which would alter the absorption/metabolism of
vitamin D by the body. These discrepancies make it difficult
to compare study outcomes. Consensus needs to be reached
in this area to improve methodologic quality and comparability
of future study findings. Furthermore, the majority of
studies reviewed were conducted in time spans of