**1. Introduction**

Vitamin D is a fat-soluble vitamin utilised by human beings for normal bone development; maintenance by increasing the absorption of calcium, magnesium and phosphate; as well as potential pleiotropic effects on the prevention of cancer, heart disease, autoimmune disease, type 2 diabetes and depression [1].

A circulating level of 25-hydroxyvitamin D greater than 30 ng/mL (50 nmol/L) is considered a healthy level of vitamin D to maintain. About 1 billion people in the world have vitamin D deficiency, ranging from children (leading most notably to rickets), pregnant and lactating women, certain racial groups, post-menopausal women and the elderly (leading to osteoporosis) [2].

Vitamin D deficiency arises from multiple causes including inadequate dietary intake and inadequate exposure to sunlight. About 50–90% of vitamin D is absorbed through the skin via sunlight, whilst the rest comes from the diet.

Vitamin D clinical trials have increased over the past decade. Despite these trials, studies (including meta-analyses) have provided inconsistent conclusions. Some meta-analyses make the assumption that vitamin D is a pharmacological agent, and not as a nutrient. Bolland [3, 4] and Zhao's [5] findings suggest that vitamin D supplementation does not prevent fractures or falls (and in fact one study suggested it may contribute to falls in community-dwelling older women at higher dosages [6],

#### **Figure 1.**

*In many countries, there is an abundance of sunlight throughout the entire year. Ironically, the sun-protective messages for the primary prevention of skin cancers become stronger than those of the moderate benefits of sunlight.*

or have clinically meaningful effects on bone mineral density, and suggested 'little justification to use vitamin D supplements to maintain or improve musculoskeletal health'). A recent umbrella review did conclude that most RCTs have been carried out in populations that are not vitamin D deficient and, because of this, possible beneficial effects from vitamin D supplementation cannot be excluded [7].

Further, complementary medicine supplementation receives less stringent regulation for approval to the general public. For example, in Australia, vitamin D or other supplements which claim that it 'may' improve bone health and general well-being are approved on the Therapeutic Goods Administration as an ARTG [8] compared with vitamin D as a medication.

Given the urgent need for the general population, clinicians and consumers will benefit from the latest evidence for vitamin D supplementation (through diet, extraneous supplementation and sunlight therapy), including its efficacy and potential harm (**Figure 1**).
