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Vitamin D and Cancer<br />
But relationships between vitamin D and ethnicity are complex and cannot be simply explained<br />
by lower capacity to synthesise vitamin D in the skin. A study utilizing a single fixed dose of ultraviolet<br />
B radiation noted that baseline circulating vitamin D3 levels in African, white, Oriental and south Asian<br />
Americans were similar but that white and Oriental Americans had significantly higher levels of vitamin<br />
D3 and of 25-hydroxyvitamin D3 post-radiation than black or south Asian Americans (Hypponen and<br />
Power, 2007). However, post-radiation serum levels of 1α,25-dihydroxyvitamin D3 were similar.<br />
African and white Americans exposed to sequentially increasing minimal erythemal doses of UVB had<br />
similar elevations of circulating 25-hydroxyvitamin D3 although baseline levels were much lower in<br />
African than white Americans (Brazerol et al.,1988; Barnes et al.,2006).<br />
In whites, decreasing vitamin D status is associated with increasing body mass index (see 7.3.3),<br />
low bone mass density (Cauley et al., 2008) and diabetes (Scragg et al., 1995). In blacks, vitamin D<br />
status is not associated with obesity (Epstein et al.,1986; Looker, 2005), bone mass density (Hannan<br />
et al.,2008) and diabetes (Scragg et al.,2004).<br />
Limited data also suggests possible ethnic differences in terms of dietary vitamin D absorption or<br />
its endogenous conversion to 25-hydroxyvitamin D. A small study comparing circulating 25hydroxyvitamin<br />
D levels between African and white Americans matched by age, gender and socioeconomic<br />
status showed that African Americans had significantly lower basal levels but a much higher<br />
percentage increase after a single large oral bolus of vitamin D2 (Matsuoka et al.,1995). These results<br />
suggest that in young healthy subjects with darker skin the reduced availability of circulating vitamin D<br />
may be compensated for by higher conversion rates of vitamin D into 25-hydroxyvitamin D by hepatic<br />
enzymes. It may be speculated that, low vitamin D concentrations may enhance the activity of the<br />
hepatic 25-hydroxylase (CPYP27A1), and that low 25-hydroxyvitamin D concentration may enhance<br />
the activity of renal 1α-hydroxylase.<br />
These complex findings from a number of studies suggest notable ethnic differences in the<br />
vitamin D endocrine system. So, overall, if in general, Africans as a group have lower circulating levels<br />
of 25-hydroxyvitamin D than whites, it is not clear whether these differences have a clinical impact in<br />
adults. For example, although circulating 25-hyroxyvitamin D in African Americans could imply an<br />
increased risk for osteoporosis, compared to white Americans, their bone density is greater and rates<br />
of osteoporotic fracture is lower (Perry et al.,1996; Barrett et al.,1999; Aloia et al.,2008).<br />
Thus, it remains to be determined whether circulating 25-hyroxyvitamin D level is a reliable<br />
indicator of ethnic variations in vitamin D status, and if there are different adaptive responses to<br />
varying vitamin D levels by ethnicity. Limited evidence suggests the likelihood of such adaptive<br />
responses (Harris, 2006). PTH promotes renal transformation of 25-hydroxyvitamin D in 1α,25dihydroxyvitamin<br />
D and calcium resorption from bones. Black people are observed to have higher<br />
circulating levels of PTH and greater bone resistance against action of PTH. Black people therefore<br />
tend to have relatively high serum 1α,25-dihydroxyvitamin D levels that, together with greater bone<br />
resistance to PTH, would result in bone mineral content greater than that observed in other ethnic<br />
groups. A greater resistance to intestinal action of 1α,25-dihydroxyvitamin D would prevent<br />
hypercalcemia (Harris, 2006).<br />
Evidence for adaptive mechanisms was also suggested by a 3-year randomised, double-blind,<br />
placebo-controlled trial with vitamin D supplementation at the level of 20 µg per day for the first 2 years<br />
and 50 µg per day for the third year. These supplementations had no effect on bone loss, bone<br />
turnover, or serum PTH levels in postmenopausal (50- to 75-year-old) black women (Aloia et<br />
al.,2005). Mean baseline 25-hydroxyvitamin D concentrations in placebo and vitamin D supplemented<br />
women were 17.2 and 19.2 ng/mL, respectively, but the lack of a vitamin D effect was observed even<br />
in the subset of women with lowest baseline 25-hydroxyvitamin D. Women in both the vitamin D and<br />
placebo groups were given calcium supplements to achieve total daily calcium intakes of 1.2 to 1.5 g<br />
per day, and it has been speculated that the consumption of calcium may have inhibited formation of<br />
the active metabolite of vitamin D and suppressed bone turnover independent of vitamin D status<br />
(Harris, 2006).<br />
If different adaptive mechanisms exist according to ethnicity, their significance for vitamin D<br />
actions other than calcium and bone metabolism remain unknown.<br />
7.4 Interferences with dietary sources<br />
39