Contents - IARC

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