We found that supplementation with 10 μg vitamin D3/day administered for four weeks in late winter led to vitamin D sufficiency (s-25(OH)D > 50 nmol/l) in healthy adults of ethnic Norwegian background, but not completely so in Tamils living in Oslo (59°N). This was due to a very low baseline vitamin D status in Tamils, which we have also observed previously . In spite of the strict exclusion criteria, vitamin D status at baseline was not very low in ethnic Norwegian participants. A large proportion of the study sample were medical and nursing students, probably more health-conscious than the general population, and may have had a relatively high intake of vitamin D intake during winter. However, the supplementation was equally efficient regardless of ethnic background, and led to an overall mean increase in s-25(OH)D of 34.1 nmol/l in the course of four weeks. We found no effect of type of vitamin D supplement (solid multivitamin tablet or fish oil capsule containing n-3 fatty acids) on either increase in vitamin D status  or in the change in parathyroid hormone, the active vitamin D hormone, or osteoclast activity. As far as we are aware, this is the first randomised trial to compare the effect of two different modes of administration of vitamin D3 on markers of the vitamin D endocrine system and bone turnover.
S-25(OH)D above 50 nmol/l has been associated with optimal musculoskeletal function in elderly , and this level is commonly used as cut-off for vitamin D sufficiency in European populations [17–19]. However, some advocate a target s-25(OH)D in the adult European and US population of at least 75 nmol/l for optimal fall and fracture prevention, and this requires daily supplementation of at least 18–25 μg (700–1000 IU) . However, few intervention studies have focused on supplementation in young adult individuals and its influence on markers of bone health.
Unlike other intervention studies with a similar dose of vitamin D [8, 11–13], we observed a significant suppression of PTH as well as increased bone turnover in our study.
Vitamin D supplementation is expected to prevent bone loss mainly by slowing bone resorption. Due to the short follow-up time, we expected any effect on bone turnover likely to be observed in resorption rather than formation. The TRACP isoform 5b is released by osteoclasts early in their differentiation, and this enzyme is specifically a marker of number of osteoclasts rather than of their resorptive activity. In healthy individuals, however, the osteoclast number and their resorptive activity is expected to be highly correlated . TRACP 5b was also studied in the recent Finnish supplementation study in healthy men aged 21–49 years (n = 48) . The investigators observed a decrease in this marker throughout winter (6 months) regardless of whether the participants received placebo, 10, or 20 μg vitamin D. The bone formation marker (bone alkaline phosphatase) decreased in the groups receiving vitamin D supplementation but was unaltered in the placebo group.
The suggested increase in osteoclast activity in our data in spite of the improvement in vitamin D status and corresponding PTH suppression is surprising, but may represent a transient high turnover state with increased bone turnover as a result of increased bone formation. The participants were relatively young, and 40% were below 25 years of age. Peak bone mass may not have been acquired in all participants . An increased bone turnover may thus suggest an improved rate of bone growth in young adults. As bone formation and resorption are coupled processes, increased formation will follow the increase in resorption. A decrease in PTH would be expected to suppress bone resorption, but bone resorption is also stimulated by 1,25(OH)2D. Local production of 1,25(OH)2D in the tissues may have increased, while we have only measured circulating 1,25(OH)2D. There is evidence that 25(OH)D affects osteoclastogenesis and bone turnover (and thereby TRACP production) by local activation in bone cells . In a recent review, it was considered that due to this local activation, circulating levels of 25(OH)D, and not 1,25(OH)2D, may represent the better correlate to parameters of bone health .
There is ongoing research concerning the role of n-3 fatty acids in bone health, as bone loss due to increased osteoclast activity may be mediated by inflammatory cytokines, and some studies suggest a protective effect of n-3 fatty acids on bone resorption [25, 26]. In bone marrow from ovarioectomized mice, addition of eicosapentaenoic acid and docosahexaenoic acid led to significantly reduced TRACP activity and osteoclastogenesis . A protective effect of n-3 fatty acids (0.5 g/day) in fish oil on bone resorption was not supported in our data in young adults, as subjects’ s-TRACP increased regardless of being randomised to ingest fish oil capsules containing omega-3 fatty acids, or solid tablets. The increase in s-TRACP was even slightly higher in the group ingesting fish oil capsules (not shown/not significant).
It cannot be excluded that an independent effect of the supplementation on bone turnover regardless of the observed decrease in PTH could be brought about by factors other than vitamin D. Both intervention supplements contained vitamin A (retinol). Although the evidence of an effect of vitamin A on bone health is inconsistent, some in vitro studies have shown that retinoic acid directly stimulate osteoclastic bone resorption, and high intakes and serum levels of retinol have been associated with reduced bone mineral density or increased fracture risk in some population-based studies .
Our study was a parallel-group intervention and we did not include a placebo group. Due to the late wintertime intervention period we did not expect any notable background increase in vitamin D status. Moreover, any background change in vitamin D status would be the consequence of any unexpected behaviour in relation to diet or sun exposure in both parallel-groups regardless of supplement. Our study sample was restricted to not take a regular supplement other than the intervention supplement, use a tanning bed, or travel to sunny areas. Any extraordinary dietary intake (e.g. large fatty fish dinner) would be noted on the participants’ individual compliance form, and no such behaviour was reported. Also, only those who were compliant defined as consuming at least 26 of the 28 tablets were included in the analyses. Thus, there is reason to expect that any observed change in the vitamin D endocrine system is an effect of the supplementation.
A limitation of the ethnic comparison was the different age distributions between participants with ethnic Norwegian and Sri Lankan Tamil background. Mean age in the two groups differed by 14 years. While more than half of the subjects with ethnic Norwegian background were below 30 years of age, none of those with Sri Lankan Tamil background were below that age. Age is an important predictor of serum PTH , but limited overlap in age between the ethnic groups in our study makes it difficult to rule out the influence of age and ethnic background, respectively.