In the present study we have found a negative association between calcium intake and serum 25(OH)D, except in females with a low vitamin D intake where the association was positive. Furthermore, these associations were seen in subjects with low as well as high serum PTH levels.
Our results are at difference with previous observational studies on calcium intake and serum 25(OH)D [14,15,16,17,18]. There are two likely explanations for this. Firstly, there was an interaction between calcium and vitamin D intakes as well as between genders regarding serum 25(OH)D, and analysing the cohorts without proper stratification as in our study could possibly mask the true relations. Furthermore, the difference in serum 25(OH)D level between those with high and low calcium intakes was approximately 10%, and accordingly, a large number of subjects need to be included, as in our study, to show significant effects.
It is difficult to find a plausible explanation for the mainly negative association between calcium intake and serum 25(OH)D. One could hypothesize that in a situation with a low calcium intake it would be important for the body to conserve its 25(OH)D stores and therefore reduce the 24-hydroxylation of 25(OH)D to the inactive 24,25(OH)2D form. However, low calcium intake and thereby low serum calcium levels stimulate the PTH secretion which increases the renal 1-hydroxylation of 25(OH)D to the active form 1,25(OH)2D which in turn increases the intestinal calcium absorption. This process cause removal of 25(OH)D from the circulation. Additionally, 1,25(OH)2D activates the 24-hydroxylase which starts the degradation and thus the elimination of 25(OH)D . One would therefore expect that a low calcium intake should reduce, and not increase the serum 25(OH)D level. These PTH related mechanisms could therefore theoretically offer an explanation for the positive association between calcium intake and serum 25(OH)D in the females with low vitamin D intake. However, since the same patters of relation between calcium intake and serum 25(OH)D were seen when stratifying these females according to serum PTH levels, this cannot be the explanation.
The main calcium source is dairy products and subjects with a high calcium intake therefore also have a high phosphate intake, which will trigger the secretion of fibroblast growth factor 23 (FGF23). FGF23 will reduce the intestinal phosphate absorption by reducing the 1,25(OH)2D level through increased catabolism of 1,25(OH)2D as well as 25(OH)D . Unfortunately we did not measure serum FGF23 nor serum phosphate and this explanation therefore purely speculative. Another explanation could be that the absorption of vitamin D was reduced by a high calcium intake which thereby would be associated with a lower serum 25(OH)D level. The absorption of vitamin D is assumed to be through passive diffusion as well as by membrane carriers, and may be increased by concomitant fat ingestion . Furthermore, intestinal calcium ingestion may, by the formation of calcium-fatty acid soaps, which are excreted in the faeces, cause a slight reduction in intestinal fat absorption . However, this reduction is less than 5 g/d , which is probably too low to significantly affect the vitamin D absorption and can therefore hardly explain our findings. Additionally, we have no plausible hypotheses for why the positive association between calcium intake and serum 25(OH)D in subjects with low vitamin D intake was seen in women only, since there was no major gender differences regarding calcium and vitamin D intakes, age, BMI and PTH. If our findings are not the results of unaccounted for confounding, we therefore have to postulate the existence of unknown physiological process that regulate the vitamin D metabolism in response to calcium intake. In this regard the concept of the personal vitamin D response index should be considered . According to this concept, the molecular response to supplementation with vitamin D, such as changes in the epigenetic status and the respective transcription of genes, varies considerably between individuals. Hence, the measured serum 25(OH)D may not necessarily reflect the true impact of vitamin D and calcium supplementation on vitamin D metabolism in a given individual.
Our study was purely observational and we can therefore not imply causality between calcium intake and the serum 25(OH)D level. This has to be tested in randomized controlled trials (RCTs) for which there are only a few relevant ones. Thus, in a study by Goussous et al., 52 subjects were given 800 IU vitamin D/d and randomized to 500 mg calcium twice daily or placebo for 90 days. In the calcium group the mean serum 25(OH)D increased 16.2 nmol/L versus 17.4 nmol/L in the control group . Similarly, in a study by McCullough et al. 92 subjects were randomized to 800 IU vitamin D/d, 2000 mg calcium, both or placebo for 6 months. The increase in serum 25(OH)D was almost identical, 25 and 26 nmol/L respectively, in those with and without additional calcium . In a study by Grant et al. on prevention of low trauma fractures, 5292 subjects were randomized to 800 IU vitamin D/d, 1000 mg calcium/d, both or placebo, and serum 25(OH)D measured before and after one year in a subgroup of 60 subjects. In those allocated to combined vitamin D and calcium, serum 25(OH)D rose by 24.0 nmol/L, whereas the increase in those given vitamin D alone was 24.3 nmol/L . And finally, in a study by Cashman et al. including 125 subjects stratified according to calcium intake and given 800 IU vitamin D/d, there was no evidence of a 25(OH)D sparing effect of high calcium intake. However, in the subjects with a low calcium intake (< 700 mg/d) the increase in mean serum 25(OH)D was non-significantly higher than in those with a high calcium intake (> 1000 mg/d) (26.1 nmol/L vs 20.0 nmol/l) .
All of these studies have included relatively few subjects. Given that the effect probably is small and may be different in subgroups, larger studies are needed to settle this question. It is unlikely that a large RCT will be designed specifically for this purpose, but there is at least one large study performed with vitamin D intervention with or without additional calcium and with serum 25(OH)D measurements. Thus, in the study by Baron et al. 2259 participants with recently diagnosed adenomas and no known colorectal polyps remaining after colonoscopy, were assigned to vitamin D 1000 IU/d, calcium 1200 mg/d, both or neither, with recurrence of adenoma as primary endpoint. Serum 25(OH)D was measured at baseline and after 1 year. However, the serum 25(OH)D increase in the vitamin D alone vs the vitamin D plus calcium groups was not (as far as we have been able to find) specifically reported in their publications .
Our study has several limitations. Firstly, as an observational study we cannot conclude about causality. We did not measure serum 1,25(OH)2D and 24,25(OH)2D, nor did we do gene-expression studies, which would have been important from a mechanistically point of view. Our subjects had a mean age of 58 years and only 29.6% were vitamin D deficient (serum 25(OH)D < 50 nmol/L). Our results are therefore not relevant for children at risk of rickets. Although our study was population based, the subjects in the sub-study were highly selected with mean age ~ 70 years and with mean serum 25(OH)D as high as 70 nmol/L. On the other hand, our study also has strengths as we included a large number of subjects, which made it meaningful to do stratified analyses. The observation of a negative association between calcium intake and serum 25(OH)D, at least in subjects with adequate vitamin D intake, was statistically highly significant and might reflect unknown aspects of vitamin D metabolism. Direct or indirect inhibition of 25-hydroxylation and/or stimulation of 25(OH)D degradation by calcium are the most obvious pathways, that potentially could be verified by gene-expression studies.