Inclusion of 95 % range in the model to account for inter-individual variability increased the predicted intake of vitamin D needed to maintain serum 25(OH)D ≥ 50 nmol/l to 930 IU/d .
Effects of Postmenopausal Hormone Replacement Therapy With and Without Vitamin D3 on Circulating Levels of 25-Hydroxyvitamin D and 1,25-Dihydroxyvitamin D
Abstract . The effects of  postmenopausal hormone replacement therapy ( HRT ) and vitamin D3 on vitamin D metabolites ( 25OHD and 1,25(OH)2D ) were studied in a population -based prospect i ve 1-year study . The serum concentrations of intact parathyroid hormone ( PTH ) , calcium , and phosphate were also studied . A total of 72 women were r and omized into four treatment groups : HRT group ( sequential combination of 2 mg estradiol valerate and 1 mg cyproterone acetate ) , Vit D3 group ( vitamin D3 300 IU/day + calcium lactate 500 mg/day ) , HRT + Vit D3 group ( both above ) and placebo group ( calcium lactate 500 mg/day ) . Serum sample s were taken in March – April , when vitamin D formation from sunlight in Finl and is minimal after the dark winter . Serum concentrations of 25OHD increased in the Vit D3 group ( 33.5 % , P < 0.001 ) and in the HRT + Vit D3 group ( 38.2 % , P < 0.001 ) but had not changed significantly in the HRT and placebo groups at the 1-year follow-up examination . Serum concentrations of calcitriol ( 1,25(OH)2D ) increased , however , only in the HRT group ( 23.7 % , P < 0.05 ) , and remained unchanged in other groups . Serum concentrations of PTH decreased by 23.2 % ( P < 0.05 ) in the placebo group , but did not change significantly in the other three groups . The concentrations of serum calcium increased in the nonhormone groups ( P < 0.001 ) , whereas serum phosphate concentrations decreased in the hormone groups ( P < 0.05 and 0.001 ) . Our results confirm the positive effect of 1 year of HRT on serum calcitriol .  Vitamin D3 supplementation increased 25OHD concentrations , but did not affect calcitriol concentrations even though the initial levels were low . Interestingly , the combination of HRT and vitamin D3 did not increase serum calcitriol concentrations as much as HRT alone
Inclusion of 95 % range in the model to account for inter-individual variability increased the predicted intake of vitamin D needed to maintain serum 25(OH)D ≥ 50 nmol/l to 930 IU/d .