Is there a threshold where the body stops absorbing calcium, based on the amount of vitamin D being ingested? (A placebo-controlled, dose-response, randomized, double-blind study)

Based on study:

“Vitamin D supplementation increases calcium absorption without a threshold effect”

John F Aloia, Ruban Dhaliwal, Albert Shieh, Mageda Mikhail, Melissa Fazzari, Louis Ragolia, and Steven A Abrams

The American Journal of Clinical Nutrition (2014)

99(3):624-31

March

Summary:

Maximal calcium absorption in response to vitamin D has been suggested to be a biomarker for vitamin D sufficiency. Currently, there is no evidence of a threshold for calcium absorption based on vitamin D levels. This randomized, double-blind study was conducted to answer the question: “Is there a serum 25(OHD concentration or intake of vitamin D3 above which calcium absorption no longer increases?” 71 healthy postmenopausal women completed this study. The findings indicate that increasing 25(OH)D concentrations may minimally increase calcium absorption. Calcium absorption with a serum 25(OH)D range from 40 to 130 nmol/L is not useful to determine nutritional recommendations for vitamin D.

Key words:

Calcium absorption, postmenopausal, vitamin D

It has been suggested that once calcium absorption is maximized, vitamin D intake or a serum 25-hydroxyvitamin D [25(OH)D] concentration has no further influence. It has also been suggested that declined calcium absorption only occurs when there is a deficiency of 25(OH)D [<10 ng/mL (25nmol/L)]. The link between calcium absorption and vitamin D supplementation remains unclear. For measurement of calcium absorption, double-isotope technique is used as it corrects for calcium recycling. The aim of this study was to answer the question: “Is there a serum 25(OH)D concentration or intake of vitamin D3 above which calcium absorption no longer increases?” (1)

The dual-isotope technique with a calcium intake of 300 mg was used. Initially, the 76 healthy postmenopausal women in this study were assigned randomly to placebo or 800 IU (20 μg), 2000 IU (50 μg) or 4000 IU (100 μg) vitamin D3 for 8 weeks but 5 subjects were lost to follow-up. Mean ± SD age was 58.8 ± 4.9 years. Calcium absorption was performed at baseline and after 8 weeks. At baseline, serum 25(OH)D was 63 ± 14 nmol/L and the mean calcium intake was 1142 ± 509 mg/day. Results showed that 10-week calcium absorption and 10-week serum 25(OH)D concentrations had a similar linear trend with no nonlinearity (P-quadratic effect = 0.35). The variability of the 25(OH)D response was different in terms of dose group (P = 0.002) and it was highest in the 4000-IU/day group. There was a positive association between an increase in serum 1,25(OH)₂D, serum 25(OH)D (P = 0.07) and calcium absorption.

In summary, there was no evidence of a vitamin D threshold for calcium absorption, although increasing 25(OH)D concentrations may increase calcium absorption minimally. Calcium absorption with a serum 25(OH)D range from 40 to 130 nmol/L is not useful to determine nutritional recommendations for vitamin D.

Reference

1. Aloia JF, Dhaliwal R, Shieh A, Mikhail M, Fazzari M, Ragolia L, et al. Vitamin D supplementation increases calcium absorption without a threshold effect. The American journal of clinical nutrition. 2014;99(3):624-31. Epub 2013/12/18.