Vitamin D has longer than expected half-life: Tests may not show true 'plateau'

By Nathan Gray

- Last updated on GMT

Vitamin D has longer than expected half-life: Tests may not show true 'plateau'

Related tags Bone mineral density Vitamin d

Tests to assess levels of vitamin D status months or years after supplementation finishes may not be showing a true rise - or plataeu - in vitamin D status, according to new research that suggests it may take up to three years for stocks of the vitamin to be fully utilised.

Writing in Nutrition Journal​, the team reported on the findings of a single-arm clinical trial that assessed the effect of 12 months consumption of high-dose vitamin D3-fortified bread in institutionalised seniors over a three year follow-up period.

Led by Veronica Mocanu from Grigore T. Popa University of Medicine and Pharmacy in Romania, the research team measured serum 25-hydroxyvitamin D (25(OH)D), parathyroid hormone (PTH) and bone mineral density at one year and three years after stopping consumption of the fortified bread - finding that although measures of vitamin D status remained high after one year, serum levels of vitamin D returned to baseline levels over the course of three years.

"The present report is to our knowledge the longest follow-up after discontinuation of vitamin D, and it does show a sustained biochemical benefit to the one-year point, in that the serum 25(OH)D exceeded 50 nmol/L. However, by follow-up Year 3, the benefits of vitamin D fortification, were essentially gone,"​ said Mocanu and her colleagues.

"Previous studies suggests that when vitamin D supplementation is stopped, the serum 25(OH)D concentrations exhibit an apparent half-life of two months. However, the decline in serum 25(OH)D observed here exceeded 1 year ... therefore, the apparent half life can be substantially longer than what has been previously estimated,"​ the team added.

Study details

The single-arm clinical trial initially enrolled 45 patients (28 women and 17 men), aged between 58 and 89, all residing in a nursing home located in Iasi, Romania.

At the end of the one-year supplementation protocol (consumption of bread buns fortified with 5,000 IU vitamin D3 and 320mg elemental calcium per daily serving) 33 patients were assessed by Dual-energy X-ray absorptiometry (DXA). The initial findings from this study were reported in 2005 and can be found here​.

The researchers then followed the 23 of these participants, and re-tested serum 25-hydroxyvitamin D (25(OH)D), parathyroid hormone (PTH) and bone mineral density (BMD) at one year and three years after the fortification protocol had been completed.

Mocanu and her colleagues reported that at the end of the one-year fortification phase, average serum 25(OH)D was 127.3 nmol/L. At the one-year follow-up, serum 25(OH)D was 64.9 nmol/L; while three-year follow-up it was 28.0 nmol/L.

Similar findings were true for serum PTH and bone mineral density, with PTH levels recorded to be 18.8pg/ml at the end of fortification while at year three values were reported at an average of 48.4 pg/ml.

Lumbar spine BMD did not change from baseline to year three, however, by year three hip BMD had decreased significantly (from 0.927 g/cm2 to 0.907).

The researchers said their findings offered two important lessons for the future: 

"Firstly, that vitamin D nutritional status exhibits a long half-life in the body, whereby follow-up testing even at 1 year after a change in dose may not reflect a true plateau (i.e. steady-state) value."

"And secondly, that when a need for nutrient is established, even if this recognition is as part of a research protocol, then the appropriate action is to institute corrective measures beyond the extent of the research protocol."

Source: Nutrition Journal
Published online ahead of print, doi: 10.1186/1475-2891-12-137
"Three-year follow-up of serum 25-hydroxyvitamin D, parathyroid hormone, and bone mineral density in nursing home residents who had received 12 months of daily bread fortification with 125 mug of vitamin D3"
Authors: Veronica Mocanu and Reinhold Vieth

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