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Providing perspectives on recent research into vitamins and nutritionals


Seasonal Changes in Vitamin D Status and Physiological Norms

By Michael McBurney

Watching the news drives home the fact that the northern hemisphere is deep into winter. Barcelona, Spain and Boston, USA are both experiencing snow. Because of inclement weather conditions (and sun block for those in warmer climates), our ability to synthesize vitamin D3 in skin is limited. Without sun exposure, serum 25(OH)D3 concentrations fall  during the winter months.

Scientists wondered if a single very large dose (bolus) of vitamin D might maintain serum 25(OH)D3 concentrations over an entire winter. Kearns and colleagues had 28 healthy adults (78% female) take a  a large single dose of vitamin D3 (250,000 IU) (N = 14) or placebo  (N = 14) orally in the fall (November in Georgia, USA). Serum 25(OH)D3 concentrations were followed from baseline (~41 nmol/L). Vitamin D supplementation increased serum 25(OH)D3 concentrations to ~100 nmol/L within 5-10 days which reverted back to baseline at 90 and 365 days.  For safety purposes, serum calcium and parathyroid hormone concentrations were monitored and did not change. No signs of hypervitaminosis were observed in the high dose group. The study confirms the conclusion by Hathcock et al (2007) that  100,000 IU of vitamin D3 (250 µg) is a safe no-observed-adverse-effect-level (NOAEL).

What more was learned? Although safe, a single bolus does not maintain serum 25(OHD)3 concentrations.  Our bodies need vitamin D to function normally. It makes little sense to allow cells to become deprived of a nutrient for months and then ingest a large bolus to overcome deficits. Excessive swings in serum 25(OH)D3 concentrations doesn’t encompass the concept of balance and moderation.

Vieth and colleagues report it is safe to consume 4000 IU daily (100 µg/d) for months. This dose effectively increases 25(OH)D3 concentrations to high-normal levels within physiologic range in practically all adults. Using 1,000 IU daily (25 µg),  increases serum 25(OH)D3 levels to about 70 nmol/L.  At both intake levels, daily vitamin D supplementation reached steady-state 25(OH)D3 concentration by 2-3 months.

It is much more sensible to supplement vitamin D daily then to expose the body to extreme variations in serum 25(OH)D3 concentrations. Maintain serum 25(OH)D3 levels >50 nmol/L and preferably higher.

Main Citation

Kearns MD, Binongo JNG, Watson D, Alvarez JA, Lodin D, Ziegler TR, Tangpricha V. The effect of a single, large bolus of vitamin D in healthy adults over the winter and following year: a randomized, double-blind, placebo-controlled trial. 2015 EJCN doi: 10.1038/ejcn.2014.209

Other Citations

Vieth R, Chan P-C R, MacFarlane GD. Efficacy and safety of vitamin D3 intake exceeding the lowest observed adverse effect level. 2001 Am J Clin Nutr  73:288-294

Hathcock JN, Hattan DG, Jenkins MY, McDonald JT, Sundaresan PR, Wilkening VL. Evaluation of vitamin D toxicity. 1990 Am J Clin Nutr 52(2):183-202