A new RCT published by the journal Medicine and Science in Sports and Exercise determined weekly doses of 70,000 IU vitamin D3 reduced serum 1,25(OH)2D3 (calcitriol) levels among athletes, thereby hindering its ability to carry out its biological functions.
Vitamin D is continuing to gain recognition for its role in optimizing health. No longer merely acknowledged for regulating mineral balance for bone health, vitamin D has proven to initiate a physiologic response in 36 tissues of the body. This is because vitamin affects transcription, which is how proteins are made from your DNA. Vitamin D affects the production of thousands of proteins.
A topic of particular interest among the health-conscious community involves the role of vitamin D in muscle strength and athletic performance. Some research has discovered that vitamin D increases the rate of protein synthesis, enlarges muscle fiber size and improves exercise capacity. As a result, vitamin D supplementation has become widely accepted as a necessary tool for promoting peak physical performance.
There is a lack of consensus regarding the ideal dosing of vitamin D for maintaining optimal health. Despite research showing daily dosing is more beneficial in raising vitamin D levels, large weekly vitamin D doses are routinely provided for professional athletes to improve vitamin D status and optimize physical performance. Since research has not established the effect of high dose supplementation on the vitamin D endocrine system, this common practice has become an area of concern. The vitamin D endocrine system consists of the following major metabolites:
- Calcidiol or 25(OH)D: The form of vitamin D that has undergone the first activation step in the liver to become a prehormone. This is the form of vitamin D widely measured to determine vitamin D status.
- Calcitriol or 1,25(OH)2D3: The hormonally active form of vitamin D.
- 24,25-hydroxylase: An enzyme that breaks down circulating 25(OH)D and 1,25(OH)2D3
A new randomized controlled trial hypothesized that high dose weekly vitamin D supplementation negatively regulates vitamin D metabolites among elite athletes.
A total of 46 professional European athletes were included in the study. The participants were randomly divided into two groups: Group 1 received 35,000 IU/week, while group 2 received 70,000 IU/week for 12 weeks. The researchers measured the participant’s parathyroid levels and three major vitamin D metabolites (25(OH)D, 1,25(OH)2D3 and 24,25-hydroxylase) at baseline, weeks 6, 12 and 18.
Here is what the researchers found:
- None of the participants experienced adverse side effects from supplementation.
- Average baseline 25(OH)D levels of the participants was 34 ng/ml.
- Both supplement doses provided a significant increase in serum 25(OH)D and 1,25(OH)2D3 (p = 0.008).
- Four weeks after withdrawal from supplementation, the 25(OH)D levels in group 1 returned to levels similar to baseline, whereas group 2 did not (p = 0.007).
- Group 2 experienced a significant increase in 24,25-hydroxylase at weeks 6 and 12. This remained elevated at week 18, despite the subsequent decrease in 1,25(OH)2D3 at week 12 (p < 0.0001).
- As suspected, intact parathyroid hormone was decreased in both groups by week 6, which remained throughout the duration of the study.
According to these findings, blanket high dose supplementation of 70,000 IU/week generated a threshold effect. Although 25(OH)D levels remained in the healthy range, this large dose caused the body to start increasing production of the enzyme that degrades vitamin D in an effort to maintain balance in the body. As a result, the higher supplementation group experienced a subsequent decrease in activated vitamin D, thereby inhibiting its availability to carry out important biological functions in the body.
This study supports our stance that 5,000 IU/day (35,000 IU/week) is the ideal dosage to maintain healthy vitamin D levels and adequate intracellular supply of activated vitamin D. Remember, 1,25(OH)2D3 acts as a steroid hormone in the body, meaning it regulates gene expression and facilitates proper cellular function in target tissues. Chronic inadequate intracellular activated vitamin D, whether from deficiency or over supplementation, may place individuals at an increased risk for health complications.
The researchers stated,
“These data imply that lower doses of vitamin D3 ingested frequently may be most appropriate and gradual withdrawal from supplementation as opposed to rapid withdrawal may be favorable.”
RCTs comparing various daily vitamin D supplementation doses are needed to determine the ideal supplement routine for maintaining optimal vitamin D status without stimulating 24,25-hydroxylase to break down vitamin D.
Sturges, M. & Cannell, JJ. Large bolus vitamin D dosing decreases activated vitamin D levels among elite athletes. The Vitamin D Council Blog & Newsletter, 2/2017.
Owens, D. J., J. C. Y. Tang, W. J. Bradley,, A. S. Sparks, W. D. Fraser, J. P. Morton, and G. L. Close. Efficacy of High-Dose Vitamin D Supplements for Elite Athletes. Med. Sci. Sports Exerc., Vol. 49, No. 2, pp. 349–356, 2017.