Track Coach editor Russ Ebbets offers a comprehensive account of the important review of the research on Vitamin D and its effect on athletic performance by Dylan Dahlquist, Brad Dieter and Michael Koehle. That original article, by Dahlquist, et al, appeared in the Journal of the International Society of Sports Nutrition, 2015.
By Russ Ebbets, Editor, Track Coach
Vitamin D is one of the four essential fat soluble vitamins along with vitamins A, E and K. Utilized by the body in over 900 gene variants Vitamin D has the unique ability to be produced by the body (Wang, 2005).
Vitamin D is widely used by the body for a multitude of conditions. Studies have shown that Vitamin D can play a role in tumor suppression, neurologic function and cardio-vascular health. Other areas where vitamin D makes an important contribution are with bone health, glucose metabolism and the impacts of exercise induced inflammation (Smith, 2012; Alvarez-Diaz, 2009; Dhesi, 2004; Reddy, 2010; Sukuman, 2015; Schoenmakers, 2013; Close, 2013).
Deficiencies in vitamin D have been linked to psychological problems as diverse as depression (Grudet, 2014), suicidal ideation (Polek, 2014) and cognitive decline (Chei, 2014). Physiologic problems in the body have also been linked to low vitamin D levels. Researchers have seen an increased risk of cancer (Holick, 2006), the long bone softening disease of rickets (McCollum, 1922; Welsh, 2000) and spinal spondyloarthritis (Guillot, 2014). In that vitamin D is intimately linked to bone health it is not surprising that low levels are seen with an increased risk of fractures (Bikle, 2014; Ogan, 2013). Finally, low levels can cause a catabolic effect on muscle tissue that has been linked to decreased strength and subsequently decreased performance (Sato, 2005).
Of particular focus in Dahlquist’s review (2015) are the influences vitamin D can have on athletic participation. It bears repeating that anything that decreases the ability of the muscular system to exhibit strength, metabolize glucose or effects cardio-vascular health would logically have an impact on athletic performance. Numerous studies have researched the role vitamin D may have in athletic performance with both promising and confounding results.
The cause of the endogenous production of vitamin D is exposure to sunlight (Heaney, 2008). It seems almost beyond belief that in spite of this Bendik (2014) found that 88.1% of the world’s population is deficient in vitamin D. This is startling as it was theorized that as little as 20 minutes per day sun exposure to 5% of the body will produce upwards of 10,000-20,000IU per day, an amount well exceeding the daily dosages recommended below (Webb, 2006).
Farrokhyar’s (2014) meta-analysis further confirmed this rampant deficiency with an extensive study of over 2300 athletes in 23 studies and found that 56% of those athletes surveyed were low in vitamin D. One would think that athletic populations would have better access to foodstuffs and play closer attention to their diets. These 81% and 56% deficient statistics are particularly surprising especially with vitamin D produced endogenously.
Vitamin D was specifically reviewed in three areas regarding athletic performance: maximum O2 uptake, recovery and force and power production. There are vitamin D receptors in the heart and vascular network of the body. Both these locations hint that healthy “receptors” bathed in vitamin D’s pre-hormone 25-hydroxyvitamin D (25(OH)D) would be able to utilize oxygen at a greater rate than deficient ones (Reddy, 2010). Several studies (Gregory, 2013; Mowry, 2009; Ardestani, 2011) found a positive correlation between VO2 max and serum 25(OH)D concentrations. One of Dahlquist’s criticisms of these studies is that “confounding influences were not addressed” referring to the fact that the potential influence of other supplements or the ingestion of a multi-vitamin were not controlled for and may potentially have skewed the results.
Maximum O2 Uptake
Some of the studies on athletics have been promising. Fitzgerald (2014) found that higher 25(OH)D serum concentration levels helped maximum O2 uptake in males but not females. Jastrzebski (2014) found that vitamin D supplementation helped improve rower’s maximum O2 uptake slightly (12.8% v 10.3) but the question was raised as to whether this finding would transfer to other sports? Koundourakis (2014) found a positive correlation between vitamin D ingestion and soccer players’ abilities to perform squat jumps, counter movement jumps and noted improved sprint times over 10m and 20m distances.
Recovery is another critical area of concern in competitive sport. Increased levels of vitamin D have been linked to an increased myogenic differentiation and proliferation. This differentiation and proliferation aids muscle protein synthesis (MPS) and decreases the myostatin response that inhibits MPS (Garcia, 2013; Garcia, 2011). A muscle crush study was noted where Wistar rats were given high dose versus low dose (332,000IU/kg v. 33,200IU/kg) amounts of vitamin D. The results showed a significant influence with vitamin D supplementation.
The amounts of vitamin D per kilogram of body weight ingested by the Wistar rats was well above human daily recommendations bringing into question the “transferability” of the study. The high dose group evidenced significantly more phagocytic activity, evidenced improved recovery time, increased tetanic force production and increased twitch force. While it was emphasized that the dosage levels do not relate to humans, in principle the results of a “high” vitamin D dosage for injury recovery seems to be supported (Stratos, 2013).
A second study by Barker, (2013) looked at 28 healthy, moderately active adult males. Participants were given 4000IU of vitamin D daily for 35 days. Their test exercise was to perform 10 sets of 10 depth jumps. While both the vitamin D group and placebo groups lost power output following the eccentric jumps the vitamin D group lost significantly less than the placebo taking control group (-6% v. -32%).
Force and Power Production
Vitamin D has also been shown to have a positive impact on force and power production (Ogan, 2013). Three authors found an increase in muscle size and an increase in type II muscle fibers (Sato, 2005; Todd, 2015; Ceglia, 2013). The problem with these findings have only been confirmed on studies using 65+ year-old females (Ceglia, 2013).
A second study on force and power looked at the effects of vitamin D supplementation on 10 male soccer players (Close, 2013). Participants were able to increase their vertical jump performance and reduce 10m sprint times. Interestingly one had to have the increased supplementation. Those that were given moderate doses showed no significant benefit. (Close, 2010; Fitzgerald, 2014; Forney, 2014)
Low testosterone levels (aka low T) is a reality for aging males. Low T is seen as causing decreased protein anabolism, decreased strength, decreased fat metabolism and leading to an increase in fat deposition (Mauras, 1998). Wehr (2010) conducted a large study (N = 2299) on males 62 +/- 11 years and found only 11% had adequate vitamin D levels. The participant population also had significantly lower mean levels of vitamin D than the rest of the population.
A second testosterone study by Pilz (2011) was a 12-month double-blind study with 54 non-diabetic males who consumed 3332IU/day. Results showed that levels of 25(OH)D were raised and that total testosterone, bioactive testosterone and free testosterone were all elevated. It was inferred that the presence of serum 25(OH)D may enhance endogenous testosterone production.
The mechanism of the 25(OH)D on testosterone is seen as inhibiting the gradual decrease of testosterone and the enhancing androgen binding that takes place. Ultimately this increased binding leads to increased concentration of the hormone with the result being increased muscle hypertrophy, strength and power (Kinuta, 2000).
Sources of Vitamin D
Vitamin D comes from two sources–endogenously produced from exposure to the sun and from the diet. The recommendations for sun exposure are that one get 5-20 minutes exposure to 5% of the body. This should be done 2-3 times per week. It was noted that 15 minutes of exposure could endogenously produce 10-20,000IU of D3 (Holick, 2001; Heaney, 2008). Dahlquist’s review noted several “complicating” factors that may affect this number including: seasonal variations, altitude, cloudy climates, darker skin, obesity and the use of sunblock.
Attaining vitamin D from the diet can come in two ways, from the foods one eats and from supplementation. D2 is the less desirable way to ingest vitamin D. Vitamin D2 is for the most part plant based and is not as well utilized by the body. D2 is seen as less stable, less bioavailable with age and less well absorbed (Tripkovic, 2012; Houghton, 2006; Logan, 2013). Various “fortified” foods are available (Bikle, 2014; Holick, 2007) that include: milk, cereal, margarine and synthetic analogues.
Vitamin K, one of the four fat soluble vitamins mentioned above is regulated by vitamin D. Vitamin K plays a key role in healthy bones and works synergistically with vitamin D (Kidd, 2010). Low levels of vitamin K have been linked to increased calcium release from the bones with its deposition in the vascular soft tissues leading to arterial calcification and hypercalcemia (Akiyara, 1994; Masterjohn, 2007; El Asmar, 2014; Hamidi, 2014; Iwamoto, 2014). Vitamin D toxicity is only possible if there is a concomitant vitamin K deficiency (Hamidi, 2014; Iwamoto, 2014).
The recommended daily dose of vitamin K ranges from 50mcg to 1000mcg (Binkley, 2002). There are two forms of vitamin K, K1 and K2. Vitamin K1 is found in vegetables, fruits, oils and beans. Vitamin K1 has been shown to effectively help blood clotting proteins (Fusaro, 2011) and prevent bone loss in female marathoners (Craciun, 1998).
Vitamin K2 is found in fish, offal, meat, dairy, blue cheese and fermented soybeans. Vitamin K2 helps prevent soft tissue calcification (Fusaro, 2011). Mega doses of a vitamin K2 variant, MK4 has been shown to prevent osteoporosis in menopausal women when used in conjunction with vitamin D3 (Suda, 2003; Akiyara, 1994). It is Dahlquist’s conclusion that the combination of these two vitamins warrants further research to determine the ideal dosing levels.
As mentioned above one of the caveats with taking the fat-soluble vitamins is the problem of toxicity. Traditionally it was practiced that chronic ingestion of greater than 10,000IU of vitamin daily would lead to hypercalcemia (Heaney, 2008; Cannell, 2008). To date, due to the ethical concerns of human experimentation, no studies have proven this point.
Van den Ouweland (2014) reported a case study of an accidental overdose of 2,000,000IU by two elderly patients with the only side effect being elevated calcium levels. It should be noted that this overdose was a “one-off’ time and not a regular occurrence. Hypercalcemia has been reported when daily ingestion reached 40,000IU (>200nmol/L) while serum levels below 140nmol/L (28,000IU) did not cause hypercalcemia (Suda, 2003).
Daily recommendations have been given by the Institute of Medicine (IOM) and by Endocrine Society (ES). The IOM recommends a more general 400-800IU/day (50nmol/L) for children, adults and those older than 70 years of age. The ES breaks down their recommendations as follows: infants 400-800IU/day, child 600-100IU/day, adult 1500-2000IU/day to maintain serum levels of 75nmol/L. It was recommended that 70nmol/L be the lowest serum level to prevent noticeable health effects and it has recommended ingestion of 90-120nmol/L to replicate conditions of “sunlight rich environments” (Veith, 1999; Bischoff-Ferrari, 2006).
While specific studies are sparse on athletic populations and vitamin D usage there are some inferences that can be made from the available data. Decreased levels of vitamin D have been equated with an increased incidence of stress fractures (Heaney, 2011). Optimal bone health requires between 2000-5000IU/day creating a serum concentration level of 75-80nmol/L of 25(OH)D. Ingestion of 1000-3000IU/day is seen as inadequate (Logan, 2013). Maintenance of serum 25(OH)D levels ≥100nmol/L would require 6450+IU/day which is significantly more than was used in the studies mentioned in Dahlquist’s review.
The old caveats about taking too much fat-soluble vitamins seems to have lost the threat of toxic overdose. With one researcher reporting 88.1% of the population deficient in vitamin D the conversation seems to have shifted towards increasing supplementation.
While research is in its early stages indications seem to point out that vitamin D does have a positive impact on the oxygen uptake for endurance sports and muscle protein synthesis for speed and power sports.
Further research should also look to better understand the synergism between vitamin D and vitamin K. The synergism between vitamin D and vitamin K can play a central role in the prevention of stress fractures (Kidd, 2010) and the prevention of osteoporosis in menopausal women (Suda, 2003; Akiyara, 1994), a concern for the masters athlete. No doubt both vitamin D and vitamin K will come to be seen as important components in the prevention of the Female Athletic Triad (amenorrhea, osteoporosis and anorexia).
A final note would be that if one’s life is viewed as a “endurance sport” vitamin D may come to be seen as an “anti-aging” vitamin as it potentially allows one to pursue one’s goals longer and more completely due the enjoyment of improved health.
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