Endurance athletes are meticulous trainees, spending countless hours planning their training programs and nutrition strategies to successfully complete the “miles” required to achieve their goals. It takes focus and dedication to be a successful distance athlete, yet there is one vitamin in particular even the most studious athlete will likely not replenish adequately to meet their performance goals.
Vitamin D has been gaining significant momentum recently in the research for its ability to influence over 1,000 different genes in the body and subsequently some important performance parameters. Failing to get sufficient vitamin D regularly can negatively impact many endurance parameters, including maximal aerobic capacity (VO2 max), susceptibility to colds and flu, inflammation, recovery, and stress fractures.
A Paleo diet provides the ideal foundation for getting all the vitamins, minerals and essential nutrients you need to perform your best. However, during winter months even athletes following a strict regime will likely fail to meet their vitamin D requirements. If you live north of the 37th parallel, in a city or country with a true winter climate, you can’t get sufficient vitamin D from the sun alone between the months of November to March, a time when endurance athletes are in full training to prepare for the competition season.
In the general population, vitamin D levels are able to predict maximal aerobic capacity or VO2max in adults. The relationship is strongest amongst those with the lowest levels of activity.1 This is significant because half the American population is deficient in vitamin D and low levels are associated with increased cardiovascular disease (CVD) and mortality.2,3,4
Athletes require greater amounts of vitamin D than the general population. Vitamin D receptors are found in muscle, heart and vascular tissue and seem to play an important role in aerobic fitness. Recent studies showed athletes with levels above 35 ng/mL have a much higher VO2max levels than those with levels below 32 ng/mL.5,6 Less than 32 ng/mL is considered insufficient in the general population, while less than 20ng/mL is frank deficiency. While research on vitamin D supplementation (6,000 IU per day) in elite rowers did demonstrate a 12% improvement in VO2max (an extremely significant increase considering the subjects were elite trainees), other studies have failed to find a similar effect.7
Vitamin D’s potential role in performance seems difficult to isolate to one factor alone. In the case of endurance athletes, the large volume of training required lends itself to athletes constantly combatting chronic inflammation, high cortisol stress levels (and subsequently lower testosterone), suppressed immunity and increased risk of stress fractures which can all negatively impact performance.
Vitamin D & Muscular Function
Intense training and long hours on the road or in the pool take their toll on your muscles. Vitamin D plays a key role in calcium signaling in muscle tissue, amino acid absorption, as well as the production of contractile proteins, all crucial for optimal performance.8 Preventing insufficient or low vitamin D levels seems especially important for senior athletes, as vitamin D status is positively associated with increased strength and performance.8
Vitamin D & Immunity
Intense training also takes its toll on your immune system, as endurance athletes classically have lower levels of innate immune cells like macrohpages and neutrophils that act as your “first line of defense” against infection from bacteria and viruses. There is no worse feeling than getting sick before a competition you’ve trained hard for and vitamin D supplementation has been shown to prevent suppression of interleukin-5, a key player in your immune system that helps protect against viral and bacterial infections.9
Vitamin D & Stress Fractures
Stress fractures are a potential performance risk for all levels of runners, occurring in 49% of very irregular runners, 39% of irregular runners, and 29% of active runners.10 It’s especially problematic in female runners. A Scandinavian study of over 14,000 female military personnel showed mild vitamin D supplementation (800 IU daily) was able to reduce stress fracture rates by over 20%.11
In elite athletes, it has been shown that it takes a regular vitamin D intake between 2,000-5,000 IU per day to maintain blood levels above 32ng/mL, a measure shown to reduce bone turnover and risk of fractures12 It would be extremely difficult to achieve these levels in competitive athletes without supplementation, as food sources of vitamin D are sparse and not highly concentrated.
Where To Get Your Vitamin D?
An ancestral approach means looking first to dietary, exercise and lifestyle changes to improve your health and performance. The sun is hands down the best source of vitamin D, with a mere 15 minutes of exposure on only 5% of your skin capable of producing 10,000-20,000 IU of vitamin D. However, the sun is not high enough in the sky north of the 42nd parallel between the months of November and March to adequately raise vitamin D.
Therefore, experts suggest athletes take regular vitamin D3 supplementation during winter to support muscular function, immunity and prevent stress fractures while training intensely in preparation for competition. While the exact dose will vary from person to person, the research suggests approximately 4,000 IU per day is best. Remember, if supplementing at this dose, you must get regular blood tests done with your doctor to accurately monitor levels.
Lastly, adding more Paleo-friendly foods containing vitamin D can help further. You’ll notice in Figure 1 that not many foods contain high concentrations. Your best bets to round out your nutritional arsenal would be herring, sardines, egg yolks, mackerel, beef liver, and mushrooms. Unfortunately, if you live in a city with a true winter climate, food alone won’t meet the needs for athletes.
Figure 1 –
If your goal this year is hitting a new personal best time or qualifying for a key race, then supporting optimal blood vitamin D status greater than 40ng/mL will ensure your muscles, immunity and body are in peak shape come race day. More is not better, but ensuring you meet the daily needs for an athlete is crucial for future performance and preventing injury. Assess your vitamin D status, add more vitamin D rich foods, and find the right supplement strategy to meet your needs. The research shows it might make all the difference!
Dr. Marc Bubbs ND, CISSN, CSCS
(This article originally appeared @ThePaleoDiet.com)
 Ardestani, A et al. Relation of Vitamin D Level to Maximal Oxygen Uptake in Adults. Am J Cardiol. 2011 Apr 15;107(8):1246-1249.
 Zadshir A et al. The prevalence of hypovitaminosis D among US adults: data from the NHANES III. Ethn Dis. 2005 Autumn; 15(4 Suppl 5):S5-97-101. Giovannucci E, et al. 25-hydroxyvitamin D and risk of myocardial infarction in men: a prospective study. Arch Intern Med. 2008 Jun 9; 168(11):1174-80.
 Dobnig H et al. Independent association of low serum 25-hydroxyvitamin d and 1,25-dihydroxyvitamin d levels with all-cause and cardiovascular mortality. Arch Intern Med. 2008 Jun 23; 168(12):1340-9.
 Fitzgerald J, Peterson B, Warpeha J, Wilson P, Rhodes G, Ingraham S. Vitamin D status and VO2peak during a skate treadmill graded exercise test in competitive ice hockey players. J Strength Cond Res. 2014;28:3200–5 Forney L, Earnest CC, Henagan T, Johnson L, Castleberry T, Stewart L. Vitamin D Status, Body Composition, and Fitness Measures in College-Aged Students. J Strength Cond Res. 2014;28:814–24
 Jastrzębski Z. Effect of vitamin D supplementation on the level of physicalfitness and blood parameters of rowers during the 8-week high intensity training. Facicula Educ Fiz şi Sport. 2014;2:57–67.
 Ceglia L. Vitamin D and its role in skeletal muscle. Curr Opin Clin Nutr Metab Care. 2009 Nov; 12(6):628-33.
 Barker T et al. Different doses of supplemental vitamin D maintain interleukin-5 without altering skeletal muscle strength: a randomized, double-blind, placebo-controlled study in vitamin D sufficient adults. Nutr Metab (Lond). 2012 Mar 9:9(1):16
 Barrow G, Saha S. Menstrual irregularity and stress fractures in collegiate female distance runners. Am J Sports Med June 1988 vol. 16 no. 3 209-216.
 Lappe J et al. Calcium and vitamin d supplementation decreases incidence of stress fractures in female navy recruits. J Bone Miner Res. 2008 May;23(5):741-9.
 Dahlquist D et al. Plausible ergogenic effects of vitamin D on athletic performance and recovery. Journal of the International Society of Sports Nutrition (2015) 12:33