Vitamin D is a fat-soluble vitamin that is naturally synthesized by the skin following exposure to the ultraviolet rays of the sun.  However, exposure to the sun depends on time outside, the amount of skin exposed, use of sunscreens, geographical location, and the season, so supplementing can provide a consistent dose. 

Vitamin D can give you the ability to lift better, run better, swim better - and aid in recovery.  With most athletes testing low in Vitamin D, why not use it to your advantage?  Be the minority ready to win, give your body the competitive edge and experience less injuries!

Exposure to the sun and the important ultraviolet spectrum that produces vitamin D does not simply happen by being outside.  Exposure is diminished by clouds, the use of sun screens, by the clothing worn that covers your body or your location.  For example, from October through April, above the 45^th parallel, (North Dakota in the USA, or Germany in Europe), the tilt of the earth means that the ultraviolet rays that produce vitamin D are not present. 

If you are training below the 45^th parallel, the window for vitamin D production expands as you move toward the equator.  Every 5 degree change in latitude provides you an additional month of ultraviolet exposure.  For example, if you train in Kenya, you have exposure all year round between 11-1. 

Regardless of geographical location, best exposure is between 11-2.

In the US, vitamin D levels are measured in ng/ml (nano-grams per milliliter).  Outside the US, vitamin D levels are measured in nmol (nano-mols per liter).  The conversion factor between the two measurements is 2.5.  As an example, when converting 120 nmol to ng/ml you would divide by 2.5 (120 nmol/2.5= 48ng/ml).

• Improve muscle function  

A study of NFL athletes found that 81% had levels at or below deficient and those with lower levels had more muscle injuries (Shindle et al, 2011).

Vitamin D levels predicted muscle weakness.  Males with low D levels participating in intense exercise had more weakness three and four days after exercise (Barker et al, 2013).

• Reduce stress fractures  

Vitamin D is the primary regulator of calcium absorption and without sufficient vitamin D there is less calcium available for fracture healing and suboptimal fracture healing.  The administration of vitamin D accelerates the initial mineralization around a fracture (Ray, 2014). 

A study of military recruits in Finland found that a low vitamin D level (<30 nmol or 12 ng/ml) significantly increased the risk of stress fracture (Valimaki et al, 2004; Ruohola et al, 2006).

Royal Marine recruits with a vitamin D level <50 nmol (or 20 ng/ml)  showed an increased risk of stress fractures (Davey et al, 2016)

Japanese male university soccer athletes with vitamin D levels <30 ng/ml (or 12 ng/ml) had a significantly higher risk of stress fracture (Shimaski et al 2016).

A Japanese skateboarder fractured her femur.  Despite surgery to insert titanium nails, after three months, the fracture had not united.  Lab tests confirmed a vitamin D and K deficiency.  She was given 15 mg of MK4 three times a day and 1 iu of vitamin D a day.  Within four months the vitamins united the bones and at 10 months after the fracture, she had no leg length discrepancy or rotation deformity (Igarashi et al, 2024). 
 

• Aid in recovery

Men exercised and were given 4000 iu/day of D3 for six weeks (2 and 7 days post exercise) showed a 14% improved strength recovery (Owens et al, 2015).

Active males engaged in 30 minutes of physical activity at least 3 times per week and were given 4000 ius (100 mcg) of vitamin D for 35 days.  They showed an enhanced muscle strength recovery (Barker et al, 2013).

• Reduce total body inflammation 

CRP (c-reactive protein) is a measure of inflammation.  A British study showed that residents with a deficient level of vitamin D had a significantly elevated CRP score, indicating more inflammation.

Irish residents with a low-deficient level of vitamin D showed higher levels of CRP, which modulates inflammation (Laird et al, 2023; Zhou & Hypponen, 2023).

• Improve cardiac function

A study of national athletes in Qatar showed that 78% had D levels below 30 nmol or below 12 ng/ml.  Those with the lowest levels of vitamin D presented with significantly smaller hearts (Allison et al, 2015).

39 active college students, men and women were studied.  Those with the higher amounts of vitamin D levels, >35 nmol (or 14 ng/ml) had a significantly higher VO2 max than the students with low D levels (Forney et al, 2014).

Elite lightweight rowers were given 6000 iu/day (150 mcg) of vitamin D3 during an 8-week training cycle, and demonstrated a 12.1% improvement in aerobic metabolism (Jastrzebski et al, 2014).

Young soccer players were given 20,000 iu/per week ( 500mcg) of vitamin D for 8 weeks. The soccer players showed a significantly higher VO2max, compared to those receiving a placebo (Brzezia?ski' et al, 2022).

• Improve performance

UK athletes were given 5000 ius/day (125 mcg) of vitamin D for 8 weeks, resulting in a significant increase in their 10 meter sprint times and vertical jumps (Close et al, 2013).

Elite ballet dancers were given 2000 ius/day ( 50 mcg) of vitamin D3 and showed an 18.7% increase in isometric strength and a 7.1% increase in vertical jump, while sustaining significantly few injuries (Wyon et al, 2014).

A study of adolescent Chinese students found that their vitamin D levels were associated with muscle power and force, as measured in jump velocity, height, power and force (Ward et al, 2009).

• Strong immune system

There is mounting scientific evidence suggesting that increasing vitamin D intake decreases the risks of developing chronic diseases, including common cancers (Bodiwala et al, 2003), multiple sclerosis (Hernan et al, 1999), diabetes (Hypponen et al, 2001), rheumatoid arthritis (Merlino et al, 2004), and hypertension (Krause et al, 1998).

A prospective study revealed that, if 25(OH)D concentrations were <50 nmol/L (<20 ng/mL) then there was a 2-fold increased risk of developing colon cancer (Holick, 2004).

• Deficiency

Vitamin D deficiency results in the impairment of calcium absorption and dysfunctional bone remodeling, leading to reduced bone density, fragile bones, and a higher risk of fracture (Bischoff-Ferrari et al, 2009; Cauley et al, 2008).

A 2011 survey of NFL players found that black players had a higher rate of vitamin D deficiency than white players, and those with higher vitamin D levels were more likely to obtain a contract position, while those who were deficient were at greater risk for bone fractures (Maroon et al, 2015).

A meta-analysis of research on athletes showed a prominent vitamin D inadequacy, with 56% of those in the review 56% being low, which varied by geographical location, and which increased for winter and spring seasons and indoor sport activities.  The prevalence of injuries cited was 43% bone related and 37.5% soft tissue related (Farrokhyar et al, 2025).

Chinese adolescent girls were tested for their vitamin D levels and 57.8% were found to be deficient and 31.2% were severely deficient, and presented with significantly lower bone mass and muscle strength (Foo et al, 2009). 

• Levels

There is no agreement on the desired vitamin D levels among professionals. Medical professionals call for ranges of 30 nmol/l up to 110 nmol/l.

Caucasian lifeguards working at least 4 weeks at an open-air swimming pool in St Louis, during May and June, and Hawaiians receiving more than 3 hours of sun exposure per day more than 5 days per week both had D levels above 100 nmol/L (Hollis et al, 2007; Haddad & Chyu, 1971). 

Members of the Masaii and Hadzabe tribes in East Africa with lifelong exposure to abundant sunlight had average vitamin D levels of 115nmol/l, with the highest values being 167 and 171 nmol/l.  These samples were based on traditional populations living near the equator and having much skin exposed (Luxwolda et al, 2012).

However, there are unsettled questions about genetic differences in vitamin D metabolism, which may lead to different desirable levels in different populations (Masterjohn, 2013).

The importance of vitamin D has been highlighted in numerous studies conducted worldwide, and each year brings forth new positive findings. Nutrients work as integral parts of complex metabolic networks, and adequate intake of Vitamin D is essential for all athletes. 

Vitamin K is part of the network for vitamin D.  Vitamin K2 plays an important role in maintaining calcium balances in the body through its activation of proteins that help manage calcium deposits.  A low intake of vitamin K can create an imbalance in how calcium is metabolized in the body.  Adequate intake of vitamin K would be a cofactor for vitamin D (Schurgers et al, 2008).  Vitamin K in conjunction with vitamin D stimulates the transformation of fracture site stem cells to bone building osteoblasts (Gigante et al, 2014).

Testing your vitamin D level is inexpensive and is routinely available at all labs.  It is highly recommended that all athletes assess their vitamin D levels and monitor intake to ensure that your level is coordinated with your training and workouts.

Keep in mind that vitamin D is activated and metabolized by magnesium.  If magnesium levels are low, vitamin D is just stored and is inactive, regardless of how much you take.  Plus, intake of vitamin D without magnesium can increase calcium and phosphate levels which can lead to heart disease. (Unwitonze & Razzaque, 2018).  Either take the recommended dose of magnesium daily, or have your levels tested to ensure that you have sufficient amounts to make your vitamin D work.

 

References

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