~ Annie Bothma, Elite Athlete, IOPN Performance Sports Nutritionist (EQF Level 7. Masters), Running & Strength Coach
Vitamin D, often called the “sunshine vitamin,” has been known for its role in bone health for decades. But recent research highlights that this powerful nutrient does far more than protect bones. Vitamin D is essential for muscle function, immune defense, and reducing inflammation, making it especially valuable for athletes. Low levels of vitamin D are linked to increased risks for health issues like hypertension, arthritis, and even certain cancers, all which can impact athletic performance and longevity.
In my practice as a sports nutritionist, I often encounter athletes with vitamin D levels in the 20s ng/mL—far below the optimal range for health and performance. Many of these athletes have been told by their doctors that their levels are “fine” and nothing to worry about because they haven’t dropped below 20 ng/mL, the clinical threshold for deficiency. However, this cut-off was originally established as a baseline to prevent severe outcomes like osteoporosis, not to support a strong immune system, maintain healthy bones, or enhance recovery as an athlete.
As a healthcare professional with a master’s degree in sports nutrition, it’s frustrating to see these athletes given such limited guidance. Often, they come to me already dealing with bone stress injuries, recurring illnesses, or persistent recovery struggles. There’s a big difference between being within a “normal” range on a lab form—which often reflects the general population, including those with health issues—and being at an optimal level for athletic performance and resilience.
This blog post reviews why vitamin D is so vital for athletes and provides practical steps on assessing and optimizing vitamin D levels.
Vitamin D Synthesis, Sources, and Status
Synthesis and Metabolism
Vitamin D can be produced by the body through sun exposure. Specifically, UVB rays trigger a process in the skin that converts a compound called 7-dehydrocholesterol into cholecalciferol (vitamin D3). This form of vitamin D then travels to the liver, where it is converted into 25-hydroxyvitamin D (25(OH)D), the main biomarker used to assess vitamin D status. In the kidneys, 25(OH)D is further converted to its active form, 1,25-dihydroxyvitamin D (calcitriol), which supports bone health, calcium absorption, and immune function.
Dietary sources of vitamin D, such as fatty fish and fortified foods, contribute about 20% of the body’s vitamin D requirements. Various factors, including age, skin color, sun exposure, and certain health conditions, can impact vitamin D synthesis and absorption, making it important for some individuals, particularly athletes, to monitor their levels and consider supplementation.
Impact of Low-Fat Diets and Under-Fueling on Vitamin D Levels
Dietary habits also play a significant role in vitamin D levels, particularly for athletes who may follow low-fat diets or chronically under-fuel. Vitamin D is a fat-soluble vitamin, meaning it requires dietary fat for proper absorption. When athletes limit their fat intake too strictly, they may inadvertently reduce their ability to absorb vitamin D, leading to lower blood levels.
Additionally, under-fueling—whether due to calorie restriction, intense training, or unintentional energy deficits—can compromise vitamin D status over time. Consistent under-fueling may limit the body’s access to the necessary nutrients for optimal performance and health, including vitamin D. For athletes, it’s important to maintain balanced nutritional intake, ensuring adequate fat intake to support vitamin D absorption and overall well-being.
Dietary Sources
Measuring Vitamin D Status
The best way to measure vitamin D levels is through a blood test measuring 25(OH)D.
The Endocrine Society, the National and International Osteoporosis Foundation, and the American Geriatric Society define vitamin D deficiency as the level of 25-hydroxyvitamin (25 OH D) of less than 30 ng/mL. The Endocrine Society recommends a preferred range of 40 to 60 ng/mL. Levels below this threshold are associated with increased risks for health issues that can interfere with training, recovery, and performance.
Nevertheless, I would never recommend an athlete to begin taking vitamin D supplements without first having their levels tested in a laboratory. In my practice, I follow the principle of "Test don't guess." This approach allows me to provide targeted supplement recommendations and ensure that athletes are taking the appropriate dose according to the severity of their deficiency.
Functions of Vitamin D
Vitamin D serves multiple roles in the body, making it a crucial nutrient for overall health, athletic performance, and recovery. Here are some of its primary functions:
Bone Health
Vitamin D is essential for bone health because it helps the body absorb calcium, a mineral critical for bone strength. Studies have shown that vitamin D levels at least above 30 ng/mL are associated with better bone density. In athletic populations, low vitamin D can increase the risk of stress fractures and other bone injuries, especially in high-impact sports. For example, Finnish military recruits with low vitamin D levels experienced a threefold higher risk of stress fractures, illustrating its importance in physically demanding environments.
Injury Prevention
Recent research suggests that vitamin D is essential not only for bone health but also for muscle strength and the prevention of musculoskeletal injuries. A review by Yoon, Kwon, and Kim (2021) indicates that athletes with adequate vitamin D levels experience improved muscle function and reduced fatigue, which can enhance overall physical performance. Additionally, vitamin D deficiency has been linked to a higher risk of musculoskeletal injuries, including fractures and muscle strains. By maintaining sufficient vitamin D levels, athletes may reduce their injury risk and potentially improve recovery times, as vitamin D’s anti-inflammatory properties can support faster rehabilitation from injuries.
Muscle Function
Muscle weakness and pain are well-known symptoms of vitamin D deficiency, which can affect an athlete’s performance and recovery. Vitamin D is involved in the growth and repair of muscle fibers, especially the fast-twitch fibers needed for explosive movements like sprints and jumps. Research shows that when deficient athletes increase their vitamin D levels, they experience improvements in muscle strength, sprint performance, and recovery. This makes vitamin D essential not only for reducing injury risk but also for optimizing performance.
Immunity and Inflammation
Vitamin D plays an important role in the immune system by enhancing the body’s defenses against infections and by managing inflammation, a key factor in recovery. It activates antimicrobial peptides in the respiratory system, potentially reducing the risk of respiratory infections—common in athletes undergoing intense training. Research has found that athletes with higher vitamin D levels experience fewer respiratory infections, while those with lower levels are more prone to illnesses during winter.
Additionally, vitamin D helps regulate inflammation, which is crucial for athletes who regularly stress their muscles and joints. By producing anti-inflammatory compounds and reducing pro-inflammatory markers, vitamin D can help reduce pain and accelerate recovery after strenuous exercise.
Populations at Risk of Vitamin D Deficiency
Certain groups are at a higher risk of vitamin D deficiency due to lifestyle, health conditions, or medications that interfere with vitamin D synthesis or absorption. For these populations, monitoring and possibly supplementing vitamin D is essential to maintain health and reduce the risk of complications.
Older Adults: Age reduces the skin’s ability to synthesize vitamin D from sunlight, putting older individuals at higher risk.
Housebound or Institutionalized Individuals: People who are confined indoors, such as disabled or institutionalized individuals, often have limited sunlight exposure.
Indoor Workers and Athletes: Professions and athletic training requiring long hours indoors limit sunlight exposure, increasing deficiency risk. This includes:
Office workers
Factory or warehouse workers
Taxi drivers
Night-shift workers
Athletes who train predominantly indoors, such as swimmers, gymnasts, or indoor cyclists.
Runners who train very early in the morning before sunrise or late at night after sunset.
People with Dark Skin: Higher levels of melanin reduce the skin’s ability to produce vitamin D from sunlight.
Individuals Training at High Altitude: Athletes or individuals living or training at high altitudes may have reduced UV exposure, especially during colder months, limiting vitamin D synthesis.
Individuals with Low Physical Activity: Reduced outdoor activities limit sunlight exposure, impacting vitamin D synthesis.
People with Chronic or Debilitating Diseases: Certain diseases can hinder vitamin D absorption or metabolism, such as:
Diabetes
Chronic kidney disease
Gastrointestinal malabsorptive syndromes
Parathyroid disorders
Liver diseases
Individuals with Obesity: People with higher waist circumferences often have lower circulating vitamin D levels, possibly due to sequestration in fat tissue.
Post-Bariatric Surgery Patients: Surgery affecting the digestive system can impair vitamin D absorption.
Babies of Vitamin D-Deficient Mothers: Infants born to mothers with low vitamin D levels may also be deficient, highlighting the need for monitoring during pregnancy.
People Taking Medications that Increase Vitamin D Catabolism: Some medications accelerate vitamin D breakdown, increasing the risk of deficiency. These include:
Phenobarbitone
Carbamazepine
Dexamethasone
Rifampicin
Nifedipine
Spironolactone
Ritonavir
Cyproterone acetate
Requirements and Recommendations
The recommended daily allowance (RDA) for vitamin D in the U.S. is 600 IU for adults under 70. However, many experts suggest that this dosage might not be adequate for active individuals, especially those with limited sun exposure. In a study titled "Vitamin D in athletes: focus on physical performance and musculoskeletal injuries" by Yoon S, Kwon O, and Kim J, it is mentioned that athletes who are deficient may need 2,000-6,000 IU daily to address deficiencies, promote optimal recovery, and prevent injuries. To achieve sufficient levels, a combination of sun exposure, dietary intake, and supplements is generally recommended, especially for athletes training in regions with limited sunlight during winter.
Key Points to Remember
Vitamin D is essential for athlete health and performance, impacting bone strength, muscle function, recovery, immune health, and inflammation control.
Evaluating the blood levels of 25(OH)D is the most effective method to determine the status of vitamin D. Target levels should be at least 40 ng/mL, with additional advantages seen at higher levels up to 60 ng/mL.
Low vitamin D increases risks for stress fractures, muscle pain, and respiratory infections, which can interfere with training and recovery.
Combination of sun exposure, diet, and supplements is essential for athletes to reach optimal levels of vitamin D. During winter, athletes in areas with minimal sunlight may require a daily supplement ranging from 1,000 to 4,000 IU. A higher dosage might be necessary in cases of severe deficiency, a pre-existing medical condition affecting vitamin D absorption, or increased requirements for bone metabolism support, such as osteoporosis or low bone density.
Conclusion
In summary, vitamin D is more than just a "sunshine vitamin"; it’s a vital component of health and performance for athletes. Ensuring optimal levels can help support muscle function, reduce the risk of injuries, and strengthen the immune system—all of which are crucial for those who push their bodies to excel. Prioritizing vitamin D isn’t just about meeting a number on a lab report—it's an investment in your long-term health and performance.
If you are an athlete, it is advisable to assess your vitamin D levels, especially if you belong to any of the high-risk groups. Feel free to contact me to learn how to include vitamin D-rich foods in your diet or to choose the right supplement if you are already deficient. Reach out to me today at anniesathletes@gmail.com to get started.
References
Holick, M.F. (2007). Vitamin D deficiency. New England Journal of Medicine, 357, 266-281. PMID: 17634462.
Yoon S, Kwon O, Kim J. Vitamin D in athletes: focus on physical performance and musculoskeletal injuries. Phys Act Nutr. 2021 Jun;25(2):20-25. doi: 10.20463/pan.2021.0011. Epub 2021 Jun 30. PMID: 34315203; PMCID: PMC8342187.
Chauhan K, Shahrokhi M, Huecker MR. Vitamin D. 2023 Apr 9. In: StatPearls. Treasure Island (FL): StatPearls Publishing; 2024 Jan–. PMID: 28722941.
Giustina, A., Bilezikian, J.P., Adler, R.A., et al. (2024). Consensus Statement on Vitamin D Status Assessment and Supplementation: Whys, Whens, and Hows. Endocrine Reviews, 45(5), 625-654. doi: 10.1210/endrev/bnae009. PMID: 38676447.
Cannell, J.J., et al. (2008). Diagnosis and treatment of vitamin D deficiency. Expert Opinion on Pharmacotherapy, 9(1), 107-118. PMID: 18076342.
Bischoff-Ferrari, H.A., et al. (2004). Positive association between 25-hydroxy vitamin D levels and bone mineral density. American Journal of Medicine, 116(9), 634-639. PMID: 15121495.
Halliday, T., et al. (2011). Vitamin D status relative to diet, lifestyle, injury, and illness in college athletes. Medicine & Science in Sports & Exercise, 42, 335-343. PMID: 20581792.
Raimundo, F., et al. (2011). Vitamin D and absorption enhancement. Journal of Nutritional Biochemistry, 22, 233-239. PMID: 20620065.
Girgis, C.M., et al. (2013). Vitamin D’s role in muscle function. Endocrine Reviews, 34, 33-83. PMID: 23169677.
Close, G.L., et al. (2013). Effects of vitamin D on muscle performance in athletes. Journal of Sports Science, 31, 344-353. PMID: 23092016.
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