Vitamin D: Are you getting enough?

Introduction

Vitamin D is a fat soluble vitamin which is stored in the body’s fat, liver and muscle. The major sources of vitamin D for humans are the exposure of skin to sunlight and from the diet (Chen et al, 2007).

Foods like oily fish, such as salmon, and fortified foods, such as milk and margarine, contain vitamin D. Wild fish contains more than farmed fish, although the proposed content of some fortified foods may not always be accurate (Chen et al, 2003).

Vitamin D plays an essential role in the growth and maintenance of healthy bones by increasing calcium absorption. Deficiency can lead to rickets in children and osteomalacia and osteoporosis in adults (Holick, 2006).

Sufficient vitamin D can be synthesised from midday sunlight exposure during the summer months. However, due to differing climates and the risk of skin cancers due to sun exposure, dietary vitamin D supplementation is advisable (Chen et al, 2007). Those with darker skin pigmentation also need prolonged exposure to sunlight or a higher dose of vitamin D supplementation (Owens et al, 2014).

Why Your Vitamin D Status Matters

For the athletic population, vitamin D status is highly important due to the interactions of vitamin D with muscle function, bone health and immune function. Athletes and the sporting population can suffer from low vitamin D status, particularly during winter months, despite living in sun-rich environments (Owens et al 2014).

Muscle Function

Vitamin D deficiency can impair muscle function and muscle adaptations after exercise, according to Owens et al (2014). There are mixed reports regarding vitamin D supplementation and the improvement of muscle function in elite athletes. Athletes generally have lower margins for improvement due to their highly trained status.

However, high dose supplementation in vitamin D deficient youth athletes has shown improvements in sprint times, jump height and muscle adaptation and function (Close et al, 2013). Therefore, those that are advising or caring for youth athletes should be highly mindful of vitamin D status and the impact on athletic performance.

Muscle Regeneration

There are mixed reports with regards to muscle regeneration and vitamin D status. Most of the positive findings refer to vitamin D deficiency and older populations (Owens et al, 2018). However, Barker and colleagues (2013) reported that vitamin D status of over 75nmol/L is better for optimal force recovery after muscle damaging exercise.

While it may be unclear whether vitamin D supplementation can aid muscle function in athletes at elite or non-elite levels, there is sufficient data to suggest that vitamin D deficiency leads to poorer muscle function and therefore monitoring your vitamin D status can be an important factor in overall performance.

Bone Health

The most well-known function of vitamin D status is how it relates to bone mineral density due to the role it plays in calcium absorption. As reported by Owens et al (2014), vitamin D status classifications laid out by the IoM were done mainly to positively affect the bone mineral density of the general population.

Holick (2004) reported that the major function of vitamin D is to maintain serum calcium concentrations and that dietary calcium absorption can range from 10-15% in a deficient state to 15-30% in a sufficient state. Calcium absorption can reach as high as 60-80% of dietary calcium absorption during periods of growth, pregnancy or lactation. Therefore, not only is vitamin D status a crucial factor in bone health and performance for the general athlete, it is particularly important for females and developing athletes.

Weight bearing sporting activities have positive effects on bone health and this may mitigate some lower vitamin D effects on athletes bone health. However, it is still prudent to ensure sufficient vitamin D status, particularly in non-weight bearing activities such as swimming, and in sporting activities that involve weight-making or those at risk of low energy availability, such as gymnastics and combat sports (Ackerman et al, 2012).

While environmental factors such as nutrition profile and training load, as well as genetic profile, play important parts in the risk of stress fractures, there may also be a link between vitamin D status and the risk of bone injuries in athletic populations (McCabe et al, 2012). Therefore, vitamin D supplementation should also be considered for this reason.

Immune Function

Another primary role of vitamin D is the ability to affect immunity. Vitamin D deficiency can compromise optimal immune function and lead to recurring or increased infection frequency (Owens et al 2014).

While moderate training schedules can reduce the frequency of illness, athletes are at a greater risk of illness due to the training intensity and frequency leading to overreaching and reduced recovery (Gleeson et al, 2007).

He and colleagues (2013) have shown the negative effects of lower vitamin D status compared to optimal vitamin D status on the frequency of URTI symptoms. Maintaining a sufficient vitamin D status should be a high priority for athletes, especially during the winter months when the risk of illness is higher.

Measuring Vitamin D Status

Measuring vitamin D status is generally done through the extraction of serum from blood samples, focusing on circulating 25[OH]D as the best indicator of vitamin D status. The Institute of Medicine guidelines describe vitamin D status in different classifications. The categories are described as serum vitamin D levels of 12.5nmol/L (very deficient), 12-5-30nmol/L (deficient), 30-50nmol/L (insufficient) and >50nmol/L as adequate or sufficient.

O’ Malley & Mulkerrin (2006) have reported that over 75% of the older, female, Irish population have vitamin D deficiency, while deficiency affects all age groups in the Irish population. Hill et al (2004) have shown that all age groups presented 11% deficiency in summer and 45% deficiency in winter months, showing that winter months present a greater need for vitamin D supplementation at latitudes similar to Ireland.

The guidelines laid out by the IoM are often debated due to the development of these classifications based on the Estimated Average Requirement (EAR) of vitamin D for bone health, failing to take into account other functions of vitamin D within the human body (Manson et al, 2016).

Holick and colleagues (2004) recommend a higher vitamin D status of 75-125nmol/L to maintain optimal health, claiming that vitamin D status at these levels does not cause any adverse health effects. It has also been reported by Owens and colleagues (2018) that 25[OH]D concentrations of greater than 75nmol/L are more beneficial for athletes, showing that the IoM guidelines may not be fit for purpose.

Supplementing with Vitamin D

When it comes to vitamin D supplementation, it is best to first understand the units associated with vitamin D. International Units (IU) and micrograms (μg) are both commonly used when discussing vitamin D supplementation. It’s important to understand that 100iu = 2.5μg, and confusing the unit of measurement could lead to toxicity.

There are also two forms of supplemental vitamin D, D2 and D3. Heaney and colleagues (2011) showed that vitamin D3 is much more effective than vitamin D2 and therefore vitamin D3 is the more favoured source of vitamin D supplementation. Kinetica Sports vitamin D oral spray uses vitamin D3 for a more effective supplementation protocol.

Owens et al (2025) have also reported that vitamin D supplementation should be individualised, as different sporting environments, dietary preferences, clothing and athlete ethnicity can alter their vitamin D status (Chen et al, 2007). For example, it was shown that indoor sports such as basketball and dancing can lead to vitamin D deficiency.

The Institute of Medicine in USA and the European Food Safety Authority have both recommended 600iu of vitamin D per day and an upper limit of 4,000iu per day. Many research groups that have studied the effectiveness of vitamin D supplementation on muscle function, bone health and immune function are reporting benefits from higher supplemental doses than that.

Owens et al (2018) recommend 2,000-4,000iu of vitamin D per day for anyone that is showing vitamin D levels of below 75nmol/L during winter months when sun exposure is lower. Kinetica Sports vitamin D oral spray offers the perfect solution to this recommendation with a 3,000iu dose per serving.

How much is too much

While vitamin D deficiency reduces various aspects of athletic performance, you can reach levels of toxicity when it comes to supplemental vitamin D intake, due to the fat soluble nature of the vitamin. However, Owens et al (2018) and Zitterman (2003) show that toxicity levels are far above the target for sufficient vitamin D status.

Conclusion

Vitamin D status is important for bone health, immune function and muscle function, especially for athletic populations that can be more prone to injury and illness as a result of intense training.

Vitamin D supplementation needs are very individual and can depend on many factors like sun exposure, clothing, training environment and nutritional intake. Due to the individuality of supplement needs, and the potential for toxicity with over-supplementation, it would be recommended to get your vitamin D status tested by your doctor.

Blanket supplementation for athletes doesn’t always provide the optimal outcome, however, there is evidence for taking higher vitamin D doses than the recommended guidelines.

2,000-4,000iu of vitamin D can be recommended for anyone that is vitamin D deficient, with deficiencies highlighted in large numbers of the population.