Importance of Vitamin D

Vitamin D is also one of the most underrated, underused and perhaps misunderstood vitamins and its use in supplementation. Undoubtedly, the role of Vitamin D within the body serves as a foundation for numerous physiological processes. Vitamin D is one of the four fat-soluble vitamins (A, D, E, K).  Ultraviolet radiation (the sun) assists in the conversion of 7-dehydrocholesterol to pre-vitamin D3, which is converted to vitamin D3 (cholecalciferol) in the skin. Now, in the liver, vitamin D3 is converted into 25-hydroxyvitamin D, and the active form, calcitriol (Larson 2010, Udowenko 2010) is formed in the kidney.

Calcitriol is what’s called a  ‘secosteroid’ hormone, in that its transported in the blood, bound to a vitamin D binding protein.  Its function is to signal gene transcription (particular segment of DNA is copied) as research reports it may affect athletic performance by regulating the action of neurons, bones, muscles, and cardiorespiratory activity (Udowenko 2010). Vitamin D is also required for bone growth and remodeling by osteoblasts and osteoclasts (IOM 2010), and serves other roles including modulation of cell growth, neuromuscular and immune function, and reduction of inflammation (IOM 2010, Holick 2006).

A major assumption we all have is that because many of our foods via milk, cereals and bread are fortified with vitamin D that poor or deficient levels are either for underprivileged individuals or those who may live in a second or third world country. However, numerous studies reported that in different parts of the world that vitamin D levels in our blood are habitually lower than known thresholds.  In addition, these levels have been reported in young and old people as well as in several different groups of athletes (Willis 2008).

Therefore, the importance and awareness of Vitamin D is important for strength and conditioning professionals, lifters, and personal trainers to know concerning the many functional implications within the body, as Vitamin D plays a crucial role in numerous health and performance outcomes.

Status and Intake

Scientific evidence has reported a link between poor vitamin D status to a variety non-skeletal chronic and autoimmune diseases, including hypertension, cardiovascular disease, diabetes, inflammatory bowel disease, depression, multiple sclerosis, and rheumatoid arthritis (Halliday 2011, Holick 2007, Zitterman 2003).  It is also believed to act as a powerful antioxidant protecting against free radical damage, and may induce cellular differentiation, via possible anticancer activity (Hutchinson 2010, Verstuyf 2010, Holick 2008).

Vitamin D is found in some fortified foods, including oily fish (which has significant natural levels of VD), milk, breads, some juices and cereals, mushrooms, and egg yolks (Willis 2008). Unfortunately, most people consume little natural vitamin D.  Although one of the best ways to obtain Vitamin D is exposure to sunlight, this really depends on where you live and the specific time of year. Most of the people in the U.S. live above Atlanta, where exposure to cold and decreased exposure to sunlight is regular during the winter months. Studies report that endogenous vitamin D synthesis and bioavailability decrease at wintertime in the northern and southern hemispheres (Halliday 2011), and is affected by other factors such as sunscreen, skin pigmentation, and even body composition (Webb 2006, Snijder 2005, Parkikh 2004, Holick 2004). Studies by Holick (2002, 2007) have suggested, that approximately 5–30 minutes of sun exposure between 10 am and 3 pm at least twice a week to the face, arms, legs, or back without sunscreen usually leads to sufficient vitamin D synthesis and that even the moderate use of commercial tanning beds that emit 2%–6% ultraviolet radiation is also effective.

RDAs for vitamin D are 600IU’s for those 13-70, and 800IU’s for those 70 or older. However, although sunlight is a major source of vitamin D for, the vitamin D RDAs are actually set on the basis of minimal sun exposure (IOM 2010). Further, the daily value (DV) for vitamin D is currently set at 400 IU for adults and children age 4 and older. Interestingly, food labels however, are not mandated to list vitamin D content unless a food has been fortified with it.


The true definition of vitamin D deficiency and what comprises an adequate supplementation strategy is often debatable. Hollick (2007) reports that Vitamin D Levels of 20–30 ng/mL are generally noted as insufficient, whereas levels below 20 and 10 ng/mL represent deficiency and severe deficiency, and further reported deficiency in 36% of otherwise healthy young adults and in up to 57 % of general hospital patients in the USA. Like most vitamin or mineral deficiencies, this typically results from either lack of consumption or intake, decreased absorption, or increased excretion, or perhaps intake just decreases over time. Unfortunately, the kidneys cannot convert 25(OH)D to its active form, or if absorption of vitamin D is inadequate.

One of the classic cases of vitamin D deficiency is “Rickets”, which is mainly caused in children. Rickets is described by a failure of bone tissue to properly mineralize, resulting in soft bones and skeletal abnormalities (Wharton 2003). Historically, The fortification of milk with vitamin D in the early 1930’s has made rickets a rare disease in the US, although it is still reported among African American infants and children (Wharton 2003, AAD 2008), and more common from those in Asia, Africa, and the Middle East, possibly due to genetic factors.

For adults, vitamin D deficiency can lead to weak bones (osteomalacia) (IOM 2010). Symptoms of bone pain and muscle weakness can indicate inadequate vitamin D levels, but may initially go undetected. There are specific groups of people who are at greater risk for developing vitamin d deficiency. Older adults are at increased risk of developing vitamin D deficiency in part due to age, less time outside, and because the skin cannot synthesize vitamin D as efficiently, and/or have inadequate intakes. Research has reported that as many as half of older adults in the United States with hip fractures could have serum vitamin D levels <30 nmol/L (<12 ng/mL) (Cranney 2007). Those that have dark skin are also more at risk for Vitamin D deficiency. Greater amounts of the pigment melanin in the epidermal layer produce darker skin and decrease the skin’s ability to produce vitamin D from sunlight (IOM 2010). Furthermore, because vitamin D is one of four-fat soluble vitamins, it requires some dietary fat in the gut for absorption. In addition, as you might expect, those who are very overweight and obese are at greater risk for deficiency due to greater amounts of both visceral and subcutaneous fat, which isolate more of the vitamin and can augment release into circulation.

Vieth and colleagues (2007) published a controversial and provocative paper arguing that the desirable concentration of 25(OH)D was ≥75 nmol/L (≥30 ng/ml).  They noted that approximately 1,700 IU/day of vitamin D are needed to raise serum 25(OH)D concentrations from 50 to 80 nmol/L (20–32 ng/mL).

 Vitamin D Health, Performance and Use of Supplementation
AthletesLittle is known about the direct effects of vitamin D deficiency on athletic performance. However, evidence suggests that supplementation of vitamin D-deficient athletes may be beneficial. Cannel et al. (2009) concluded that the greatest improvements in performance occur in those with the lowest baseline levels, as a significant improvement in athletic performance may occur when levels rise from 15 to 30 ng/mL, but a less noticeable improvement when levels increase from 30 to 50 ng/mL.If vitamin D affects athletic performance (which evidence indicates) physical performance should peak in late summer, when VD levels are highest, and start to decline in early fall as levels drop and are lowest in the winter. The association between peak physical performance and summer season is significant, even when physical conditioning is regular.

Studies in various populations have shown that people with low blood levels of vitamin D have a greater risk for bone fractures.  Studies also indicate the risk for stress fractures (something many athletes must be concerned about) is linked to the amount of vitamin D in your blood (Willis 2008).  In addition to bone health, optimal vitamin D levels can help to control the inflammation process that occurs after every stressful training session and also aids in strengthening your immune system.

Strength, Force and Power Development

Vitamin D status is associated with reductions in strength and poor physical function (Willis 2008).  Meaning, those who had the lowest levels of vitamin D also had the lowest strength levels and the poorest indicators of physical function.

Optimal levels of vitamin D have been reported to increase muscle power development and jump height. Ward et al (2009) found that the ability of the muscles to contract and produce force is affected by vitamin D status. Subjects with low concentrations of vitamin D produced less power vs. those with higher concentrations, leading to the researchers to conclude that vitamin D is highly associated with power and force. Further, it was suggested that sub-optimal force production has negative implications for long-term bone development.

Diabetes and Insulin Resistance

Vitamin D supplementation has been shown to increase insulin sensitivity and lower insulin resistance, suggesting an effective way to offset the symptoms of diabetes. Von Hurst (2009) reported that women with insulin insensitivity (in a pre-diabetes state) found that taking 4,000 IUs a day resulted in significant decreases in insulin insensitivity. Additional research by Pittas (2007) also provides supporting evidence and concluded that vitamin D and calcium insufficiency may negatively influence glycemia, whereas combined supplementation with both nutrients can be beneficial in optimizing glucose metabolism.

Cardiovascular Health

Vitamin D deficiency is associated with cardiovascular disease and high levels of vitamin D are associated with heart health. Wang (2011) followed 1739 Caucasian individuals with an average age of 60 without prior cardiovascular disease. Participants’ vitamin D levels and cardiovascular health was assessed at the beginning of the study and 5 years later. The follow up study identified 120 individuals who had developed a first cardiovascular event. Additionally, for individuals with high blood pressure and low vitamin D levels there was a two-fold risk of cardiovascular incidence. The study indicates that maintaining optimal vitamin D levels is crucial in preventing cardiovascular disease and that vitamin D supplementation could contribute to offsetting cardiovascular disease.


Based on other health markers and the impact of low Vitamin and health, it’s safe to say that Low vitamin D levels probably makes you fat. Reports show that body fat mass is higher in individuals with vitamin D deficiency and that this shortage is associated with elevated levels of parathyroid hormone and intracellular calcium. The increased calcium levels activate various pathways that promote the accumulation of fat tissue. It was previously thought that decreased vitamin D levels were consequences of obesity but a recent science reports that reduced levels can play a role in the onset of obesity (Valine-Toth 2010).


Currently, no definitive answer exists. However, early studies in competitive athletes reported that supplementing for three months with 400 IU of vitamin D per day, (the same dose in a multi-vitamin) had no impact over increasing vitamin D levels in the athlete’s blood (Lehtonen-Veromaa 1999). Further, this amount (i.e. 400) won’t even budge your blood levels. In contrast, studies have reported that taking 10,000 IU per day for up to 5 months did not cause toxicity and appear to be safe (Willis 2008), but this should NOT be interpreted as a recommended dose.  Willis (2008) reported recommended supplementation levels of 5,000 IU/day with the most commonly recommended range being 1000- 2,000 IU/day.

Bottom Line

Vitamin D is an extremely important nutrient for health and performance outcomes. Based on current research findings, the majority of people are not at optimal levels for Vitamin D. When vitamin D is low, performance measures such as strength, and muscle function decrease, along other parameters by either helping to prevent or increasing the risk for numerous variables of health. Lastly, exposure to sunlight and, consuming foods with adequate vitamin D, and optimal supplementation are a surefire way to obtain appropriate levels of Vitamin D.


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