A Closer Look At Vitamin D

January 29, 2015
By Randy Bird, MS, RD, CSSD, CSCS

Recently, vitamin D has been getting a lot of publicity. Previously believed to be conquered, vitamin D deficiency is making a comeback. It is currently considered an epidemic in the United States [39]. This article breaks down the different variations of vitamin D and provides recommended intake dosages.
The implications of this deficiency far exceed rickets. It can lead to osteopenia, osteoporosis, osteomalacia, muscle weakness, and weakened immune function. Even with all of the recent publicity, vitamin D is still largely ignored in collegiate athletes. This article will review the roles and functions of vitamin D, how it affects athletes, and how to get adequate vitamin D daily.


Vitamins are organic components in food that are essential in minute amounts for growth and for maintaining good health. Vitamin D was discovered in the early 1900s by E.V. McCollum. He determined a factor in cod liver oil was an effective medicine for treating rickets in children.

There are two major forms of vitamin D, Vitamin D2 (ergocalciferol) and Vitamin D3 (cholecalciferol). It is classified as a fat-soluble vitamin because it is necessary for calcium absorption, but also functions as a hormone because the body produces it in response to the skin's exposure to ultraviolet-B (UVB) rays. After vitamin D is produced in the skin or consumed in food, it is converted in the liver and kidney to form dihydroxyvitamin D, the physiologically active form of vitamin D. This metabolically active form of vitamin D is known as calcitriol.

A blood calcidiol (25-hydroxyvitamin D) level is the accepted way to determine vitamin D nutritional status. Optimal levels of serum 25-hydroxyvitamin D have not been established [20] and are likely to vary at each stage of life. In March 2007, a group of vitamin D and nutrition researchers published an editorial contending that the desirable concentration of 25-hydroxyvitamin D is ≥30 ng/mL[36]. It seems that a good target for vitamin D is 50 ng/mL[5]. Vieth, et al. [36] noted that supplemental intakes of 400 IU/day of vitamin D increase 25-hydroxyvitamin D concentrations by only 2.8-4.8 ng/mL and that daily intakes of approximately 1,700 IU are needed to raise these concentrations from 20 to 32 ng/mL.

Vitamen D Table 1.jpg


Deficiency
Some people are at greater risk of vitamin D deficiency than others. First the list are people with limited sun exposure. Athletes who practice and play indoors, live in northern latitudes, or keep their skin covered preventing sun exposure are unlikely to obtain adequate vitamin D from sunlight [38].

Another factor that affects the production of vitamin D is melanin. People with darker skin color have reduced ability to produce vitamin D from exposure to sunlight. Oral vitamin D requires some dietary fat in the gut for absorption. Lastly, people who have a reduced ability to absorb dietary fat might are at risk [25]. Fat malabsorption is associated with a variety of medical conditions including pancreatic enzyme deficiency, Crohn's disease, cystic fibrosis, celiac disease, some forms of liver disease, and surgical removal of part of the stomach or intestines [23].

One side effect of deficiency is impaired bone mineralization that leads to bone softening diseases including rickets, osteomalacia, and osteoporosis. For athletes, the concern is osteomalacia. This disorder occurs is characterized by proximal muscle weakness and bone fragility. This can set an athlete up for injury.

Osteomalacia is also thought to contribute to chronic musculoskeletal pain. Insufficient amounts of vitamin D may also be linked to an increased susceptibility to several chronic diseases such as cardiovascular disease, hypertension, tuberculosis, chronic pain, seasonal affective disorder [13], cognitive impairment which includes memory loss,[24] and several autoimmune diseases including type 1 diabetes.[19, 33]

Benefits of Adequate Vitamin D
One benefit of obtaining adequate vitamin D for athletes is that it strengthens the immune system in two different ways. The vitamin D-Vitamin D receptor complex appears to control lymphocyte and cytokine production. Also, researchers have found a link between vitamin D status and the release of antimicrobial peptides that attack the cell membrane of pathogens [39]. Some researchers also suggest that these antimicrobial peptides help fight of the influenza virus and that seasonal fluctuations in vitamin D status could explain the higher incidences of colds and flu during the winter [39].

Another area of interest for athletes is inflammation. Intense exercise leads to elevated levels of proinflammatory cytokines. These are associated with overtraining and other sports-related injuries. Vitamin D has been shown to reduce the production of these cytokines, while increasing the production of anti-inflammatory cytokines [39]. So, it is possible that an adequate supply of vitamin D may speed recovery from intense exercise.

There is limited research on vitamin D and athletic performance. Studies were done in Russia and Germany during the early part of the 20th century that indicated a possible link between vitamin D and athletic performance [5]. Most current studies have found a link between vitamin D deficiency and loss of muscle strength in the elderly [20]. One study looked at post-menarchal adolescent females [37]. They found a significant association between vitamin D status and muscular power and force in these adolescent girls. More research is needed, but the implications are that performance worsens with a decline in vitamin D status.

Toxicity
It is possible to get too much vitamin D. Toxicity can cause nonspecific symptoms such as nausea, vomiting, poor appetite, constipation, weakness, and weight loss [7]. More seriously, it can also raise blood levels of calcium, causing mental status changes such as confusion and heart rhythm abnormalities [12]. Excessive vitamin D levels can also lead to deposition of calcium and phosphate in the kidneys and other soft tissues [11]. A serum 25-hydroxyvitamin D concentration consistently >200 ng/mL is considered to be potentially toxic [23]. Vitamin D toxicity is treated by discontinuing vitamin D supplementation, and restricting calcium intake. In some cases kidney damage may be irreversible.

The exact long-term safe dose of vitamin D is not known. In 1997 the U.S. Dietary Reference Intake Tolerable Upper Intake Level (UL) of vitamin D for children and adults was set at 2,000 IU. Many view this as outdated.

A 2007 risk assessment suggested that 10,000 IU /day in healthy adults should be adopted as the tolerable upper limit [15]. In adults, sustained intake of 100,000 IU /day can produce toxicity within a few months [33]. Normal food and pill vitamin D concentration levels are generally too low to be toxic in adults.

One thing we have to be careful of is the mindset that if a little is good, more must be better. With this mindset, athletes could potentially take toxic levels of vitamin D. Most officially-recorded historical cases of vitamin D overdose have occurred due to manufacturing and industrial accidents [35]. Exposure to sunlight for extended periods of time does not normally cause vitamin D toxicity [35]. This is because within about 20 minutes of exposure to UVB radiation in light-skinned individuals (one to three hours for dark skin), any further vitamin D that is produced is degraded [17]. The most endogenous production with full body exposure to sunlight is 10,000 IU per day. [35]

Endogenous Production From Sunlight
Most people can meet their vitamin D needs through exposure to sunlight [16]. UVB radiation penetrates uncovered skin and converts cutaneous 7-dehydrocholesterol to previtamin D3, which is converted to vitamin D3 [20]. As mentioned previously, there are many factors that affect UV radiation exposure and vitamin D synthesis. For latitudes above 35 degrees north (Atlanta), little or no vitamin D synthesis occurs from November through February; in far northern latitudes, this reduced intensity lasts for up to six months. Latitudes below 35 degrees north allow for cutaneous production of vitamin D throughout the year [20]. Therefore, athletes competing in the northern half of the United States are relying on their summer stores of vitamin D and their diet.

It has been suggested by some vitamin D researchers that approximately five to 30 minutes of sun exposure between 10 a.m. and 3 p.m. at least twice a week to the face, arms, legs, or back without sunscreen usually lead to sufficient vitamin D synthesis [18, 20]. However, UV energy is reduced by complete cloud cover, shade, and pollution; thus reducing the synthesis of vitamin D. UVB radiation does not penetrate glass, so exposure to sunshine indoors through a window does not produce vitamin D [4]. Sunscreens also block vitamin D-producing UV rays [40].

Even though the sun is of vast importance to vitamin D synthesis, it is wise to limit exposure of skin to sunlight. UV radiation is responsible for most of the estimated 1.5 million skin cancers and the 8,000 deaths due to metastatic melanoma that occur annually in the United States [40]. The safest way to get adequate vitamin D is through fortified foods and supplements.

Food Sources
One problem that is a direct cause of vitamin D insufficiency is that very few foods in nature contain vitamin D. Fish and fish liver oils are some of the best sources [11]. Small amounts of vitamin D are found in beef liver, cheese, and egg yolks. Vitamin D in these foods is primarily in the form of vitamin D3 [30]. Some mushrooms provide vitamin D2 [3, 26, 29].

Most of the vitamin D in the American diet comes from fortified foods [3, 11]. The best example is milk. Almost all of the U.S. milk supply is fortified with 100 IU/cup of vitamin D. Milk fortification was implemented in the 1930's in the United States to combat rickets. Foods allowed to be fortified with vitamin D include cereal flours and related products, milk and products made from milk, and calcium-fortified fruit juices and drinks [3]. Other dairy products made from milk, such as cheese and ice cream, are generally not fortified. Table 2 shows quantities of vitamin D found in food.

Vitamen D Table 2.jpg


Supplements
Unless someone eats oily fish frequently and drinks a substantial quantity of milk, obtaining sufficient vitamin D from natural food sources alone can be difficult. For many people, dietary supplements might be required to meet the daily need for vitamin D. Both vitamin D2 and vitamin D3 are available. Both forms are effective at raising serum 25-hydroxyvitamin D levels, but vitamin D3 appears to be more than three times as effective as vitamin D2 [1, 21].

When choosing a supplement, it is always important to choose products from reputable companies. Look for certification from an agency such as USP, NSF, or Informed Choice. Also, look at the company's entire product line to ensure they don't handle products banned by the governing body of your sport. For vitamin D, many vegetarians will not want vitamin D3, because it is manufactured from animal fat. Vitamin D2 is synthesized from plants.

Summary
Vitamin D has been ignored too long. Given what is now known about vitamin D, it is essential that athletes achieve adequate vitamin D status. Many athletes may be deficient.

Remember that athletes who practice and play indoors, live in northern latitudes, or have dark skin are at higher risk for deficiency. Since this deficiency can be associated with stress fractures, chronic musculoskeletal pain, weakened immune function, and inflammation, it is important that deficiency is diagnosed and treated without delay.

To maintain adequate vitamin D levels, athletes should get 1700-2000 IU per day. This can be achieved with a good multivitamin and a vitamin D supplement.


Randy Bird, MS, RD, CSSD, CSCS is Sports Nutritionist for the University of Kansas Athletic Department. He can be reached at: [email protected]


References

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