Vitamin D has an important role in skeletal muscles. Previously recognized for its effects on bone, it is now known that vitamin D has a much wider spectrum of usefulness for muscle. Studies indicate that vitamin D deficiency is pandemic. Those affected include the young and otherwise healthy members of the population, including athletes. Controversy exists regarding the amount of supplementation required to reverse deficiency and the relative effect of such a reversal on overall health. This article reviews current data on the role of vitamin D on muscle function, and explores the potential implications of its deficiency and supplementation on physical fitness and athletic performance.
Vitamin D is currently a topic of great interest among researchers. Increasing evidence suggests that exposure to a few minutes of sun daily may strengthen the immune system, maintain cardiovascular health, protect against certain cancers, and possibly even enhance athletic performance. Researchers propose that these are among the nonskeletal benefits of vitamin D. Recent data also indicate that vitamin D deficiency is pandemic; the healthy and the young are not excluded.
Muscle tissue was among the first nontraditional vitamin D target organs to be identified. Much attention has since been paid to the effects of vitamin D on muscle strength, exercise capacity, and physical performance. Numerous studies have reported a correlation between vitamin D and muscle function. These findings raise many important clinical questions, especially in the area of sports medicine.
If vitamin D does improve muscle function, what implications does this have on the performance of athletes who have vitamin D deficiency? Is routine screening necessary? What is the role of vitamin D supplementation to enhance performance in these athletes? Although the issue of vitamin D deficit may have been recognized, controversy exists regarding what constitutes normal serum levels, the amount of supplementation required to restore normal levels, and the relative effect of such supplementation on our overall health. This review summarizes the current understanding of the functions of vitamin D, describes its role in muscle function, and explores the possible implications of its deficiency and supplementation on exercise performance in individuals who are vitamin D deficient and nondeficient.
Search strategy
The authors performed a literature search using PubMed/Medline, Ovid, EMBASE, and ScienceDirect databases indexed under the Medical Subject Heading (MeSH) terms; “Vitamin D OR Vitamin D supplementation OR Vitamin D deficiency” combined with the terms; “athletic performance” OR “performance enhancement” OR “strength” OR “exercise” OR “muscle strength” OR “muscle function” OR “physical performance” OR “in normal individuals” OR “chronic fatigue” OR “myalgia.” The literature search was limited to articles dating back to January 2000 and written in the English language. All types of articles were reviewed, based on their direct relevance to the aims of this review. In addition, we searched the reference lists of all identified articles, and latest editions of standard texts. Additional sources written before the year 2000 were included for completeness as part of background information on the topic.
The basics about vitamin D
Sources
Vitamin D is a steroid hormone that regulates various tissue processes by way of specific cell receptors. 25-Hydroxy vitamin D (25-OHD) is the main storage form of vitamin D, whereas 1,25-dihydroxy vitamin D (1,25-DiOHD or 1,25(OH) 2 D) is the main biologically active metabolite responsible for calcium homeostasis. The main forms of vitamin D are listed in Table 1 . The main source of vitamin D in the human body is cutaneous synthesis via the effects of ultraviolet radiation on the vitamin D precursor, 7-dehydrocholesterol. This mechanism is well regulated and accounts for most of vitamin D synthesis in the body. Anything that reduces the number of solar UVB photons penetrating the skin or alters the amount of 7-dehydrocholesterol in the skin affects production of vitamin D 3 . Examples of factors that interfere with 1 or both of these processes, thus causing vitamin D deficiency, are listed in Table 2 . On the contrary, protective mechanisms such as pigment in skin and formation of inactive isomers (luminosterol, tachysterol) prevent its overproduction.
| Type of Vitamin D | Source |
|---|---|
| Vitamin D 2 (ergocalciferol) | Plant |
| Vitamin D 3 (cholecalciferol) | Animal |
| Dihydrotachysterol | Synthetic |
| Dark skin |
| Inadequate sun exposure |
| Sun exposure at dawn and dusk |
| Insufficient surface area of skin exposed to sun |
| Cloudy (typically winter) outdoors |
| Northern latitudes |
| Consistent use of sunscreens or sun block lotions |
| Low dietary intake |
The alternative source of vitamin D is from dietary intake ( Table 3 ). However, most modern diets contain inadequate vitamin D; even those rich in fortified dairy or cereal products. Vegetarian and vegan diets are lacking in this regard. Thus, whenever sufficient cutaneous production is poor, vitamin D deficiency likely ensues.
| For Every 100 g (3.5 ounces) of: | IU of Vitamin D: |
|---|---|
| Cod liver oil (∼2 teaspoons) | 10,000 |
| Lard (pork fat) | 2800 |
| Atlantic herring (pickled) | 680 |
| Eastern oysters (steamed) | 642 |
| Catfish (steamed/poached) | 500 |
| Skinless sardines (water packed) | 480 |
| Mackerel (canned/drained) | 450 |
| Smoked chinook salmon | 320 |
| Sturgeon roe | 232 |
| Shrimp (canned/drained) | 172 |
| Egg yolk (fresh) | 148 (one yolk contains about 24) |
| Butter | 56 |
| Lamb liver (braised) | 20 |
| Beef tallow | 19 |
| Pork liver (braised) | 12 |
| Beef liver (fried) | 12 |
| Beef tripe (raw) | 12 |
| Beef kidney (simmered) | 12 |
| Chicken livers (simmered) | 12 |
| Small clams (steamed/cooked moist) | 8 |
| Blue crab (steamed) | 4 |
| Crayfish/crawdads (steamed) | 4 |
| Northern lobster (steamed) | 4 |
Vitamin D Requirements and Normal Levels
The present recommendation for adequate daily vitamin D for all age groups is 400 IU. Despite clear recommendations, there is still controversy about the normal serum level of serum vitamin D. From studies looking at the physiology of the vitamin D cycle ( Fig. 1 ), it is surmised that serum levels of less than 30 ng/mL are consistent with relative vitamin D deficit, whereas levels of 30 ng/mL or more may be considered to be sufficient. Using these guidelines, it is estimated that 50% of children and adolescents have vitamin D deficiency. The importance of adequate vitamin D levels for the maintenance of good bone health and prevention of osteoporosis is clearly documented. The presence of insufficient vitamin D levels in such a large proportion of the population therefore has significant implications for prevalence of disorders of calcium, phosphorus, bone metabolism, and the resultant pathology.
Mechanism of Vitamin D Action
Vitamin D exerts its effects by binding to the vitamin D receptor (VDR), which is a nuclear hormone receptor. VDRs belong to the family of nuclear hormone receptors and are structurally homologous to the other members of the group, including retinoic acid, retinoid X, and thyroid hormone receptors. The VDR acts by forming a heterodimer with retinoid-X receptor, binding to the DNA elements, and recruiting coactivators in a ligand-dependent fashion. Vitamin D acts mainly to amplify transcription by way of upregulatory response elements, although some of its effects may also include transcription repression. In addition, the hormone promotes DNA protein interactions of various other transcription factors.
VDRs are found in many tissues and organs, including the small intestine, colon, osteoblasts, activated T and B lymphocytes, pancreatic β islet cells, brain, heart, skin, gonads, prostate, breast, mononuclear cells, as well as skeletal and smooth muscle.
Functions of Vitamin D
Vitamin D has been long-recognized for its calciotropic properties, acting at the level of the kidney, intestine, and bone. Vitamin D is a pleiotropic hormone, influencing other body processes in addition to calcium metabolism. In the last few years, new data have revealed a correlation between vitamin D levels and several significant diseases, including hypertension, diabetes, depression, and cancer. Studies suggesting a likely protective role of vitamin D in these conditions have placed it at the frontline of current scientific endeavors, earning it a potential claim to being the new miracle drug.
The concept of vitamin D pleiotropism originated in 1985 with the identification of VDRs outside bone from cultured rat myoblast cells, proving that, in addition to bone, muscle is also a direct target organ for 1,25(OH)2D. The VDR has subsequently been described in tissues such as smooth muscle, heart muscle, liver, lung, colon, gonads, and skin, and was also recently isolated from human skeletal muscle.
The expanded role of vitamin D was further supported by the discovery of an independent photoendocrine system in epidermal keratinocytes. In addition to synthesizing the prehormone vitamin D destined for kidney activation, epidermal keratinocytes were also found to produce their own, local vitamin D 24-hydroxylase enzyme capable of activating it, as well as epidermal VDRs. These findings suggest local paracrine and autocrine effects of vitamin D in addition to its role as an endocrine hormone. This intracellular autocrine function has also been identified in various organs including bone, brain, muscle, pituitary gland, and liver. The main functions of vitamin D are summarized in Table 4 .
| Site | Function |
|---|---|
| Intestine | Production of calbindin (the calcium-binding protein in the intestine) |
| Promotes calcium and phosphorus absorption in the intestine | |
| Regulates gene transcription and cell proliferation in the parathyroid gland | |
| Bone | Role in synthesis of type 1 collagen: evidence is equivocal |
| Stimulates synthesis of osteocalcin | |
| Promotes differentiation of osteoclasts | |
| Muscles | Increases amino acid uptake in muscles |
| Alters phospholipid metabolism in muscles | |
| Vitamin D deficiency causes myopathy | |
| Nongenomic effects on muscles | |
| Increases troponin C in muscles |
Vitamin D is believed to affect the body’s immune system, endocrine system, cardiocirculatory system, neuromuscular performance, and neuropsychological functioning. It is also believed to act as a potent antioxidant protecting against free radical damage, as well as being an inducer of cellular differentiation, protecting against carcinogenesis.
The basics about vitamin D
Sources
Vitamin D is a steroid hormone that regulates various tissue processes by way of specific cell receptors. 25-Hydroxy vitamin D (25-OHD) is the main storage form of vitamin D, whereas 1,25-dihydroxy vitamin D (1,25-DiOHD or 1,25(OH) 2 D) is the main biologically active metabolite responsible for calcium homeostasis. The main forms of vitamin D are listed in Table 1 . The main source of vitamin D in the human body is cutaneous synthesis via the effects of ultraviolet radiation on the vitamin D precursor, 7-dehydrocholesterol. This mechanism is well regulated and accounts for most of vitamin D synthesis in the body. Anything that reduces the number of solar UVB photons penetrating the skin or alters the amount of 7-dehydrocholesterol in the skin affects production of vitamin D 3 . Examples of factors that interfere with 1 or both of these processes, thus causing vitamin D deficiency, are listed in Table 2 . On the contrary, protective mechanisms such as pigment in skin and formation of inactive isomers (luminosterol, tachysterol) prevent its overproduction.
| Type of Vitamin D | Source |
|---|---|
| Vitamin D 2 (ergocalciferol) | Plant |
| Vitamin D 3 (cholecalciferol) | Animal |
| Dihydrotachysterol | Synthetic |
| Dark skin |
| Inadequate sun exposure |
| Sun exposure at dawn and dusk |
| Insufficient surface area of skin exposed to sun |
| Cloudy (typically winter) outdoors |
| Northern latitudes |
| Consistent use of sunscreens or sun block lotions |
| Low dietary intake |
The alternative source of vitamin D is from dietary intake ( Table 3 ). However, most modern diets contain inadequate vitamin D; even those rich in fortified dairy or cereal products. Vegetarian and vegan diets are lacking in this regard. Thus, whenever sufficient cutaneous production is poor, vitamin D deficiency likely ensues.
| For Every 100 g (3.5 ounces) of: | IU of Vitamin D: |
|---|---|
| Cod liver oil (∼2 teaspoons) | 10,000 |
| Lard (pork fat) | 2800 |
| Atlantic herring (pickled) | 680 |
| Eastern oysters (steamed) | 642 |
| Catfish (steamed/poached) | 500 |
| Skinless sardines (water packed) | 480 |
| Mackerel (canned/drained) | 450 |
| Smoked chinook salmon | 320 |
| Sturgeon roe | 232 |
| Shrimp (canned/drained) | 172 |
| Egg yolk (fresh) | 148 (one yolk contains about 24) |
| Butter | 56 |
| Lamb liver (braised) | 20 |
| Beef tallow | 19 |
| Pork liver (braised) | 12 |
| Beef liver (fried) | 12 |
| Beef tripe (raw) | 12 |
| Beef kidney (simmered) | 12 |
| Chicken livers (simmered) | 12 |
| Small clams (steamed/cooked moist) | 8 |
| Blue crab (steamed) | 4 |
| Crayfish/crawdads (steamed) | 4 |
| Northern lobster (steamed) | 4 |
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