Abstract
Vitamin D is a fat-soluble vitamin with properties similar to those of a
steroid hormone. Sufficient vitamin D levels are essential for maintaining
overall body homeostasis and well-being, while vitamin D deficiency is
associated with a higher prevalence of several diseases and a higher incidence of injuries
and bone fractures. This review summarizes the most relevant aspects of
vitamin D metabolism, examines the different dietary sources of vitamin D (including supplements)
and provides recommendations on vitamin D intake for both
sedentary and physically active individuals, including athletes. It also discusses
when vitamin D supplementation should be considered to achieve optimal levels.
The available evidence indicates that oral vitamin D supplementation may have
a notable impact on physical performance, particularly in individuals with serum
25(OH)D levels—the marker of serum vitamin D concentration—that are deficient (<50 nmol/L) or
insufficient (50–75 nmol/L). Raising vitamin D levels to sufficient values (75 to <125
nmol/L) in the general population may support overall health, while increasing
vitamin D to optimal levels (≥125 nmol/L) in athletes may optimize physical
performance.
Toxicity from excessive vitamin D intake is rare in athletes. However,
taking high doses of vitamin D for many years can cause toxicity, defined
as serum 25(OH)D levels above 180 nmol/L (72 ng/mL), which would lead to
hypercalcemia. A daily intake of up to 10,000 IU is generally considered the
tolerable upper intake level.
Introduction
Vitamin D is nutritionally classified as a fat-soluble vitamin, but it acts
as a steroid hormone (through genomic and non-genomic mechanisms).
Vitamin D controls the function of many non-skeletal tissues and cells, affecting human
health and quality of life. In fact, sufficient vitamin D levels are essential for
maintaining body homeostasis, well-being and overall health, from fetal development
through old age (7), while vitamin D deficiency is associated with an increase in the
prevalence of numerous diseases (for example, diabetes, cancer, and autoimmune
and cardiovascular disorders), including skeletal-muscle diseases (3), as well
as a greater incidence of injuries and bone fractures (14). Vitamin D's broad effect
stems from the widespread presence of vitamin D receptors (VDR), which, although
present at varying concentrations, are expressed in all human tissues, including
skeletal muscles (6). Limited skin exposure to sunlight and poor nutrition
contribute, among other factors, to insufficient vitamin D levels observed in
athletes (15). In addition, obesity, through the accumulation of vitamin D metabolites in
adipose tissue, is a cause of vitamin D deficiency (13), which has subsequently been
linked to muscle weakness, myalgia and poor physical performance (8).
Vitamin D Sources, Metabolism and Levels in Humans
The main source of vitamin D in humans is ultraviolet-B (UV-B) radiation (sunlight,
which provides between 80–90%), inducing vitamin D synthesis from its precursor
7-dehydrocholesterol in the skin, while natural dietary sources of vitamin D
play a smaller role (providing between 10 and 20%) (17). Two
forms of vitamin D can be obtained from the diet: vitamin D3 (cholecalciferol) and vitamin D2
(ergocalciferol). Vitamin D3 is found in foods of animal origin, such as
cod liver oil, salmon and egg yolk, as well as fortified products such as
milk, cereals and orange juice, while vitamin D2 is present in some
plants and fungi (10). Although these dietary sources may be relevant, the process of
absorbing dietary vitamin D is only approximately 50% efficient; therefore,
much of its nutritional value is lost during digestion (15). For this reason, to achieve
optimal blood vitamin D levels, adequate intake from
the diet, exposure to ultraviolet B (UVB) radiation and, if necessary, following a
supplementation protocol are essential.
Vitamin D3 plays a fundamental role in bone metabolism and has important effects
on muscle function (8). Both forms (vitamin D3 and D2) are converted into 25-
hydroxyvitamin [25(OH)D] in the liver, and the latter can be measured in
blood samples. In the kidney, 25(OH)D is converted into the biologically active compound
calcitriol, also known as 1,25-dihydroxyvitamin D3 (1,25(OH)2 D3) (7). Calcitriol
stimulates intestinal calcium and phosphate absorption and promotes the formation of new
bone tissue (9).
Although the optimal serum 25(OH)D level remains under debate, in sports
an optimal blood level of around ~125 nmol/L (50 ng/mL) is recommended, with no
additional benefits observed above this value (19). Although this level has been
considered excessive for the general population (7), it is estimated that, to optimize
physical performance in athletes, reaching this 25(OH)D concentration is necessary for
vitamin D to be stored in adipose tissue and skeletal muscle (3,17). In
general, serum 25(OH)D levels of ≥ 75 -100 up to 125 nmol/L (≥ 30 – 40 up to 50 ng/mL),
although perhaps not optimal for athletes, are considered sufficient for sedentary
or less-active people (21). However, values between ~ 50 and < 75 nmol/L (20 to < 30 ng/mL) are considered inadequate or insufficient; when blood levels fall below 50 nmol/L (<20 ng/mL), this is considered vitamin D deficiency (21). In fact, levels below 10 nmol/L indicate severe deficiency (14).
In trained individuals, maintaining adequate 25(OH)D levels may positively affect athletic performance, whereas insufficient vitamin D levels may limit adaptation to exercise and prevent optimal performance. Animal studies suggest that increased 1,25(OH)2D and the presence of VDR in muscle cells could appreciably affect calcium-binding efficiency during muscle activation and might even contribute to the enlargement and multiplication of fast-twitch muscle fibers (16, 22). In addition, in vitro studies indicate stimulatory effects of elevated 1,25(OH)2D and intracellular VDR on lipolysis and Krebs cycle activity (the tricarboxylic acid cycle) (2).
Athletes who live above 35° latitude or primarily train and compete indoors are at greater risk of vitamin D insufficiency or even deficiency (21). Other lifestyle factors and habits—such as dark skin, high body-fat content, training in the early morning or at dusk when UVB levels are low, and aggressively blocking exposure to UVB radiation through clothing, equipment and sunscreen use—increase the risk of vitamin D insufficiency and deficiency. In fact, the recommended exposure window for obtaining enough ultraviolet (UV) radiation for vitamin D synthesis is considered to be 5 to 30 minutes between 10 a.m. and 4 p.m. (4).
Vitamin D Intake and Physical Performance
It is estimated that the body requires 3000 to 5000 IU of vitamin D per day to meet the needs of nearly all tissues and cells in the human body (11). ). The Endocrine Society recommends a daily vitamin D intake of 400–1000 IU for infants, 600–1000 IU for children (1–18 years) and 1500–2000 IU for adults, together with adequate sun exposure (12). However, higher intakes would be necessary if serum levels begin to fall into the insufficient range (15). Intake recommendations increase with age, pregnancy and breastfeeding. In addition, various supplementation protocols have been proposed to support athletic performance and restore optimal levels when necessary. For example, the literature describes doses ranging from 2000 IU to 200,000 IU (in patients with severe deficiency) until optimal 25(OH)D levels are reached, followed by 1000 to 2000 IU daily for maintenance (15). For athletes with low serum 25(OH)D levels (< 75 nmol/L or < 30 ng/mL), an intake of 5000 IU daily for 8 weeks is recommended, followed by a maintenance intake of 1000 to 2000 IU (15). Nevertheless, it is worth remembering that vitamin D only begins to be stored in muscle and fat for future use when 25(OH)D levels exceed 100 to 125 nmol/L (more than 40 to 50 ng/mL) (3). Therefore, at values < 125 nmol/L (50 ng/mL), the body would depend primarily on daily vitamin D replenishment to meet its requirements, something the usual diet generally does not provide. Although concentrations of 75 to 100 up to 125 nmol/L of 25(OH)D are considered sufficient by most health associations (21), and may be just enough to cover immediate metabolic needs, this amount does not ensure that stored vitamin D is available for the advanced processes involved in critical autocrine pathways, which probably have a greater influence on physical performance. Thus, even when vitamin D levels are clinically normal, they remain insufficient to sustain physical performance and adaptation to exercise in athletes (17). It is essential to stress that once serum 25(OH)D concentrations reach adequate levels through supplementation, no relevant improvements in physical performance are observed (9). In particular, when study participants began interventions with 25(OH)D values above ~50 nmol/L (20 ng/L), vitamin D3 supplementation generally showed no significant effects (8). It is worth noting that some randomized controlled trials (1) may have overlooked changes that were not statistically significant but could still influence athletic performance when participants started with insufficient 25(OH)D levels (50 to 75 nmol/L or 20 to 30 ng/L) and rose toward > 75 to 125 nmol/L (>30 to 50 ng/L).
In summary, increasing dietary vitamin D3 intake, including
supplement sources, is more useful for preventing declines in muscle function and optimizing
training-induced adaptation than for continuously driving increases in
physical performance.
Vitamin D Supplementation in Physically Active People
Commercially available forms of vitamin D supplements include tablets, capsules,
softgels, chewable tablets (which may sometimes be marketed interchangeably as
tablets or capsules), sprays (oral or sublingual), drops or liquid solutions, and gummies
(which generally contain a considerable amount of sugar).
Vitamin D2 has markedly lower biological activity than vitamin
D3. Consequently, vitamin D3 is the most effective form for raising the total
25(OH)D concentration (14). Effective daily doses of vitamin D3 may range from 1000 IU to
5000 IU (21). Recent research recommends 2000 IU (50 μg) of vitamin D3 per day
as a simple, effective and safe dose for preventing and treating vitamin D deficiency in
the general adult population (18). During winter, athletes who train indoors or
at high latitudes may fall at the upper end of this range (2000–5000 IU daily).
In addition, short-duration studies (from a few weeks to 2–3 months of intervention)
show rapid improvements in physical performance in deficient individuals when
vitamin D levels are normalized, whereas longer intervention periods
generally do not show significant increases in physical performance. As
mentioned earlier, once appropriate vitamin D levels have been restored,
additional supplementation provides no ergogenic benefits, and performance improvements
plateau, being particularly limited by optimal adaptation to training (8).
For physically active individuals, to maintain appropriately stable
vitamin D levels throughout training cycles, consistent
supplementation (daily or weekly) should be considered. Vitamin D3 is better absorbed when
taken with food, particularly with a meal or snack containing
some fat (for example, eggs, avocado, nuts or dairy products), which stimulates
bile secretion and micelle formation, thereby facilitating vitamin D absorption (5).
Indeed, because vitamin D is fat-soluble, although many protocols
recommend daily intake, it is not strictly necessary to take it every day. Large doses
(> 5000 IU) could be taken once a week; the nutrient would then be stored
and gradually used by the body. Consistency and the food context
are more important than the exact time or weekly frequency of intake (5).
Other aspects to consider are dietary composition and ethnicity. Although there is no
evidence that vegetarians metabolize vitamin D differently,
some studies indicate that vitamin D levels in vegans may be slightly
lower if they do not take supplements (23). It is also worth remembering that most
recommendations are based on White populations, while recent research
suggests that much higher intakes may be required in people from other
ethnic groups; moreover, vitamin D needs may vary considerably among different
regions or continents, suggesting that some populations may require doses
higher than initially estimated (18).
In summary, an appropriate vitamin D3 supplementation protocol for
healthy, physically active individuals, adjusted for diet and personal factors, could be as follows:
- Assess serum 25(OH)D level: Measuring the
25(OH)D concentration (especially at the end of winter) is strongly recommended to adjust the dose. - If levels are optimal (in athletes, 125 nmol/L, ≥50 ng/mL) or sufficient for the
sedentary general population (≥ 75 nmol/L, 30 to 40 ng/L), a
maintenance dose may be enough (or even sensible sun exposure alone in summer). For athletes with
levels below 125 nmol/L, a dose of 3000 to 5000 IU for 8
weeks is suggested, followed by a maintenance dose of 2000 IU. - For insufficiency (50 to 75 nmol/L, 20–30 ng/mL), ~2000–4000 (or
5000 in athletes) IU per day for 8–12 weeks is recommended. - For deficiency (< 50 nmol/L, <20 ng/mL), ~7000 IU per day may be prescribed
(~50,000 IU once a week) for 6–8 weeks (8). - Maintenance dose: After reaching the desired 25(OH)D concentration: ~1000–
2000 IU per day (in omnivores this may be closer to 1000; in vegetarians, often
2000 if there is no dietary intake). Athletes with high training loads or
high body mass may require 2000–5000 IU daily, even for
maintenance, especially in winter (23). This can be administered daily or as its
weekly equivalent (for example, 7000 IU once a week) (20).
Toxicity and Possible Side Effects Caused by Vitamin D Intake
According to the Institute of Medicine, there are limited data on the negative effects of
high 25(OH)D concentrations, with values above 125 nmol/L in the sedentary
general population. In athletes, levels above 375 nmol/L should alert
healthcare professionals to possible adverse effects, including kidney and other
tissue damage (14). Vitamin D toxicity is very rare in athletes. Taking high doses of
vitamin D for years can cause toxicity (>180 nmol/L, 72 ng/mL) and hypercalcemia.
Although studies of prolonged oral dosing indicate that 10,000 IU daily is a
tolerable upper intake level with no reported cases of toxicity (19), consistently taking such a high dose
for years could trigger a negative
feedback effect, reducing 1,25(OH)2D concentrations (17).
Conclusions
Oral vitamin D3 supplementation influences physical performance, particularly in
situations of vitamin D deficiency or insufficiency. Scientific evidence indicates that
achieving sufficient vitamin D levels in the general population (or optimal levels in
athletes) is important for avoiding limitations in physical performance. Ultimately,
a serum 25(OH)D concentration of 75 nmol/L (30 ng/mL) or lower requires
vitamin D supplementation, and when the concentration ranges between >75 and <175 nmol/L (31–
40 ng/L), physically active individuals should consider
supplementation strategies for this nutrient.
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AUTHOR
Dr Fernando Naclerio
Professor of Strength Training and Sports Nutrition
Centre for Exercise Activity and Rehabilitation
Institute for Lifecourse Development
School of Human Sciences
University of Greenwich
Email: f.j.naclerio@gre.ac.uk
REFERENCES
- Beaudart, C, Buckinx, F, Rabenda, V, et al. The effects of vitamin d on skeletal muscle
strength, muscle mass, and muscle power: A systematic review and meta-analysis of
randomized controlled trials. J Clin Endocrinol Metab 99: 4336–4345, 2014. - Chang, E and Kim, Y. Vitamin D decreases adipocyte lipid storage and increases
NAD-SIRT1 pathway in 3T3-L1 adipocytes. Nutrition 32: 702–708, 2016.
doi.org/10.1016/j.nut.2015.12.032 - Chiang, C, Ismaeel, A, Griffis, RB, and Weems, S. Effects of vitamin D
supplementation on muscle strength in athletes: A systematic review. J Strength Cond
Res 31: 566–574, 2017. - Crescioli, C. Vitamin d restores skeletal muscle cell remodeling and myogenic
program: Potential impact on human health. Int J Mol Sci 22: 1–12, 2021. - Dawson-Hughes, B, Harris, SS, Lichtenstein, AH, et al. Dietary fat increases vitamin
d-3 absorption. J Acad Nutr Diet 115: 225–230, 2015.
doi.org/10.1016/j.jand.2014.09.014 - Dzik, KP and Kaczor, JJ. Mechanisms of vitamin D on skeletal muscle function:
oxidative stress, energy metabolism and anabolic state. Eur J Appl Physiol 119: 825–
839, 2019. doi.org/10.1007/s00421-019-04104-x - Farrokhyar, F, Sivakumar, G, Savage, K, et al. Effects of Vitamin D Supplementation
on Serum 25-Hydroxyvitamin D Concentrations and Physical Performance in Athletes:
A Systematic Review and Meta-analysis of Randomized Controlled Trials. Sports
Medicine 47: 2323–2339, 2017. - Gunton, JE and Girgis, CM. Vitamin D and muscle. Bone Rep 8: 163–167, 2018.
doi.org/10.1016/j.bonr.2018.04.004 - Han, Q, Xiang, M, An, N, et al. Effects of vitamin D3 supplementation on strength of
lower and upper extremities in athletes: an updated systematic review and metaanalysis of randomized controlled trials. Front Nutr 11: 1–15, 2024. - He, CS, Fraser, WD, Tang, J, et al. The effect of 14 weeks of vitamin D3
supplementation on antimicrobial peptides and proteins in athletes. J Sports Sci 34:
67–74, 2016. doi.org/10.1080/02640414.2015.1033642 - Holick, MF. The vitamin D epidemic and its health consequences. J Nutr 135, 2005.
- Holick, MF, Binkley, NC, Bischoff-Ferrari, HA, et al. Evaluation, treatment, and
prevention of vitamin D deficiency: An endocrine society clinical practice guideline. J
Clin Endocrinol Metab 96: 1911–1930, 2011. - Hyppönen, E and Boucher, BJ. Adiposity, Vitamin D requirements, and clinical
implications for obesity-related metabolic abnormalities. Nutr Rev 76: 678–692, 2018. - Moran, DS, McClung, JP, Kohen, T, and Lieberman, HR. Vitamin D and physical
performance. Sports Med. 43: 601–611, 2013. - Ogan, D and Pritchett, K. Vitamin D and the athlete: Risks, recommendations, and
benefits. Nutrients 5: 1856–1868, 2013. - Oku, Y, Tanabe, R, Nakaoka, K, et al. Influences of dietary vitamin D restriction on
bone strength, body composition and muscle in rats fed a high-fat diet: Involvement of
mRNA expression of MyoD in skeletal muscle. J Nutr Biochem. 32: 85–90,
2016.Available from: http://dx.doi.org/10.1016/j.jnutbio.2016.01.013 - Owens, DJ, Allison, R, and Close, GL. Vitamin D and the Athlete: Current
Perspectives and New Challenges. Sports Medicine 48: 3–16, 2018.
doi.org/10.1007/s40279-017-0841-9 - Pludowski, P, Grant, WB, Karras, SN, Zittermann, A, and Pilz, S. Vitamin D
Supplementation: A Review of the Evidence Arguing for a Daily Dose of 2000
International Units (50 µg) of Vitamin D for Adults in the General Population.
Nutrients 16, 2024. - Shuler, FD, Wingate, MK, Moore, GH, and Giangarra, C. Sports Health Benefits of
Vitamin D. Sports Health 4: 496–501, 2012. - Takács, I, Tóth, BE, Szekeres, L, et al. Randomized clinical trial to comparing efficacy
of daily, weekly and monthly administration of vitamin D3. Endocrine 55: 60–65, - doi.org/10.1007/s12020-016-1137-9
- Thomas, T, Erdman, KA, and Burke, LM. American College of Sports Medicine Joint
Position Statement: Nutrition and athletic performance. Med Sci Sports Exerc 48: 543–
68, 2016. - Trovato, FM, Castrogiovanni, P, Szychlinska, MA, Purrello, F, and Musumeci, G.
Impact of western and mediterranean diets and vitamin D on muscle fibers of
sedentary rats. Nutrients 10: 1–15, 2018. - Yoon, S, Kwon, O, and Kim, J. Vitamin D in athletes: focus on physical performance
and musculoskeletal injuries. Phys Act Nutr 25: 20–25, 2021
