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    What’s the deal with vitamin D?

    What is vitamin D?

    Vitamin D is a series of organic chemical compounds, the most important of which are cholecalciferol (vitamin D3) and ergocalciferol (vitamin D2). They differ chemically only in their side-chain structures, but both play the same role in the body. Both vitamin D3 and D2 are inactive, and only in the body are they transformed into biologically active forms: calcitriol and ercalcitriol [6,9,23].

    The biologically active form is a hormone that affects most of the physiological processes in the human body and mainly controls the body’s calcium and phosphate metabolism.

    Vitamin D promotes calcium absorption in the gut and maintains adequate serum calcium and phosphate concentrations to enable normal bone mineralization and prevent hypocalcemic tetany (involuntary muscle contractions leading to cramps and spasms). It is also needed for bone growth and remodeling by osteoblasts and osteoclasts. Bones can become thin, brittle, and misshapen without sufficient vitamin D. Vitamin D sufficiency prevents rickets in children and osteomalacia in adults. Together with calcium, vitamin D also helps protect older adults from osteoporosis [22,24].

    In addition to the most well-known ones described above, vitamin D also has several other functions, including reduction of inflammation as well as modulation of cell growth, neuromuscular and immune function, and glucose metabolism. Many genes encoding proteins that regulate cell proliferation, differentiation, and apoptosis are modulated in part by vitamin D. Furthermore, vitamin D is an important factor in muscle protein synthesis. This vitamin, due to its antioxidant and anti-inflammatory properties and activity in brain tissue, is essential for mood disorders preventions or treatments, such as depression and anxiety. There are also reports that vitamin D has anti-cancer properties regarding some types of cancer. Such a diverse functionality of vitamin D results from the fact that there are receptors for this vitamin in the cells of many tissues [2,4,18].

    How much vitamin D do we need for proper functioning?

    Given that vitamin D is involved in many processes in the body, and its need depends on many factors, it is difficult to provide a precise and universal recommendation for a daily dose. Organizations issuing clinical guidelines, however, make some estimates, although it should be noted that these often differ from one another.

    The Panel on Dietetic Products, Nutrition, and Allergies (EFSA 2016) defined an adequate vitamin D intake of 600 IU per day for healthy individuals over one year of age, including pregnant and lactating women. The dietary reference values for healthy infants aged 7-11 months have been set at 400 IU per day.

    In turn, the upper safe limit of vitamin D is 4000 IU /day for adults (including pregnant and lactating women) and adolescents aged 11–17 . For children aged 1–10 , the upper safe limit is 2000 IU.

    The extensive range between the appropriate intake and the maximum safe dose may be because vitamin D is relatively safe. Also, the demand for this vitamin varies, depending on many factors, which will be discussed later.

    Sources of vitamin D

    FOOD

    Food products rich in vitamin D are primarily:

    • Oily fish – such as eel, salmon, sardines, herring, and mackerel
    • Cod liver oil
    • Red meat
    • Egg yolks and cheddar cheese

    The richest source of vitamin D is fresh eel, whose content can reach up to 1200 IU/100 g, or cod liver oil, which contains up to 1000 IU/tbs. This vitamin is found negligible in other foods, such as cow milk, vegetables, and fruits. Vegetable oils do not contain it at all [21].

    The average person’s most varied diet is not enough to provide the body with optimal doses of vitamin D. Even in people who do not follow an elimination diet, vitamin D absorption may be insufficient. People who do not eat fish or meat are particularly vulnerable to vitamin D deficiencies, as a plant-based diet is extremely poor in this vitamin [20].

    SUN

    It is also possible to obtain sufficient amounts of vitamin D through exposure to sunlight. Vitamin D is produced endogenously when sunlight ultraviolet type B (UVB) rays strike the uncovered skin and trigger its synthesis. Cutaneous 7-dehydrocholesterol turns into previtamin D3, which becomes inactive vitamin D3. It is then metabolized in the body into its active form [20].

    For this reason, moderate sun exposure is recommended.

    But what does moderate mean? It is estimated that about 30 minutes of sun exposure per day (particularly between 10 a.m. and 4 p.m.) covering the face, arms, hands, and legs without sunscreen usually leads to sufficient vitamin D synthesis, i.e., 2000-4000 IU [21,22].

    This amount of sun exposure may seem easy to achieve, but many factors influence the ability to produce vitamin D in the skin. The season, time of day, day length, cloud cover, smog, melanin content in the skin, and sunscreen are some factors that affect UVB exposure and vitamin D synthesis. Older people and people with dark skin have a lower ability to produce vitamin D from sunlight. Significantly, UVB radiation does not penetrate glass, so exposure to sunlight indoors through a window does not produce vitamin D [20].
    On the other hand, excessive sun exposure will not be beneficial. UV radiation is a carcinogen, and the use of photoprotective agents to reduce the risk of skin cancer is necessary. Creams with high SPF factors are highly recommended –>click to see our article about UV filters.

    Unfortunately, the very same UVB wavelengths (290-320 nanometers, or nm) that make the body synthesize vitamin D also produce sunburn and genetic mutations that can lead to skin cancer [20].
    Is this already an impasse? While the sun’s vitamin D synthesis is beneficial, we should use sunscreens with a high SPF. Let’s take a closer look at this.
    Sunscreens with an SPF of only eight block the UVB rays that produce vitamin D. However, this is a purely theoretical situation that may occur under ideal laboratory conditions rather than in real life. Studies have never found that everyday sunscreen use leads to vitamin D insufficiency.

    In fact, people who use sunscreen daily can maintain their vitamin D levels. One of the explanations for this may be that no matter how much sunscreen you use or how high the SPF is, some of the sun’s UV rays reach your skin. An SPF 15 sunscreen filters out 93 percent of UVB rays, SPF 30 keeps out 97 percent, and SPF 50 filters out 98 percent. This leaves anywhere from 2 to 7 percent of solar UVB reaching your skin, even with high-SPF sunscreens. And that’s if you use them perfectly all over your skin – an appropriately thick layer of cream for every centimeter of exposed skin [19]. However, covering the entire body with a suitably thick layer of sunscreen is not very realistic. Moreover, sunscreen can wear off during the day, for example, when sweating or touching the skin. Either way, vitamin D production is still going on!

    SUPPLEMENTATION

    So, is there a chance we can get as much vitamin D as we need without supplementation? For healthy people who eat a balanced diet and spend time outdoors during spring and summer, this should not be a problem. However, vitamin D supplementation is highly recommended in the fall and winter. It is also recommended that young children and the elderly take supplements all year round.

    Many products containing vitamin D are available on the market, and manufacturers are competing with each other by offering increasingly higher doses. But is the highest over-the-counter dose the best?

    Remember that supplementation for a healthy person is intended to PREVENT deficiencies and should not be based on high doses. High doses of vitamin D are intended for people with diagnosed deficiencies or diagnosed diseases in which a high supply of vitamin D is particularly necessary.

    Recommendations for vitamin D supplementation according to the current Polish guidelines issued last year are as follows [22] :

    GroupAge and recommended dose
    Neonates born at term and infantsAge 0–6 months: 400 IU of vitamin D3/day

    Age 6–12 months: 400–600 IU of vitamin D3/day
    Children (1–10 years) Age 1–3 years: 600 IU of vitamin D3/day

    Age 4–10 years: 600–1000 IU of vitamin D3/day (starting from May until the end of September; supplementation is not necessary, although still recommended and safe)
    Adolescents (11–18 years) and adults (19–65 years)1000–2000 IU of vitamin D3/day (starting from May until the end of September; supplementation is not necessary, although still recommended and safe)
    Pregnancy and lactationWomen planning pregnancy should receive adequate cholecalciferol supplementation – the same as in the general adult population.
    When pregnancy is confirmed until the end of breastfeeding, cholecalciferol supplementation should be carried out under the control of serum concentration to achieve and maintain optimal concentrations within the ranges of >30–50 ng/mL OR 2000 IU of vitamin D3/day.
    Younger seniors (>65–75 years)1000–2000 IU of vitamin D3/day
    Older seniors (>75 years)2000–4000 IU of vitamin D3/day
    These guidelines are designed for the general population without an increased risk of significant vitamin D deficiency. People with significant deficiencies are usually under medical care and often require much higher doses of vitamin D.

    These guidelines are designed for the general population without an increased risk of significant vitamin D deficiency. People with significant deficiencies are usually under medical care and often require much higher doses of vitamin D.

    How do we recognize symptoms of vitamin D deficiency?

    ConditionReason
    Elderly peopleBoth synthesis and metabolism of vitamin D change with aging due to reduced synthesis capacity of the skin (aged skin produces approximately 40% less vitamin D than younger skin) and age-related renal function reduction [11].
    Vegans, vegetariansReduced dietary intake [10].
    Overweight and obese peopleVitamin D is fat-soluble, so in obese people, it accumulates in large quantities in adipose tissue, which is not available to the body.
    During effective weight loss, vitamin D is released from adipose tissue [1].
    DepressionSerum vitamin D levels inversely correlate with clinical depression; the association appears to be driven by the homeostatic, trophic, and immunomodulatory effects of vitamin D [16,28].
    Diabetes type 1 and 2Vitamin D alleviates the course of the disease. There is a significant relationship between vitamin D deficiency and the development of type 1 and type 2 diabetes. Inflammation has a significant role in the pathogenesis of type 1 diabetes, and vitamin D acts as an immunomodulator. Low vitamin D is associated with insulin resistance, and so with type 2 diabetes [3,22].
    Atopic dermatitisVitamin D alleviates the course of the disease. Atopic dermatitis is a chronic inflammatory skin disease that involves a dysregulated immune response. Vitamin D is an immunomodulator but is also associated with the proliferation and differentiation of keratinocytes and the production of antimicrobial peptides-endogenous antibiotics [12,17].
    Rheumatoid arthritisVitamin D alleviates the course of the disease; rheumatoid arthritis manifests with chronic inflammation of synovial joints, and vitamin D concentration is lower in these patients than in healthy individuals. Vitamin D is important for immunomodulation, as its active form suppresses autoimmunity by inhibiting the production of pro-inflammatory cytokines [13,15].
    Hyperthyroidism, hypothyroidism and parathyroidismPoor absorption of vitamin D from the intestine and the body may not activate vitamin D properly [7,14,25].
    Renal failureReduced amount of active form of vitamin D in serum [24].
    Liver failure, hepatic steatosisReduced amount of active form of vitamin D in serum [24].
    Inflammatory bowel diseasesReduced absorption from the gut [24].

    Vitamin D levels that are too low are quite common-about 50% of the population has vitamin D insufficiency. Unfortunately, recognizing it is not easy because most patients with vitamin D insufficiency (or even deficiency) are asymptomatic, and routine testing of serum vitamin D levels in healthy populations is not recommended [20,24].

    Only patients with prolonged and severe vitamin D deficiency can experience symptoms associated with secondary hyperparathyroidism, including bone pain, arthralgias, myalgias, fatigue, muscle twitching (fasciculations), and weakness. Fragility fractures may result from chronic vitamin D deficiency, leading to osteoporosis. In children, irritability, lethargy, developmental delay, bone changes, or fractures can be vitamin D deficiency symptoms [24].
    It means that significantly low vitamin D levels become noticeable only in the long term, but the effects on the body may be severe and irreversible.

    This data shows us that vitamin D supplementation is extremely important. It is important, however, to use it wisely. As with everything, you can overdo it in this area too.

    How to approach vitamin D supplementation?

    Follow the current guidelines for vitamin D

    Healthy people do not need to use too high doses – vitamins can also be overdosed. Although vitamin D is quite safe, in very high doses used for a long time, it may be toxic [24].

    Acute intoxication can lead to acute hypercalcemia that can cause confusion, anorexia, vomiting, polyuria, polydipsia, and muscle weakness. Chronic intoxication can lead to nephrocalcinosis and bone pain [20]. How high doses are we talking about? It is worth being careful when using vitamin D in doses higher than 10 000 IU/day for a long time – then the concentration of calcium in the serum should be monitored [20]. Currently, many products on the market contain this vitamin, sometimes even in very high doses – remember, you may not need the highest dose.

    Vitamin D3 is more efficacious than vitamin D2

    At raising serum concentrations, and thus, vitamin D3 is the preferred choice for supplementation [26].

    Vitamin D cannot be metabolized without adequate magnesium level

    Ensure you get enough magnesium from food or products containing magnesium – magnesium lactate has one of the best absorption rates. Taking recommended doses of vitamin D may not be beneficial if magnesium levels are too low [27].

    Vitamin D may be harmful in the presence of magnesium deficiency; then, the use of high doses of vitamin D may lead to a dangerous depletion of magnesium resources in the body, which may lead to arterial calcification or atherosclerosis.

    The presence of zinc is also important

    Zinc deficiency inhibits processes related to the activation of vitamin D in the kidneys [5].

    Calcium Intake in Vitamin D Deficiency Prevention

    During the prevention of vitamin D deficiency, an appropriate dietary calcium intake should be assured [22].

    Vitamin D supplementation is very important, but it should be approached responsibly.

    References:

    1. Abdullah Thani NSI, Khairudin R, Ho JJ, Muhamad NA, Ismail H. Vitamin D supplementation for overweight or obese adults. Cochrane Database Syst Rev. 2019 May 23;2019(5):CD011629. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6532760/
    2. Abiri B, Vafa M. Vitamin D and Muscle Sarcopenia in Aging. Methods Mol Biol. 2020;2138:29-47. https://pubmed.ncbi.nlm.nih.gov/32219739/
    3. Abugoukh TM, Al Sharaby A, Elshaikh AO, Joda M, Madni A, Ahmed I, Abdalla RS, Ahmed K, Elazrag SE, Abdelrahman N. Does Vitamin D Have a Role in Diabetes? Cureus. 2022 Oct 18;14(10):e30432. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9671203/
    4. Akpınar Ş, Karadağ MG. Is Vitamin D Important in Anxiety or Depression? What Is the Truth? Curr Nutr Rep. 2022 Dec;11(4):675-681. doi: 10.1007/s13668-022-00441-0. Epub 2022 Sep 13. PMID: 36097104; PMCID: PMC9468237. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9468237/
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    7. Czarnywojtek A, Florek E, Pietrończyk K, Sawicka-Gutaj N, Ruchała M, Ronen O, Nixon IJ, Shaha AR, Rodrigo JP, Tufano RP, Zafereo M, Randolph GW, Ferlito A. The Role of Vitamin D in Autoimmune Thyroid Diseases: A Narrative Review. J Clin Med. 2023 Feb 11;12(4):1452. https://pubmed.ncbi.nlm.nih.gov/36835987/
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    10. García-Maldonado E, Gallego-Narbón A, Zapatera B, Alcorta A, Martínez-Suárez M, Vaquero MP. Bone Remodelling, Vitamin D Status, and Lifestyle Factors in Spanish Vegans, Lacto-Ovo Vegetarians, and Omnivores. Nutrients. 2024 Feb 2;16(3):448. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC10857037/
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