Components of vitamin D: Prospect of over-absorbing vitamin D

First isolated in 1918, **vitamin D** encompasses a trio of biologically active secosteroid compounds: **ergocalciferol (D₂)**, derived from plant and fungal sources; **cholecalciferol (D₃)**, synthesized endogenously in animal tissues; and **7-dehydrocholesterol**, a cutaneous precursor that undergoes photochemical conversion to active vitamin D upon exposure to ultraviolet B (UVB) radiation. While the former two forms are obtained through dietary intake, the latter is produced *in situ* via a sunlight-dependent process whose efficiency is contingent upon multiple variables, including duration of solar exposure, latitude-specific UVB intensity, skin melanin concentration, genetic predispositions, and environmental impediments such as atmospheric pollution and persistent cloud cover. In temperate climatic zones—such as those prevalent across Central and Northern Europe—suboptimal UVB irradiation for much of the year significantly curtails cutaneous vitamin D synthesis. Additional inhibitory factors include the routine application of broad-spectrum sunscreens, urban particulate pollution, and prolonged overcast conditions. By contrast, dietary vitamin D exhibits a modest bioavailability of approximately 50%. Current dietary reference intakes (DRIs) stipulate a daily allowance of 10 micrograms for infants up to 12 months of age and 15 micrograms for all other demographic groups. Maintaining adequate vitamin D status is imperative owing to its pleiotropic physiological roles, which encompass the facilitation of intestinal calcium and phosphate absorption, the regulation of serum calcium homeostasis, the promotion of skeletal and dental mineralization, the modulation of vascular blood pressure, the enhancement of innate and adaptive immune responses, the preservation of muscular strength, and the optimization of neuromuscular coordination. Emerging epidemiological evidence further suggests a protective association between sufficient vitamin D levels and reduced incidence of certain malignancies (e.g., colorectal and breast cancers), as well as a lowered susceptibility to bacterial vaginosis during pregnancy. Prolonged vitamin D insufficiency may precipitate severe metabolic derangements, including rickets in pediatric populations, osteomalacia in adults, hypertensive disorders, neurocognitive impairments, an elevated risk of type 2 diabetes mellitus, autoimmune pathologies (e.g., multiple sclerosis), and chronic inflammatory conditions. Milder deficiencies manifest as nonspecific symptomatology, such as irritability, heightened susceptibility to viral and bacterial infections, persistent fatigue, myalgia, sleep architecture disturbances, psychomotor agitation, and depressive affect. Populations at heightened risk for deficiency include the elderly, pregnant and lactating women, individuals with obesity, and patients afflicted with chronic illnesses—such as rheumatoid arthritis, congestive heart failure, hypoparathyroidism, hepatic cirrhosis, end-stage renal disease, type 2 diabetes mellitus, tuberculosis, psoriasis, and multiple sclerosis.

The most abundant dietary sources of vitamin D are primarily fatty fish varieties, including smoked herring, eel, Atlantic herring, and mackerel. Eggs—particularly the yolks—also contain substantial amounts of calciferol. Other foods rich in this vitamin include calf liver, pork liver, beef liver, natural butter, aged yellow cheeses, and whole milk. Despite the widespread availability of vitamin D-rich foods, deficiencies remain a significant public health concern in Poland, affecting between 50% and 80% of the population. Consequently, at low concentrations, systematic supplementation is recommended from September through April, when sunlight exposure is limited. The market offers a variety of supplements, such as cod liver oil fortified with vitamins A, D, and K, micronized fish bone preparations, and combined D2 + D3 complexes. However, before initiating supplementation, it is essential to undergo laboratory testing to determine the current vitamin D levels in the body. For mild deficiencies, regular sun exposure can be an effective remedy: just 20 minutes of unprotected UV exposure to the face, hands, and forearms may significantly improve vitamin D status. It is crucial, however, to avoid sunbathing during peak hours (11:00 AM to 3:00 PM) to prevent sunburn and accelerated skin aging. Learn more about how to effectively prevent and correct vitamin D deficiencies in the body!

Excessive vitamin D accumulation in the body is an exceptionally rare occurrence, particularly given the widespread deficiencies observed in the Polish population. Hypervitaminosis D arises solely from the uncontrolled ingestion of high-dose pharmaceutical supplements, rather than from prolonged sun exposure or consumption of vitamin D-rich foods. Symptoms of calciferol overdose include gastrointestinal disturbances such as diarrhea, vomiting, and abdominal pain, along with headaches, loss of appetite, weight reduction, persistent fatigue, muscle weakness, renal calculus formation, and developmental delays in children. The most reliable and safest method of obtaining adequate vitamin D remains a balanced diet, which negates the necessity for supplemental intake while ensuring all essential nutrients are provided in optimal quantities.