Vitamin D is a fat-soluble vitamin, and it includes vitamins D3 (cholecalciferol) and D2 (ergocalciferol). Calciferol can be found in its active form after UV light exposure and after synthesis from its precursor 7-dehydrocholesterol. We can get the rest of D3 and D2 from certain foods, including Solanaceae plants and fish oils.

Vitamin D is crucial for bone health because it plays a role in calcium metabolism. Lately, more attention has been given to its other roles, such as maintaining a healthy immunity system.

Low concentrations of this vitamin are associated with a higher risk of developing diseases such as cancer or multiple sclerosis.

No fear of the sun

Exposure to UV light can’t cause vitamin D toxicity because its excess is degraded to compounds without calcemic activity. When the concentration is low, they can be converted into active vitamin forms.

Vitamin D is stored in the liver where its concentration is regulated by D-25-dehydrogenase. During a vitamin overdose, the enzyme is unable to complete its function and the receptors are saturated.

Vitamin D toxicity hypotheses

There are several hypotheses available to explain the mechanism of vitamin D toxicity, but its exact cause has not been determined yet. A high concentration of vitamin D generates numerous metabolites. Active metabolite 1,25-dihydroxy vitamin D (1,25(OH)2D) exhibits high affinity towards vitamin D binding receptors leading to increased gene expression.

Inactive metabolites are 25(OH)D, 24,25(OH)2D, 25,26(OH)2D and 25(OH)D-26,23-lactone. In the bloodstream, they are transported by vitamin D-binding protein. As a result of its saturation, free 1,25(OH)2D is released, which, along with 25(OH)D, binds to vitamin D receptors, affecting transcription within the cell.

Vitamin D toxicity is manifested as hypercalcemia and imbalance in bone metabolism regulation which leads to clinical symptoms.

Overdose symptoms

Overdose symptoms are weakness, anorexia, polyuria and dehydration, constipation, sickness, vomiting, annoyance, concentration difficulties, irritability, fatigue, and sometimes coma.

Causes of overdose

Vitamin D can be found in supplements that are either prescribed or available for free sale. Diseases such as osteoporosis, renal osteodystrophy, psoriasis, celiac disease, and some IBDs require vitamin D supplementation.

Overdose is common due to insufficient therapy monitoring. In addition, patients frequently take doses higher than the prescribed ones, either intentionally or out of ignorance. Another reason for overdose can be high prescribed doses.

The role of healthcare workers

Vitamin D overdose can be a life-threatening condition, so healthcare workers need to recognize symptoms on time and direct the patient to additional check-ups. Besides therapy monitoring, patient education is also important, especially for ones who have to ingest high supplement doses.