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Uncontrolled and excessive use of vitamin D supplements may lead to vitamin D toxicity and serious, potentially life-threatening health problems, particularly during the autumn and winter months when sunlight exposure is limited, said Prof. Dr. Mustafa Altay, Endocrinology and Metabolic Diseases specialist at the University of Health Sciences Ankara Training and Research Hospital.
In a statement to Anadolu Agency, Prof. Dr. Altay said that blood vitamin D levels above 100 ng/mL significantly increase the risk of poisoning. He explained that vitamin D production decreases during autumn and winter due to insufficient sunlight exposure, which has led to increased and sometimes unregulated use of vitamin D supplements.
According to Altay, a vitamin D level of 30–50 ng/mL is considered sufficient for musculoskeletal health in healthy adults. Levels between 12 and 20 ng/mL indicate insufficiency, while levels below 12 ng/mL are classified as deficiency. Excessive intake, however, may cause vitamin D to accumulate in the body and produce toxic effects.
Prof. Dr. Altay emphasized that vitamin D deficiency is more common among individuals who have limited sun exposure, such as those working indoors. Typical symptoms include fatigue, muscle weakness, difficulty walking, and bone or muscle pain. Long-term and severe deficiency may lead to osteoporosis and increased fracture risk.
While vitamin D can be obtained from foods such as fatty fish, egg yolk, butter, liver, and sun-dried mushrooms, Altay stated that dietary sources alone are generally insufficient. He stressed that vitamin D supplements should not be used in therapeutic doses without medical supervision.
“Supplementation and medical treatment should not be confused,” Altay said. “Treatment requires individualized dosing based on laboratory results and clinical evaluation.”
Prof. Dr. Altay noted that vitamin D toxicity rarely occurs from sun exposure or natural dietary sources. Instead, it is primarily associated with the excessive use of over-the-counter vitamin D supplements. He stated that the tolerable upper daily intake is 4,000 international units (IU), and that consuming more than 40,000 IU at one time or using high doses over a prolonged period can raise vitamin D levels to toxic ranges.
Vitamin D toxicity is more frequently observed in older adults, young children and women, he added.
Older adults, individuals with obesity, people who work in enclosed environments, those with osteoporosis, malabsorption disorders, chronic illnesses, or those taking certain medications as high-risk groups for vitamin D deficiency. He recommended that these individuals have their vitamin D levels monitored under physician supervision, particularly during the autumn and winter months.
He also noted that routine vitamin D testing and supplementation are not necessary for individuals without risk factors and that there is insufficient scientific evidence to support additional health benefits from supplementation in people with normal vitamin D levels.
Prof. Dr. Altay stated that sunlight is suitable for vitamin D synthesis between March and October. During this period, he recommended sun exposure three to four times per week between 10:00 a.m. and 4:00 p.m. Fair-skinned individuals may require 10–15 minutes of exposure, while darker-skinned individuals may need up to 35 minutes of direct sunlight on uncovered skin.
He also warned that sunscreens with an SPF of 15 or higher block UV-B rays necessary for vitamin D production and cautioned that prolonged sun exposure increases the risk of skin cancer.
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