From the Guidelines
Hypovitaminosis D can occur despite taking high-dose Vitamin D supplements due to various factors, including poor absorption, certain medications, liver or kidney disease, genetic variations, and obesity. These factors can impingle on the body's ability to synthesize and utilize Vitamin D, even with sufficient supplementation. According to the espen guideline on clinical nutrition in the intensive care unit 1, Vitamin D3 can be synthesized in sufficient amounts by the human body with exposure to sunlight and good liver and renal function, but hypovitaminosis D is common in the general population and has been associated with poor outcomes in critically ill patients.
Some key factors to consider include:
- Poor absorption due to gastrointestinal conditions like celiac disease, Crohn's disease, or gastric bypass surgery
- Interference with vitamin D metabolism by certain medications, such as anticonvulsants, glucocorticoids, and cholesterol-lowering drugs
- Liver or kidney disease, which can impair the conversion of vitamin D to its active form
- Genetic variations in vitamin D binding protein or vitamin D receptors
- Obesity, as vitamin D is fat-soluble and can become sequestered in adipose tissue
To improve absorption, it is recommended to take vitamin D supplements with a meal containing some fat and consider a more bioavailable form like vitamin D3 (cholecalciferol) rather than D2 (ergocalciferol) 1. Additionally, ensuring proper dosing timing, such as taking smaller daily doses rather than extremely high doses once weekly or monthly, may help maintain steady blood levels. If levels remain low despite these adjustments, it is essential to consult a healthcare provider to investigate underlying medical conditions.
From the Research
Causes of Hypovitaminosis D Despite High-Dose Vitamin D Supplements
- Inadequate absorption of vitamin D due to intestinal malabsorption syndromes, such as cystic fibrosis, celiac disease, short bowel syndrome, and inflammatory bowel disease 2
- Insufficient exposure to sunlight, which is essential for vitamin D synthesis in the skin 2
- Inflammation, which can contribute to vitamin D deficiency in patients with intestinal malabsorption syndromes 2
- High-dose vitamin D supplementation may not always be effective in raising serum 25-hydroxyvitamin D levels, and may even cause adverse effects such as hypercalcemia, hypercalciuria, and hyperphosphatemia 3, 4, 5
- Individual variability in response to vitamin D supplementation, with some people requiring higher or lower doses to achieve adequate serum levels 4
- Potential underlying mechanisms for harmful vitamin D effects at circulating 25(OH)D levels between 125 and 375 nmol/l, which are not yet fully understood 4
Factors Influencing Vitamin D Supplementation
- Baseline 25(OH)D levels and body weight, which can affect the increment in circulating 25(OH)D levels in response to supplementation 4
- Dosage and duration of supplementation, with high-dose regimens potentially causing adverse effects 3, 4, 5
- Individual tolerance and sensitivity to vitamin D, which can vary widely 4, 5