Why can very large doses of vitamins A and D be toxic?

Vitamins A and D are fat-soluble, meaning they are stored in the body’s tissues—primarily the liver—rather than being readily eliminated in the urine like water-soluble vitamins. Because of this storage capacity, excessive intake can lead to the accumulation of these substances to levels that become toxic, a condition known as hypervitaminosis.123

The toxicity of these vitamins is primarily a concern when pharmaceutical preparations or high-dose supplements are consumed in excess of normal requirements.13

• Vitamin A: Excess intake, such as 150,000 micrograms daily over several months, can cause symptoms including nausea, hair loss, skin scaling, bone pain, and liver enlargement. It is particularly dangerous during pregnancy, as it can cause birth defects.12
• Vitamin D: Toxicity, or hypervitaminosis D, leads to abnormally high concentrations of calcium in the blood (hypercalcemia), which can result in the widespread deposition of calcium phosphate throughout the body, particularly in the kidneys. Symptoms include weakness, fatigue, nausea, and vomiting.3

It is important to adhere to recommended intake levels to avoid these adverse health effects.4