Which vitamin influences calcium absorption?

• A. Vitamin A
• B. Vitamin D
• C. Vitamin C
• D. Vitamin E

Answer: (B)

Vitamin D influences calcium absorption. When activated to calcitriol, it enters intestinal cells and boosts the production of calcium-transporting proteins, such as TRPV6 channels on the cell surface and calbindin in the cytoplasm. These proteins work together to move calcium from the gut lumen into the blood, increasing how much calcium the body can absorb. This mechanism is especially important when dietary calcium is low. Without sufficient vitamin D, calcium absorption declines, contributing to weaker bones. The other vitamins listed don’t have a direct role in regulating intestinal calcium transport.