Why is calcium not absorbed if there is no vitamin D?

Vitamin D plays a key role in regulating calcium and phosphorus metabolism. Its active form — calcitriol (1,25-dihydroxyvitamin D₃) — acts as a hormone, influencing three main systems: the intestine, bones, and kidneys.

How vitamin D affects calcium absorption

Activates calcium transport in the intestine. Vitamin D increases the synthesis of the protein calbindin in the cells of the intestinal mucosa. Calbindin binds calcium from food and transports it through the intestinal epithelium into the bloodstream.

Without sufficient vitamin D, this mechanism hardly works, and most of the calcium is simply excreted with the feces.

Regulates the function of calcium channels. Vitamin D stimulates the expression of calcium channels (TRPV6) on the surface of enterocytes, allowing calcium to enter the intestinal cells.

Maintains calcium balance in the blood through bones and kidneys. Vitamin D, together with parathyroid hormone (PTH), regulates how much calcium will be retained in the blood and how much will be deposited in bone tissue.

What happens with vitamin D deficiency

Calcium from food is hardly absorbed. Even with a calcium-rich diet (dairy products, fish with bones, nuts), it will pass through the system.

The body compensates for the deficiency by drawing from the bones. To maintain calcium levels in the blood (vital for the functioning of the heart and nerves), PTH increases the resorption of bone tissue.

Osteopenia and osteoporosis develop. Prolonged deficiency leads to brittle bones, increased risk of fractures, and deformities (rickets in children, osteomalacia in adults).

Conclusion

Without sufficient levels of vitamin D, calcium is indeed hardly absorbed from the gastrointestinal tract and begins to leach from the bones, which is one of the key reasons for osteoporosis and muscle weakness.