Vitamin D

The blood calcium level stays stable thanks to three hormones: Vitamin D, parathyroid hormone, and calcitonin.

We’ll focus on Vitamin D, which along with parathyroid hormone, helps increase calcium levels, whereas calcitonin helps lower them.

The majority of the extracellular calcium, the calcium in the blood and interstitium, is split almost equally into calcium that’s diffusible and calcium that’s not diffusible.

Diffusible calcium is small enough to diffuse across cell membranes and there are two subcategories.

The first is free-ionized calcium, which is involved in all sorts of cellular processes like neuronal action potentials, contraction of skeletal, smooth, and cardiac muscle, hormone secretion, and blood coagulation, all of which are tightly regulated by enzymes and hormones.

The second category is complexed calcium, which is where the positively charged calcium is ionically linked to tiny negatively charged molecules like oxalate and phosphate, which are small anions, that are found in our blood.

The complexed calcium forms a molecule that’s electrically neutral but unlike free-ionized calcium it’s not useful for cellular processes.

Finally there’s the non-diffusible calcium which is bound to large negatively charged proteins like albumin.

The resulting protein-calcium complex is too large and charged to cross membranes, so the non-diffusible calcium is also uninvolved in cellular processes.

Now, after parathyroid hormone, the metabolically active form of vitamin D, also called calcitriol, is the second most important hormone involved in regulating blood calcium.

Vitamin D is a steroid hormone, which means that it’s made from cholesterol and it’s fat-soluble.

Active vitamin D starts out as one of two metabolically inactive molecules.

Either vitamin D2, or ergocalciferol, which comes from plant sources in our diet, and vitamin D3, or cholecalciferol, which can either come from animal products in our diet, but can also be made in skin cells that are exposed to sunlight.

But since both ergocalciferol and cholecalciferol are physiologically inactive molecules to vitamin D, they have to be modified a bit by the body before they can be used.