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Figure 79-8

Plasma calcium ion concentration-----

Figure 79-6

Activation of vitamin D3 to form 1,25-dihydroxycholecalciferol and the role of vitamin D in controlling the plasma calcium concentration.

Effect of plasma calcium concentration on the plasma concentration of 1,25-dihydroxycholecalciferol. This figure shows that a slight decrease in calcium concentration below normal causes increased formation of activated vitamin D, which in turn leads to greatly increased absorption of calcium from the intestine.

and yet the concentration of 25-hydroxycholecalcif-erol remains nearly normal. This high degree of feedback control prevents excessive action of vitamin D when intake of vitamin D3 is altered over a wide range.

Second, this controlled conversion of vitamin D3 to 25-hydroxycholecalciferol conserves the vitamin D stored in the liver for future use. Once it is converted, it persists in the body for only a few weeks, whereas in the vitamin D form, it can be stored in the liver for many months.

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