Which of these vitamins is produced in the human body?

Vitamin K is unique among the vitamins in several respects. It is the only vitamin that can be produced within the human body, but not by the body (to be defined as a vitamin, a substance cannot be produced by human tissue). Beneficial bacteria in the human intestine produce about 75% of the vitamin K the body absorbs each day, with the other 25% coming from dietary sources. The amount of vitamin K absorbed each day from both sources usually is equal to the minimum amount required for normal bodily function.

Like the body’s absorption of other fat-soluble vitamins (A, D, and E), vitamin K absorption depends on healthy liver and gallbladder function. Unlike the other fat-soluble vitamins, however, vitamin K is not stored in the body. Taken together, these factors explain why the net daily balance of vitamin K is so delicate. As people live longer and vitamin K-dependent processes are discovered in more and more tissues, more scientists are suggesting that vitamin K is needed in larger quantities than what was once thought, particularly in aging adults.