Nutrition Science and Everyday Application - beta v 0.1

450 ALICE CALLAHAN, PHD, HEATHER LEONARD, MED, RDN, AND TAMBERLY POWELL, MS, RDN
synthesized in the body. There are two ways the body can make vitamins: certain vitamins
can be made from a provitamin, or a precursor substance that can be converted into the
active form of a vitamin; other vitamins can be synthesized by bacteria living in the intestinal
tract.
Vitamins Made From Precursors
Vitamins made in the body from precursors include vitamin A, vitamin D, and niacin, one of
the B vitamins.
• The active form of vitamin A, called retinol, is found in animal foods, but plants
contain beta-carotene, a provitamin that can be converted to vitamin A in the body.
This red-orange pigment found in fruits and vegetables is converted to vitamin A
primarily in the small intestine. 2 We will discuss this conversion in more depth later
in this unit.
• Vitamin D can be made when ultraviolet light from sunlight strikes cholesterol in
the skin. Cholesterol, which our body can make, is a precursor for vitamin D. This
process of making vitamin D from cholesterol is limited by geographic location
(both latitude and altitude) and seasonal changes, both of which influence the
quality, quantity, and intensity of ultraviolet rays that reach the skin. 3 We will
discuss this conversion in more depth later in this unit.
• Niacin can be made in the liver from the amino acid tryptophan, when tryptophan
is available in quantities greater than needed for protein synthesis. The efficiency
in which tryptophan is converted to niacin varies greatly in individuals. 4