Agriculture Reference
In-Depth Information
v
i t a m i in
k
Naturally occurring forms of vitamin K include phylloquinone, produced by green
plants, and at least eight menaquinones, produced by anaerobic bacteria in the lower
digestive tract. Menadione is a synthetic form of vitamin K that must be alkylated in
the liver to become active. Dietary phylloquinone is absorbed from the small intestine
via dissolution in micelles and passive diffusion into the enterocyte. Menaquinones
synthesized by bacteria in the lower tract are absorbed by passive diffusion from
the ileum and colon. In the enterocyte, vitamin K is incorporated into chylomicrons
and carried via lymph into the general circulation. Vitamin K is involved in blood
clotting and in bone mineralization by promoting carboxylation of glutamyl residues
in specific proteins required for these processes. There also is evidence of activity
of vitamin K in promoting nerve growth and neuronal survival. Phylloquinone con-
centrations are particularly high (>200 µg/100 g) in broccoli, collards, kale, spinach,
Swiss chard, and watercress. Recommended adequate intakes range from 30 µg per
day for 1- to 3-year-old children to 120 µg per day for adult men.
t
h i a m i n
Thiamin (vitamin B
1
), when phosphorylated to thiamin diphosphate (TDP, also
known as thiamin pyrophosphate, TPP), plays an essential role as a coenzyme in
energy metabolism. It promotes conversion of pyruvate (a three-carbon acid) to
acetate (a two-carbon acid), which then enters the citric acid cycle, a key cycle in
interconversions of energy metabolites in the body. TDP/TPP is also involved in
synthesis of five-carbon sugars (pentoses) and nicotinamide adenine dinucleotide
phosphate (NADP), in metabolism of branched-chain amino acids, and in oxida-
tion of certain branched-chain fatty acids. Thiamin, as thiamin triphosphate (TTP),
appears to activate ion transport in nerve membranes and may be involved in nerve
impulse transmission. Thiamin is absorbed from the intestine by an energy- and
sodium-dependent transport mechanism at low dietary concentrations, but when
intakes are high, absorption is mostly by passive transport (diffusion). Thiamin in
the blood occurs either in the free form, bound to the protein albumin, or as thiamin
monophosphate (TMP). However, most of blood thiamin is in the red cells, not the
plasma. Free thiamin is taken up by the liver and phosphorylated. Most of the thia-
min in the liver and other (extrahepatic) tissues is in the form of TDP/TPP. Thiamin is
widely distributed in foods of animal and plant origin, including liver, muscle meats,
legumes, whole grains, and fortified breads and cereals. Thiamin supplements are
usually in the form of thiamin hydrochloride or thiamin mononitrate. Recommended
dietary allowances range from 0.5 mg per day for 1- to 3-year-old children to 1.4 mg
per day for pregnant and lactating women.
r
i b of f L a v i in
Riboflavin (vitamin B
2
) exists in free form or as part of two coenzymes, flavin mono-
nucleotide (FMN) or flavin adenine dinucleotide (FAD). These coenzymes function
as prosthetic groups (nonprotein constituents) for flavoprotein enzymes involved in
