Introduction to Human Nutrition

156 Introduction to Human Nutrition

the vitamin; urinary excretion of ribofl avin after
moderately high doses can be two- to threefold greater
than the glomerular fi ltration rate.
Under normal conditions about 25% of the urinary
excretion of ribofl avin is as the unchanged vitamin,
with a small amount as glycosides of ribofl avin and
its metabolites.

Ribofl avin balance
There is no signifi cant storage of ribofl avin; apart
from the limitation on absorption, any surplus
intake is excreted rapidly, so that once metabolic
requirements have been met urinary excretion of
ribofl avin and its metabolites refl ects intake until
intestinal absorption is saturated. In depleted animals,
the maximum growth response is achieved with
intakes that give about 75% saturation of tissues,
and the intake to achieve tissue saturation is that

at which there is quantitative excretion of the

vitamin.

There is very effi cient conservation of ribofl avin in

defi ciency, and almost the only loss from tissues will

be the small amount that is covalently bound to

enzymes and cannot be salvaged for reuse. There is

only a fourfold difference between the minimum con-

centration of fl avins in the liver in defi ciency and the

level at which saturation occurs. In the central nervous

system there is only a 35% difference between defi -

ciency and saturation.

Metabolic functions of the fl avin

coenzymes

The metabolic function of the fl avin coenzymes is as

electron carriers in a wide variety of oxidation and

reduction reactions central to all metabolic processes,

including the mitochondrial electron transport chain,

N

N

NH

N

O

O

CH 2 C

H

OH

C

H

OH

C

H

OH

CH 2 OH

N

N

NH

N

O

O

CH 2 C

H

OH

C

H

OH

C

H

OH

CH 2 O P O

N

N

NH

N

O

O

CH 2 C

H

OH

C

H

OH

C

H

OH

CH 2 O P POCH 2

N

N N

N

NH 2

O O

OH OH

OH

OH OH

OH

OH

OH

Riboflavin

Riboflavin monophosphate

(flavin mononucleotide)

Flavin adenine dinucleotide

H 3 C

H 3 C

H 3 C

H 3 C

H 3 C

H 3 C

Figure 8.9 Ribofl avin (vitamin B 2 ) and the fl avin coenzymes, ribofl avin monophosphate and fl avin adenine dinucleotide.