Structurally vitamin B2 consists of
a ribitol moiety and a substituted isoalloxazine ring (Fig. 8.6).
Riboflavin is a constituent of a group of
enzymes called “flavoproteins”. As with thiamine, the enzymes are necessary in
the break down of the glucose to form energy. Riboflavin is essential for a
healthy skin and for good vision in bright light. If the individual ingest more
riboflavin than their body needs, the urinary excretion will increase, if the
intake is inadequate, the body maintains its supply very carefully and the
urinary excretion will practically stop.
About half of the intake of riboflavin daily is
furnished by milk alone and cheese is a good source, although some of the
vitamin has been lost in the whey.
A more generous estimate of requirements is the
level of intake at which there is normalisation of the activity of the red cell
enzyme glutathione reductase, which
is a flavoprotein whose activity is especially sensitive to riboflavin
Infants - 0.4 - 0.6 mg / day
Children - 0.8 - 1.2 mg / day
Adults male - 1.5 -1.8 mg/day
Adults female - 1.1 - 1.4 mg/day
Pregnant women - 1.4 - 1.7 mg/day
Lactating women - 1.6 - 1.9 mg / day
The vitamin is phosphorylated in the intestinal
mucosa during absorption. It is absorbed from the small intestine through the
portal vein and is passed to all tissues being stored in the body. The major
part is excreted in urine and a small part is metabolized in the body.
Riboflavin deficiency leads to cheilosis, a
cracking of the skin at the corners of the lips and scaliness of the skin
around the ears and nose. There may be redness and burning as well as itching
of the eyes, and extreme sensitivity to strong light.