The Vitamins 155dose of glucose and mild exercise. The test is not
specifi c for thiamin defi ciency since a variety of other
conditions can also result in metabolic acidosis.
Although it may be useful in depletion/repletion
studies, it is little used nowadays in assessment of
nutritional status.
Whole blood total thiamin below 150 nmol/l is
considered to indicate defi ciency. However, the
changes observed in depletion studies are small. Even
in patients with frank beriberi the total thiamin con-
centration in erythrocytes is only 20% lower than
normal, so whole blood thiamin is not a sensitive
index of status.
Although there are several urinary metabolites of
thiamin, a signifi cant proportion is excreted either
unchanged or as thiochrome, and therefore the
urinary excretion of the vitamin (measured as thio-
chrome) can provide information on nutritional
status. Excretion decreases proportionally with intake
in adequately nourished subjects, but at low intakes
there is a threshold below which further reduction in
intake has little effect on excretion.
The activation of apo-transketolase in erythrocyte
lysate by thiamin diphosphate added in vitro has
become the most widely used and accepted index of
thiamin nutritional status. Apo-transketolase is
unstable both in vivo and in vitro, so problems may
arise in the interpretation of results, especially if
samples have been stored for any appreciable time. An
activation coeffi cient >1.25 is indicative of defi ciency,
and <1.15 is considered to refl ect adequate thiamin
status.
8.7 Vitamin B 2 (ribofl avin)
Ribofl avin defi ciency is a signifi cant public health
problem in many areas of the world. The vitamin has
a central role as a coenzyme in energy-yielding
metabolism, yet defi ciency is rarely, if ever, fatal, since
there is very effi cient conservation and recycling of
ribofl avin in defi ciency.
The structures of ribofl avin and the ribofl avin-
derived coenzymes are shown in Figure 8.9.
Milk and dairy products are important sources,
providing 25% or more of total ribofl avin intake in
most diets, and it is noteworthy that average ribofl a-
vin status in different countries refl ects milk con-
sumption to a considerable extent. Other rich sources
are eggs, meat, and fi sh. In addition, because of its
intense yellow color, ribofl avin is widely used as a
food color.Photolytic destruction
Photolysis of ribofl avin leads to the formation of
lumifl avin (in alkaline solution) and lumichrome (in
acidic or neutral solution), both of which are biologi-
cally inactive. Exposure of milk in clear glass bottles
to sunlight or fl uorescent light can result in the loss
of signifi cant amounts of ribofl avin. This is poten-
tially nutritionally important. Lumifl avin and lumi-
chrome catalyze oxidation of lipids (to lipid perox-
ides) and methionine (to methional), resulting in the
development of an unpleasant fl avor, known as the
“sunlight” fl avor.Absorption and metabolism
Apart from milk and eggs, which contain relatively
large amounts of free ribofl avin bound to specifi c
binding proteins, most of the vitamin in foods is as
fl avin coenzymes bound to enzymes, which are
released when the protein is hydrolyzed. Intestinal
phosphatases then hydrolyze the coenzymes to liber-
ate ribofl avin, which is absorbed in the upper small
intestine. The absorption of ribofl avin is limited and
after moderately high doses only a small proportion
is absorbed.
Much of the absorbed ribofl avin is phosphorylated
in the intestinal mucosa and enters the bloodstream
as ribofl avin phosphate, although this does not seem
to be essential for absorption of the vitamin.
About 50% of plasma ribofl avin is free ribofl avin,
which is the main transport form, with 44% as fl avin
adenine dinucleotide (FAD) and the remainder as
ribofl avin phosphate. The vitamin is largely protein-
bound in plasma; free ribofl avin binds to both
albumin and α- and β-globulins; both ribofl avin and
the coenzymes also bind to immunoglobulins.
Uptake into tissues is by passive carrier-mediated
transport of free ribofl avin, followed by metabolic
trapping by phosphorylation to ribofl avin phosphate,
and onward metabolism to FAD.
Ribofl avin phosphate and FAD that are not bound
to proteins are rapidly hydrolyzed to ribofl avin, which
diffuses out of tissues into the bloodstream. Ribofl a-
vin and ribofl avin phosphate that are not bound to
plasma proteins are fi ltered at the glomerulus; renal
tubular resorption is saturated at normal plasma con-
centrations. There is also active tubular secretion of