Significance and sources of thiamin (B1 vitamin), thiamin antagonists

1. Significance and sources of thiamin
Domina Petric, MD
April 14, 2018 1

2. Significance of thiamin
Thiamin is essential in carbohydrate
metabolism and neural function.
Severe thiamin deficiency results
in the nerve and heart disease
beriberi.
Less severe deficiency results in
nonspecific signs: malaise, loss of
weight, irritability and confusion.
April 14, 2018
Combs GF. The Vitamins. Fundamental Aspects in
Nutrition and Health. Elsevier Inc. 2008.
2

3. Significance of thiamin
• Thiamin deficiency has been
prevalent among people dependent
on polished rice as the predominant,
if not sole, source of food.
• Demographic trends indicate that for
many people dependence on rice is
likely to increase in the future.
April 14, 2018
Combs GF. The Vitamins. Fundamental Aspects in
Nutrition and Health. Elsevier Inc. 2008.
3

4. Significance of thiamin
Whole-grain rice and other
cereals are not particularly
deficient in thiamin.
The removal of their thiamin-containing
aleurone cells renders the polished grains,
which consist of little more than the
carbohydrate-rich endosperm, nearly
devoid of thiamin as well as other vitamins
and essential inorganic elements.
April 14, 2018
Combs GF. The Vitamins. Fundamental Aspects in
Nutrition and Health. Elsevier Inc. 2008.
4

5. Distribution in foods
• Thiamin is widely distributed in foods, but
most contain only low concentrations of
the vitamin.
• The richest sources are yeasts (dried
brewer’s and baker’s yeasts) and liver
(especially pork liver).
• Cereal grains comprise the most
important dietary sources of the vitamin
in most human diets.
April 14, 2018
Combs GF. The Vitamins. Fundamental Aspects in
Nutrition and Health. Elsevier Inc. 2008.
5

6. Distribution in foods
• The greatest concentrations of thiamin in
grains are typically found in the scutellum (the
thin layer between the germ and the
endosperm) and the germ.
• The endosperm (the starchy interior) is quite
low in the vitamin.
• Milling to degerminate grain, which, because it
removes the highly unsaturated oils associated
with the germ, yields a product that will not
rancidify and, thus, has a longer storage life,
also results in very low thiamin content.
April 14, 2018
Combs GF. The Vitamins. Fundamental Aspects in
Nutrition and Health. Elsevier Inc. 2008.
6

7. Distribution in foods
• In foods derived from plants, thiamin
occurs predominantly as free thiamin.
• Thiamin occurs in animal tissues
almost entirely (95-98%) in
phosphorylated forms (thiamin mono-,
di- and triphosphates), the
predominant form (80-85%) being the
coenzyme thiamin diphosphate also
called thiamin pyrophosphate (TPP).
April 14, 2018
Combs GF. The Vitamins. Fundamental Aspects in
Nutrition and Health. Elsevier Inc. 2008.
7

8. Stability in foods
• Thiamin is susceptible to destruction
by several factors including neutral
and alkaline conditions, heat,
oxidation and ionizing radiation.
• It is stable at low pH (pH<7), but
decomposes when heated,
particularly under nonacidic
conditions.
April 14, 2018
Combs GF. The Vitamins. Fundamental Aspects in
Nutrition and Health. Elsevier Inc. 2008.
8

9. Stability in foods
• Protein-bound thiamin, found in
animal tissues, is more stable to
such losses.
• Thiamin is stable during
frozen storage.
• Substantial losses occur during
thawing owing mainly to removal via
drip fluid.
April 14, 2018
Combs GF. The Vitamins. Fundamental Aspects in
Nutrition and Health. Elsevier Inc. 2008.
9

10. Thiamin antagonists
• Thiamin in foods can be
destroyed by sulfites added in
processing and by thiamin-
degrading enzymes
(thiaminases) and other
antithiamin compounds that
may occur naturally.
April 14, 2018
Combs GF. The Vitamins. Fundamental Aspects in
Nutrition and Health. Elsevier Inc. 2008.
10

11. Types of thiaminases
Type Present in: Mechanism
I Fresh fish,
shellfish, ferns,
some bacteria
Displaces pyrimidine
methylene group with a
nitrogenous base or SH
compound, to eliminate
the thiazole ring.
II Certain bacteria Hydrolytic cleavage of
methylene-thiazole-
nitrogen
bond to yield the
pyrimidine and thiazole
moieties.
April 14, 2018
Combs GF. The Vitamins. Fundamental Aspects in
Nutrition and Health. Elsevier Inc. 2008.
11

12. Thiamin antagonists
Sulfites react with thiamin to cleave
its methylene bridge between the
pyrimidine ring and thiazole ring.
The reaction is slow at high pH, but is
rapid in neutral and acidic conditions.
April 14, 2018
Combs GF. The Vitamins. Fundamental Aspects in
Nutrition and Health. Elsevier Inc. 2008.
12

13. Thiamin antagonists
• The thiaminases are heat labile, but can
be effective antagonists of the vitamin
when consumed without heat treatment.
• Heat-stable thiamin antagonists occur in
several plants (ferns, tea, betel nut).
• These include o- and p-hydroxypolyphenols
(caffeic acid, chlorogenic acid, tannic acid),
which react with thiamin to oxidize the
thiazole ring, yielding the nonabsorbable form
thiamin disulfide.
April 14, 2018
Combs GF. The Vitamins. Fundamental Aspects in
Nutrition and Health. Elsevier Inc. 2008.
13

14. Thiamin antagonists
Some flavonoids
(quercetin, rutin) have
been reported to
antagonize thiamin.
April 14, 2018
Combs GF. The Vitamins. Fundamental Aspects in
Nutrition and Health. Elsevier Inc. 2008.
14

15. Literature
• Combs GF. The Vitamins. Fundamental Aspects in
Nutrition and Health. Elsevier Inc. 2008.
April 14, 2018 15