Dietary Sources of Thiamin

Thiamin is present in most foods but cereal products provide most thiamin for most people in the world, although the source is fundamentally different in developing and more industrialized countries. In the developing world, unrefined cereal grains and/or starchy roots and tubers provide 60-85% of dietary thiamin, whereas most dietary thiamin in industrialized countries is obtained from fortified cereal products. In the United Kingdom, for example, wheat flour is fortified with 2.4 mg thiamin per kilogram and many breakfast cereals contain 30% or more of the daily thiamin requirement per portion. Thiamin is present in greatest amounts in brewers yeast, the germ and aleuron layers of fresh wheat, egg yolk, and mammalian liver. It is also present in meat flesh, particularly pork, and vegetables, nuts, and legumes (Table 1). Milk from both humans (0.49-0.79 mmol/l; 0.23 mg/4.2MJ (1000 kcal)) and cows (1.18-1.48 mmol/l) is a poor source of thiamin. Thiamin is actively secreted into milk by the lactating mother, and it is of interest that the amount of thiamin in human milk is not increased by supplements, but the concentration

.CH3

C CH2 CH2

OH = thiamin

OH OH

Figure 1 Thiamin and thiamin diphosphate (asterisk). Thiamin monophosphate and triphosphate are formed by the similar addition of one or three phosphate groups at the asterisk.

Table 1 Thiamin content of common foods

Food group

Food item

Thiamin content (mg/100g)

Bread

Wholemeal

0.26

White

0.18

Hovis

0.52

Breakfast cereals

Cornflakes (fortified)

1.8

Rice Krispies

2.3

Weetabix

1.0

Flour

Wholemeal (100%a)

0.46

Brown (85%)

0.42

White (fortified) (70%)

0.28-0.33

Milk, cheeses

0.03-0.06

Eggs

Cooked (various)

0.07-0.09

Yolk raw

0.30

Vegetables

Various leaf and root

0.02-0.07

(cooked)

types

Dahl, chick peas,

0.05-0.14

green, beans, etc.

Pork products

Gammon rashers (lean)

1.0

Bacon (various)

0.36-0.55

Pork meat

0.5-0.88

Liver (stewed)

0.21

Other meats

Beef (various)

0.03-0.09

Lamb (various)

0.04-0.14

Lamb liver

0.56

Chicken (various)

0.04-0.10

Game

~0.30

Yeast (dried)

2.33

aPercentages indicate the level of extraction in flour preparation. Source: Paul AA, Southgate DAT (1978) McCance & Widdowson's The composition of food, 4th edn. London: HMO.

aPercentages indicate the level of extraction in flour preparation. Source: Paul AA, Southgate DAT (1978) McCance & Widdowson's The composition of food, 4th edn. London: HMO.

and of course the volume consumed increase during the first 6 weeks of lactation.

Refined foods in general, such as fat, sugar, and alcohol, are poor sources of thiamin. Polished rice is particularly low in thiamin (80 mg/100 g) and is especially important because of its widespread consumption and importance as a source of calories. Cereal grains lose thiamin during refining, but the process of parboiling rice before milling enables most of the thiamin to be retained (190 mg/100 g) since it migrates into the starchy endosperm during the procedure. Proper storage of cereal grains is also important to maintain thiamin activity. Studies in The Gambia, West Africa, found that old season millet, which had been stored under thatch and in high humidity, when consumed in the middle of the rainy season had thiamin concentrations (11 mg/ 100 g) that were 6-12 times lower than cooked samples obtained immediately postharvest. Imported rice used in the village likewise only contained 10 mg/100 g at the time of consumption.

Because of the water-soluble properties of thiamin, it can be leached from food during cooking. Thiamin is stable in slightly acid water up to boiling point but is unstable in alkaline solution that oxidizes it

CH2CH2OH

CH2CH2OH

Figure 2 Structures of thiamin and thiochrome.

Thiamin

Thiochrome

CH2CH2OH

Figure 2 Structures of thiamin and thiochrome.

quantitatively to thiochrome (Figure 2). In addition, anti-thiamin factors in food can accelerate thiamin losses. Paralysis in foxes fed raw carp led to the discovery of the thiaminase enzymes. Two thiami-nases are found in food. Thiaminase I is found in fish, shellfish, ferns, and some bacteria and catalyzes a base exchange reaction between thiazole and another base. Thiaminase II is a hydrolytic enzyme that cleaves the vitamin at the methylene bridge and is found mainly in bacteria. The thiaminases are heat labile, so only food that is eaten raw or fermented may loose thiamin during its preparation or in the gastrointestinal tract. There are also heat-stable anti-thiamin factors that are found in ferns, tea, betel nuts, large numbers of plants and vegetables, and some animal tissues. Anti-thiamin factors bind with varying degrees of attachment to thiamin and may or may not interfere with the bioavailability of thiamin. Diphenols, especially those with the hydroxyl groups in the ortho position, tend to react to give products that are both thiochrome negative and microbiologi-cally inactive (i.e., thiamin is deactivated). Thus, in areas of northern and northeastern Thailand where tea drinking, chewing fermented tea leaves, chewing betel nuts, and consuming raw/fermented fish are common practices, thiamin deficiency still occurs despite thiamin intakes of 0.44-0.50 mg/4.2MJ.

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