Thiamin (or thiamine) is one of the water-soluble B vitamins. It is also known as vitamin B1. Thiamin is naturally present in some foods, added to some food products, and available as a dietary supplement. This vitamin plays a critical role in energy metabolism and, therefore, in the growth, development, and function of cells .
Ingested thiamin from food and dietary supplements is absorbed by the small intestine through active transport at nutritional doses and by passive diffusion at pharmacologic doses . Most dietary thiamin is in phosphorylated forms, and intestinal phosphatases hydrolyze them to free thiamin before the vitamin is absorbed . The remaining dietary thiamin is in free (absorbable) form [1,2]. Humans store thiamin primarily in the liver, but in very small amounts . The vitamin has a short half-life, so people require a continuous supply of it from the diet.
About 80% of the approximately 25–30 mg of thiamin in the adult human body is in the form of thiamin diphosphate (TDP; also known as thiamin pyrophosphate), the main metabolically active form of thiamin. Bacteria in the large intestine also synthesize free thiamin and TDP, but their contribution, if any, to thiamin nutrition is currently unknown . TDP serves as an essential cofactor for five enzymes involved in glucose, amino acid, and lipid metabolism [1,3].
Levels of thiamin in the blood are not reliable indicators of thiamin status. Thiamin status is often measured indirectly by assaying the activity of the transketolase enzyme, which depends on TDP, in erythrocyte hemolysates in the presence and absence of added TDP . The result, known as the “TDP effect,” reflects the extent of unsaturation of transketolase with TDP. The result is typically 0%–15% in healthy people, 15%–25% in those with marginal deficiency, and higher than 25% in people with deficiency. Another commonly used measure of thiamin status is urinary thiamin excretion, which provides data on dietary intakes but not tissue stores . For adults, excretion of less than 100 mcg/day thiamin in urine suggests insufficient thiamin intake, and less than 40 mcg/day indicates an extremely low intake .
Sources of Thiamin
Food sources of thiamin include whole grains, meat, and fish . Breads, cereals, and infant formulas in the United States and many other countries are fortified with thiamin .The most common sources of thiamin in the U.S. diet are cereals and bread . Pork is another major source of the vitamin. Dairy products and most fruits contain little thiamin . About half of the thiamin in the U.S. diet comes from foods that naturally contain thiamin; the remainder comes from foods to which thiamin has been added .
Heating foods containing thiamin can reduce their thiamin content. For example, bread has 20%–30% less thiamin than its raw ingredients, and pasteurization reduces thiamin content (which is very small to begin with) in milk by up to 20% . Because thiamin dissolves in water, a significant amount of the vitamin is lost when cooking water is thrown out . Processing also alters thiamin levels in foods; for example, unless white rice is enriched with thiamin, it has one tenth the amount of thiamin in unenriched brown rice .
Several food sources of thiamin are listed in Table 2.
|Table 2: Selected Food Sources of Thiamin |
|Breakfast cereals, fortified with 100% of the DV for thiamin, 1 serving||1.5||100|
|Rice, white, long grain, enriched, parboiled, ½ cup||1.4||73|
|Egg noodles, enriched, cooked, 1 cup||0.5||33|
|Pork chop, bone-in, broiled, 3 ounces||0.4||27|
|Trout, cooked, dry heat, 3 ounces||0.4||27|
|Black beans, boiled, ½ cup||0.4||27|
|English muffin, plain, enriched, 1 muffin||0.3||20|
|Mussels, blue, cooked, moist heat, 3 ounces||0.3||20|
|Tuna, Bluefin, cooked, dry heat, 3 ounces||0.2||13|
|Macaroni, whole wheat, cooked, 1 cup||0.2||13|
|Acorn squash, cubed, baked, ½ cup||0.2||13|
|Rice, brown, long grain, not enriched, cooked, ½ cup||0.1||7|
|Bread, whole wheat, 1 slice||0.1||7|
|Orange juice, prepared from concentrate, 1 cup||0.1||7|
|Sunflower seeds, toasted, 1 ounce||0.1||7|
|Beef steak, bottom round, trimmed of fat, braised, 3 ounces||0.1||7|
|Yogurt, plain, low fat, 1 cup||0.1||7|
|Oatmeal, regular and quick, unenriched, cooked with water, ½ cup||0.1||7|
|Corn, yellow, boiled, 1 medium ear||0.1||7|
|Milk, 2%, 1 cup||0.1||7|
|Barley, pearled, cooked, 1 cup||0.1||7|
|Cheddar cheese, 1½ ounces||0||0|
|Chicken, meat and skin, roasted, 3 ounces||0||0|
|Apple, sliced, 1 cup||0||0|
*DV = Daily Value. DVs were developed by the U.S. Food and Drug Administration (FDA) to help consumers compare the nutrient contents of products within the context of a total diet. The DV for thiamin is 1.5 mg for adults and children age 4 and older. Foods providing 20% or more of the DV are considered to be high sources of a nutrient.
The U.S. Department of Agriculture’s (USDA’s) Nutrient Database website  lists the nutrient content of many foods and provides a comprehensive list of foods containing thiamin arranged by nutrient content and by food name.