Vitamin B1: Thiamin

Thiamin, vitamin B1, is structurally composed of a pyrimidine group and a thiazole ring.¹ Thiamin is found freely within plant sources but is bound to phosphate as thiamin pyrophosphate (TPP), thiamin monophosphate (TMP), thiamin diphosphate (TDP), and thiamin triphosphate (TTP) in most animal products.¹ TPP and TMP may also be referred to as TDP. It is considered an essential nutrient as deficiency can develop.  Thiamin is inactivated by copper, tannic acid, and caffeic acid.

Digestion/Absorption: Thiamin is freed from phosphate and absorbed within the small intestine rapidly. Transport proteins THTR2 and THTR1 are responsible for the transport of free thiamin into enterocytes (THTR2) and blood (THTR1).¹ If phosphorylated within enterocytes, thiamin can be converted into TPP (or TDP). TPP, TDP, and TTP are converted into thiamin monophosphate (TMP).² TMP is considered a major form of thiamin. TMP is carried by a transporter called reduced folate carrier (RFC) into the blood. Thiamin is found primarily in the skeletal tissue followed by the heart, liver, brain, and kidneys.¹

Functions: Thiamin functions as a molecule necessary for carbohydrate and nutrient metabolism as follows:

Glycolysis - TPP is used to aid in the conversion of pyruvate to acetyl CoA. 

Krebs cycle - TPP is used to aid in the conversion of a-ketoglutarate to succinyl CoA

Pentose Phosphate Pathway - TPP is used as a coenzyme for an enzymic reaction that allows the pentose phosphate pathway to produce NADPH (used for lipid and nucleic acids) and ribose (used in RNA and DNA).²

Thiamin is also thought to be important for impulse conduction in nervous tissue through the regulation of sodium channels.¹ A-ketoacid dehydrogenase, an enzyme responsible for the amino acid metabolism of leucine, isoleucine, and valine, requires thiamin.¹ The production of thiazole from thiamin may be neuroprotective.¹

Dietary Reference Intake (DRI) - The recommended dietary allowance (RDA) for adult men and women over 19 years old is 1.2 and 1.1 milligrams/day.²

Deficiency - Severe thiamin deficiency is known to cause beriberi whose symptoms include mental confusion and muscle cramps. An individual can have wet beriberi (edemic) or dry beriberi (cachexic).² Heart enlargement is common. Mild thiamin deficiency includes fatigue, irregular heartbeat, muscle cramps, and tingling of feet.¹

Toxicity - Uncommon but can cause edema, fast heart rate, tremors, etc. 

Dietary Sources of Thiamin 

Thiamin is found in a variety of foods, including grains, meat/animal products, fruits, and vegetables. Thiamin is enriched into refined grains after processing due to loss of B vitamins, including thiamin. It is required by the U.S. government in a legislative act known as the Enrichment Act.¹ Thiamin is found in cereals, breads, and pork. 

Vegetables - peas, lima beans, corn 

Fruits - oranges

Starches - Beans, pasta, bread, rice 

Nuts/seeds - sunflower seeds, peanuts, almonds 

Meat/Animal Products - Pork, fish, beef, milk 

Source(s):

1. Denise M Medeiros and Robert E.C. Wildman, Advanced Human Nutrition, 4th ed. (Burlington, MA: Jones & Bartlett, 2019).

2. https://ods.od.nih.gov/factsheets/Thiamin-HealthProfessional/