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By Mike Murray, RDN, CSCS, CISSN

 

Thiamin (Vitamin B1)

Thiamin consists of pyrimidine and thiazole rings joined by a methylene bridge. Its primary functions include acting as a coenzyme in branched-chain amino acid and carbohydrate metabolism.Thiamin is also critical for brain function; it plays a role in the propagation of nerve impulses and aids in myelin sheath maintenance.

Absorption of thiamin occurs mainly in the jejunum of the small intestine. In higher concentrations, thiamin is absorbed via passive diffusion. In lower concentrations, it is absorbed via an active, carrier-mediated system that involves phosphorylation. It has been estimated that the adult human body stores about 30 mg of thiamin, and the biological half-life of the vitamin is in the range of 9 to 18 days. 

The RDA for thiamin is 1.2 mg/day and 1.1 mg/day for adult men and women. The EAR is 1.0 mg/day for men and 0.9 mg/day for women. However, it may be more appropriate to base thiamin requirements on energy intake.

In accordance with its role in the metabolism of glucose, thiamin requirements increase proportionally with carbohydrate intake. Evidence suggests that increasing carbohydrate intake from 55% to 65% and 75% of total caloric intake causes a decrease in thiamin status.

The energy-adjusted EAR is set at 0.34 mg/1000 kcal for men and 0.41 mg/1000 kcal for women. From the energy-adjusted EAR, an energy-adjusted RDA can be calculated, which is 0.41 mg/1000 kcal for men and 0.49 mg/1000 kcal for women. 

Thiamin is found in fortified processed flours and ready-to-eat cereals, pork, seafood, whole grains, legumes, nuts, and seeds. The following foods are some of the most efficient to meet the daily value (i.e., RDA) for thiamin:

Flax seeds 0.47 mg per serving (28 g/1 oz)
Pistachio nuts 0.19 mg per serving (28 g/1 oz)
Ground pork (cooked) 0.60 mg per serving (3 oz)
Whole wheat bagel  0.40 mg per serving (100 g)
Green peas 0.25 mg per serving (100 g)

As a consequence of being water-soluble and stored in relatively low amounts, thiamin reserves can be depleted in 2 to 3 weeks without adequate intake. 

Inadequate intake may result in symptoms such as anorexia, weight loss, apathy, confusion, short-term memory issues, muscular weakness, and enlargement of the heart. Chronic thiamin deficiency manifests as beriberi or Wernicke encephalopathy (which is typically associated with chronic alcoholism). 

Factors that can increase the risk of thiamin deficiency, include anti-thiamin factors in foods and beverages such as raw fish, shellfish, coffee, and tea; strenuous physical activity; diets high in simple sugars; and conditions that increase requirements (e.g., pregnancy, breastfeeding) or facilitate excessive losses (e.g., kidney disease that requires dialysis). 

Due to insufficient data on the adverse effects of excess thiamin consumption, a UL for thiamin has not been established. There have only been a few reports of adverse effects. In each, thiamin was administered through either the intramuscular or parenteral route in a dose of 100–500 mg.  

The lack of toxicity may be explained by the dramatic decline in absorption and rapid urinary excretion of thiamin that occurs at intakes above 5 mg.

Thiamin status is measurable via urinary thiamin excretion, erythrocyte thiamin, and erythrocyte transketolase activity. Erythrocyte transketolase activity is considered the gold standard functional test of thiamin status. However, this method is expensive and not widely available. Therefore, it’s most practical to monitor daily intake and to be aware of signs and symptoms of inadequacy mentioned above.

Indicator Deficiency
Erythrocyte transketolase activity > 1.25
Erythrocyte thiamin (nmol/L) < 70
Urinary thiamin (nmol [µg]/d) < 40