Thiamin, or vitamin B1, is central to both energy metabolism and antioxidant defense. While its deficiency causes many problems, out of all the B vitamins its deficiency is most neurological in nature, because energy metabolism of the brain becomes severely compromised, and because neurotransmitters derived from protein cannot be produced. In its most severe form, beriberi, it can cause loss of muscle control, twitching, muscles freezing into awkward positions, and seizures.

Carbohydrates require twice as much thiamin as fat. This means, on the one hand, that high-carbohydrate diets increase the need for thiamin, and on the other hand, that people who are deficient in thiamin may have neurological symptoms that resolve when they go on a low-carbohydrate, high-fat diet.

Thiamin is widely distributed in foods. Historically, diets high in refined grain produced beriberi, but now refined grains are enriched in thiamin, so its deficiency is rarely caused by diet. However, certain foods such as raw fish and moths that are eaten in some cultures contain thiamin antagonists, gastrointestinal microbes can degrade thiamin, and there are various environmental causes of thiamin deficiency, such the algae that grow in dead zones. As such, environmental exposure to thiamin antagonists may be a more common cause of thiamin deficiency than poor dietary intake.

Still, some foods are much higher in thiamin than most others, with whole grains, legumes, yeast, and red meat being among the best sources.

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