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Why can a diet based mainly on polished white rice lead to thiamin deficiency?

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USDA MyPlate dietary guidelines — human nutrition MyPlate, a revised set of dietary guidelines introduced by the U.S. Department of Agriculture (USDA) in 2011, divides the four basic food groups (fruits, grains, protein, and vegetables) into sections on a plate, the size of each section representing the relative dietary proportions of each food group. The small blue circle shown at the upper right illustrates the inclusion and recommended proportion of dairy products in the diet.

A diet based primarily on polished white rice can lead to thiamin (vitamin B1) deficiency because the essential nutrients are removed during the milling process.12

In cereal grains, B vitamins are concentrated in the outer layers, specifically the bran and the scutellum of the germ. When brown rice is processed into polished white rice, these nutrient-rich outer coverings are removed, significantly reducing the thiamin content of the final product.21

Thiamin is a critical coenzyme required for carbohydrate metabolism. When a person consumes a diet consisting almost exclusively of polished rice, they lack sufficient intake of this vital nutrient. This deficiency prevents the proper metabolism of carbohydrates, leading to the accumulation of pyruvic acid and lactic acid in the tissues. These metabolic byproducts are believed to cause the neurological and cardiac damage characteristic of beriberi, a condition marked by nerve impairment, muscle wasting, and heart failure.13

To prevent this, many regions now utilize enriched rice or encourage the consumption of parboiled rice, which helps retain more nutrients.145