Thiamine is vital to every living thing, both plant and animal. It is an essential component of dozens of enzymes that metabolize food. In particular, thiamine is necessary to derive energy from glucose, the preferred food of nerve cells, and from other carbohydrates. It is more indirectly involved in the metabolism of the amino acids isoleucine, leucine, and valine.
An enzyme is a catalyst for chemical reactions. It consists of a protein and a coenzyme that attaches to the target substance, activates the chemical change, and then detaches to be ready again for the target substance. In its most important role, thiamine, as thiamine pyrophosphate, is the coenzyme of carboxylase. Among other things, it causes carboxyl (COOH) groups to be oxidized into carbon dioxide and water, releasing energy to body cells. Because it functions as part of a reusable enzyme catalysis system and is not an integral part of tissue structure itself, thiamine is needed in only small amounts of 1 to 3 micrograms a day.
Thiamine is a water-soluble vitamin that is found widely in foods. It is most concentrated in whole grains, yeast, and legumes; in liver, heart, and kidneys of most mammals; and in oysters. It is available also in most green vegetables and pork. An antagonistic enzyme produced by bacteria - thiaminase - is found in a few diverse foods such as raw fish and tea. The symptoms of beriberi are caused by a deficiency of thiamine, which may be slight or severe, temporary or long term. The deficiency usually results from the shortage of thiamine in a restricted and monotonous diet, but it can sometimes be exacerbated by the consumption of large amounts of foods high in thiaminase.
The epidemiology of beriberi follows from the role of thiamine in energy metabolism and its deficiency in restricted diets. The population at highest risk for beriberi have been (1) people engaged in heavy labor, such as farmers plowing their fields and soldiers and construction workers; (2) pregnant women; and subsequently, (3) their nursing infants. The populations in which beriberi has been most prevalent have been of two kinds: people confined to institutions, such as prisons, asylums, and naval ships, who are limited to monotonous and restricted diets such as bread and water or fish and rice; and people who derive a large portion of their calories from rice from which milling has removed most of the bran in which the thiamine is found.
Beriberi is in large part a disease of rice culture. When rice is the staple food, it is eaten in very large quantities and commonly provides 80 percent or more of the caloric energy. When the hull is removed with a mortar and pestle at home, enough bran remains on the rice to provide the necessary thiamine. When the rice is milled efficiently in modern plants, however, it is polished into white rice and thiamine is almost entirely eliminated.
Cooking methods are also important in the etiology of the disease. In northern China, Korea, and Japan, the rice hulls were traditionally removed before shipment in order to reduce bulk. When the rice reached the cities, it was so crawling with weevils that the subsequently highly-milled rice was covered in weevil juices and thus often treated with talc. Cooking procedures called for the rice to be thoroughly washed several times. The first washing alone removed half of the thiamine. In Burma and other parts of Southeast Asia, the custom has been to cook rice with excess water and throw away the water that is not absorbed, which contains most of the thiamine. Other peoples either cook the rice so as to absorb all the water, or use the extra water for drinking purposes or for other cooking. In the lower Ganges Valley in India, in Bihar and Bengal, the custom has been to parboil rice. Steaming the rice for partial cooking before drying it and then milling it for distribution preserves most of the thiamine in white rice, and is protective against beriberi. However, the labor or costs required in the process and the different taste and texture produced have not been widely acceptable among other peoples in Asia.
Numerous cultural beliefs and practices are involved in the regional etiology of beriberi. In northeastern Thailand and Laos, for example, people usually steam their glutinous variety of rice - a protective behavior. But, unlike other people in Thailand, they have very limited supplies of fresh fruit or vegetables or of meat for consumption. They eat fish, most of which is in the form of a fermented raw paste and is high in thiaminase, the destroyer of thiamine. Throughout the region, rice is so central to the cultures that it is synonymous with food itself. One result is that invalids, weanlings, and other susceptible people may eat nothing except rice. As milling spreads throughout the region, beriberi has resulted.
There is now indication that in eighteenth- and nineteenth-century Brazil, the disease was endemic among slaves and members of the working classes. The deficiency was usually the result of diets consisting mainly of manioc flour and a little dried meat. This flour actually contains less thiamine than does milled rice, and the preparation of the lean dried meat not only eliminated most of its natural thiamine but also increased the body's need for the vitamin (Kiple 1989).
A new population at risk has recently been recognized. In urbanized and industrial countries, beriberi occurs most frequently among alcoholics. At the same time that chronic alcohol consumption impairs the absorption of thiamine by the intestine and its storage and utilization in the liver, it increases the metabolic rate and requires relatively enormous amounts of thiamine for metabolism of the alcohol. When the alcoholic substitutes alcohol for other foods in his or her diet and curtails consumption of thiamine, Wernicke's encephalopathy and other neuropsychiatric disorders may occur, involving loss of motor coordination, loss of feeling in the hands and feet, and inability to remember or learn.
There are a few other specific groups of individuals at risk of beriberi. These include people suffering from renal failure and under long-term dialysis, and people under long-term intravenous feeding.
Because beriberi is a deficiency disease, it is entirely preventable by the consumption of adequate amounts of thiamine. In the United States, enrichment of white bread - replacing the thiamine that had been removed in the milling and bleaching of wheat — caused clinical disease virtually to disappear except among alcoholics. More recently, rice enrichment has also proved beneficial.
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