Tuesday, February 17, 2009

Vitamin B

The B Vitamins are a group of water-soluble substances which are not otherwise related to each other. There are eight of them, but claims for vitamin status are made for another 23. A vitamin is a substance that is essential for healthy living that cannot be synthesized by teh body. The other 23 eithercan be synthesized by teh human body or are not essential for health. I will deal with teh eight true vitamins and then explain why some of the others are not true vitamins.

Vitamin B1 or thiamine is the substance that is necessary in the diet to prevent beriberi. Chinese medical texts referred to beriberi as early as 2700 BC. Symptoms of beriberi include severe lethargy and fatigue, together with complications affecting the cardiovascular, nervous, muscular, and gastrointestinal systems. The origin of the word is from a Sinhalese phrase meaning "I cannot, I cannot", the word being doubled for emphasis. Two types of beriberi are recognized: the 'wet-type' affects the heart; through a combination of heart failure and weakening of the capillary walls, it causes the peripheral tissues to become edematous. Dry beriberi causes wasting and partial paralysis resulting from damaged peripheral nerves.

In 1884, Takaki Kanehiro, a British-trained Japanese doctor of the Japanese Navy, observed that beriberi was endemic among low-ranking crew who often ate nothing but rice, but not among crews of Western navies and officers who consumed a Western-style diet. Kanehiro initially believed that lack of protein was the chief cause of beriberi. He conducted one of the first cliical trials using the crews of two battleships; one crew was fed only white rice, while the other was fed a diet of meat, fish, barley, rice, and beans. The group that ate only white rice reported 161 crew with beriberi and 25 deaths, while the latter group had only 14 cases of beriberi and no deaths. This convinced Kanehiro and the Japanese Navy that diet was the cause of beriberi. But Kanehiro wrongly thought that protein was the missing element in the diet (Kwashiorkor, which is caused by protein deficiency also causes heart failure).

In 1897 Christian Eijkman, a military doctor in the Dutch Indies, discovered that chickens fed on a diet of cooked, polished rice developed paralysis, which could be reversed by discontinuing rice polishing (he mistakenly attributed that to a nerve poison in the endosperm of rice, from which the outer layers of the grain gave protection to the body. In 1897, Christiaan Eijkman in the Dutch East Indies, discovered that feeding unpolished rice instead of the polished variety to chickens prevented paralysis in the chickens. In 1898, Sir Frederick Hopkins postulated that some foods contained "accessory factors" — in addition to proteins, carbohydrates and fats, — that were necessary for the functions of the human body.

Eijkman was awarded a Nobel Price in Psysiology and Medicine in 1929, because his observations led to the discovery of vitamins). An associate, Gerrit Grijns, correctly interpreted the connection between excessive consumption of polished rice and beriberi in 1901: he concluded that rice contained an essential nutrient in the outer layers of the grain that was removed in polishing. In 1911 Casimir Funk isolated an antineuritic substance from rice bran that he called a “vitamine” (on account of its containing an amino group). Dutch chemists, Jansen and Donath, went on to isolate and crystallize the active agent in 1926, whose structure was determined by R.R.Williams, a US chemist, in 1934. Thiamin (“sulfur-containing vitamin”) was synthesized in 1936 by the same group. It was first named “aneurin” (for anti-neuritic vitamin).

Thiamine occurs naturally in unrefined cereals and fresh foods, particularly whole grain bread, fresh meat, legumes, green vegetables, fruit, and milk. Beriberi is therefore common in people whose diet excludes these particular types of nutrition.

Beriberi may be found in people whose diet consists mainly of polished white rice, which is very low in thiamine because the thiamine-bearing husk has been removed. It can also be seen in chronic alcoholics with an inadequate diet (Wernicke-Korsakoff syndrome), as well as being a rare side effect of gastric bypass surgery. If a baby is mainly fed on the milk of a mother who suffers from thiamine deficiency then that child may develop beriberi as well.

Wernicke’s encephalopathy (WE) is the type of thiaqmine deficiency most frequently seen in Western society. Although it may also occur in patients with impaired nutrition from other causes, it is usually seen in alcoholics. It is a striking neuro-psychiatric disorder characterized by paralysis of eye movements, abnormal stance and gait, and markedly deranged mental function.

Alcoholics may have thiamin deficiency because of the following: 1) inadequate nutritional intake: alcoholics tend to intake less than the recommended amount of thiamin. 2) decreased uptake of thiamin from the GI tract: active transport of thiamin into enterocytes is disturbed during acute alcohol exposure. 3) liver thiamin stores are reduced due to hepatic steatosis or fibrosis. 4) impaired thiamin utilization: magnesium, which is required for the binding of thiamin to thiamin-using enzymes within the cell, is also deficient due to chronic alcohol consumption. The inefficient utilization of any thiamin that does reach the cells will further exacerbate the thiamin deficiency. 5) Ethanol per se inhibits thiamin transport in the gastrointestinal system and blocks phosphorylation of thiamin to its cofactor form, TDP.

Korsakoff Psychosis is generally considered to occur with deterioration of brain function in patients initially diagnosed with WE. This is an amnestic-confabulatory syndrome characterized by retrograde and anterograde amnesia, impairment of conceptual functions, and decreased spontaneity and initiative. Because of their loss of memory they invent the most fantastic explanations for their circumstances.

However, most people on normal diets do not require vitamin B1 supplements.

1 comment:

Brian Koffman said...

Great post.

I well remember my first examination in medical history taking.

My test patient had Korsakoff's psychosis, but weaved such a beautiful story about why he was in hospital, he completely fooled me, a naive med student, until I was rechecking some dates with him and discovered the whole history was confabulated. In the last 5 minutes of the exam, I quickly did a mental status exam and I learned a unforgettable lesson about needing to make sure you and the patient are on the same page.