Among nutritional disorders, those of the nervous system occupy a position of special interest and importance. The early studies of beriberi at the turn of the twentieth century were largely responsible for the discovery of thiamine and consequently for the modern concept of deficiency disease. A series of notable achievements in the science of nutrition followed the discovery of vitamins. Despite such progress, a number of diseases caused by nutritional deficiency, and particularly those of the nervous system, continue to represent a worldwide health problem of serious proportions. In some communities, where the diet consists mainly of highly milled rice, there is still a significant incidence of beriberi. In some developing countries, deficiency diseases are endemic, the result of chronic dietary deprivation. And the ultimate effects on the nervous system of intermittent mass starvation, involving large portions of the African continent, are alarming to contemplate.
It comes as a surprise to many physicians that deficiency diseases still occur in the United States and other parts of the developed world. To some extent this is attributable to the prevalence of alcoholism. Relatively less common causes are dietary faddism and impaired absorption of dietary nutrients, which occurs in patients with celiac sprue, pernicious anemia, or surgical exclusion of portions of the gastrointestinal tract for treatment of obesity, and the wasting syndromes of cancer and AIDS. Finally, there are iatrogenic deficiencies induced by the use of vitamin antagonists or certain drugs, such as isonicotinic acid hydrazide (INH), which is used in the treatment of tuberculosis and interferes with the enzymatic function of pyridoxine or methotrexate.
The term deficiency is used throughout this chapter in its strictest sense to designate disorders that result from the lack of an essential nutrient or nutrients in the diet or from a conditioning factor that increases the need for these nutrients. The most important of these are the vitamins, more specifically, members of the B group—thiamine, nicotinic acid, pyridoxine, pantothenic acid, riboflavin, folic acid, and cobalamin (vitamin B12). However, some deficiency diseases cannot be related to the lack of a single vitamin. Usually the effects of several vitamin deficiencies can be recognized (thiamine deficiency causing Wernicke disease and subacute combined degeneration [SCD] of the cord, as a result of vitamin B12 deficiency, are notable exceptions that are due to single deficiencies).
Furthermore, nutritional diseases of the nervous system are not simply a matter of vitamin deprivation. The general signs of undernutrition, such as circulatory abnormalities and loss of subcutaneous fat and muscle bulk, are usually associated and conversely, a total lack of vitamins, as in starvation, is rarely associated with the classic deficiency syndromes of beriberi or pellagra. In other words, a certain amount of food is necessary to produce them. In a similar way, excessive intake of carbohydrates relative to the supply of thiamine favors the development of a thiamine-deficiency state. All deficiency ...