The lack of any essential nutrient in the diet produces a state of deficiency called malnutrition, and chronic malnutrition leads to a characteristic deficiency disease. An example is kwa-shiorkor, the disease that results from protein deficiency. A shortage of any of the vitamins also results in specific deficiency symptoms (see Table 50.2). We have already described scurvy, which results from a lack of vitamin C. Another deficiency disease, beriberi, was directly involved in the discovery of vitamins. Beriberi means "extreme weakness." It became prevalent in Asia in the nineteenth century, after it became standard practice to mill rice to a high, white polish and discard the hulls that are present in brown rice. A critical observation was that birds—chickens and pigeons—de veloped beriberi-like symptoms when fed only polished rice. In 1912, Casimir Funk cured pigeons of beriberi by feeding them the discarded hulls.
At the time of Funk's discovery, all diseases were thought to be either caused by microorganisms or inherited. Funk suggested the radical idea that beriberi and some other diseases are dietary in origin and result from deficiencies in specific substances. Funk coined the term "vitamines" because he mistakenly thought that all these substances were amines (compounds with amino groups) vital for life. In 1926, thiamin (vitamin B1)—the substance lost in the rice milling process—was the first vitamin to be isolated in pure form.
Deficiency diseases can also result from an inability to absorb or process an essential nutrient even if it is present in the diet. Vitamin B12 (cobalamin), for example, is present in all foods of animal origin. Since plants neither use nor produce vitamin B12, a strictly vegetarian diet (not supplemented by vitamin pills) can lead to a B12 deficiency disease called pernicious anemia, characterized by a failure of red blood cells to mature. The most common cause of pernicious anemia, however, is not a lack of vitamin B12 in the diet, but an inability to absorb it. Normally, cells in the stomach lining secrete a peptide called intrinsic factor, which binds to vitamin B12 and makes it possible for it to be absorbed in the small intestine. Conditions that damage the stomach lining can therefore cause pernicious anemia.
Inadequate mineral nutrition can also lead to deficiency diseases. Iodine, for example, is a constituent of the hormone thyroxine, which is produced in the thyroid gland. If there is insufficient iodine in the diet, the thyroid gland grows larger in an attempt to compensate for inadequate production of thyroxine. The swelling of the neck that results is called a goiter. The introduction of iodized table salt has greatly reduced the incidence of goiter in the United States.
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