In the adult, some cell types are extremely long-lived (e.g., neuronal cells), whereas others last only a few days and therefore need constant replacement (epithelial cells, e.g., intestine and skin). This synthesis requires a number of micronutrients, protein, and energy. Should any of these be in short supply, symptoms of malnutrition will be observed, especially in those cell types that have very short half-lives. Typical of niacin deficiency (pellagra), for example, are skin lesions. As epithelial cells die and must be replaced, niacin is needed for this replacement. All the components of the new cells including DNA must be synthesized. The purines and pyrimidines that comprise DNA must be synthesized and this requires energy as well as micronutrients (niacin, riboflavin, pyridoxine, folic acid, vitamin B12, copper, iron, sulfur, zinc, magnesium, and phosphorus). Anemia is another characteristic of malnutrition. Not only must new blood cells be made but also the essential ingredient of these cells, hemoglobin, must be synthesized. Among the nutrients needed for red blood cell synthesis are iron, copper, magnesium, folic acid, vitamin B12, vitamin B6, and, of course, energy and protein sufficient to support this synthesis.
The nutritional requirements for new cell synthesis are much greater in growing individuals than in adults because growth and cell division are much greater. Thus, energy and protein deficiency can be particularly detrimental. In addition, an adequate supply of specific micronutrients is crucial. For example, zinc deficiency was first described in teenage boys who were stunted and also sexually undeveloped. This report showed that zinc was required for both growth in general and the development of specific organ systems. Folic acid is required for DNA synthesis. Meeting this requirement is crucial during embryonic development. Inadequate folate intake by prepregnant and pregnant women can result in neural tube defects due to insufficient cell division during this time period. Not all women are so affected; there may be genetic differences in the need for folate that in turn determine whether the embryo is affected.
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