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Nutrition is the complex series of events by which living organisms consume and assimilate foods and other nutrients in order to live, grow, and maintain homeostasis. Proper nutrition involves the consumption of key macronutrients in balanced tandem with essential micronutrients. Consisting of carbohydrates, proteins, and lipids, macronutrients are those that are needed in large quantities by an organism to both fuel metabolic processes and provide the substrate for building and maintaining cellular structure. By contrast, micronutrients signify vitamins and minerals, which while necessary to good health, are required in relatively minute quantities. Because humans are unable to synthesize these molecules, clinical disease results when disturbances occur in that equilibrium—most commonly from nutrient deficiencies, but also from an unbalanced ratio of consumed nutrients, or less commonly from nutrient excesses.
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With an improved understanding of the role of diet in health and the advent of nutritional supplements, long-standing named scourges such as scurvy, beriberi, and pellagra have largely become diseases of historical interest. Even so, nutritional diseases remain problematic in developing countries and in settings of war, famine, and poverty. In industrialized countries, nutritional diseases may still arise among the disenfranchised: the homeless and those suffering from alcoholism or other forms of substance abuse. Individuals at risk also include those with derangements in their normal diets such as might be encountered with eating disorders or unusual dietary habits as well as with parenteral nutrition. Hypercatabolic states, exemplified by those with cancer, AIDS, hepatic or renal disease, and certain disease states such as carcinoid syndrome, may develop nutrient deficiencies even in the face of normal intake due to increased metabolic requirements. Excessive nutrient losses may occur as a result of decreased absorption arising from gastrointestinal diseases such as cystic fibrosis, inflammatory bowel disease, celiac disease, or following gastrointestinal surgery. Those receiving chronic medications such as anticonvulsants or antibiotics may experience impaired utilization of their nutrients when their medications interfere with gastrointestinal absorption and normal metabolism. Impaired utilization may derive from underlying genetic metabolic defects, enzyme deficiencies, hepatic disease, or drug–nutrient interactions. By contrast, syndromes of nutrient excess generally stem from dietary surplus or iatrogenic therapeutic intake.
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Because macronutrients and micronutrients are intrinsically involved in multiple biochemical pathways, disorders of nutrition may produce extracutaneous consequences. Moreover, those at risk for one nutrient deficiency may be suffering from other concomitant deficiencies as well. Appropriate evaluation of the patient with suspected nutritional disease should commence with a detailed history of dietary and medication ...