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In worldwide terms nutritional disorders are responsible for much morbidity and mortality. The three main categories of nutritional disorders are under nutrition (which is dominated by insufficient food energy), producing the features of starvation, malnutrition, which is deficiency, of one or more of the essential nutrients; and obesity, which is excessive positive energy balance. Disease is also possible as a result of nutrient excess

Nutritional issues, directly or indirectly, impinge upon many of the tests undertaken in clinical biochemistry. Many analytes are altered by nutritional status. For e.g. diet exerts important short-term effects on plasma (trigycerides, glucose) and longer term effects on plasma cholesterol. Certain inborn errors of metabolism may demand special diets, which are monitored biochemically (e.g. phenylalanine in phenylketonuria). Less obviously, diagnostic tests may only be valid if certain nutritional requirements are met. For e.g. faecal fat measurements are only valid if the patients is taking adequate fat intake(70 – 100g daily); measurement of 5-hydroxy in dole acetic acid requires exclusion of rich sources of serotonin from the diet; screening for hypercalciuria requires a high normal calcium intake, etc.

Laboratory measurements are also necessary in the management of patients receiving nutritional support, especially total parenteral nutrition, and in the assessment of malabsorption. Suspected nutritional deficiencies, ranging from possible iron deficiencies to vitamin or trace metal deficiencies, also require specialist laboratory tests.